Plasticizing aluminum by deformable and phase-transformable nitinol alloys

Plasticizing aluminum by deformable and phase-transformable nitinol alloys | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Plasticizing aluminum by deformable and phase-transformable nitinol alloys Ying Hu, Wangshu Zheng, Shuangyue Jia, Xintu Liu, Xuyang Feng, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8547474/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 8 You are reading this latest preprint version Abstract Secondary rigid fillers reinforce the metals, yet usually at the cost of strain localization and reduced plasticity/toughness. Nitinol (NiTi) alloy, as a representative of shape memory materials, exhibits reversible phase transformation ability and considerable deformability upon loading. Here, we introduced age-treated NiTi particles into aluminum (Al) via powder metallurgy, where the sufficient matrix constraint maintains metastable R phase in the NiTi particles. The resultant composite with 30 weight ratio NiTi particles achieves the optimal ultimate tensile strength (UTS) and uniform elongation, 8% and 48% surpassing those of pure Al without NiTi filler. Such salient property gains are attributed to the stress-induced R-phase reorientation and particle deformation, thus effectively accommodating strain localization, alleviating stress concentration near the NiTi/Al interface, and activating micro-crack toughening effect. Our strategy can be readily extended to other matrices with deformable or phase-transformable fillers, offering broad potential for applications in energy dissipation, damping, and intelligent material systems. Composites Phase Transformation Mechanical Properties Nitinol Interface Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction The increasing demands from both scientific study and industrial upgrading have been calling for lightweight, strong-yet-tough composites, especially in use under specific service conditions ( e.g. , high temperatures, low temperatures, and vibrations) [ 1 ]. Incorporating secondary fillers into metals/alloys represents an effective approach towards high-performance composites with structural and functional integrations [ 2 ]. Traditional rigid yet brittle ceramics, such as TiB₂, SiC, TiC, WC, AlN, Al₂O₃, Al₃BC, and Si₃N₄, are prone to induce strain localization and stress concentration at the ceramic/metal interface, which significantly undermines the mechanical robustness of metal matrix composites (MMCs) [ 3 ]. Hitherto, several efforts have been made to tackle the issue, such as interface engineering [ 4 ] and heterostructured design [ 5 ]. Wherein the core ideas are to facilitate strain delocalization and stress partitioning to ensure sustainable plastic stability. Recently, metastability compositing has emerged as a feasible approach to regain the mechanical robustness [ 6 – 8 ]. In our previous work [ 9 – 12 ], we introduced phase-transformable zirconia-based ceramics into the aluminum (Al). This metastability-engineered composite exhibited twice the strength and energy dissipation of the standalone matrix, enabled by strain hardening and synergistic deformation from the constrained phase transformation. Likewise, we integrated deformable CrCoNi medium entropy alloys into the Al matrix, presenting enhanced strength-toughness synergy [ 13 , 14 ]. Nickel-titanium alloy (NiTi, nitinol), a representative of shape memory alloys (SMAs), combines high deformability with phase-transformation ability, making it a promising candidate for achieving structural–functional integration in composites. Specifically, it can undergo reversible martensitic phase transformation between the cubic (B2 phase, austenite), rhombohedral (R phase, martensite) and monoclinic (B19' phase, martensite) structures. In addition, its energy absorption ability and high damping coefficient (above 0.1) offer significant application potential in fields such as shock absorption [ 15 , 16 ] and damping system [ 17 , 18 ]. Of note, the B2→R phase transformation exhibits distinct characteristics of small shear strain (< 1%), low thermal hysteresis (< 5°C) and superior functional fatigue resistance than the direct B2→B19' transformation during thermal cycling [ 19 – 21 ]. Previous studies have made several attempts to investigate the effect of NiTi on metal matrices. These studies typically reported enhanced strength and damping capacity but concurrently observed reductions in plasticity, exemplified by NiTi/1060Al lamellar composites [ 22 ], and NiTi nanoparticle- reinforced Al composites [ 23 ]. Critically, stress-induced phase transformation and deformation of NiTi particles in the confined state has never been elucidated, greatly limiting the application of shock absorption, damping and self-healing. In this study, we dig into these unsolved issues using NiTi/Al composites as a model material, where the age-treated NiTi particles are stabilized into R phase under sufficient Al matrix constraint. The resultant composites with 30 wt.% NiTi achieve the optimal ultimate tensile strength and uniform elongation, 8% and 48% surpassing those of pure Al. The synergistic enhancement mechanism is elucidated by systematically characterizing microstructures, phase constituents and transformation characteristics of the composites. Our study also provides a design framework for composites with deformable and phase-transformable fillers and is anticipated to guide their implementation in energy dissipation, damping and intelligent material systems. 2. Experimental 2.1 Start materials and fabrication routes Near-spherical NiTi alloy powders (Ni 50 Ti 50 , in atomic ratio) with a diameter range of 0–20 µm were purchased from Foshan Chengfeng Material Technology Co., Ltd.. Al powders with an average size of 10 µm were purchased from Henan Yuanyang Powder Technology Co., Ltd. As-received NiTi powders were pre-aged at 550°C for 3 h in a muffle furnace (argon atmosphere, 99.99% purity) to suppress the generation of B19' phase and obtain the metastable R phase when they are cooled down to room temperature [ 24 – 26 ]. The resultant R phase is expected to enhance the plasticity and damping capacity of the composites. As-aged NiTi and Al powders were pre-mixed with NiTi weight fraction of 10, 20, 30%. 1 wt.% stearic acid was added as the process control agent. The pre-mixed composite powders were then milled at 200 rpm for 12 h and 400 rpm for 2 h (QM-DY2 planetary) with hardened stainless-steel vials and balls (ball-to-powder ratio of 10:1). Subsequently, as-milled NiTi/Al composite powders were hot-pressed at 450°C for half an hour, followed by hot extrusion at 350°C into rod-like samples (extrusion ration 9:1). Both ball milling and densification processes were conducted under the protection of high-purity argon. Table 1 provides the terminology of bulk NiTi/Al composites in this work. Pure Al without NiTi addition was also purposely prepared using the same fabrication process for comparison. Table 1 Terminology and definitions of NiTi/Al composites reported in this work. Sample NiTi aging time/h NiTi weight percent/% Al - 0 10NT 3 10 20NT 3 20 30NT 3 30 2.2 Characterization techniques The sample surfaces were first mechanically ground and polished, followed by ion milling (SEM Mill, Fischione Model 1060) to eliminate contaminants and stress layers. Phase constituents of NiTi powders and NiTi/Al composites were examined by X-ray diffraction (XRD, Mini Flex 600) with Cu K α radiation (λ = 0.15406 nm). XRD conducted at room temperature was scanned in a 2θ range of 25–65° at a scanning rate of 2°/min and 41–44° at a scanning rate of 0.5°/min. The characteristic peaks (112) and (300) of R phase at 41–44° were fitted using Pseudo-Voigt Function Type (a linear combination of Gaussian and Lorentzian functions) in OriginPro Lab software (version 2025b). Transformation temperatures of NiTi powders and NiTi/Al composites were determined by differential scanning calorimetry (DSC, Perkin Elmer DSC800 calorimeter) performed between − 90°C and 100°C at a cooling/heating rate of 5°C/min. Internal frictions (tan δ) were measured by dynamic mechanical analysis equipment (DMA, TA Q800), a fixed strain of 0.01% and frequency of 1 Hz. The samples, with dimensions of 30 mm × 4 mm × 1 mm, were tested over a temperature range of − 100°C to 100°C, with cooling and heating rates of 5°C/min. Heat treatment under flowing argon gas conditions was particularly used to eliminate hydrogen in the NiTi specimen to measure the intrinsic internal friction value without the influence of hydrogen-related relaxation behavior. Scanning electron microscope (SEM, FEI Scios) attached with energy-dispersive X-ray spectrometer (EDS, Oxford X-Max80T) was utilized to characterize microstructures and elemental distributions. Uniaxial tensile tests were carried out on an INSTRON 3340 testing machine at a strain rate of 5 × 10 − 4 s − 1 and room temperature, along the extrusion direction. The specimens had a gauge length of 10 mm, width of 2 mm and thickness of 1 mm. At least three tensile tests were performed for each sample set. The strain hardening rate ( \(\:\theta\:\) ) was determined from \(\:\theta\:\:=\:\partial\:\sigma\:/\partial\:\epsilon\:\) , where \(\:\sigma\:\) and \(\:\epsilon\:\) represent the true stress and true strain obtained from the tensile data. To obtain smoother results, the true stress–strain curves were fitted using polynomial regression. Digital Image Correlation (DIC, Instron 5966, Bluehill) with an infrared camera was used to capture the in situ strain distribution during loading. X-ray computed tomography (XCT, Xradia 620 Versa) was performed at 60 kV and 6.5 W with a 4× objective lens to quantitatively characterize the microcracks and NiTi particle deformation via absorption contrast tomography. Three-dimensional visualization and analysis were conducted using Dragonfly software. 3. Results 3.1 Phase constituents and transformation temperatures of NiTi powders Figure 1 a shows the XRD patterns of the as-received and as-aged NiTi powders. As-received NiTi is composed entirely of the monoclinic B19' phase. After aging treatment for 3 h, the B19' phase mostly transforms into the cubic B2 phase, with minor quantities of B19' left. DSC curves in Fig. 1 b display that as-received NiTi present one-step B19'↔B2 phase transformation over heating and cooling cycles, with a single characteristic peak each way. In stark contrast, as-aged NiTi exhibits a multi-step phase transformation characteristic (B19'→R→B2 during heating and B2→R→B19' during cooling) and a wider temperature range (~ 75°C) for transformation compared to that of the as-received NiTi (~ 30°C). This can be well explained by the fact that aging treatment suppresses the large lattice shear (~ 10%) in B2→B19' transformation [ 27 ], thus preferring B2→R transformation with a relatively small lattice shear (~ 1%, [ 28 ]) before the eventual formation of B19' phase. Echoed with XRD observation, a small amount of B19' phase in the as-aged NiTi indicates that the B2→B19' transformation is not completely suppressed and just initiated when cooling down to the room temperature (~ 25°C). 3.2 Microstructures of NiTi/Al composites Figure 2 a-c depicts the typical SEM images of the NiTi/Al composites. The regions with grey contrast represent the Al matrix and the white contrast for NiTi particles. In all three sets, NiTi particles appear as randomly distributed in the two-dimensional cross-section, without noticeable particle agglomeration or large pore defects. Specifically, more elongated NiTi particles are identified in composites with relatively low NiTi content, such as 10NT (Fig. S1 b, average aspect ratio: 2.2 ± 1.0). With higher NiTi content, as manifested in 30NT, NiTi particles tend to maintain a more near-sphere shape (Fig. S1 d, average aspect ratio: 1.6 ± 0.6), resembling the as-aged NiTi particles (Fig. S1 a, average aspect ratio: 1.4 ± 0.6). This discrepancy could probably arise from the stronger resistance to shear and deformation during ball milling and hot extrusion with higher NiTi content. Figure 2 d shows the representative EDS elemental mapping images from 20NT. Apparently, the interfaces between the Al matrix and NiTi particles are clear and free from cracks, pores, or other visible defects. In other words, interfacial elemental diffusion or intermetallic compound formation is well controlled for all three sets. 3.3 Phase constituents of NiTi/Al composites Figure 3 a presents the XRD patterns of 10NT, 20NT and 30NT. In addition to the Al phase, no B19' phase (martensite) was detected in these three sets. Such observation on austenite stabilization by matrix constraint is indeed in line with our previous studies [ 29 ]. Figure 3 b highlights the XRD patterns at the scanning range of 41–44°, where the R phase can be identified and fitted by the characteristic \(\:{\left(112\right)}_{R}\) and \(\:{\left(300\right)}_{R}\) diffraction peaks [ 20 ]. As shown, increasing NiTi content from 10 to 20 to 30 wt.% leads to a decrease in the area ratio of \(\:{\left(300\right)}_{R}\) peak from approximately 73% to 56% to 35%. Of particular note, the two characteristic peaks corresponding to the R phase in 10NT exhibit the broadest peak widths among all. These findings suggest a strong correlation between the NiTi content and the resulting phase constituents, likely stemming from the higher deformation degree (Fig. 2 a-c and Fig. S1 ) observed at lower NiTi contents ( e.g. , 10NT). Consistent with the EDS results (Fig. 2 d), no (or less than 1%, if any) intermetallic phases (such as Al 3 Ti, Ti 3 Al and TiAl) are present in the XRD spectrums, indicating the absence of interfacial reaction byproducts formed during the fabrication process. 3.4 Transformation characteristics of NiTi/Al composites Figure 4 a illustrates the DSC curves of 10NT, 20NT, and 30NT. Different from the as-aged NiTi powders with a multistep transformation (B2→R→B19' during cooling and B19'→R→B2 during heating), all three NiTi/Al composites show a single-step transformation (B2→R during cooling and R→B2 during heating). The peak intensity (heat flow) increases with increasing NiTi content, indicating a stronger transformation response at higher NiTi contents. Figure 4 b compares the martensite start and finish temperatures (M s and M f ) of the NiTi/Al composites (under constraint) with those of the as-aged NiTi powders (at monolithic), determined from Fig. 4 a and Fig. 1 b using the tangent method. In particular, the transformation temperatures of the NiTi/Al composites are evidently higher than those of the as-aged NiTi powders at their monolithic state. The M f values of the composites (~ 21–27°C) are close to room temperature, implying that R phases can exist at ambient conditions—consistent with the microstructural observations (Fig. 3 b). This elevation in transformation temperatures under constraint can be attributed partly to the stress fields or local stresses generated during fabrication and by the matrix constraint, which stabilize the parent austenite phase and hinder the formation of martensitic variants. Within the NiTi/Al composites, increasing the NiTi content from 10 wt.% to 30 wt.% results in only a slight, monotonic increase in transformation temperatures (M s : 43.2→46.6°C; M f : 21.4→27.3°C), confirming that the NiTi content has a negligible influence on the thermally induced phase transformation. 3.5 Mechanical properties Figure 5 a shows the representative engineering stress–strain curves of NiTi/Al composites and pure Al. Detailed mechanical property data are listed in the Table 2 . With increasing NiTi particle content, the composites show a consistent enhancement in ultimate tensile strength (UTS) accompanied by a reduction in total elongation. Interestingly, the uniform elongation (UE) displays the opposite trend (Fig. 5 b): it increases progressively with NiTi addition, reaching a maximum of 4.0 ± 0.2% in 30NT, approximately 48% higher than that of pure Al (2.7 ± 0.2%). Within the 0–30 wt.% reinforcement range, the 30NT composite achieves the optimal combination of UTS (214 ± 4 MPa) and UE (4.0 ± 0.2%). Further increasing the NiTi content to 40 wt.%, however, results in pronounced particle agglomeration, which undermines plastic stability (Fig. S2). To further elucidate the role of aging, un-aged NiTi/Al composites (20 wt.%) were fabricated for comparison (Fig. S3). Compared to 3 h-aged 20NT, NiTi/Al absent in aging treatment enhances the UTS to 239 ± 12 MPa, albeit at the considerable expense of UE (2.3 ± 0.1%) and total elongation (6.0 ± 1.7%). Figure 5 c compares the relative gains in UTS and UE of NiTi/Al composites in this work with those reported in previous studies [ 22 , 23 , 31 – 35 ], using pure Al as the benchmark. A distinctive feature of this work is the remarkable enhancement in UE achieved without compromising strength. The maximum UE gain reaches up to 50%, markedly exceeding that of previously reported NiTi/Al systems. Of note, earlier studies primarily focused on thermally induced phase transformations, with limited attention to stress-induced phase transformations or fully R-phase-based NiTi/Al composites. The exceptional plasticity improvement observed here is likely to associate with the unique deformation characteristics of the R phase, and will be discussed in section 4.2 . Table 2 Mechanical properties of NiTi/Al composites. Sample Yield Strength (MPa) Ultimate Tensile Strength (MPa) Uniform Elongation (%) Total Elongation (%) Al 163 ± 7 199 ± 7 2.7 ± 0.2 20.5 ± 3.8 10NT 166 ± 9 199 ± 10 2.4 ± 0.0 17.5 ± 3.4 20NT 169 ± 11 207 ± 2 3.8 ± 0.3 16.1 ± 3.1 30NT 178 ± 3 214 ± 4 4.0 ± 0.2 13.4 ± 0.1 4. Discussion 4.1 Effect of aging treatment and matrix constraint on the stability of R phase Considering that the presence of R phase and its orientation preference are the main differences among the NiTi/Al composites (Fig. 3 b), their underlying mechanisms of formation and deformation are crucial to the mechanical properties. As shown in Fig. 1 , we first confirmed that the aging treatment promoted the formation of R phase (small lattice shear ~ 1%) in freestanding NiTi alloy particles, consistent with previous studies [ 25 , 26 , 36 ]. There is a common acceptance that this ageing-induced B2→R transformation behaviour is affected by the stress field of precipitates and dislocations, which act as a much stronger obstacle to the B2→B19' transformation than that to the B2→R transformation because the former has a much larger transformation strain. Such stabilization effect is also analogous to the matrix constraint effect from the viewpoint of thermally-induced phase transformation: only one endothermic/exothermic peak (B2→R) is found in the DSC results (Fig. 4 a), which differs from the freestanding NiTi of two-step phase transformation (B2→R→B19') with relatively large lattice strain (> 10%; Fig. 1 b). When cooling down to the room temperature (~ 20°C, lower than martensite finish temperatures in all samples, Fig. 4 b), the as-fabricated NiTi/Al composites underwent full B2→R phase transformation and therefore fell into the full R phase structure, as also evidenced by the room-temperature XRD patterns (Fig. 1 and Fig. 3 ). This confined R phase presents ultrahigh damping coefficient for its abundant twin boundaries and phase-transformation ability as an intermediate martensite [ 37 , 38 ]. Compared to the freestanding NiTi alloy with widely reported reversible phase transformation under temperature fields [ 26 ], thermally-induced R (rhombohedral)→B19' (monoclinic) phase transformation is absent in the NiTi/Al composites either with or without aging treatment (Fig. 4 b and Fig. S3b). A possible explanation might be the matrix constraint stress near the NiTi/Al interface caused by the compounding process, which is reported to lower the Gibbs free energy of high-temperature or highly symmetric phase [ 10 ], and thus suppress large-scale lattice shear [ 38 ]. Herein, the synergistic effect of the two factors (aging treatment and matrix constraint) stabilized the NiTi in fully R-phase state, which was further proven to regain the plasticity of the NiTi/Al composites (Fig. S3). 4.2 Stress partitioning and stress-induced R-phase reorientation In light of the key role of R phase in the present NiTi/Al system, we applied the rule-of-mixture (ROM) to derive the average principal stress of NiTi particles ( \(\:{\sigma\:}_{NiTi}\) ) within Al matrix at every principal strain, $$\:{\sigma\:}_{c}=(1-{V}_{f})\bullet\:{\sigma\:}_{Al}+{V}_{f}\bullet\:{\sigma\:}_{NiTi}$$ where \(\:{V}_{f}\) is the volume fraction of NiTi particles, \(\:{\sigma\:}_{c}\) and \(\:{\sigma\:}_{Al}\) and are the engineering stress of NiTi/Al composites and pure Al measured in Fig. 5 a. As shown in Fig. 6 a, the average principal stresses of NiTi particles derived from 20NT and 30NT first gradually increase with the strain, and afterwards decline once reaching the peak stress. In the composite with relatively low NiTi content (10 wt.%), the calculated stress onto NiTi particles constantly decreases with the strain. As previously noted in Fig. 2 and Fig. S1 , the NiTi particles in the as-fabricated 10NT already experienced substantial deformation during fabrication, and therefore its deformation was naturally restricted upon loading. This pre-existed deformation state provides a mechanistic explanation for the decreased load transferring efficiency throughout the tensile process. Conversely, the NiTi particles in the as-fabricated 20NT and 30NT with relatively higher NiTi contents endured lower pre-existed deformation; consequently, their synergistic deformation with Al matrix well contributed to the stress partitioning, as manifested by the promoted load transferring efficiency with strain upon tension (Fig. 6 a). Prominently, 30NT with the highest stress partitioning in NiTi presents the best combination of uniform elongation and strength among all composite sample sets (Fig. 5 ). These traits stand in stark contrast with the freestanding R-phase NiTi alloy wires frequently reported in the literatures [ 39 ] (Fig. 6 b). With increasing stress, NiTi alloy typically undergoes R-phase reorientation (section a-b, < 100 MPa) and martensitic transformation from the R phase to the B19' phase (section b-c, typically ~ 200–400 MPa). Apparently, the average principal stresses of NiTi by stress partitioning in this work can clearly reach the critical stress of the reorientation; however, the critical stress of the R→B19' martensitic transformation in NiTi alloys varies widely (typically from several tens to a few hundred MPa) depending on the alloy composition, heat treatment strategy, microstructure, and testing temperature [ 19 , 40 ]. It is noteworthy that the actual stress partitioning between Al and NiTi may not strictly follow the ROM model as outlined above; moreover, the actual principal stress of each NiTi particles may be lower than the calculation results, depending on the nuances of the matrix constraint, interfacial bonding and particle size distribution. Nonetheless, the direct application of ROM here indicates the feasibility of R-phase reorientation and the explanation for the absence of subsequent R-B19' phase transformation, from the perspective of stress-induced transformation. To further substantiate the occurrence of the R-phase reorientation, the phase composition before and after the tensile test were analyzed using fine-scanning XRD (Fig. 6 c) combined with muti-peak fitting (Fig. S4). The most pronounced differences appear in the R phase (112) and (300) diffraction peaks. As summarized in Fig. 6 d, the area fraction of \(\:{\left(300\right)}_{R}\) peak increases notably after tension, while the \(\:{\left(112\right)}_{R}\) peak correspondingly weakens. Quantitatively, the \(\:{\left(300\right)}_{R}\) area fraction rises from 73% to 81% in 10NT, from 56% to 64% in 20NT, and from 35% to 45% in 30NT, strong indicators of stress-induced R-phase reorientation. We note that the overall fraction of reoriented R phase is relatively low (~ 8–10%), likely due to the spatial averaging inherent to large-area XRD measurements. It is reasonable to infer that the local stress and strain concentration in the necking micro-region promote a much higher degree of R-phase reorientation—consistent with the transformation-induced heterogeneity widely observed in transformation-/twinning-induced plasticity (TRIP/TWIP) steels [ 41 ], high-entropy alloys (HEAs) [ 42 ], and transformation-toughened ceramic [ 43 ]. We also emphasize that, although stress-induced R-phase reorientation has been previously observed primarily in monolithic NiTi [ 44 – 46 ], the present work is the first to explore this phenomenon on NiTi under constraint. Such R-phase reorientation, essentially a detwinning process caused by the shear of the lattice under stress [ 47 ], renders three distinct stages in NiTi/Al composites (Fig. 6 a). At stage Ⅰ (engineering strain 0–4%): elastic deformation, yielding and work hardening. In this stage, the partitioned stress onto NiTi particles elevated with increasing strain and reached the critical stress of the R-phase reorientation (Fig. 6 b). The composite subsequently underwent sustained plastic deformation at stage Ⅱ (strain 4–12%), followed with necking at stage Ⅲ (strain 12% to fracture) as the partitioned stress decreased. Of particular note, no B19' phase was observed in this work (Fig. 6 c), even though the partitioned stress might reach the critical value to trigger the R→B19' phase transformation (Fig. 6 b). This restricted transformation behavior under stress resembles that under temperature as discussed in Section 4.1 , which may be attributable to the formation of the inhomogeneity of the internal stress field [ 48 – 50 ]. 4.3 Transformation-induced plasticity effects To elucidate the origin of the robust plastic deformation, the strain-hardening rates were calculated from the engineering stress–strain curves presented in Fig. 5 a. As demonstrated in Fig. 7 a, both 20NT and 30NT exhibited higher strain-hardening rates than pure Al within the true strain range of 1–4% after yielding. Whilst 10NT with insufficient deformation ability exerted a detrimental effect on the strain-hardening behavior ( e.g. , 10NT), consistent with the prediction from the ROM analysis (Fig. 6 a). Such distinct plasticity gains in 20NT and 30NT over 10NT are also in concert with the strain delocalization and necking delay along the gauge length, as observed from the DIC results (Fig. 7 b). We further conducted quasi- in situ tensile test via XCT to visualize the three-dimensional particle deformation in 30NT. As displayed in Fig. 7 c, the typical NiTi particle in stage Ⅰ experienced almost unchanged local strain ( i.e. , the deformation experienced at the specific point within the composite, can be obtained by calculating the displacement of the image recognition points along the loading direction) whilst its aspect ratio raised from 1.52 to 1.67, indicating the significant deformation in stage Ⅰ. These structural evolutions correlate with the prominent R-phase reorientation at stage I (Fig. 6 a). As the engineering strain increased from 4% to 12%, the stress levels varied only moderately and the NiTi particles deformed little (aspect ratio almost unchanged). At the final stage III, the severe strain localization reached its maximum and thus induced pronounced deformation of the NiTi particles (aspect ratio: 1.66→1.74). In this regard, the increasing aspect ratio by approximately 14% and ~ 30% principal strain during tension demonstrates the excellent deformability of NiTi under constraint. Such deformation is therefore expected to enable effective synergistic deformation between Al and NiTi and strain delocalization at the NiTi/Al interfaces, and redistribute local stresses via lattice shearing (R-phase reorientation). These mechanisms also explain the enhanced stress partitioning onto NiTi in 30NT (Fig. 6 a) and improved plasticity and strain hardening of NiTi/Al composites by aging treatment (Fig. S3a) and increasing NiTi content (Fig. 5 b). 4.4 Transformation-induced toughening effects The top-view fractured surfaces further verified the tight NiTi/Al interfacial bonding and load transferring efficiency within the composites (Fig. 8 a). Specifically, along the tension direction, two-dimensional morphologies of the 10NT sample were examined at various distances from the fracture surface (X = 0, 500, 1000, 1500, 2000 µm). At all measured positions, small NiTi fragments were consistently observed in regions near the NiTi/Al interfaces, akin to that shown in Fig. 7 c. Importantly, no such breakage fragments were present in the as-fabricated sample, confirming that the breakage of these fine NiTi particles arose solely from tensile loading rather than from fabrication. Moreover, the number of tiny fragments decreased progressively with increasing distance from the fracture surface (corresponding to regions of lower principal strain, Fig. 7 b). This trend further supports the notion that tensile stress not only deformed the NiTi particles but also fractured them into fine debris. Intriguingly, no obvious NiTi/Al interface debonding was observed, which can be explained by the good chemical infinity between NiTi and Al [ 51 ], as well as small transformation-induced lattice strain (typically less than 1%) [ 20 ]. These findings also agree with the XCT findings (Fig. 8 b) that the mean volume of NiTi decreased from 556.0 µm 3 to 529.4 µm 3 upon tension. As discussed in Section 4.2 , the excellent deformability of NiTi and stress-induced R-phase reorientation are expected to significantly suppress the crack initiation and propagation, thereby contributing to the observed improvement in toughness. To validate this, we characterized three-dimensional microcrack distribution in post-mortem 10NT and 30NT tensile specimens (Fig. 8 c-d) at locations ~ 200 µm from the fracture surface. The crack volume fraction ρ v and crack number density ρ n in 30NT (ρ v : 0.45%, ρ n : 13 × 10 17 m 3 ) were nearly an order of magnitude greater than those in 10NT (ρ v : 0.04%, ρ n : 1.0 × 10 17 m 3 ). In this context, the aging treatment and matrix constraint confined NiTi in the metastable R phase, imparting it with distinct cycling stability and deformability. These features promoted R-phase reorientation, particle deformation/breakage and inhibition of microcracks, collectively toughening the composite. 4.5 Implications and outlooks Beyond the aforementioned benefits from mechanical properties, the combination of lightweight Al matrices with the unique functional characteristics of NiTi offer other engineering and intelligent potentials: (1) Damping capacity . As shown in Fig. 9 a, the damping capacity of 20NT (0.051) in this work successfully achieves the enhancement of 48% over the pure Al (0.035), likely attributed to the rich twin boundaries of R phase, numerous NiTi/Al interfaces, and the R→B2 phase transformation. Such strength-damping synergy also surpasses other existing NiTi/Al composites (Fig. 9 b). (2) Impact resistance : Previous studies have showed the substantial improvement on the impact resistance of freestanding NiTi [ 52 – 54 ]. The exceptional energy-dissipation capacity of NiTi, derived from its stress-induced phase transformation and reversible lattice shear, offers a compelling pathway to enhance impact resistance. Under high-rate loading, NiTi particles can undergo rapid martensitic transformation, thereby absorbing impact energy, mitigating stress concentrations, and delaying crack initiation. This will be reported as a separate work in the future. (3) Training by cyclic loading : The intrinsic fatigue resistance of R phase enables the composites to progressively stabilize mechanical response under cyclic loading. Through repeated transformation cycles, the interfacial compatibility and deformation coordination between NiTi particles and the Al matrix can be further optimized, improving fatigue resistance and maintaining load transferring efficiency over long service periods. (4) Self-healing . As a typical type of intelligent materials, NiTi alloys exhibit reversible martensitic transformation with shape-memory/superelastic effects, enabling them to autonomously adapt to external stimuli. Moreover, the shape-memory effect offers a unique pathway for self-healing: upon thermal activation, NiTi can partially recover their pre-deformed geometry, thereby repair microcracks, and recover load transferring efficiency in the surrounding Al matrix, as unveiled in Fig. 7 – 8 . Such tunable transformation characteristics highlight its potential for developing structurally embedded sensing, damage mitigation, and smart actuation functions. These attributes position NiTi/Al composites as a compelling platform for self-adaptive and self-healing systems. 5. Conclusions In summary, we successfully incorporated deformable and phase-transformable nitinol (NiTi) particles into aluminum (Al) matrix. The constrained phase transformation and deformation behavior of NiTi under loading were elucidated, and the mechanisms that couple these structural evolutions to enhanced mechanical performance were revealed. The most salient findings are summarized below: The age-treated NiTi particles were effectively embedded into the Al matrix via powder metallurgy, where sufficient matrix constraint lowered the transformation temperature of NiTi, thus stabilizing the metastable R phase and preserving its reorientation capability. The resultant composite with 30 wt.% NiTi achieves the optimal ultimate tensile strength and uniform elongation, 8% and 48% surpassing those of pure Al without NiTi filler. The enhanced plasticity was attributable to the stress-induced R-phase reorientation and substantial particle deformability within the NiTi/Al composites, which effectively accommodated localized strain, alleviated stress concentration near the NiTi/Al interface and activated micro-crack toughening effect. The results highlight the advantages of using deformable or phase-transformable fillers as an alternative to traditional brittle reinforcements, offering a new pathway toward high-performance metal matrix composites with exceptional plasticity, impact toughness and damping capacity. Moreover, the phase-transformable nature of NiTi unleashes the opportunities for developing intelligent composites with adaptive response, cyclic loading stability, and self-healing functionalities. Declarations Competing interests Authors declare that they have no competing interests. Data and materials availability All data are available in the main text or the supplementary materials. Funding This work is financially supported by the National Natural Science Foundation of China (Nos. 52192595, 523B2005 and 52571174). Author Contribution Y. H., W. Z., and L. Z. conceptualized the study.Y. H. conducted methodology design, investigation, validation, and wrote the original manuscript draft.W. Z., L. Z., and Q. G. acquired funding, supervised the project, and contributed to manuscript review and editing.S. J., X. L., X. F. and S. M. performed investigation.Q. G. provided resources and additional validation.All authors reviewed the manuscript. Acknowledgement The authors would like to thank Dr. Zhongxin Zhao and Dr. Linghai Li from Shanghai Jiao Tong University (SJTU) for their assistance in fabrication and XCT experiments. 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Mater Today Commun 25:101306. https://doi.org/10.1016/j.mtcomm.2020.101306 Saletti D, Pattofatto S, Zhao H (2013) Measurement of phase transformation properties under moderate impact tensile loading in a NiTi alloy. Mech Mater 65:1–11. https://doi.org/10.1016/j.mechmat.2013.05.017 Additional Declarations No competing interests reported. Supplementary Files SM.docx Cite Share Download PDF Status: Under Revision Version 1 posted Reviewers agreed at journal 04 Mar, 2026 Reviewers agreed at journal 04 Mar, 2026 Reviewers agreed at journal 04 Mar, 2026 Reviewers agreed at journal 02 Mar, 2026 Reviewers invited by journal 02 Mar, 2026 Editor assigned by journal 02 Mar, 2026 Submission checks completed at journal 08 Jan, 2026 First submitted to journal 08 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8547474","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":600759261,"identity":"45aac5db-f5c0-431c-9924-e1a6b91e2b49","order_by":0,"name":"Ying Hu","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Ying","middleName":"","lastName":"Hu","suffix":""},{"id":600759271,"identity":"a1c1614f-441c-489b-97af-213800ac9946","order_by":1,"name":"Wangshu Zheng","email":"","orcid":"","institution":"Nanyang Technological University","correspondingAuthor":false,"prefix":"","firstName":"Wangshu","middleName":"","lastName":"Zheng","suffix":""},{"id":600759272,"identity":"49dee622-ef43-48a4-a100-3e193266f1dc","order_by":2,"name":"Shuangyue Jia","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Shuangyue","middleName":"","lastName":"Jia","suffix":""},{"id":600759273,"identity":"8a2e8353-1b53-4005-b9be-d8dddfe9c69a","order_by":3,"name":"Xintu Liu","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Xintu","middleName":"","lastName":"Liu","suffix":""},{"id":600759279,"identity":"7eef6d2d-f0c8-4652-b753-46b7e9f321ce","order_by":4,"name":"Xuyang Feng","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Xuyang","middleName":"","lastName":"Feng","suffix":""},{"id":600759285,"identity":"3c67ff4c-bb9f-4549-84f9-212453009fb0","order_by":5,"name":"Shangnan Mo","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Shangnan","middleName":"","lastName":"Mo","suffix":""},{"id":600759287,"identity":"bbb7ddb9-eb50-45c3-958b-5e020b612fad","order_by":6,"name":"Lei Zhao","email":"","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":false,"prefix":"","firstName":"Lei","middleName":"","lastName":"Zhao","suffix":""},{"id":600759288,"identity":"78f860ec-42c8-4112-ab26-359af43564d7","order_by":7,"name":"Qiang Guo","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAuUlEQVRIiWNgGAWjYLCCDwxsYFqCaB2MMxjYJEjTwswDVU2cFvmI5IO3bf7w1RkcYD54m4fBLo+gFsMbacnWuW1sEgYH2JKteRiSiwlrmZFjJp3bANLCYybNw3AgsYEoLRZ/QFr4vxGnRV4CqIWBDWwLG3FaDHieJVv2trFJzjzMZmw5xyCZCFvakw/e+PHnGD/f8eaHN95U2BFhywFwdBwDxg6YS0g9yJYGsJYaIpSOglEwCkbBiAUAmHA0NaHa2iwAAAAASUVORK5CYII=","orcid":"","institution":"Shanghai Jiao Tong University","correspondingAuthor":true,"prefix":"","firstName":"Qiang","middleName":"","lastName":"Guo","suffix":""}],"badges":[],"createdAt":"2026-01-08 06:09:05","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8547474/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8547474/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":103972708,"identity":"e8f76018-5bc6-4635-8f18-cb4f44a9190b","added_by":"auto","created_at":"2026-03-05 07:57:41","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":356675,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhase constituents and transformation temperatures of NiTi powders\u003c/strong\u003e. (a) XRD patterns and (b) DSC curves for as-received NiTi and as-aged NiTi powders. Endo. – Endothermic.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/7cf44abd4209b4af266dd63a.png"},{"id":103972661,"identity":"224c4f8d-f49e-4315-82ad-d6644c8fb878","added_by":"auto","created_at":"2026-03-05 07:57:29","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":828713,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMicrostructures of NiTi/Al composites\u003c/strong\u003e. (a-c) Typical low-magnified SEM images of (a) 10NT, (b) 20NT, and (c) 30NT. The typical magnified view of the three composite sample sets are shown in (a\u003csub\u003e1\u003c/sub\u003e), (b\u003csub\u003e1\u003c/sub\u003e) and (c\u003csub\u003e1\u003c/sub\u003e), respectively. The statistics of the particles’ aspect ratio are manifested in Fig. S1. (d) Typical EDS mapping of Al, Ni and Ti element from 20NT.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/80de3b7bc1de3aa0c0e79320.png"},{"id":103972711,"identity":"4bcdc4eb-b311-4672-bee7-711a45b77e44","added_by":"auto","created_at":"2026-03-05 07:57:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":441940,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePhase constituents of NiTi/Al composites.\u003c/strong\u003e (a) XRD patterns for 10NT, 20NT and 30NT. (b) Fine scanned XRD patterns at the scanning range of 41-44° boxed in (a), with peak fitting of (112) and (300) R phases. R ̅\u003csup\u003e2\u003c/sup\u003e and χ\u003csup\u003e2\u003c/sup\u003e \u0026nbsp;stand for Adjusted Coefficient of Determination and Chi-squared Statistic.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/d0732796a19c4d598bb72646.png"},{"id":103972646,"identity":"8f8125ce-5e63-4b19-8526-b6e571b3b7e7","added_by":"auto","created_at":"2026-03-05 07:57:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":368809,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTransformation characteristics of NiTi/Al composites (NiTi under constraint)\u003c/strong\u003e. (a) DSC curves of the 10NT, 20NT and 30NT. (b) Comparison on the transformation temperatures of NiTi/Al composites with varying NiTi contents. The dashed lines denote the transformation temperatures of the as-aged NiTi powders. Note: Room temperature – 25°C. M\u003csub\u003es\u003c/sub\u003e and M\u003csub\u003ef\u003c/sub\u003e are the martensite start/finish temperatures, respectively. Under constraint – NiTi in the composites; At monolithic – as-aged freestanding NiTi powders. The M\u003csub\u003ef\u003c/sub\u003e value of the as-aged NiTi powders here is obtained from the first stage B2→R transformation.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/ec1eda75da2794c9c627edd4.png"},{"id":103972725,"identity":"2c40dc32-2974-4be2-82af-f51cef4934c0","added_by":"auto","created_at":"2026-03-05 07:57:45","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":435735,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMechanical properties of NiTi/Al composites\u003c/strong\u003e. (a) Typical Engineering stress–strain curves of the Al (gray), 10NT (green), 20NT (purple) and 30NT (red). (b) Enlarged region boxed in (a). Uniform elongation values are labeled on each curve. (c) Comparison on the uniform elongation (UE) gain and ultimate tensile strength (UTS) gain of the NiTi/Al composites in this work and in previous studies [22, 23, 31–35]. Pure Al is set as the benchmark.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/2c8f14f51834d68e3ade3825.png"},{"id":103972703,"identity":"2aa3ca58-5aab-4bd2-b661-e435efbd7e96","added_by":"auto","created_at":"2026-03-05 07:57:37","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":556422,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eStress\u003c/strong\u003e \u003cstrong\u003epartitioning and stress-induced R-phase reorientation.\u003c/strong\u003e (a) Engineering stress–strain curves of 10NT (green), 20NT (purple) and 30NT (red), and NiTi calculated by ROM. The dashed lines represent the bearing load of NiTi calculated by ROM, respectively. (b) Comparison on the stress-strain curve of the NiTi under constrained state and freestanding state. Constrained NiTi (this work) – NiTi calculated by ROM from 30NT; Freestanding NiTi – initial R-phase NiTi in previous literature [39]. (c) Fine scanned XRD patterns for 10NT, 20NT and 30NT before and after tension, at the scanning range of 41-44°, with peak fitting of (112) and (300) R phases in Fig. S4. (d) The calculated area percentages of (300) peak in all three NiTi/Al composites before and after tension, originated from (c).\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/a074ae161cb6f3b1529d0527.png"},{"id":103972730,"identity":"aa416253-dfbf-4493-adfe-a83537448385","added_by":"auto","created_at":"2026-03-05 07:57:47","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":445555,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTransformation-induced plasticity effects.\u003c/strong\u003e (a) Strain hardening rates of the pure Al, 10NT, 20NT and 30NT samples. (b) Strain partitioning of the pure Al, 10NT, 20NT and 30NT samples upon tension obtained from DIC method. (c) The evolutions of aspect ratio of the typical NiTi particle and the corresponding principal strains (determined by (b) at the same position) in 30NT, with respective to the engineering strains of 0, 2%, 4%, 6%, 8%, 12% and fractured point. The inside NiTi particle images originated from the quasi-\u003cem\u003ein situ\u003c/em\u003eXCT experiments after each 2% engineering strain increment.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/79f1a5498633766fe315540c.png"},{"id":103972716,"identity":"96a3caa4-851c-42c3-972a-aeedf96045c3","added_by":"auto","created_at":"2026-03-05 07:57:44","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":977207,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTransformation-induced toughening effects. \u003c/strong\u003e(a) Fractured surface (top of view) of the NiTi/Al composite. From left to right, before tension, 0, 500, 1000, 1500 and 2000 μm away from the fracture surface along the tensile direction respectively. The red arrows mark the tiny NiTi particles near the NiTi/Al interfaces. (b) The relative frequency statistics of NiTi particles volume before (blue) and after (red) tension. The schematic diagram of the NiTi/Al composite is drawn inside, with the grey particles representing NiTi and the white background representing the Al matrix. (c-d) X-ray computed tomography (XCT) 3D reconstructions of microcracks near the fracture surfaces of 10NT (c) and 30NT (d). The crack volume fraction ρ\u003csub\u003ev\u003c/sub\u003e and crack number density ρ\u003csub\u003en\u003c/sub\u003e are labeled.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/d6eabcdc95f6eaf9bd07737a.png"},{"id":103972713,"identity":"3f7698b9-d706-4196-aae3-ecc7290e7b13","added_by":"auto","created_at":"2026-03-05 07:57:43","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":88753,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eDamping property. \u003c/strong\u003e(a) Temperature-internal friction curves of Al (gray) and 20NT (red), while heating from -100°C to 100°C. (b) Comparison on the internal friction and tensile strength of the NiTi/Al composites in this work and previous studies [22,31,33–35], at the same test frequency and room temperature, during the heating process.\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/f82e90252fd5bc785ff42c75.png"},{"id":104779321,"identity":"dc29c7ed-985a-4a30-96d1-e98e10896864","added_by":"auto","created_at":"2026-03-17 07:38:58","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5556866,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/ef2f2fdb-ba4d-47ac-81d5-ef4ca163c47d.pdf"},{"id":103972670,"identity":"8fe0908e-c5ce-4026-9c6c-e2c51c646b5c","added_by":"auto","created_at":"2026-03-05 07:57:30","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":890531,"visible":true,"origin":"","legend":"","description":"","filename":"SM.docx","url":"https://assets-eu.researchsquare.com/files/rs-8547474/v1/a06615842c18011c89999e86.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Plasticizing aluminum by deformable and phase-transformable nitinol alloys","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe increasing demands from both scientific study and industrial upgrading have been calling for lightweight, strong-yet-tough composites, especially in use under specific service conditions (\u003cem\u003ee.g.\u003c/em\u003e, high temperatures, low temperatures, and vibrations) [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Incorporating secondary fillers into metals/alloys represents an effective approach towards high-performance composites with structural and functional integrations [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Traditional rigid yet brittle ceramics, such as TiB₂, SiC, TiC, WC, AlN, Al₂O₃, Al₃BC, and Si₃N₄, are prone to induce strain localization and stress concentration at the ceramic/metal interface, which significantly undermines the mechanical robustness of metal matrix composites (MMCs) [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Hitherto, several efforts have been made to tackle the issue, such as interface engineering [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] and heterostructured design [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Wherein the core ideas are to facilitate strain delocalization and stress partitioning to ensure sustainable plastic stability. Recently, metastability compositing has emerged as a feasible approach to regain the mechanical robustness [\u003cspan additionalcitationids=\"CR7\" citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In our previous work [\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], we introduced phase-transformable zirconia-based ceramics into the aluminum (Al). This metastability-engineered composite exhibited twice the strength and energy dissipation of the standalone matrix, enabled by strain hardening and synergistic deformation from the constrained phase transformation. Likewise, we integrated deformable CrCoNi medium entropy alloys into the Al matrix, presenting enhanced strength-toughness synergy [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eNickel-titanium alloy (NiTi, nitinol), a representative of shape memory alloys (SMAs), combines high deformability with phase-transformation ability, making it a promising candidate for achieving structural\u0026ndash;functional integration in composites. Specifically, it can undergo reversible martensitic phase transformation between the cubic (B2 phase, austenite), rhombohedral (R phase, martensite) and monoclinic (B19' phase, martensite) structures. In addition, its energy absorption ability and high damping coefficient (above 0.1) offer significant application potential in fields such as shock absorption [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] and damping system [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Of note, the B2\u0026rarr;R phase transformation exhibits distinct characteristics of small shear strain (\u0026lt;\u0026thinsp;1%), low thermal hysteresis (\u0026lt;\u0026thinsp;5\u0026deg;C) and superior functional fatigue resistance than the direct B2\u0026rarr;B19' transformation during thermal cycling [\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Previous studies have made several attempts to investigate the effect of NiTi on metal matrices. These studies typically reported enhanced strength and damping capacity but concurrently observed reductions in plasticity, exemplified by NiTi/1060Al lamellar composites [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e], and NiTi nanoparticle- reinforced Al composites [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Critically, stress-induced phase transformation and deformation of NiTi particles in the confined state has never been elucidated, greatly limiting the application of shock absorption, damping and self-healing.\u003c/p\u003e \u003cp\u003eIn this study, we dig into these unsolved issues using NiTi/Al composites as a model material, where the age-treated NiTi particles are stabilized into R phase under sufficient Al matrix constraint. The resultant composites with 30 wt.% NiTi achieve the optimal ultimate tensile strength and uniform elongation, 8% and 48% surpassing those of pure Al. The synergistic enhancement mechanism is elucidated by systematically characterizing microstructures, phase constituents and transformation characteristics of the composites. Our study also provides a design framework for composites with deformable and phase-transformable fillers and is anticipated to guide their implementation in energy dissipation, damping and intelligent material systems.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Start materials and fabrication routes\u003c/h2\u003e \u003cp\u003eNear-spherical NiTi alloy powders (Ni\u003csub\u003e50\u003c/sub\u003eTi\u003csub\u003e50\u003c/sub\u003e, in atomic ratio) with a diameter range of 0\u0026ndash;20 \u0026micro;m were purchased from Foshan Chengfeng Material Technology Co., Ltd.. Al powders with an average size of 10 \u0026micro;m were purchased from Henan Yuanyang Powder Technology Co., Ltd. As-received NiTi powders were pre-aged at 550\u0026deg;C for 3 h in a muffle furnace (argon atmosphere, 99.99% purity) to suppress the generation of B19' phase and obtain the metastable R phase when they are cooled down to room temperature [\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. The resultant R phase is expected to enhance the plasticity and damping capacity of the composites.\u003c/p\u003e \u003cp\u003eAs-aged NiTi and Al powders were pre-mixed with NiTi weight fraction of 10, 20, 30%. 1 wt.% stearic acid was added as the process control agent. The pre-mixed composite powders were then milled at 200 rpm for 12 h and 400 rpm for 2 h (QM-DY2 planetary) with hardened stainless-steel vials and balls (ball-to-powder ratio of 10:1). Subsequently, as-milled NiTi/Al composite powders were hot-pressed at 450\u0026deg;C for half an hour, followed by hot extrusion at 350\u0026deg;C into rod-like samples (extrusion ration 9:1). Both ball milling and densification processes were conducted under the protection of high-purity argon. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e provides the terminology of bulk NiTi/Al composites in this work. Pure Al without NiTi addition was also purposely prepared using the same fabrication process for comparison.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003e\u003cb\u003eTerminology and definitions of NiTi/Al composites reported in this work.\u003c/b\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eNiTi aging time/h\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNiTi weight percent/%\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10NT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20NT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e30NT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Characterization techniques\u003c/h2\u003e \u003cp\u003eThe sample surfaces were first mechanically ground and polished, followed by ion milling (SEM Mill, Fischione Model 1060) to eliminate contaminants and stress layers. Phase constituents of NiTi powders and NiTi/Al composites were examined by X-ray diffraction (XRD, Mini Flex 600) with Cu K\u003csub\u003eα\u003c/sub\u003e radiation (λ\u0026thinsp;=\u0026thinsp;0.15406 nm). XRD conducted at room temperature was scanned in a 2θ range of 25\u0026ndash;65\u0026deg; at a scanning rate of 2\u0026deg;/min and 41\u0026ndash;44\u0026deg; at a scanning rate of 0.5\u0026deg;/min. The characteristic peaks (112) and (300) of R phase at 41\u0026ndash;44\u0026deg; were fitted using Pseudo-Voigt Function Type (a linear combination of Gaussian and Lorentzian functions) in OriginPro Lab software (version 2025b). Transformation temperatures of NiTi powders and NiTi/Al composites were determined by differential scanning calorimetry (DSC, Perkin Elmer DSC800 calorimeter) performed between \u0026minus;\u0026thinsp;90\u0026deg;C and 100\u0026deg;C at a cooling/heating rate of 5\u0026deg;C/min. Internal frictions (tan δ) were measured by dynamic mechanical analysis equipment (DMA, TA Q800), a fixed strain of 0.01% and frequency of 1 Hz. The samples, with dimensions of 30 mm \u0026times; 4 mm \u0026times; 1 mm, were tested over a temperature range of \u0026minus;\u0026thinsp;100\u0026deg;C to 100\u0026deg;C, with cooling and heating rates of 5\u0026deg;C/min. Heat treatment under flowing argon gas conditions was particularly used to eliminate hydrogen in the NiTi specimen to measure the intrinsic internal friction value without the influence of hydrogen-related relaxation behavior. Scanning electron microscope (SEM, FEI Scios) attached with energy-dispersive X-ray spectrometer (EDS, Oxford X-Max80T) was utilized to characterize microstructures and elemental distributions.\u003c/p\u003e \u003cp\u003eUniaxial tensile tests were carried out on an INSTRON 3340 testing machine at a strain rate of 5 \u0026times; 10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and room temperature, along the extrusion direction. The specimens had a gauge length of 10 mm, width of 2 mm and thickness of 1 mm. At least three tensile tests were performed for each sample set. The strain hardening rate (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\)\u003c/span\u003e\u003c/span\u003e) was determined from \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\theta\\:\\:=\\:\\partial\\:\\sigma\\:/\\partial\\:\\epsilon\\:\\)\u003c/span\u003e\u003c/span\u003e, where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sigma\\:\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\epsilon\\:\\)\u003c/span\u003e\u003c/span\u003e represent the true stress and true strain obtained from the tensile data. To obtain smoother results, the true stress\u0026ndash;strain curves were fitted using polynomial regression. Digital Image Correlation (DIC, Instron 5966, Bluehill) with an infrared camera was used to capture the \u003cem\u003ein situ\u003c/em\u003e strain distribution during loading. X-ray computed tomography (XCT, Xradia 620 Versa) was performed at 60 kV and 6.5 W with a 4\u0026times; objective lens to quantitatively characterize the microcracks and NiTi particle deformation via absorption contrast tomography. Three-dimensional visualization and analysis were conducted using Dragonfly software.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Phase constituents and transformation temperatures of NiTi powders\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea shows the XRD patterns of the as-received and as-aged NiTi powders. As-received NiTi is composed entirely of the monoclinic B19' phase. After aging treatment for 3 h, the B19' phase mostly transforms into the cubic B2 phase, with minor quantities of B19' left. DSC curves in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb display that as-received NiTi present one-step B19'\u0026harr;B2 phase transformation over heating and cooling cycles, with a single characteristic peak each way. In stark contrast, as-aged NiTi exhibits a multi-step phase transformation characteristic (B19'\u0026rarr;R\u0026rarr;B2 during heating and B2\u0026rarr;R\u0026rarr;B19' during cooling) and a wider temperature range (~\u0026thinsp;75\u0026deg;C) for transformation compared to that of the as-received NiTi (~\u0026thinsp;30\u0026deg;C). This can be well explained by the fact that aging treatment suppresses the large lattice shear (~\u0026thinsp;10%) in B2\u0026rarr;B19' transformation [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], thus preferring B2\u0026rarr;R transformation with a relatively small lattice shear (~\u0026thinsp;1%, [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]) before the eventual formation of B19' phase. Echoed with XRD observation, a small amount of B19' phase in the as-aged NiTi indicates that the B2\u0026rarr;B19' transformation is not completely suppressed and just initiated when cooling down to the room temperature (~\u0026thinsp;25\u0026deg;C).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Microstructures of NiTi/Al composites\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-c depicts the typical SEM images of the NiTi/Al composites. The regions with grey contrast represent the Al matrix and the white contrast for NiTi particles. In all three sets, NiTi particles appear as randomly distributed in the two-dimensional cross-section, without noticeable particle agglomeration or large pore defects. Specifically, more elongated NiTi particles are identified in composites with relatively low NiTi content, such as 10NT (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eb, average aspect ratio: 2.2\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0). With higher NiTi content, as manifested in 30NT, NiTi particles tend to maintain a more near-sphere shape (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ed, average aspect ratio: 1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6), resembling the as-aged NiTi particles (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea, average aspect ratio: 1.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.6). This discrepancy could probably arise from the stronger resistance to shear and deformation during ball milling and hot extrusion with higher NiTi content. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed shows the representative EDS elemental mapping images from 20NT. Apparently, the interfaces between the Al matrix and NiTi particles are clear and free from cracks, pores, or other visible defects. In other words, interfacial elemental diffusion or intermetallic compound formation is well controlled for all three sets.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Phase constituents of NiTi/Al composites\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea presents the XRD patterns of 10NT, 20NT and 30NT. In addition to the Al phase, no B19' phase (martensite) was detected in these three sets. Such observation on austenite stabilization by matrix constraint is indeed in line with our previous studies [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb highlights the XRD patterns at the scanning range of 41\u0026ndash;44\u0026deg;, where the R phase can be identified and fitted by the characteristic \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(112\\right)}_{R}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(300\\right)}_{R}\\)\u003c/span\u003e\u003c/span\u003e diffraction peaks [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. As shown, increasing NiTi content from 10 to 20 to 30 wt.% leads to a decrease in the area ratio of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(300\\right)}_{R}\\)\u003c/span\u003e\u003c/span\u003e peak from approximately 73% to 56% to 35%. Of particular note, the two characteristic peaks corresponding to the R phase in 10NT exhibit the broadest peak widths among all. These findings suggest a strong correlation between the NiTi content and the resulting phase constituents, likely stemming from the higher deformation degree (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea-c and Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) observed at lower NiTi contents (\u003cem\u003ee.g.\u003c/em\u003e, 10NT). Consistent with the EDS results (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), no (or less than 1%, if any) intermetallic phases (such as Al\u003csub\u003e3\u003c/sub\u003eTi, Ti\u003csub\u003e3\u003c/sub\u003eAl and TiAl) are present in the XRD spectrums, indicating the absence of interfacial reaction byproducts formed during the fabrication process.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Transformation characteristics of NiTi/Al composites\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea illustrates the DSC curves of 10NT, 20NT, and 30NT. Different from the as-aged NiTi powders with a multistep transformation (B2\u0026rarr;R\u0026rarr;B19' during cooling and B19'\u0026rarr;R\u0026rarr;B2 during heating), all three NiTi/Al composites show a single-step transformation (B2\u0026rarr;R during cooling and R\u0026rarr;B2 during heating). The peak intensity (heat flow) increases with increasing NiTi content, indicating a stronger transformation response at higher NiTi contents. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb compares the martensite start and finish temperatures (M\u003csub\u003es\u003c/sub\u003e and M\u003csub\u003ef\u003c/sub\u003e) of the NiTi/Al composites (under constraint) with those of the as-aged NiTi powders (at monolithic), determined from Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb using the tangent method. In particular, the transformation temperatures of the NiTi/Al composites are evidently higher than those of the as-aged NiTi powders at their monolithic state. The M\u003csub\u003ef\u003c/sub\u003e values of the composites (~\u0026thinsp;21\u0026ndash;27\u0026deg;C) are close to room temperature, implying that R phases can exist at ambient conditions\u0026mdash;consistent with the microstructural observations (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). This elevation in transformation temperatures under constraint can be attributed partly to the stress fields or local stresses generated during fabrication and by the matrix constraint, which stabilize the parent austenite phase and hinder the formation of martensitic variants. Within the NiTi/Al composites, increasing the NiTi content from 10 wt.% to 30 wt.% results in only a slight, monotonic increase in transformation temperatures (M\u003csub\u003es\u003c/sub\u003e: 43.2\u0026rarr;46.6\u0026deg;C; M\u003csub\u003ef\u003c/sub\u003e: 21.4\u0026rarr;27.3\u0026deg;C), confirming that the NiTi content has a negligible influence on the thermally induced phase transformation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.5 Mechanical properties\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the representative engineering stress\u0026ndash;strain curves of NiTi/Al composites and pure Al. Detailed mechanical property data are listed in the Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. With increasing NiTi particle content, the composites show a consistent enhancement in ultimate tensile strength (UTS) accompanied by a reduction in total elongation. Interestingly, the uniform elongation (UE) displays the opposite trend (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb): it increases progressively with NiTi addition, reaching a maximum of 4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2% in 30NT, approximately 48% higher than that of pure Al (2.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2%). Within the 0\u0026ndash;30 wt.% reinforcement range, the 30NT composite achieves the optimal combination of UTS (214\u0026thinsp;\u0026plusmn;\u0026thinsp;4 MPa) and UE (4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2%). Further increasing the NiTi content to 40 wt.%, however, results in pronounced particle agglomeration, which undermines plastic stability (Fig. S2). To further elucidate the role of aging, un-aged NiTi/Al composites (20 wt.%) were fabricated for comparison (Fig. S3). Compared to 3 h-aged 20NT, NiTi/Al absent in aging treatment enhances the UTS to 239\u0026thinsp;\u0026plusmn;\u0026thinsp;12 MPa, albeit at the considerable expense of UE (2.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1%) and total elongation (6.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7%).\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec compares the relative gains in UTS and UE of NiTi/Al composites in this work with those reported in previous studies [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan additionalcitationids=\"CR32 CR33 CR34\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e], using pure Al as the benchmark. A distinctive feature of this work is the remarkable enhancement in UE achieved without compromising strength. The maximum UE gain reaches up to 50%, markedly exceeding that of previously reported NiTi/Al systems. Of note, earlier studies primarily focused on thermally induced phase transformations, with limited attention to stress-induced phase transformations or fully R-phase-based NiTi/Al composites. The exceptional plasticity improvement observed here is likely to associate with the unique deformation characteristics of the R phase, and will be discussed in section \u003cspan refid=\"Sec13\" class=\"InternalRef\"\u003e4.2\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eMechanical properties of NiTi/Al composites.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYield Strength (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eUltimate Tensile Strength (MPa)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eUniform Elongation (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTotal Elongation (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e163\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e199\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e2.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e20.5\u0026thinsp;\u0026plusmn;\u0026thinsp;3.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10NT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e166\u0026thinsp;\u0026plusmn;\u0026thinsp;9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e199\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e2.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e17.5\u0026thinsp;\u0026plusmn;\u0026thinsp;3.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20NT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e169\u0026thinsp;\u0026plusmn;\u0026thinsp;11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e207\u0026thinsp;\u0026plusmn;\u0026thinsp;2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e3.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e16.1\u0026thinsp;\u0026plusmn;\u0026thinsp;3.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e30NT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e178\u0026thinsp;\u0026plusmn;\u0026thinsp;3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e214\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e \u003cp\u003e4.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e13.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4.1 Effect of aging treatment and matrix constraint on the stability of R phase\u003c/h2\u003e \u003cp\u003eConsidering that the presence of R phase and its orientation preference are the main differences among the NiTi/Al composites (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb), their underlying mechanisms of formation and deformation are crucial to the mechanical properties. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, we first confirmed that the aging treatment promoted the formation of R phase (small lattice shear\u0026thinsp;~\u0026thinsp;1%) in freestanding NiTi alloy particles, consistent with previous studies [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. There is a common acceptance that this ageing-induced B2\u0026rarr;R transformation behaviour is affected by the stress field of precipitates and dislocations, which act as a much stronger obstacle to the B2\u0026rarr;B19' transformation than that to the B2\u0026rarr;R transformation because the former has a much larger transformation strain.\u003c/p\u003e \u003cp\u003eSuch stabilization effect is also analogous to the matrix constraint effect from the viewpoint of thermally-induced phase transformation: only one endothermic/exothermic peak (B2\u0026rarr;R) is found in the DSC results (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), which differs from the freestanding NiTi of two-step phase transformation (B2\u0026rarr;R\u0026rarr;B19') with relatively large lattice strain (\u0026gt;\u0026thinsp;10%; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). When cooling down to the room temperature (~\u0026thinsp;20\u0026deg;C, lower than martensite finish temperatures in all samples, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), the as-fabricated NiTi/Al composites underwent full B2\u0026rarr;R phase transformation and therefore fell into the full R phase structure, as also evidenced by the room-temperature XRD patterns (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This confined R phase presents ultrahigh damping coefficient for its abundant twin boundaries and phase-transformation ability as an intermediate martensite [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eCompared to the freestanding NiTi alloy with widely reported reversible phase transformation under temperature fields [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e], thermally-induced R (rhombohedral)\u0026rarr;B19' (monoclinic) phase transformation is absent in the NiTi/Al composites either with or without aging treatment (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and Fig. S3b). A possible explanation might be the matrix constraint stress near the NiTi/Al interface caused by the compounding process, which is reported to lower the Gibbs free energy of high-temperature or highly symmetric phase [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e], and thus suppress large-scale lattice shear [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Herein, the synergistic effect of the two factors (aging treatment and matrix constraint) stabilized the NiTi in fully R-phase state, which was further proven to regain the plasticity of the NiTi/Al composites (Fig. S3).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e4.2 Stress partitioning and stress-induced R-phase reorientation\u003c/h2\u003e \u003cp\u003eIn light of the key role of R phase in the present NiTi/Al system, we applied the rule-of-mixture (ROM) to derive the average principal stress of NiTi particles (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{NiTi}\\)\u003c/span\u003e\u003c/span\u003e) within Al matrix at every principal strain,\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{\\sigma\\:}_{c}=(1-{V}_{f})\\bullet\\:{\\sigma\\:}_{Al}+{V}_{f}\\bullet\\:{\\sigma\\:}_{NiTi}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{V}_{f}\\)\u003c/span\u003e\u003c/span\u003e is the volume fraction of NiTi particles, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{c}\\)\u003c/span\u003e\u003c/span\u003e and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\sigma\\:}_{Al}\\)\u003c/span\u003e\u003c/span\u003e and are the engineering stress of NiTi/Al composites and pure Al measured in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea, the average principal stresses of NiTi particles derived from 20NT and 30NT first gradually increase with the strain, and afterwards decline once reaching the peak stress. In the composite with relatively low NiTi content (10 wt.%), the calculated stress onto NiTi particles constantly decreases with the strain. As previously noted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e, the NiTi particles in the as-fabricated 10NT already experienced substantial deformation during fabrication, and therefore its deformation was naturally restricted upon loading. This pre-existed deformation state provides a mechanistic explanation for the decreased load transferring efficiency throughout the tensile process. Conversely, the NiTi particles in the as-fabricated 20NT and 30NT with relatively higher NiTi contents endured lower pre-existed deformation; consequently, their synergistic deformation with Al matrix well contributed to the stress partitioning, as manifested by the promoted load transferring efficiency with strain upon tension (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). Prominently, 30NT with the highest stress partitioning in NiTi presents the best combination of uniform elongation and strength among all composite sample sets (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThese traits stand in stark contrast with the freestanding R-phase NiTi alloy wires frequently reported in the literatures [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e] (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). With increasing stress, NiTi alloy typically undergoes R-phase reorientation (section a-b, \u0026lt;\u0026thinsp;100 MPa) and martensitic transformation from the R phase to the B19' phase (section b-c, typically\u0026thinsp;~\u0026thinsp;200\u0026ndash;400 MPa). Apparently, the average principal stresses of NiTi by stress partitioning in this work can clearly reach the critical stress of the reorientation; however, the critical stress of the R\u0026rarr;B19' martensitic transformation in NiTi alloys varies widely (typically from several tens to a few hundred MPa) depending on the alloy composition, heat treatment strategy, microstructure, and testing temperature [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. It is noteworthy that the actual stress partitioning between Al and NiTi may not strictly follow the ROM model as outlined above; moreover, the actual principal stress of each NiTi particles may be lower than the calculation results, depending on the nuances of the matrix constraint, interfacial bonding and particle size distribution. Nonetheless, the direct application of ROM here indicates the feasibility of R-phase reorientation and the explanation for the absence of subsequent R-B19' phase transformation, from the perspective of stress-induced transformation.\u003c/p\u003e \u003cp\u003eTo further substantiate the occurrence of the R-phase reorientation, the phase composition before and after the tensile test were analyzed using fine-scanning XRD (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec) combined with muti-peak fitting (Fig. S4). The most pronounced differences appear in the R phase (112) and (300) diffraction peaks. As summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed, the area fraction of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(300\\right)}_{R}\\)\u003c/span\u003e\u003c/span\u003e peak increases notably after tension, while the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(112\\right)}_{R}\\)\u003c/span\u003e\u003c/span\u003e peak correspondingly weakens. Quantitatively, the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(300\\right)}_{R}\\)\u003c/span\u003e\u003c/span\u003e area fraction rises from 73% to 81% in 10NT, from 56% to 64% in 20NT, and from 35% to 45% in 30NT, strong indicators of stress-induced R-phase reorientation. We note that the overall fraction of reoriented R phase is relatively low (~\u0026thinsp;8\u0026ndash;10%), likely due to the spatial averaging inherent to large-area XRD measurements. It is reasonable to infer that the local stress and strain concentration in the necking micro-region promote a much higher degree of R-phase reorientation\u0026mdash;consistent with the transformation-induced heterogeneity widely observed in transformation-/twinning-induced plasticity (TRIP/TWIP) steels [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e], high-entropy alloys (HEAs) [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e], and transformation-toughened ceramic [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWe also emphasize that, although stress-induced R-phase reorientation has been previously observed primarily in monolithic NiTi [\u003cspan additionalcitationids=\"CR45\" citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e], the present work is the first to explore this phenomenon on NiTi under constraint. Such R-phase reorientation, essentially a detwinning process caused by the shear of the lattice under stress [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e], renders three distinct stages in NiTi/Al composites (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). At stage Ⅰ (engineering strain 0\u0026ndash;4%): elastic deformation, yielding and work hardening. In this stage, the partitioned stress onto NiTi particles elevated with increasing strain and reached the critical stress of the R-phase reorientation (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). The composite subsequently underwent sustained plastic deformation at stage Ⅱ (strain 4\u0026ndash;12%), followed with necking at stage Ⅲ (strain 12% to fracture) as the partitioned stress decreased. Of particular note, no B19' phase was observed in this work (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec), even though the partitioned stress might reach the critical value to trigger the R\u0026rarr;B19' phase transformation (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). This restricted transformation behavior under stress resembles that under temperature as discussed in Section \u003cspan refid=\"Sec12\" class=\"InternalRef\"\u003e4.1\u003c/span\u003e, which may be attributable to the formation of the inhomogeneity of the internal stress field [\u003cspan additionalcitationids=\"CR49\" citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.3 Transformation-induced plasticity effects\u003c/h2\u003e \u003cp\u003eTo elucidate the origin of the robust plastic deformation, the strain-hardening rates were calculated from the engineering stress\u0026ndash;strain curves presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea. As demonstrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea, both 20NT and 30NT exhibited higher strain-hardening rates than pure Al within the true strain range of 1\u0026ndash;4% after yielding. Whilst 10NT with insufficient deformation ability exerted a detrimental effect on the strain-hardening behavior (\u003cem\u003ee.g.\u003c/em\u003e, 10NT), consistent with the prediction from the ROM analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). Such distinct plasticity gains in 20NT and 30NT over 10NT are also in concert with the strain delocalization and necking delay along the gauge length, as observed from the DIC results (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003eWe further conducted quasi-\u003cem\u003ein situ\u003c/em\u003e tensile test via XCT to visualize the three-dimensional particle deformation in 30NT. As displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec, the typical NiTi particle in stage Ⅰ experienced almost unchanged local strain (\u003cem\u003ei.e.\u003c/em\u003e, the deformation experienced at the specific point within the composite, can be obtained by calculating the displacement of the image recognition points along the loading direction) whilst its aspect ratio raised from 1.52 to 1.67, indicating the significant deformation in stage Ⅰ. These structural evolutions correlate with the prominent R-phase reorientation at stage I (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea). As the engineering strain increased from 4% to 12%, the stress levels varied only moderately and the NiTi particles deformed little (aspect ratio almost unchanged). At the final stage III, the severe strain localization reached its maximum and thus induced pronounced deformation of the NiTi particles (aspect ratio: 1.66\u0026rarr;1.74).\u003c/p\u003e \u003cp\u003eIn this regard, the increasing aspect ratio by approximately 14% and ~\u0026thinsp;30% principal strain during tension demonstrates the excellent deformability of NiTi under constraint. Such deformation is therefore expected to enable effective synergistic deformation between Al and NiTi and strain delocalization at the NiTi/Al interfaces, and redistribute local stresses via lattice shearing (R-phase reorientation). These mechanisms also explain the enhanced stress partitioning onto NiTi in 30NT (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea) and improved plasticity and strain hardening of NiTi/Al composites by aging treatment (Fig. S3a) and increasing NiTi content (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.4 Transformation-induced toughening effects\u003c/h2\u003e \u003cp\u003eThe top-view fractured surfaces further verified the tight NiTi/Al interfacial bonding and load transferring efficiency within the composites (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea). Specifically, along the tension direction, two-dimensional morphologies of the 10NT sample were examined at various distances from the fracture surface (X\u0026thinsp;=\u0026thinsp;0, 500, 1000, 1500, 2000 \u0026micro;m). At all measured positions, small NiTi fragments were consistently observed in regions near the NiTi/Al interfaces, akin to that shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ec. Importantly, no such breakage fragments were present in the as-fabricated sample, confirming that the breakage of these fine NiTi particles arose solely from tensile loading rather than from fabrication. Moreover, the number of tiny fragments decreased progressively with increasing distance from the fracture surface (corresponding to regions of lower principal strain, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb). This trend further supports the notion that tensile stress not only deformed the NiTi particles but also fractured them into fine debris. Intriguingly, no obvious NiTi/Al interface debonding was observed, which can be explained by the good chemical infinity between NiTi and Al [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e], as well as small transformation-induced lattice strain (typically less than 1%) [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. These findings also agree with the XCT findings (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb) that the mean volume of NiTi decreased from 556.0 \u0026micro;m\u003csup\u003e3\u003c/sup\u003e to 529.4 \u0026micro;m\u003csup\u003e3\u003c/sup\u003e upon tension.\u003c/p\u003e \u003cp\u003eAs discussed in Section \u003cspan refid=\"Sec13\" class=\"InternalRef\"\u003e4.2\u003c/span\u003e, the excellent deformability of NiTi and stress-induced R-phase reorientation are expected to significantly suppress the crack initiation and propagation, thereby contributing to the observed improvement in toughness. To validate this, we characterized three-dimensional microcrack distribution in post-mortem 10NT and 30NT tensile specimens (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec-d) at locations\u0026thinsp;~\u0026thinsp;200 \u0026micro;m from the fracture surface. The crack volume fraction ρ\u003csub\u003ev\u003c/sub\u003e and crack number density ρ\u003csub\u003en\u003c/sub\u003e in 30NT (ρ\u003csub\u003ev\u003c/sub\u003e: 0.45%, ρ\u003csub\u003en\u003c/sub\u003e: 13 \u0026times; 10\u003csup\u003e17\u003c/sup\u003e m\u003csup\u003e3\u003c/sup\u003e) were nearly an order of magnitude greater than those in 10NT (ρ\u003csub\u003ev\u003c/sub\u003e: 0.04%, ρ\u003csub\u003en\u003c/sub\u003e: 1.0 \u0026times; 10\u003csup\u003e17\u003c/sup\u003e m\u003csup\u003e3\u003c/sup\u003e). In this context, the aging treatment and matrix constraint confined NiTi in the metastable R phase, imparting it with distinct cycling stability and deformability. These features promoted R-phase reorientation, particle deformation/breakage and inhibition of microcracks, collectively toughening the composite.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e4.5 Implications and outlooks\u003c/h2\u003e \u003cp\u003eBeyond the aforementioned benefits from mechanical properties, the combination of lightweight Al matrices with the unique functional characteristics of NiTi offer other engineering and intelligent potentials:\u003c/p\u003e \u003cp\u003e(1) \u003cem\u003eDamping capacity\u003c/em\u003e. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003ea, the damping capacity of 20NT (0.051) in this work successfully achieves the enhancement of 48% over the pure Al (0.035), likely attributed to the rich twin boundaries of R phase, numerous NiTi/Al interfaces, and the R\u0026rarr;B2 phase transformation. Such strength-damping synergy also surpasses other existing NiTi/Al composites (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e(2) \u003cem\u003eImpact resistance\u003c/em\u003e: Previous studies have showed the substantial improvement on the impact resistance of freestanding NiTi [\u003cspan additionalcitationids=\"CR53\" citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. The exceptional energy-dissipation capacity of NiTi, derived from its stress-induced phase transformation and reversible lattice shear, offers a compelling pathway to enhance impact resistance. Under high-rate loading, NiTi particles can undergo rapid martensitic transformation, thereby absorbing impact energy, mitigating stress concentrations, and delaying crack initiation. This will be reported as a separate work in the future.\u003c/p\u003e \u003cp\u003e(3) \u003cem\u003eTraining by cyclic loading\u003c/em\u003e: The intrinsic fatigue resistance of R phase enables the composites to progressively stabilize mechanical response under cyclic loading. Through repeated transformation cycles, the interfacial compatibility and deformation coordination between NiTi particles and the Al matrix can be further optimized, improving fatigue resistance and maintaining load transferring efficiency over long service periods.\u003c/p\u003e \u003cp\u003e(4) \u003cem\u003eSelf-healing\u003c/em\u003e. As a typical type of intelligent materials, NiTi alloys exhibit reversible martensitic transformation with shape-memory/superelastic effects, enabling them to autonomously adapt to external stimuli. Moreover, the shape-memory effect offers a unique pathway for self-healing: upon thermal activation, NiTi can partially recover their pre-deformed geometry, thereby repair microcracks, and recover load transferring efficiency in the surrounding Al matrix, as unveiled in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. Such tunable transformation characteristics highlight its potential for developing structurally embedded sensing, damage mitigation, and smart actuation functions. These attributes position NiTi/Al composites as a compelling platform for self-adaptive and self-healing systems.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eIn summary, we successfully incorporated deformable and phase-transformable nitinol (NiTi) particles into aluminum (Al) matrix. The constrained phase transformation and deformation behavior of NiTi under loading were elucidated, and the mechanisms that couple these structural evolutions to enhanced mechanical performance were revealed. The most salient findings are summarized below:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe age-treated NiTi particles were effectively embedded into the Al matrix via powder metallurgy, where sufficient matrix constraint lowered the transformation temperature of NiTi, thus stabilizing the metastable R phase and preserving its reorientation capability.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe resultant composite with 30 wt.% NiTi achieves the optimal ultimate tensile strength and uniform elongation, 8% and 48% surpassing those of pure Al without NiTi filler.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe enhanced plasticity was attributable to the stress-induced R-phase reorientation and substantial particle deformability within the NiTi/Al composites, which effectively accommodated localized strain, alleviated stress concentration near the NiTi/Al interface and activated micro-crack toughening effect.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe results highlight the advantages of using deformable or phase-transformable fillers as an alternative to traditional brittle reinforcements, offering a new pathway toward high-performance metal matrix composites with exceptional plasticity, impact toughness and damping capacity. Moreover, the phase-transformable nature of NiTi unleashes the opportunities for developing intelligent composites with adaptive response, cyclic loading stability, and self-healing functionalities.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eAuthors declare that they have no competing interests.\u003c/p\u003e \u003c/p\u003e\u003cp\u003e \u003ch2\u003eData and materials availability\u003c/h2\u003e \u003cp\u003eAll data are available in the main text or the supplementary materials.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work is financially supported by the National Natural Science Foundation of China (Nos. 52192595, 523B2005 and 52571174).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eY. H., W. Z., and L. Z. conceptualized the study.Y. H. conducted methodology design, investigation, validation, and wrote the original manuscript draft.W. Z., L. Z., and Q. G. acquired funding, supervised the project, and contributed to manuscript review and editing.S. J., X. L., X. F. and S. M. performed investigation.Q. G. provided resources and additional validation.All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThe authors would like to thank Dr. Zhongxin Zhao and Dr. Linghai Li from Shanghai Jiao Tong University (SJTU) for their assistance in fabrication and XCT experiments. WZ also thanks Outstanding Doctoral Graduate Development Scholarship of Shanghai Jiao Tong University (2025).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eAll data are available in the main text or the supplementary materials.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eCaccia M, Tabandeh-Khorshid M, Itskos G, Strayer AR, Caldwell AS, Pidaparti S et al (2018) Ceramic\u0026ndash;metal composites for heat exchangers in concentrated solar power plants. 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Mech Mater 65:1\u0026ndash;11. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.mechmat.2013.05.017\u003c/span\u003e\u003cspan address=\"10.1016/j.mechmat.2013.05.017\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Composites, Phase Transformation, Mechanical Properties, Nitinol, Interface","lastPublishedDoi":"10.21203/rs.3.rs-8547474/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8547474/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSecondary rigid fillers reinforce the metals, yet usually at the cost of strain localization and reduced plasticity/toughness. Nitinol (NiTi) alloy, as a representative of shape memory materials, exhibits reversible phase transformation ability and considerable deformability upon loading. Here, we introduced age-treated NiTi particles into aluminum (Al) via powder metallurgy, where the sufficient matrix constraint maintains metastable R phase in the NiTi particles. The resultant composite with 30 weight ratio NiTi particles achieves the optimal ultimate tensile strength (UTS) and uniform elongation, 8% and 48% surpassing those of pure Al without NiTi filler. Such salient property gains are attributed to the stress-induced R-phase reorientation and particle deformation, thus effectively accommodating strain localization, alleviating stress concentration near the NiTi/Al interface, and activating micro-crack toughening effect. Our strategy can be readily extended to other matrices with deformable or phase-transformable fillers, offering broad potential for applications in energy dissipation, damping, and intelligent material systems.\u003c/p\u003e","manuscriptTitle":"Plasticizing aluminum by deformable and phase-transformable nitinol alloys","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-03-05 07:56:25","doi":"10.21203/rs.3.rs-8547474/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"329889251422308545078503063365399282378","date":"2026-03-04T15:19:36+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"200394452806960054533923157931128491121","date":"2026-03-04T12:51:20+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"58309872767112959595180140054451015097","date":"2026-03-04T12:26:08+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"53721674454563681704727660941909464212","date":"2026-03-02T12:55:01+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-03-02T11:45:08+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-02T11:42:22+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-09T00:56:47+00:00","index":"","fulltext":""},{"type":"submitted","content":"Advanced Composites and Hybrid Materials","date":"2026-01-08T05:49:29+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"advanced-composites-and-hybrid-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"achm","sideBox":"Learn more about [Advanced Composites and Hybrid Materials](https://link.springer.com/journal/42114)","snPcode":"42114","submissionUrl":"https://submission.nature.com/new-submission/42114/3","title":"Advanced Composites and Hybrid Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"2567f200-425d-48a5-b9f5-57da383e3747","owner":[],"postedDate":"March 5th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-03-27T17:40:00+00:00","versionOfRecord":[],"versionCreatedAt":"2026-03-05 07:56:25","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8547474","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8547474","identity":"rs-8547474","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}