Fabrication of TiN-doped Ti nanocomposites with high strength and ductility by plasma-assisted ball milling and selective laser melting

Fabrication of TiN-doped Ti nanocomposites with high strength and ductility by plasma-assisted ball milling and selective laser melting | 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 Fabrication of TiN-doped Ti nanocomposites with high strength and ductility by plasma-assisted ball milling and selective laser melting Chuanjun Zang, Jianhua Yin, Shaofeng Hong, Meiqin Zeng, Zhongchen Lu This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4487360/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 17 Oct, 2024 Read the published version in Discover Materials → Version 1 posted 10 You are reading this latest preprint version Abstract An efficient method to fabricate titanium nitride (TiN)-doped titanium (Ti) nanocomposites through a combination of plasma milling, plasma spheroidization, and selective laser melting (SLM) was developed to obtain Ti-based materials with high strength and ductility from commercially pure titanium (CP-Ti). Nitrogen plasma milling and plasma spheroidization were used to fabricate Ti-TiN nanocomposites, which were mixed with CP-Ti powder at a 1:9 ratio and printed by SLM forming. The resulting materials possessed a dual-scale morphology with both coarse lath-like and fine acicular-like grains. The Ti-TiN nanocomposites possessed higher hardness and tensile strength and lower ductility than those of the control sample without TiN. The mechanical properties of the SLM-printed Ti-TiN nanocomposites were improved compared with those of SLM-printed CP-Ti because of the TiN particles and resulting dual-scale structure. Nitrogen plasma milling provides a simple route to fabricate TiN nanophase-reinforced Ti-based nanocomposites suitable for SLM printing. Selective laser melting Commercially pure titanium Plasma milling In situ nitriding Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 1. Introduction Selective laser melting (SLM) is increasingly being used to fabricate commercially pure titanium (CP-Ti) and titanium (Ti) alloys and matrix nanocomposites [ 1 – 2 ]. However, the industrial use of CP-Ti is limited by its poor wear resistance and low hardness [ 3 – 4 ]. The hardness and wear performance of CP-Ti can be enhanced by addition of second-phase particles to induce dislocation motion [ 5 – 10 ]. Commonly used reinforcing phases for CP-Ti include ceramic particles such as TiB [ 5 ], TiB 2 [ 5 ], TiN [ 6 , 8 ],TiC [ 6 – 7 ], SiC [ 9 ], and B 4 C [ 10 ]. In general, the average cooling rate in SLM forming is about 10 4 k/s, which means that β-Ti with body-centered cubic (BCC) structure may be transformed into α'-Ti with hexagonal close packed (HCP) structure, that is, martensite, by coherent shear [ 11 ]. However, because there are no obstacles to solidification and nucleation, Ti grains grow epitaxially at high temperature during SLM, leading to Ti alloys with a microstructure of coarse β columnar crystals and disordered needle-like α' martensite [ 12 – 13 ]. Addition of reinforcing particles during the three-dimensional printing of Ti alloys can increase the concentration of nucleation sites to promote the formation of internal structures, which can raise the recrystallization rate of grains [ 14 ]. The in situ synthesis of a reinforced phase in a Ti matrix has a larger effect on the alloy microstructure than that of directly adding a reinforced phase [ 15 ]. Nitrogen (N) is an effective strengthening element in Ti alloys; it can be dissolved in HCP α-Ti crystals to form titanium nitride (TiN) interstitial solid solutions [ 16 – 17 ]. Recently, SLM forming with in situ N solid solutions has been used to fabricate Ti materials with balanced strength and ductility [ 8 , 15 , 18 ]. High-energy ball milling is an effective route to promote the direct reaction of Ti metal with molecular nitrogen (N 2 ) at room temperature [ 19 – 20 ]. Discharge plasma can accelerate the kinetics of compound synthesis and powder activation. We synthesized TiH 2 and TiN phases in a short time by ammonia plasma-assisted ball milling of Ti powder [ 21 ]. In this study, we attempt to fabricate CP-Ti nanocomposites with high strength and ductility by economical methods. The in situ synthesis of a TiN reinforcing phase in a Ti matrix using a combination of N 2 plasma milling, plasma spheroidization, and SLM forming is investigated, leading to the fabrication of Ti-TiN nanocomposites. Two different N 2 plasma milling processes accompanied with plasma spheroidization are first used to fabricate Ti-TiN nanocomposite powder samples, which are then mixed with spherical CP-Ti powder and printed by SLM forming. The microstructure and mechanical properties of the resulting Ti-TiN nanocomposites are examined. 2. Materials and methods 2.1. Material preparation The Ti-TiN nanocomposites were synthesized from raw atomized spherical CP-Ti powder with a diameter of about 15–53 µm (Changsha TIJO Metal Materials Co., Ltd., China) using a plasma milling device (PBMS, South China University of Technology, China). The working principle of this device has been reported previously [ 22 – 25 ]. In this work, mechanical ball milling was conducted during exposure to a homogeneous non-thermal N 2 gas plasma, which was generated with a pulse peak voltage of 25 kV and discharge frequency of 11.5 kHz using a single dielectric barrier discharge reactor. Ball milling was conducted at 1350 rpm. Two different N 2 plasma milling regimes were used. The first used a 30:1 weight ratio of balls to powder for 14 h (denoted as N 2 -P-milling-1) and the second used a 60:1 weight ratio of balls to powder for 10 h (denoted as N 2 -P-milling-2). For comparison, conventional milling without plasma discharge in the milling jar was carried out with a 60:1 weight ratio of balls to powder for 10 h (denoted as N 2 -C-milling). As shown in Fig. 1 , the ball-milled Ti powders were treated by plasma spheroidization to improve their flowability. The spheroidized N 2 -P-milling-1 and N 2 -P-milling-2 Ti powder samples were mixed with CP-Ti powder at a weight ratio of 1:9 and subjected to SLM processing. 2.2. Selective laser melting The Ti-TiN nanocomposite samples were fabricated by SLM of mixtures of spheroidized N 2 plasma-milled Ti samples and raw atomized pure Ti powder. An SLM machine (BLT S200, Xi’an Bright Laser Technologies Co., Ltd., China) equipped with a 500-W Gaussian beam fiber laser was used to obtain block samples with dimensions of 40×10×3 mm. The SLM process was conducted using a laser power of 350 W, scan rate of 900 mm/s, powder layer thickness of 30 µm, and scan spacing of 30 µm. Argon was used as a protective atmosphere in the working chamber during SLM. 2.3. Structural characterization and property evaluation of materials To assess the mechanical properties of the nanocomposite samples in the XY (horizontal) direction (Fig. 2 ), they were evaluated by tensile testing. Samples with a length of 40 mm, gauge width of 3 mm, and thickness of 3 mm, were prepared using an electro-discharge machining technique and subjected to tensile testing according to ISO standard 6892-1:2019. Tensile properties were assessed with a tensile testing machine (Instron 5980, USA) using a crosshead speed of 0.5 mm/min. The data for both tensile and wear tests represent the average of at least five individual measurements. X-ray diffraction (XRD; Philips X’ Pert MPD, Ireland) using Cu–K α radiation ( λ = 0.1541 nm) at 40 kV and 40 mA, scanning electron microscopy (SEM) (TESCAN GAIA3) coupled with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM; JEOL JEM-2100) at a voltage of 200 kV were used to characterize the microstructure of the samples. Microparticle size distribution measurements of the plasma-spheroidized Ti powder samples were performed using a laser diffractometer (Mastersizer 2000, Malvern Instruments, Worcestershire, UK). Microhardness testing was conducted using a digital hardness tester (HVS-1000, Guangshi Instrument Co., Guangzhou, China) by applying a load of 2.94 N. 3. Results and discussion 3.1 Microstructure evolution of Ti powder samples during ball milling Figure 3 (a) shows the XRD patterns of Ti powder samples that were prepared by plasma milling and conventional milling with different ball-to-powder ratios and milling times under N 2 atmosphere. All samples after ball milling exhibited obvious broadening of the diffraction peaks of Ti compared with those of the raw CP-Ti powder, which was caused by the grain refinement during ball milling. The XRD pattern of N 2 -P-milling-1 only contained diffraction peaks consistent with Ti; no obvious new phase was detected. In contrast, the XRD pattern of the N 2 -P-milling-2 sample contained peaks attributed to TiN. These results indicate that a higher ball-to-powder ratio facilitated the synthesis of TiN, which was attributed to the increase of mechanical energy with ball-to-powder ratio. No TiN peaks were observed in the XRD pattern of the N 2 -C-milling sample, which was prepared under the same ball milling conditions as N 2 -P-milling-2 but without plasma exposure. This finding is consistent with a previous observation that the chemical reaction between N 2 and Ti by conventional ball milling requires a very long time (e.g., 300 h) [ 20 ]. The facile synthesis of the TiN phase during the N 2 plasma milling process is attributed to charged N radicals produced by the N 2 discharge interacting with the surface of the Ti metal powder, promoting dislocation movement and grain-boundary migration [ 21 ], as shown in Fig. 3 (b). These results indicate that N 2 plasma milling might provide a simple route to fabricate suitable precursors for TiN nanophase-reinforced Ti-based nanocomposites. The SEM image in Fig. 4 (a) reveals that most of the particles in the N 2 -P-milling-2 sample were less than 10 µm in diameter. Elemental mapping results indicated that the TiN phase was uniformly dispersed in the milled Ti powder, as shown in Fig. 4 (a-c). TEM detected agglomerates with diameters of 100–300 nm in the milled Ti powder (Fig. 4 (d)). Electron diffraction patterns and high-resolution transmission electron microscopy (HRTEM) images (Fig. 4 (e) and (f), respectively) confirmed that TiN and Ti coexisted in the milled samples, consistent with the XRD analysis results. HRTEM revealed the presence of TiN phases with both nanocrystalline and amorphous structures, as shown in Fig. 4 (f). 3.2 Microstructure evolution of milled Ti nanocomposite powders during plasma spheroidization A fine powder with a narrow particle size distribution is required to achieve sufficient flowability for the SLM process. To obtain plasma-milled Ti nanocomposite powder samples with a combination of good flowability and high TiN concentration, they were treated by plasma spheroidization. The SEM image of the N 2 -P-milling-2 Ti-TiN nanocomposite powder in Fig. 5 (a) shows that the Ti particles mostly changed from irregular shapes to spherical after spheroidization treatment. The same behavior was observed for the N 2 -P-milling-1 sample, as given in Fig. S1. The mean particle diameter of the N2-P-milling-2 Ti-TiN nanocomposite powder after spheroidization was 32.5 µm, as presented in Fig. 5 (b). Chemical analysis by EDS mapping indicated that Ti and N were uniformly distributed in the spherical particles (Fig. 5 (c) and (d), respectively). Figure 6 shows the XRD patterns and corresponding Rietveld refinement results obtained for the N 2 -P-milling-1 and N 2 -P-milling-2 samples after plasma spheroidization. After plasma spheroidization, the TiN phase was detected in both N 2 -P-milling-2 and N 2 -P-milling-1 samples. This indicates that N radicals adsorbed on the surface of the milled metallic Ti powder still moved to form TiN crystal nuclei, leading to the growth of more TiN during plasma spheroidization. Previous work indicated that the TiN phase synthesized through the in-situ Ti–N reaction exhibited a high melting point of ~ 3200 K [ 26 ]. To further examine the microstructure evolution of the Ti-TiN nanocomposites after plasma spheroidization, Rietveld refinement of the XRD patterns of the samples was conducted (Fig. 6 (b) and (c)). The refinement gave residual statistics of Rp = 7.15, Rwp = 9.47, and χ 2 = 2.05 for the N 2 -P-milling-1 sample and Rp = 7.69, Rwp = 9.98, and χ 2 = 1.67 for the N 2 -P-milling-2 sample. Rietveld profile fitting indicated that the TiN concentration of the N 2 -P-milling-1 and N 2 -P-milling-2 samples after spheroidization was 30 and 20 wt%, respectively. Therefore, spheroidization increased the concentration of the TiN phase in the milled Ti-TiN nanocomposites. 3.3 Microstructure evolution of Ti-TiN nanocomposite powders during SLM processing The spheroidized N 2 -P-milling-1 and N 2 -P-milling-2 Ti-TiN nanocomposite powders were then mixed with the spherical CP-Ti powder at a weight ratio of 1:9 to form Ti-3.0 wt% TiN and Ti-2.2 wt% TiN nanocomposite powders, respectively. Subsequent SLM processing of the Ti-3.0 wt% TiN and Ti-2.2 wt% TiN nanocomposite powders gave samples denoted as as-SLM Ti-3TiN and as-SLM Ti-2.2TiN, respectively. Figure 7 shows SEM images of the typical morphology in the XY direction of the as-SLM CP-Ti and as-SLM Ti-TiN samples after chemical etching. As can be seen in Fig. 7 (a) and (b), the as-SLM CP-Ti sample formed from the CP-Ti powder mainly consisted of large lath- or lens-like martensitic α′ crystals with a maximum thickness of around 1.5 µm. The formation of this typical martensite α′ phase is attributed to the very fast cooling rate during the SLM processing enabling diffusion-controlled phase transformation [ 1 , 11 , 27 ]. According to previous work, lath-like martensite forms at cooling rates of around 10 3 K/s [ 28 – 30 ], whereas finer lath-like or acicular martensitic morphology is formed at higher cooling rates of 10 5 –10 6 K/s [ 29 – 31 ]. The microstructure of as-SLM CP-Ti is different from that of the as-SLM Ti-TiN nanocomposites, in which the as-SLM Ti-TiN regions consisted of numerous acicular-like grains and some lath-like grains, as shown in Fig. 7 (c-f). The two distinct areas of acicular- and lath-like grains should originate from the spheroidized N 2 -P-milling Ti-TiN nanocomposite and CP-Ti components, respectively, as shown in Fig. 7 (c) and (e). Thus, the Ti-TiN nanocomposite powder has dual-scale characteristics, which leads to a dual-scale morphology after SLM forming. The areas with acicular-like structure consist of fine grains, whereas that with the lath-like structure has coarse grains, although this is finer in the Ti-TiN nanocomposites than that in the as-SLM CP-Ti sample. With the increase of TiN content, the microstructure of the Ti matrix refines gradually; the as-SLM Ti-3TiN nanocomposite exhibited the most refined microstructure of the samples. This is attributed to that the TiN particles with high melting point could exist in the molten pool and serve as effective heterogeneous nucleation sites, increasing the nucleation rate of Ti grains and pinning the grain boundaries to lower the threshold for grain growth of acicular-like Ti, contributing to the grain refinement [ 8 , 15 , 32 – 33 ]. Previous studies [ 34 – 35 ] confirmed that grain size decreased when the nitrogen concentration was increased Ti matrix. These observations indicate that a small amount of TiN strongly affects the microstructural evolution of Ti-TiN alloys during SLM processing. To further evaluate the microstructure and crystal structure of the as-SLM Ti-TiN samples, TEM analysis of the as-SLM Ti-3TiN nanocomposite was performed; the results are shown in Figs. 8 and 9 . Figure 8 (a) and (b) reveal that the as-SLM Ti-3TiN nanocomposite contained both large lath-like Ti crystals with a thickness of around 700–1000 nm and fine acicular-like Ti crystals that were around 50–300 nm thick. The corresponding electron diffraction patterns of the TiN phase and HCP crystal structure of the Ti matrix are presented in Fig. 8 (b). These images confirm that the TiN phase remained in the Ti matrix after SLM processing. Previous work revealed that the reason for the formation of a fine acicular-like structure is that a ceramic reinforcing phase synthesized in the Ti matrix can inhibit the growth of martensite grains during SLM forming [ 15 , 33 , 36 ]. This is consistent with the stacking fault regions with a width of 10 nm observed near the tip of the acicular-like Ti phase, as shown in Fig. 8 (c) and (d). This observation implies that the presence of a TiN pinning force inhibits the growth of the acicular-like Ti phase [ 15 ], which leads to stress at the interfaces between the TiN particles and Ti grain boundaries during the cooling process. The generated stress segregated at the boundaries of the acicular-like Ti phase induced numerous stacking faults. The detailed microstructure of the TiN phase in the as-SLM Ti-3TiN nanocomposite was also analyzed using TEM combined with EDS, as presented in Fig. 9 . TiN nanocrystals with particle sizes of less than 200 nm and growth twins were observed in the Ti matrix, as shown in Fig. 9 (a). A corresponding selected-area electron diffraction pattern revealed that the orientation relationship between the TiN phase and Ti matrix was ( \(\stackrel{-}{1}\) 10)TiN//(010)Ti (Fig. 9 (b)). HRTEM observation of the TiN phase unveiled the presence of two twin boundaries, which are labeled A and B in Fig. 9 (c). Figure 9 (d) reveals that twin boundary B has (111) as its twinning plane (mirror symmetry). Growth twins were observed after spark plasma sintering of near-fully dense fine-grained TiN at 1500°C and can promote high hardness [ 37 ]. An annealed TiN film grown on a single-crystalline alumina substrate at 1400°C also contained low-index interfaces and twin boundaries [ 38 ]. The high temperature during SLM processing promoted the formation of twinned crystals in the TiN phase. In addition, EDS analysis of the area inside the red circle in Fig. 9 (a) revealed N accumulation in the as-SLM Ti-3TiN nanocomposite (Fig. 9 (e-g)), which further confirmed the existence of TiN in the sample. 3.4 Mechanical properties of SLM Ti/TiN-based nanocomposites The density and hardness of the as-SLM Ti-TiN nanocomposites were measured and compared with those of the as-SLM CP-Ti sample, as shown in Fig. 10 . Although both the as-SLM Ti-3TiN and as-SLM Ti-2.2TiN nanocomposites exhibited slightly lower densities than that of the as-SLM CP-Ti sample, microhardness increased from 192 HV for the as-SLM CP-Ti sample to 344 and 365 HV for the as-SLM Ti-2.2TiN and as-SLM Ti-3TiN samples, respectively. This increase of hardness was attributed to the strengthening effect of TiN particles in the nanocomposites. The nanohardness distribution in the as-SLM Ti-3TiN nanocomposite was investigated by nanoindentation tests, as shown in Fig. 11 . The statistics of the corresponding Vickers hardness data revealed a bimodal nanohardness distribution in the as-SLM Ti-3TiN nanocomposite (Fig. 11 (b)). Comparing two different indents in the as-SLM Ti-3TiN nanocomposite as an example, as shown in Fig. 11 (c and d), a lower nanohardness of 255 and higher nanohardness of 438 were obtained for the coarse-grain lath-like Ti phase and fine-grain acicular-like Ti region, respectively. These observations further confirmed the dual-scale morphology of the as-SLM Ti-3TiN nanocomposite. Obviously, both regions contribute to the average hardness of the whole nanocomposite. The combination of the refined acicular-like Ti matrix and TiN reinforcing particles in the areas with higher nanohardness increase both the microhardness and strength of the as-SLM Ti-TiN nanocomposite compared with those of the as-SLM CP-Ti sample. Nanoindentation testing of the as-SLM CP-Ti sample gave a Vickers hardness and elastic modulus of around 256 and 108 GPa, respectively, which are consistent with the nanohardness of the coarse-grain lath-like Ti phase regions of the Ti/TiN nanocomposites. To further evaluate the effect of the TiN phase on the mechanical properties of the Ti-based nanocomposites, tensile testing was performed at room temperature. Figure 12 shows the engineering stress–strain curves of the as-SLM CP-Ti and as-SLM Ti-TiN nanocomposites. The ultimate tensile strength (UTS) of the as-SLM CP-Ti, Ti-2.2TiN, and Ti-3TiN samples was 532, 967, and 1066 MPa, respectively. Thus, the UTS of the nanocomposite samples was increased by the presence of TiN. However, the total elongation-to-failure was lowered by the presence of TiN, with elongation of 6.8%, 12.2%, and 19.2% obtained for as-SLM Ti-3TiN, Ti-2.2TiN, and CP-Ti samples, respectively. According to previous work [ 8 , 15 ], the increased strength of the as-SLM Ti-TiN samples compared with that of as-SLM CP-Ti is attributed to both the increased dislocation density and refined acicular-like microstructure in the Ti matrix induced by TiN. The higher UTS of the as-SLM Ti-3TiN sample than that of the as-SLM Ti-2.2TiN sample was caused by the higher TiN concentration and smaller grain size of the former than those of the latter. In addition, the presence of coarse lath-like grains increased the plasticity of as-SLM Ti-2.2TiN compared with that of the as-SLM Ti-3TiN sample. Figure 13 shows SEM images of the tensile fracture surfaces of the as-SLM CP-Ti and T-TiN nanocomposites. The tensile fractography of the as-SLM CP-Ti sample was dominated by dimple fracture with large, deep dimples, indicating near-complete ductile fracture behavior (Fig. 13 (a) and (b)). Bimodal dimple sizes were observed for the samples containing TiN, as presented in Fig. 13 (d) and (f). The fracture surfaces of the as-SLM Ti-2.2TiN nanocomposite exhibited a typical dimpled appearance with more small dimples than was the case for as-SLM CP-Ti, also indicating a ductile fracture mechanism (Fig. 13 (c) and (d)). When the TiN content was increased to 3 wt%, as shown in Fig. 13 (e) and (f), the fracture surface displayed dimples, grooves, and cleavage planes. Therefore, it was inferred that the fracture mechanism of the as-SLM Ti-3.0TiN nanocomposite sample involved a mixed mode rather than only ductile fracture. The ductility of the as-SLM Ti-3TiN sample was lower than that of the other samples because it contained more and smaller acicular-like structures. 4. Conclusions A combination of N 2 plasma milling, plasma spheroidization, and SLM forming was used to fabricate Ti-TiN nanocomponents with high strength and ductility. The following conclusions were obtained: (1) The N 2 plasma milling process provided a simple route to fabricate a TiN nanophase-reinforced milled Ti nanocomposite powder. After subsequent spheroidization, the concentration of the TiN phase in the N 2 -P-milling-1 and N 2 -P-milling-2 nanocomposites was 22wt.% and 30wt.%, respectively. (2) The as-SLM CP-Ti sample consisted of lath-like or lens-like martensitic α′ crystals with a maximum thickness of around 1.5 µm. Mixing the CP-Ti powder with the milled Ti-TiN nanocomposites and subsequent SLM led to the formation of dual-scale morphologies containing both coarse lath-like (700–1000 nm thick) and fine acicular-like grains with thicknesses of 50–300 nm, respectively. (3) Both the as-SLM Ti-2.2TiN and Ti-3TiN samples exhibited higher UTS than that of the as-SLM CP-Ti sample ( 967, 1066, and 532 MPa, respectively), but this came at the expense of the total elongation (12.2%, 6.8%, and 19.2%, respectively). The strengthening mechanisms of the TiN particles in the Ti matrix were acicular-like grain refinement, dislocation, and TiN dispersion strengthening. Declarations ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Author Contribution Chuanjun Zang and Jianhua Yin: Conceptualization, Data curation, Formal analysis, Investigation, Methodology; Shaofeng Hong: Conceptualization, Formal analysis, Methodology; Meiqin Zeng: Conceptualization, Investigation; Zhongchen Lu: Data curation, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Writing – original draft, Writing – review & editing. All authors contributed to the general discussion. Acknowledgments This work was supported by Guangdong Provincial Natural Science Foundation (No. 2024A1515012843) and Fundamental Research Funds for the Central Universities (No. 2023ZYGXZR004). We thank Natasha Lundin, PhD, from Liwen Bianji (Edanz) ( www.liwenbianji.cn ) for editing the English text of a draft of this manuscript. Data Availability Data is provided within the manuscript or supplementary information files. References Li XP, Van Humbeeck J, Kruth JP. Selective laser melting of weak-textured commercially pure titanium with high strength and ductility: A study from laser power perspective. Mater Des. 2017;116:352–8. Gunasekaran J, Sevvel P, Solomon IJ. Metallic materials fabrication by selective laser melting: A review, Mater. Today: Proc. 37 (2021) 252–256. Shen J, Chen B, Umeda J, Kondoh K. Microstructure and mechanical properties of CP-Ti fabricated via powder metallurgy with non-uniformly dispersed impurity solutes. Mater Sci Eng A. 2018;716:1–10. 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Metall Mater Trans A. 2004;35:3071–7. Attar H, Ehtemam-Haghighi S, Kent D, Wu X, Dargusch MS. Comparative study of commercially pure titanium produced by laser engineered net shaping, selective laser melting and casting processes. Mater Sci Eng A. 2017;705:385–93. Xu W, Xiao S, Lu X, Chen G, Liu C, Qu X. Fabrication of commercial pure Ti by selective laser melting using hydride-dehydride titanium powders treated by ball milling. J Mater Sci Technol. 2019;35:322–7. Yang J, Yu H, Yin J, Gao M, Wang Z, Zeng X. Formation and control of martensite in Ti-6Al-4V alloy produced by selective laser melting. Mater Des. 2016;108:308–18. Zang CJ, Liu WF, Zeng MQ, Liu MX, Cui J, Lu ZC. The influence of NH3 plasma treatment on microstructure and mechanical property of AlSi10Mg alloy fabricated by selective laser melting. Mater Today Commun. 2023;34:105274. Greer AL. Overview: application of heterogeneous nucleation in grain-refining of metals. J Chem Phys. 2016;145:211704. Kondoh K, Sun B, Li S, Imai H, Umeda J. Experimental and theoretical analysis of nitrogen solid-solution strengthening of PM titanium. Int J Powder Met. 2014;50:35–40. Bramfitt BL. The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron. Metall Trans. 1970;1(7):1987–95. Xu W, Brandt M, Sun S, et al. Additive manufacturing of strong and ductile Ti–6Al–4V by selective laser melting via in situ martensite decomposition. Acta Mater. 2015;85:74–84. Moshtaghioun BM, Gómez-García D, Domínguez-Rodríguez A. Spark plasma sintering of titanium nitride in nitrogen: does it affect the sinterability and the mechanical properties? J Eur Ceram Soc. 2018;38(4):1190–6. Krekeler T, Rout SS, Krishnamurthy GV, et al. Unprecedented thermal stability of plasmonic titanium nitride films up to 1400° C. Adv Opt Mater. 2021;9(16):2100323. Additional Declarations No competing interests reported. Supplementary Files AppendixA.docx Cite Share Download PDF Status: Published Journal Publication published 17 Oct, 2024 Read the published version in Discover Materials → Version 1 posted Editorial decision: Revision requested 28 Jun, 2024 Reviews received at journal 26 Jun, 2024 Reviews received at journal 22 Jun, 2024 Reviewers agreed at journal 20 Jun, 2024 Reviewers agreed at journal 19 Jun, 2024 Reviewers agreed at journal 18 Jun, 2024 Reviewers invited by journal 18 Jun, 2024 Editor assigned by journal 29 May, 2024 Submission checks completed at journal 29 May, 2024 First submitted to journal 27 May, 2024 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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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-4487360","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":311465339,"identity":"0b28c801-7880-4559-9532-c15b9dca6c9b","order_by":0,"name":"Chuanjun Zang","email":"","orcid":"","institution":"South China University of Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Chuanjun","middleName":"","lastName":"Zang","suffix":""},{"id":311465340,"identity":"4d62b194-3b83-45f8-8dcd-e0bb107b94da","order_by":1,"name":"Jianhua Yin","email":"","orcid":"","institution":"South China University of Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Jianhua","middleName":"","lastName":"Yin","suffix":""},{"id":311465341,"identity":"dbd7428e-7985-4dc1-9002-7befd86c0412","order_by":2,"name":"Shaofeng Hong","email":"","orcid":"","institution":"South China University of Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Shaofeng","middleName":"","lastName":"Hong","suffix":""},{"id":311465342,"identity":"b6334f35-28ee-47e9-9ca4-6022c3750fdd","order_by":3,"name":"Meiqin Zeng","email":"","orcid":"","institution":"South China University of Technology","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Meiqin","middleName":"","lastName":"Zeng","suffix":""},{"id":311465343,"identity":"463ef0c4-c8a0-4cbb-8d45-6b2ea4354efe","order_by":4,"name":"Zhongchen Lu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAwklEQVRIiWNgGAWjYLCCBww2UBYbsVoSGNKAJDNpWg6ToEW3vf3hh8S28/b8M/IPMHwoO8zAP7sBvxazMweSJRLbbifOuJHMwDjj3GEGiTsHCGi5kXCMAaglwUAimYGZt+0wg4FEAgEt9x+2AbWcswdr+UuUlhvMbEAtBxg3gLQwEqXlTBqzRMK55MQZZx4bHOw5l84jcYOQluPHH374UGZnz9+e+PDBjzJrOf4ZBLSggANAzEOC+lEwCkbBKBgFuAAAMV1B2s+sKo0AAAAASUVORK5CYII=","orcid":"","institution":"South China University of Technology","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Zhongchen","middleName":"","lastName":"Lu","suffix":""}],"badges":[],"createdAt":"2024-05-28 01:10:31","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4487360/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4487360/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s43939-024-00120-7","type":"published","date":"2024-10-17T15:56:53+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":58110050,"identity":"d8206484-f530-4e5b-93d1-77221289fc4c","added_by":"auto","created_at":"2024-06-11 09:03:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":339140,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic representation of the in situ synthesis of TiN nanophase-reinforced Ti nanocomposites by plasma ball milling, spheroidization, and selective laser melting.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/f7584b57ce2fb783c0ea96fc.png"},{"id":58110051,"identity":"3086ba1d-b1e3-4c30-9048-3f7b12de45f2","added_by":"auto","created_at":"2024-06-11 09:03:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":474134,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic of the growth directions of a Ti-TiN nanocomposite sample.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/c9f2b742197dabb122cb84dd.png"},{"id":58110052,"identity":"c2d4e0e8-74e9-40ff-8998-80cd34fe3583","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":209773,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns of Ti powder samples prepared by plasma milling and conventional milling. (b) Schematic diagram of the formation of TiN phase during nitrogen plasma milling.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/3e42b1168c88d15e8c88bd16.png"},{"id":58110054,"identity":"7166b70f-e6cf-45af-b37e-c210b3ca0e7f","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1240676,"visible":true,"origin":"","legend":"\u003cp\u003e(a) SEM image of the N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 Ti-TiN nanocomposite powder. Associated EDS elemental mapping results showing the distributions of (b) Ti and (c) N in this sample. (d) TEM image and associated (e) diffraction rings and (f) HRTEM image of the N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 Ti-TiN nanocomposite powder.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/6d65d0d4c1056dd93f849838.png"},{"id":58110347,"identity":"7abecdb5-40bc-4348-8fe8-7110e6daea3e","added_by":"auto","created_at":"2024-06-11 09:11:08","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":848617,"visible":true,"origin":"","legend":"\u003cp\u003e(a) SEM image of the N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 Ti-TiN nanocomposite powder after plasma spheroidization. (b) Corresponding particle size distribution. Associated EDS elemental mapping results showing the distributions of (c) Ti and (d) N in the sample.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/29c49aaef6eeb79a8c314a3e.png"},{"id":58110056,"identity":"78aca148-d41f-4795-aebb-8d5d93bc1dfc","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":83545,"visible":true,"origin":"","legend":"\u003cp\u003e(a) XRD patterns of plasma-milled\u003cstrong\u003e \u003c/strong\u003eTi-TiN nanocomposite powders after plasma spheroidization. Rietveld refinement results of the XRD patterns of the plasma-spheroidized Ti-TiN nanocomposite powders obtained from (b) N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 and (c) N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 samples.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/a61e4849a9705e6bf80d0c7d.png"},{"id":58110346,"identity":"e37cb326-3cab-49b1-a50d-1978f848a4d8","added_by":"auto","created_at":"2024-06-11 09:11:08","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":3503321,"visible":true,"origin":"","legend":"\u003cp\u003eBackscattering SEM images of the transverse cross section of SLM Ti-TiN nanocomposites after chemical etching. (a, b) as-SLM CP-Ti, (c, d) as-SLM Ti-2.2TiN, and (e, f) as-SLM Ti-3TiN.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/b78a0f92960e99cc13fb0ca2.png"},{"id":58110060,"identity":"79db47fd-2b22-4b81-a66b-49796686894c","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1939003,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images of the as-SLM Ti-3TiN nanocomposite. (a) Bright-field image of the Ti matrix. Inset is an electron diffraction pattern of the lath-like Ti crystal obtained from the region in the dashed yellow circle. (b) Electron diffraction pattern for (a). (c, d) Magnified views of the acicular-like structure with stress-induced stacking faults.\u003c/p\u003e","description":"","filename":"image8.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/e956ee5ca1bace9f3b6ea30f.png"},{"id":58110057,"identity":"767c2966-8a30-4ee3-82fc-212e06a26620","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":1740096,"visible":true,"origin":"","legend":"\u003cp\u003eTEM images of the as-SLM Ti-3TiN nanocomposite. (a) Bright-field image of a TiN particle in the Ti matrix. (b) Selected-area electron diffraction pattern and (c) HRTEM image of the area in the red circle in (a). Twin boundariesin the TiN phase are labeled A and B. (d) HRTEM image of twin boundary B. (e) Higher-magnification TEM image of microstructures in the red circle in (a). Corresponding EDS maps of (f) Ti and (g) N for the TEM image in (e).\u003c/p\u003e","description":"","filename":"image9.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/c5a413bb8d20b05316346867.png"},{"id":58110063,"identity":"1758a5aa-001b-42b5-99a3-23314373cac6","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":473795,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Density and (b) microhardness of pure Ti and Ti-TiN nanocomposite samples processed by selective laser melting.\u003c/p\u003e","description":"","filename":"image10.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/91d0b8042c355b2367518102.png"},{"id":58110064,"identity":"a0597f05-e56f-44cc-b317-351618f13800","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1524212,"visible":true,"origin":"","legend":"\u003cp\u003e(a) SEM images of the as-SLM Ti-3TiN nanocomposite after nanoindentation testing. (b) Vickers hardness distribution of the as-SLM Ti-3TiN nanocomposite obtained from the nanoindentation measurements. Higher-magnification SEM images of nanoindentations in (c) coarse-grain lath-like Ti and (d) fine-grain acicular-like Ti regions of the as-SLM Ti-3TiN nanocomposite.\u003c/p\u003e","description":"","filename":"image11.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/729960f6473ddc682c199480.png"},{"id":58110062,"identity":"6d0f9505-9677-4fb1-80a2-834ca301bca2","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":56695,"visible":true,"origin":"","legend":"\u003cp\u003eStress–strain curves of the as-SLM CP-Ti and T-TiN nanocomposites.\u003c/p\u003e","description":"","filename":"image12.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/8d321e8ac454815ae1065233.png"},{"id":58110348,"identity":"066a8044-c61b-42b9-b355-4dd40cb24c50","added_by":"auto","created_at":"2024-06-11 09:11:08","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":1843116,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of the \u003ca href=\"https://www.sciencedirect.com/topics/engineering/fracture-surface\" title=\"Learn more about fracture surface from ScienceDirect's AI-generated Topic Pages\"\u003efracture surface\u003c/a\u003es of \u003ca href=\"https://www.sciencedirect.com/topics/engineering/tensile-specimen\" title=\"Learn more about tensile specimens from ScienceDirect's AI-generated Topic Pages\"\u003etensile specimens\u003c/a\u003e of (a, b) as-SLM CP-Ti, (c, d) as-SLM Ti-2.2TiN, and (e, f) as-SLM Ti-3TiN nanocomposite samples.\u003c/p\u003e","description":"","filename":"image13.png","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/18e40fb3a57296c71d5fbf85.png"},{"id":67148683,"identity":"33b50d4d-72bf-4ce1-9143-52661c5330cb","added_by":"auto","created_at":"2024-10-21 16:06:06","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":16496135,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/29bd13bb-249a-4564-a66d-14dfd2398f67.pdf"},{"id":58110055,"identity":"63ad9a3e-193b-475e-ad10-4fd2e779afca","added_by":"auto","created_at":"2024-06-11 09:03:08","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":944450,"visible":true,"origin":"","legend":"","description":"","filename":"AppendixA.docx","url":"https://assets-eu.researchsquare.com/files/rs-4487360/v1/4fe3f85e8fe7b27ccef671b8.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fabrication of TiN-doped Ti nanocomposites with high strength and ductility by plasma-assisted ball milling and selective laser melting","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSelective laser melting (SLM) is increasingly being used to fabricate commercially pure titanium (CP-Ti) and titanium (Ti) alloys and matrix nanocomposites [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. However, the industrial use of CP-Ti is limited by its poor wear resistance and low hardness [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. The hardness and wear performance of CP-Ti can be enhanced by addition of second-phase particles to induce dislocation motion [\u003cspan additionalcitationids=\"CR6 CR7 CR8 CR9\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Commonly used reinforcing phases for CP-Ti include ceramic particles such as TiB [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], TiB\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], TiN [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e],TiC [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], SiC [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], and B\u003csub\u003e4\u003c/sub\u003eC [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn general, the average cooling rate in SLM forming is about 10\u003csup\u003e4\u003c/sup\u003e k/s, which means that β-Ti with body-centered cubic (BCC) structure may be transformed into α'-Ti with hexagonal close packed (HCP) structure, that is, martensite, by coherent shear [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. However, because there are no obstacles to solidification and nucleation, Ti grains grow epitaxially at high temperature during SLM, leading to Ti alloys with a microstructure of coarse β columnar crystals and disordered needle-like α' martensite [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Addition of reinforcing particles during the three-dimensional printing of Ti alloys can increase the concentration of nucleation sites to promote the formation of internal structures, which can raise the recrystallization rate of grains [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The in situ synthesis of a reinforced phase in a Ti matrix has a larger effect on the alloy microstructure than that of directly adding a reinforced phase [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eNitrogen (N) is an effective strengthening element in Ti alloys; it can be dissolved in HCP α-Ti crystals to form titanium nitride (TiN) interstitial solid solutions [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Recently, SLM forming with in situ N solid solutions has been used to fabricate Ti materials with balanced strength and ductility [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. High-energy ball milling is an effective route to promote the direct reaction of Ti metal with molecular nitrogen (N\u003csub\u003e2\u003c/sub\u003e) at room temperature [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Discharge plasma can accelerate the kinetics of compound synthesis and powder activation. We synthesized TiH\u003csub\u003e2\u003c/sub\u003e and TiN phases in a short time by ammonia plasma-assisted ball milling of Ti powder [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this study, we attempt to fabricate CP-Ti nanocomposites with high strength and ductility by economical methods. The in situ synthesis of a TiN reinforcing phase in a Ti matrix using a combination of N\u003csub\u003e2\u003c/sub\u003e plasma milling, plasma spheroidization, and SLM forming is investigated, leading to the fabrication of Ti-TiN nanocomposites. Two different N\u003csub\u003e2\u003c/sub\u003e plasma milling processes accompanied with plasma spheroidization are first used to fabricate Ti-TiN nanocomposite powder samples, which are then mixed with spherical CP-Ti powder and printed by SLM forming. The microstructure and mechanical properties of the resulting Ti-TiN nanocomposites are examined.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Material preparation\u003c/h2\u003e \u003cp\u003eThe Ti-TiN nanocomposites were synthesized from raw atomized spherical CP-Ti powder with a diameter of about 15\u0026ndash;53 \u0026micro;m (Changsha TIJO Metal Materials Co., Ltd., China) using a plasma milling device (PBMS, South China University of Technology, China). The working principle of this device has been reported previously [\u003cspan additionalcitationids=\"CR23 CR24\" citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. In this work, mechanical ball milling was conducted during exposure to a homogeneous non-thermal N\u003csub\u003e2\u003c/sub\u003e gas plasma, which was generated with a pulse peak voltage of 25 kV and discharge frequency of 11.5 kHz using a single dielectric barrier discharge reactor. Ball milling was conducted at 1350 rpm. Two different N\u003csub\u003e2\u003c/sub\u003e plasma milling regimes were used. The first used a 30:1 weight ratio of balls to powder for 14 h (denoted as N\u003csub\u003e2\u003c/sub\u003e-P-milling-1) and the second used a 60:1 weight ratio of balls to powder for 10 h (denoted as N\u003csub\u003e2\u003c/sub\u003e-P-milling-2). For comparison, conventional milling without plasma discharge in the milling jar was carried out with a 60:1 weight ratio of balls to powder for 10 h (denoted as N\u003csub\u003e2\u003c/sub\u003e-C-milling). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the ball-milled Ti powders were treated by plasma spheroidization to improve their flowability. The spheroidized N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 and N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 Ti powder samples were mixed with CP-Ti powder at a weight ratio of 1:9 and subjected to SLM processing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Selective laser melting\u003c/h2\u003e \u003cp\u003eThe Ti-TiN nanocomposite samples were fabricated by SLM of mixtures of spheroidized N\u003csub\u003e2\u003c/sub\u003e plasma-milled Ti samples and raw atomized pure Ti powder. An SLM machine (BLT S200, Xi\u0026rsquo;an Bright Laser Technologies Co., Ltd., China) equipped with a 500-W Gaussian beam fiber laser was used to obtain block samples with dimensions of 40\u0026times;10\u0026times;3 mm. The SLM process was conducted using a laser power of 350 W, scan rate of 900 mm/s, powder layer thickness of 30 \u0026micro;m, and scan spacing of 30 \u0026micro;m. Argon was used as a protective atmosphere in the working chamber during SLM.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Structural characterization and property evaluation of materials\u003c/h2\u003e \u003cp\u003eTo assess the mechanical properties of the nanocomposite samples in the XY (horizontal) direction (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), they were evaluated by tensile testing. Samples with a length of 40 mm, gauge width of 3 mm, and thickness of 3 mm, were prepared using an electro-discharge machining technique and subjected to tensile testing according to ISO standard 6892-1:2019. Tensile properties were assessed with a tensile testing machine (Instron 5980, USA) using a crosshead speed of 0.5 mm/min. The data for both tensile and wear tests represent the average of at least five individual measurements.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eX-ray diffraction (XRD; Philips X\u0026rsquo; Pert MPD, Ireland) using Cu\u0026ndash;K\u003cem\u003eα\u003c/em\u003e radiation (\u003cem\u003eλ\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.1541 nm) at 40 kV and 40 mA, scanning electron microscopy (SEM) (TESCAN GAIA3) coupled with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM; JEOL JEM-2100) at a voltage of 200 kV were used to characterize the microstructure of the samples. Microparticle size distribution measurements of the plasma-spheroidized Ti powder samples were performed using a laser diffractometer (Mastersizer 2000, Malvern Instruments, Worcestershire, UK). Microhardness testing was conducted using a digital hardness tester (HVS-1000, Guangshi Instrument Co., Guangzhou, China) by applying a load of 2.94 N.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Microstructure evolution of Ti powder samples during ball milling\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) shows the XRD patterns of Ti powder samples that were prepared by plasma milling and conventional milling with different ball-to-powder ratios and milling times under N\u003csub\u003e2\u003c/sub\u003e atmosphere. All samples after ball milling exhibited obvious broadening of the diffraction peaks of Ti compared with those of the raw CP-Ti powder, which was caused by the grain refinement during ball milling. The XRD pattern of N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 only contained diffraction peaks consistent with Ti; no obvious new phase was detected. In contrast, the XRD pattern of the N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 sample contained peaks attributed to TiN. These results indicate that a higher ball-to-powder ratio facilitated the synthesis of TiN, which was attributed to the increase of mechanical energy with ball-to-powder ratio. No TiN peaks were observed in the XRD pattern of the N\u003csub\u003e2\u003c/sub\u003e-C-milling sample, which was prepared under the same ball milling conditions as N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 but without plasma exposure. This finding is consistent with a previous observation that the chemical reaction between N\u003csub\u003e2\u003c/sub\u003e and Ti by conventional ball milling requires a very long time (e.g., 300 h) [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The facile synthesis of the TiN phase during the N\u003csub\u003e2\u003c/sub\u003e plasma milling process is attributed to charged N radicals produced by the N\u003csub\u003e2\u003c/sub\u003e discharge interacting with the surface of the Ti metal powder, promoting dislocation movement and grain-boundary migration [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b). These results indicate that N\u003csub\u003e2\u003c/sub\u003e plasma milling might provide a simple route to fabricate suitable precursors for TiN nanophase-reinforced Ti-based nanocomposites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe SEM image in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) reveals that most of the particles in the N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 sample were less than 10 \u0026micro;m in diameter. Elemental mapping results indicated that the TiN phase was uniformly dispersed in the milled Ti powder, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a-c). TEM detected agglomerates with diameters of 100\u0026ndash;300 nm in the milled Ti powder (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d)). Electron diffraction patterns and high-resolution transmission electron microscopy (HRTEM) images (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(e) and (f), respectively) confirmed that TiN and Ti coexisted in the milled samples, consistent with the XRD analysis results. HRTEM revealed the presence of TiN phases with both nanocrystalline and amorphous structures, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(f).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Microstructure evolution of milled Ti nanocomposite powders during plasma spheroidization\u003c/h2\u003e \u003cp\u003eA fine powder with a narrow particle size distribution is required to achieve sufficient flowability for the SLM process. To obtain plasma-milled Ti nanocomposite powder samples with a combination of good flowability and high TiN concentration, they were treated by plasma spheroidization. The SEM image of the N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 Ti-TiN nanocomposite powder in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) shows that the Ti particles mostly changed from irregular shapes to spherical after spheroidization treatment. The same behavior was observed for the N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 sample, as given in Fig. S1. The mean particle diameter of the N2-P-milling-2 Ti-TiN nanocomposite powder after spheroidization was 32.5 \u0026micro;m, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b). Chemical analysis by EDS mapping indicated that Ti and N were uniformly distributed in the spherical particles (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(c) and (d), respectively).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the XRD patterns and corresponding Rietveld refinement results obtained for the N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 and N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 samples after plasma spheroidization. After plasma spheroidization, the TiN phase was detected in both N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 and N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 samples. This indicates that N radicals adsorbed on the surface of the milled metallic Ti powder still moved to form TiN crystal nuclei, leading to the growth of more TiN during plasma spheroidization. Previous work indicated that the TiN phase synthesized through the in-situ Ti\u0026ndash;N reaction exhibited a high melting point of ~\u0026thinsp;3200 K [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. To further examine the microstructure evolution of the Ti-TiN nanocomposites after plasma spheroidization, Rietveld refinement of the XRD patterns of the samples was conducted (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) and (c)). The refinement gave residual statistics of Rp\u0026thinsp;=\u0026thinsp;7.15, Rwp\u0026thinsp;=\u0026thinsp;9.47, and χ\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;2.05 for the N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 sample and Rp\u0026thinsp;=\u0026thinsp;7.69, Rwp\u0026thinsp;=\u0026thinsp;9.98, and χ\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;1.67 for the N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 sample. Rietveld profile fitting indicated that the TiN concentration of the N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 and N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 samples after spheroidization was 30 and 20 wt%, respectively. Therefore, spheroidization increased the concentration of the TiN phase in the milled Ti-TiN nanocomposites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Microstructure evolution of Ti-TiN nanocomposite powders during SLM processing\u003c/h2\u003e \u003cp\u003eThe spheroidized N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 and N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 Ti-TiN nanocomposite powders were then mixed with the spherical CP-Ti powder at a weight ratio of 1:9 to form Ti-3.0 wt% TiN and Ti-2.2 wt% TiN nanocomposite powders, respectively. Subsequent SLM processing of the Ti-3.0 wt% TiN and Ti-2.2 wt% TiN nanocomposite powders gave samples denoted as as-SLM Ti-3TiN and as-SLM Ti-2.2TiN, respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows SEM images of the typical morphology in the XY direction of the as-SLM CP-Ti and as-SLM Ti-TiN samples after chemical etching. As can be seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(a) and (b), the as-SLM CP-Ti sample formed from the CP-Ti powder mainly consisted of large lath- or lens-like martensitic α\u0026prime; crystals with a maximum thickness of around 1.5 \u0026micro;m. The formation of this typical martensite α\u0026prime; phase is attributed to the very fast cooling rate during the SLM processing enabling diffusion-controlled phase transformation [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. According to previous work, lath-like martensite forms at cooling rates of around 10\u003csup\u003e3\u003c/sup\u003e K/s [\u003cspan additionalcitationids=\"CR29\" citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], whereas finer lath-like or acicular martensitic morphology is formed at higher cooling rates of 10\u003csup\u003e5\u003c/sup\u003e\u0026ndash;10\u003csup\u003e6\u003c/sup\u003e K/s [\u003cspan additionalcitationids=\"CR30\" citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The microstructure of as-SLM CP-Ti is different from that of the as-SLM Ti-TiN nanocomposites, in which the as-SLM Ti-TiN regions consisted of numerous acicular-like grains and some lath-like grains, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(c-f). The two distinct areas of acicular- and lath-like grains should originate from the spheroidized N\u003csub\u003e2\u003c/sub\u003e-P-milling Ti-TiN nanocomposite and CP-Ti components, respectively, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e(c) and (e). Thus, the Ti-TiN nanocomposite powder has dual-scale characteristics, which leads to a dual-scale morphology after SLM forming. The areas with acicular-like structure consist of fine grains, whereas that with the lath-like structure has coarse grains, although this is finer in the Ti-TiN nanocomposites than that in the as-SLM CP-Ti sample. With the increase of TiN content, the microstructure of the Ti matrix refines gradually; the as-SLM Ti-3TiN nanocomposite exhibited the most refined microstructure of the samples. This is attributed to that the TiN particles with high melting point could exist in the molten pool and serve as effective heterogeneous nucleation sites, increasing the nucleation rate of Ti grains and pinning the grain boundaries to lower the threshold for grain growth of acicular-like Ti, contributing to the grain refinement [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Previous studies [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] confirmed that grain size decreased when the nitrogen concentration was increased Ti matrix. These observations indicate that a small amount of TiN strongly affects the microstructural evolution of Ti-TiN alloys during SLM processing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further evaluate the microstructure and crystal structure of the as-SLM Ti-TiN samples, TEM analysis of the as-SLM Ti-3TiN nanocomposite was performed; the results are shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e and \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) and (b) reveal that the as-SLM Ti-3TiN nanocomposite contained both large lath-like Ti crystals with a thickness of around 700\u0026ndash;1000 nm and fine acicular-like Ti crystals that were around 50\u0026ndash;300 nm thick. The corresponding electron diffraction patterns of the TiN phase and HCP crystal structure of the Ti matrix are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b). These images confirm that the TiN phase remained in the Ti matrix after SLM processing. Previous work revealed that the reason for the formation of a fine acicular-like structure is that a ceramic reinforcing phase synthesized in the Ti matrix can inhibit the growth of martensite grains during SLM forming [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. This is consistent with the stacking fault regions with a width of 10 nm observed near the tip of the acicular-like Ti phase, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(c) and (d). This observation implies that the presence of a TiN pinning force inhibits the growth of the acicular-like Ti phase [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], which leads to stress at the interfaces between the TiN particles and Ti grain boundaries during the cooling process. The generated stress segregated at the boundaries of the acicular-like Ti phase induced numerous stacking faults.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe detailed microstructure of the TiN phase in the as-SLM Ti-3TiN nanocomposite was also analyzed using TEM combined with EDS, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. TiN nanocrystals with particle sizes of less than 200 nm and growth twins were observed in the Ti matrix, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a). A corresponding selected-area electron diffraction pattern revealed that the orientation relationship between the TiN phase and Ti matrix was (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\stackrel{-}{1}\\)\u003c/span\u003e\u003c/span\u003e10)TiN//(010)Ti (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(b)). HRTEM observation of the TiN phase unveiled the presence of two twin boundaries, which are labeled A and B in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(c). Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(d) reveals that twin boundary B has (111) as its twinning plane (mirror symmetry). Growth twins were observed after spark plasma sintering of near-fully dense fine-grained TiN at 1500\u0026deg;C and can promote high hardness [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. An annealed TiN film grown on a single-crystalline alumina substrate at 1400\u0026deg;C also contained low-index interfaces and twin boundaries [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The high temperature during SLM processing promoted the formation of twinned crystals in the TiN phase. In addition, EDS analysis of the area inside the red circle in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a) revealed N accumulation in the as-SLM Ti-3TiN nanocomposite (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(e-g)), which further confirmed the existence of TiN in the sample.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Mechanical properties of SLM Ti/TiN-based nanocomposites\u003c/h2\u003e \u003cp\u003eThe density and hardness of the as-SLM Ti-TiN nanocomposites were measured and compared with those of the as-SLM CP-Ti sample, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. Although both the as-SLM Ti-3TiN and as-SLM Ti-2.2TiN nanocomposites exhibited slightly lower densities than that of the as-SLM CP-Ti sample, microhardness increased from 192 HV for the as-SLM CP-Ti sample to 344 and 365 HV for the as-SLM Ti-2.2TiN and as-SLM Ti-3TiN samples, respectively. This increase of hardness was attributed to the strengthening effect of TiN particles in the nanocomposites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe nanohardness distribution in the as-SLM Ti-3TiN nanocomposite was investigated by nanoindentation tests, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e. The statistics of the corresponding Vickers hardness data revealed a bimodal nanohardness distribution in the as-SLM Ti-3TiN nanocomposite (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(b)). Comparing two different indents in the as-SLM Ti-3TiN nanocomposite as an example, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003e(c and d), a lower nanohardness of 255 and higher nanohardness of 438 were obtained for the coarse-grain lath-like Ti phase and fine-grain acicular-like Ti region, respectively. These observations further confirmed the dual-scale morphology of the as-SLM Ti-3TiN nanocomposite. Obviously, both regions contribute to the average hardness of the whole nanocomposite. The combination of the refined acicular-like Ti matrix and TiN reinforcing particles in the areas with higher nanohardness increase both the microhardness and strength of the as-SLM Ti-TiN nanocomposite compared with those of the as-SLM CP-Ti sample. Nanoindentation testing of the as-SLM CP-Ti sample gave a Vickers hardness and elastic modulus of around 256 and 108 GPa, respectively, which are consistent with the nanohardness of the coarse-grain lath-like Ti phase regions of the Ti/TiN nanocomposites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further evaluate the effect of the TiN phase on the mechanical properties of the Ti-based nanocomposites, tensile testing was performed at room temperature. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e shows the engineering stress\u0026ndash;strain curves of the as-SLM CP-Ti and as-SLM Ti-TiN nanocomposites. The ultimate tensile strength (UTS) of the as-SLM CP-Ti, Ti-2.2TiN, and Ti-3TiN samples was 532, 967, and 1066 MPa, respectively. Thus, the UTS of the nanocomposite samples was increased by the presence of TiN. However, the total elongation-to-failure was lowered by the presence of TiN, with elongation of 6.8%, 12.2%, and 19.2% obtained for as-SLM Ti-3TiN, Ti-2.2TiN, and CP-Ti samples, respectively. According to previous work [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], the increased strength of the as-SLM Ti-TiN samples compared with that of as-SLM CP-Ti is attributed to both the increased dislocation density and refined acicular-like microstructure in the Ti matrix induced by TiN. The higher UTS of the as-SLM Ti-3TiN sample than that of the as-SLM Ti-2.2TiN sample was caused by the higher TiN concentration and smaller grain size of the former than those of the latter. In addition, the presence of coarse lath-like grains increased the plasticity of as-SLM Ti-2.2TiN compared with that of the as-SLM Ti-3TiN sample.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e shows SEM images of the tensile fracture surfaces of the as-SLM CP-Ti and T-TiN nanocomposites. The tensile fractography of the as-SLM CP-Ti sample was dominated by dimple fracture with large, deep dimples, indicating near-complete ductile fracture behavior (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e(a) and (b)). Bimodal dimple sizes were observed for the samples containing TiN, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e(d) and (f). The fracture surfaces of the as-SLM Ti-2.2TiN nanocomposite exhibited a typical dimpled appearance with more small dimples than was the case for as-SLM CP-Ti, also indicating a ductile fracture mechanism (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e(c) and (d)). When the TiN content was increased to 3 wt%, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e13\u003c/span\u003e(e) and (f), the fracture surface displayed dimples, grooves, and cleavage planes. Therefore, it was inferred that the fracture mechanism of the as-SLM Ti-3.0TiN nanocomposite sample involved a mixed mode rather than only ductile fracture. The ductility of the as-SLM Ti-3TiN sample was lower than that of the other samples because it contained more and smaller acicular-like structures.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eA combination of N\u003csub\u003e2\u003c/sub\u003e plasma milling, plasma spheroidization, and SLM forming was used to fabricate Ti-TiN nanocomponents with high strength and ductility. The following conclusions were obtained:\u003c/p\u003e \u003cp\u003e(1) The N\u003csub\u003e2\u003c/sub\u003e plasma milling process provided a simple route to fabricate a TiN nanophase-reinforced milled Ti nanocomposite powder. After subsequent spheroidization, the concentration of the TiN phase in the N\u003csub\u003e2\u003c/sub\u003e-P-milling-1 and N\u003csub\u003e2\u003c/sub\u003e-P-milling-2 nanocomposites was 22wt.% and 30wt.%, respectively.\u003c/p\u003e \u003cp\u003e(2) The as-SLM CP-Ti sample consisted of lath-like or lens-like martensitic α\u0026prime; crystals with a maximum thickness of around 1.5 \u0026micro;m. Mixing the CP-Ti powder with the milled Ti-TiN nanocomposites and subsequent SLM led to the formation of dual-scale morphologies containing both coarse lath-like (700\u0026ndash;1000 nm thick) and fine acicular-like grains with thicknesses of 50\u0026ndash;300 nm, respectively.\u003c/p\u003e \u003cp\u003e(3) Both the as-SLM Ti-2.2TiN and Ti-3TiN samples exhibited higher UTS than that of the as-SLM CP-Ti sample ( 967, 1066, and 532 MPa, respectively), but this came at the expense of the total elongation (12.2%, 6.8%, and 19.2%, respectively). The strengthening mechanisms of the TiN particles in the Ti matrix were acicular-like grain refinement, dislocation, and TiN dispersion strengthening.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e☐\u0026nbsp;The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e☐The authors declare the following\u0026nbsp;financial interests/personal relationships\u0026nbsp;which may be considered as potential competing interests:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eChuanjun Zang and Jianhua Yin:\u0026nbsp;Conceptualization, Data curation, Formal analysis, Investigation, Methodology;\u0026nbsp;Shaofeng Hong:\u0026nbsp;Conceptualization, Formal analysis, Methodology;\u0026nbsp;Meiqin Zeng: Conceptualization, Investigation; Zhongchen Lu:\u0026nbsp;Data curation, Funding acquisition, Investigation, Methodology, Project administration, Supervision, Writing \u0026ndash; original draft, Writing \u0026ndash; review \u0026amp; editing. All authors contributed to the general discussion.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e \u003cp\u003eThis work was supported by Guangdong Provincial Natural Science Foundation (No. 2024A1515012843) and Fundamental Research Funds for the Central Universities (No. 2023ZYGXZR004). We thank Natasha Lundin, PhD, from Liwen Bianji (Edanz) (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e\u003ca href=\"http://www.liwenbianji.cn\" target=\"_blank\"\u003ewww.liwenbianji.cn\u003c/a\u003e\u003c/span\u003e\u003cspan address=\"http://www.liwenbianji.cn\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e) for editing the English text of a draft of this manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData is provided within the manuscript or supplementary information files.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLi XP, Van Humbeeck J, Kruth JP. Selective laser melting of weak-textured commercially pure titanium with high strength and ductility: A study from laser power perspective. Mater Des. 2017;116:352\u0026ndash;8.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGunasekaran J, Sevvel P, Solomon IJ. 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Adv Opt Mater. 2021;9(16):2100323.\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":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"dime","sideBox":"Learn more about [Discover Materials](https://www.springer.com/journal/43939)","snPcode":"","submissionUrl":"","title":"Discover Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Selective laser melting, Commercially pure titanium, Plasma milling, In situ nitriding","lastPublishedDoi":"10.21203/rs.3.rs-4487360/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4487360/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAn efficient method to fabricate titanium nitride (TiN)-doped titanium (Ti) nanocomposites through a combination of plasma milling, plasma spheroidization, and selective laser melting (SLM) was developed to obtain Ti-based materials with high strength and ductility from commercially pure titanium (CP-Ti). Nitrogen plasma milling and plasma spheroidization were used to fabricate Ti-TiN nanocomposites, which were mixed with CP-Ti powder at a 1:9 ratio and printed by SLM forming. The resulting materials possessed a dual-scale morphology with both coarse lath-like and fine acicular-like grains. The Ti-TiN nanocomposites possessed higher hardness and tensile strength and lower ductility than those of the control sample without TiN. The mechanical properties of the SLM-printed Ti-TiN nanocomposites were improved compared with those of SLM-printed CP-Ti because of the TiN particles and resulting dual-scale structure. Nitrogen plasma milling provides a simple route to fabricate TiN nanophase-reinforced Ti-based nanocomposites suitable for SLM printing.\u003c/p\u003e","manuscriptTitle":"Fabrication of TiN-doped Ti nanocomposites with high strength and ductility by plasma-assisted ball milling and selective laser melting","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-11 09:03:03","doi":"10.21203/rs.3.rs-4487360/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-06-28T08:00:49+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-27T01:22:38+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-22T04:22:06+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"70408187528892554311082335144982932946","date":"2024-06-20T22:16:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"155290002561115963634538260597789559808","date":"2024-06-19T11:25:44+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"277262542865863869352369548043290524400","date":"2024-06-18T23:46:30+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-06-18T22:07:53+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-29T17:04:56+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-05-29T17:04:46+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Materials","date":"2024-05-28T01:09:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"dime","sideBox":"Learn more about [Discover Materials](https://www.springer.com/journal/43939)","snPcode":"","submissionUrl":"","title":"Discover Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"85e4463b-95be-491a-95af-bf658d17ffb8","owner":[],"postedDate":"June 11th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-10-21T15:58:40+00:00","versionOfRecord":{"articleIdentity":"rs-4487360","link":"https://doi.org/10.1007/s43939-024-00120-7","journal":{"identity":"discover-materials","isVorOnly":false,"title":"Discover Materials"},"publishedOn":"2024-10-17 15:56:53","publishedOnDateReadable":"October 17th, 2024"},"versionCreatedAt":"2024-06-11 09:03:03","video":"","vorDoi":"10.1007/s43939-024-00120-7","vorDoiUrl":"https://doi.org/10.1007/s43939-024-00120-7","workflowStages":[]},"version":"v1","identity":"rs-4487360","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4487360","identity":"rs-4487360","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}