Highly porous Hastelloy-X nickel superalloy produced by a space holder approach: microstructure and mechanical properties

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Among various methods of producing open-porous materials, a space holder approach provides number of benefits regarding economic and ecological aspects of production. In this work, the pioneering results of microstructure and mechanical properties analyses of highly porous Hastelloy-X nickel superalloy produced by the space holder approach, are presented. The materials were fabricated by using spherical fine Hastelloy-X powders and carbamide particles as batch materials. Multi-step powder metallurgy and thermomechanical processing was applied to produce open porous samples having a total volumetric porosity of 50, 60 and 70%. The produced materials were subjected to non-destructive (X-ray computed tomography) and metallographic inspections. Mechanical properties of the porous Hastelloy-X samples were examined in static room temperature compression tests, to discuss the effect of obtained porosity on compressive response. Hastelloy-X nickel superalloys metallic porous materials space holder technique X-ray computed tomography Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Metallic porous materials (MPMs) are attractive candidates for both structural and functional applications. A high surface area and open porosity of these materials provide unique properties that are specifically attractive for lightweight automotive or space applications for example as filters, catalysts, thermal management devices, acoustic panels or energy absorption units[ 1 ] [ 2 ]. The manufacturing techniques of MPMs are based on liquid-state, solid-state or gas-liquid processes, while a selection of proper fabrication technology depends on the materials-based and application-oriented requirements. A solid-state space holder approach[ 3 ] appears as attractive alternative to conventional liquid metal foaming techniques, because it allows easily controlling shape and size of pores inside the volume of processed component, under sustainable and cost-effective conditions. Among various reported examples of MPMs (including these produced by the space holder approach) most of them are focused on aluminum, copper or specifically on biomedical titanium-based foams. On the other hand, there is much more limited information regarding processing and properties of high temperature, creep and oxidation resistant MPMs. This is mostly related to arising technological issues associated with high melting points and superior mechanical properties of materials predisposed for high temperature applications. These properties make them less prone to a pressure-less processing and compaction. A good example of that are porous nickel-based superalloys that are predicted to be potentially applicable as abradable seals materials in gas turbine engines. The European Commission-funded ADSEAL project [ 4 ] provided important practical insights into the requirements for materials and structures in abradable seal technology for future devices. During the project, the same metallic alloy in the form of thin walled honeycombs, gradient fibre or hollow sphere structures, was examined in cyclic oxidation resistance and abradability tests. It has been documented that metal alloy hollow sphere structures combine very good oxidation resistance (that was superior or at least not worse than that of honeycombs) with the required abradability. Furthermore, it has been proposed that the functionality of hollow sphere structured might be further improved by reducing the sphere shell thickness and by increasing the sphere diameter. These findings led us to initiate new research efforts focused on Hastelloy-X (H-X) nickel superalloy as a novel material in the field of porous high-temperature materials. The H-X is a Ni-Cr-Fe-Mo alloy that possesses a combination of very good oxidation resistance at temperatures as high as up to 1095˚C (exceeding that of Inconel 600, Alloy 625, Alloy 800H), fabricability and high-temperature strength. Moreover, it has also been found that the H-X alloy exhibit exceptionally resistant to stress corrosion cracking in petrochemical applications [ 5 ]. Therefore, these properties predispose the H-X alloy to be applied as a porous material for many hi-tech high temperature functional and structural applications. In this work, we present for the first time the results of our research on the fabrication of highly porous H-X alloys (with a volumetric porosity up to 70%). For this purpose, we used a multi-step powder metallurgy-based approach utilizing a space holder concept. The produced materials were subjected to both non-destructive and destructive structural characterization, as well as to room temperature compression tests. 2. Materials and methods 2.1. Fabrication of highly porous H-X alloys by space holder approach Commercially gas-atomized spherical Hastelloy-X powders (Imphytek Powders, France) with diameters of D10 = 5.26 µm, D50 = 11.60 µm, D90 = 21.30 µm, were used as batch materials. The certified chemical composition of the powders was: Ni-22.1Cr-18.2Fe-9.2Mo-2.1Co (wt%). To fabricate highly porous H-X alloys, we adopted technical solutions previously reported by Unver et al. for porous 625 Ni superalloy [ 6 ]. A graphical representation of the entire process can be found in Fig. 1 . The powders were firstly mixed with a paraffin wax (3 wt %) and then mechanically stirred on a hot plate (90˚C). After that, spherical granules of carbamide (sieved down to a size of d = 2 − 1 mm), were added and mixed together with wax-impregnated H-X powders. The following volumetric content of carbamide particles were applied: 50, 60 and 70 vol. %. Next, the mixtures underwent cold compaction in a stainless steel die with a diameter of 23 mm under an isostatic pressure of 150 MPa. The cold compacted sinters were then subjected to a three-stage heat-treatment. During the first stage, a slow heating up (at 0.2˚Cmin − 1 ) and annealing at 210˚C in air was applied to remove paraffin wax binder and carbamide particles. The second stage (annealing at 600˚C/2h + 700˚C/1h) was planned as a pressure-less preliminary sintering in argon flow atmosphere to complete the removal of organics. Finally, high temperature sintering at 1300˚C/2h under vacuum of p = 10 − 2 mbar, was applied to densify the compacts. 2.1. Characterization of highly porous H-X alloys The porous H-X alloys were subjected to a structural characterization by using non-destructive and metallographic techniques. The GE V|TOMEX|L-450 computed tomography (CT) device operating under voltage of 150 kV, current of 80 µA, exposure time of 250 ms and voxel size of 18.4 µm was used for the acquisition of X-ray images. Scanned objects were digitally reconstructed using Datos—x reconstruction and VGStudio Max 2.0 commercial software. The microstructural characterization was carried out on cross-sectioned specimens by using light microscopy and scanning electron microscopy. FEI Scios™ Field Emission Gun Scanning Electron Microscope (FEG SEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS) and Electron Backscatter Diffraction system (EBSD), were utilized. Room temperature mechanical properties were examined in static compression tests carried out on in accordance to ASTM E9 standard cylindrical samples having dimensions of Φ = 6.7 mm × 10 mm. MTS 312.31 (200 kN) universal machine operating at a traverse speed rate of 0.005 min − 1 , was applied. 3. Results and discussion The results of SEM inspections have confirmed a high sphericity and particle size of the commercial H-X superalloy powders that were used as the batch materials (Fig. 2 a, b). As declared by the producers, the powder showed an average particle size below 15 µm. The powders were found to be free of shape defects (e.g. satellites). Spherical carbamide particles (Fig. 2 c) combined with the applied processing allowed replicating their shape in the final porous samples. Finally, highly porous H-X sinters were successfully produced (Fig. 2 d). 3.1. Macro- and microstructure of porous H-X alloy The results of non-destructive CT analyses (see example in Fig. 3 ) revealed homogeneous distribution of porosity within the produced sinters. As revealed by the reconstructed models, even for a porosity as high as 70%, the sinters showed a good integrity and a well-defined porous structure. The shape and size of pores replicate the morphology of used carbamide particles. The results of metallographic analyses (Fig. 4 ) revealed: (i) an average grain size of ~ 120 µm: (ii) an existence of some internal microporosity (Fig. 4 a); and (iii) a presence of skeleton-like precipitates at grain boundaries (GBs). More detailed analyses by simultaneous SEM/EDS/EBSD method (Fig. 4 b-d) allowed recognizing these structural features as Cr-rich M 23 C 6 and Mo-rich M 6 C carbides. The phase identification of GB precipitates is in line with results reported for additively manufactured (AM) H-X alloy [ 7 ], [ 8 ]. On the other hand ,a skeleton-like morphology of the GB carbides points towards a discontinous precipitation (DP) as the main governing reaction. The basic feature of this phenomenon is a “ lamellar, transformation product behind a GB advancing into a supersaturated matrix ” [ 9 ]. Over the years, the presence of DP-like products in heat treated nickel based superalloys has been documented and widely discussed by many authors [ 10 ]. Furthermore, we have also recently documented a presence of analogous structural features in another Ni-Fe-Cr-based alloy subjected to non-equilibrium oversaturation followed by aging treatments [ 11 ], [ 12 ]. For the sake of discussing the results of our present work, the model recently proposed by Atrazhev et al. [ 13 ] might be adopted. The authors have proposed that GBs mobility is the key factor governing the formation of either GB serration under low GBs mobility conditions or skeleton-type GBs structures (similar to these observed in the present work) when the mobility of grain boundaries is high. It is reasonable to assume that very high temperature applied during the final processing step (T = 1300°C = 0.96 T m ), supports both high GBs mobility and diffusion kinetics. Therefore, it is proposed that the formation of skeleton-like M 23 C6/M 6 C products is driven by a local segregation of Cr to GBs areas combined with an effective grain growth (i.e. a migration of GBs) inside the matrix of produced porous sinters and a high chemical affinity of Mo to carbon. Furthermore, organic materials used during the fabrication process (paraffin wax and carbamide) can easily serve as the carbon source supporting the DP reaction. 3.2. Room temperature mechanical properties Compressive stress-strain curves obtained for H-X samples with porosities of 50, 60 and 70%, are shown in Fig. 5 , while values of quantitative parameters are listed in Table 1 . The reference Hastelloy-X sample (marked as 0%) produced according to the same powder metallurgy-based procedure as described in the Section 2.1 , but without introducing porosity formers, was used for the sake of comparison. The results obtained for the reference H-X alloy sample show a good agreement with these recently reported in the literature for the alloy processed by additive manufacturing techniques [ 7 ], [ 14 ], [ 15 ]. However, it should be noted, that literature data regarding mechanical response of the AMed H-X alloy shows rather high scattering (Yield strength of 290–690 MPa; Ultimate Strength of 560–1060 MPa), as many variants of the processing and/or heat treatment, are applied in various laboratories. On the other hand, it is worth noting that values available in the literature regarding mechanical properties of the Hastelloy-X alloy are mostly limited to those produce in tensile tests on additively manufactured specimens, and are rather highly scattered. Table 1 The results of compression tests carried out on porous Hastelloy-X superalloy samples with 50–70% porosity (bulk alloy sample, marked as 0% was used as reference) and a comparison to reported literature data. Porosity [%] Yield Strength (0.2) [MPa] Ultimate Strength [MPa] Total strain [%] Compressive stress [MPa] at 5% strain at 10% strain at 25% strain at 50% strain 0 251 847 19 445 583 n.a n.a 50 n.a 1965 74 96 116 195 511 60 n.a 1144 73 82 100 163 375 70 n.a 1043 74 69 75 115 280 The results obtained for porous Hastelloy-X samples show typical effects of introducing a high volumetric content of porosity. With increasing porosity content in the Hastelloy-X alloy, the obtained curves became more and more “flattened”, i.e. a noticeable decrease of compressive strain was observed over the wide strain range. After reaching yielding point at a low level of ~ 100 MPa, the porous samples underwent a plastic deformation in two stages: (i) under a near linear-like course associated with a densification of sinters through deforming and breaking individual walls; and (ii) in non-linear regime characterized by a more intensive strain hardening. This destruction mechanism has been reported for example in the case of highly porous Fe-Al intermetallics [ 16 ]. It is found that a general strain hardening coefficient (expressed as a local slope of the compressive curve) decreases with the increment in the volumetric content of pores. This effect seems to be reasonable, as more pores in the sample volume, means a smaller load bearing cross-section area. Analogous mechanical behavior (and a similar shape of compressive curves and stress/strain values) has been previously reported by Unver et al. for porous 625 Ni superalloy [ 6 ]. Conclusions and future remarks In this work, we show for the first time, the results of fabrication and characterization of highly porous (50–70 vol.%) Hastelloy-X alloy. Based on the results of non-destructive and destructive structural characterization, compression tests and reported literature the following conclusions are written down: The developed powder metallurgy-based space holder approach appears as feasible and sustainable method for producing highly porous Hastelloy-X alloy. The method involves non-toxic, cheap and easily available substances as binders and space holders, while the processing involves cold-compaction and pressure-less sintering steps. Even for a porosity as high as 70%, the sinters show a good integrity and a well-defined porous structure. The shape and size of pores replicates the morphology of used carbamide particles. The walls’ microstructure consists of an fcc matrix having the average grain size of ~ 120 µm: (ii) and a presence of skeleton-like precipitates at grain boundaries. The precipitates were recognized as M 23 C6/M 6 C products of the discontinuous precipitation reaction. Their impact on mechanical properties will be further evaluated. The highly porous Hastelloy-X alloy show prominently different mechanical behavior in compression tests, as compared to the bulk counterpart. Destruction of porous sinters takes place in two stages: densification of sinters trough deforming and breaking individual walls; and a more intensive strain hardening of desified speciemens. A general strain hardening coefficient (expressed as a local slope of the compressive curve) decreases with the increment in the volumetric content of pores. Future planned works will be focused on examining application oriented performance properties. In this regards, mechanical strenght, oxidation behavior and wear resistance of highly porous Hastelloy-X alloys at high temperarures, will be experimentally investigated. Declarations Author Contribution A.B. made a processing of powders; M.P. carried out cold compaction and heat treatment; A.P. carried out structural characterization; W.P. has acquired the funding and designed the process; A.B. prepared the first draft of the paper; W.P. and G.W. edited and review the manuscript, G.W. supervised the experiments, wrote ans stuctured the manuscript. All authors reviewed the manuscript. Acknowledgments A financial support from the National Science Centre, Poland, under Grant no. UMO-2021/41/B/ST5/03525 (OPUS 21 call), is gratefully acknowledged. A support of Mr. Sławomir Czarniewicz from Łukasiewicz - Institute of Aviation (Warsaw, Poland) in conducting compression tests, is appreciated. Data Availability Related research data is available on demand from the corresponding author (at [email protected] ). References L.-Y. Gao, H.-K. Yang, X. Chen, W.-D. Tang, X.-M. Huang and Z.-Q. Liu, "The development of porous metallic materials: a short review of fabrication, characteristics, and applications," Physica Scripta, vol. 98, p. 122001, 2023. B. Zhao, A. K. Gain, W. Ding, L. Zhang, X. Li and Y. Fu, "A review on metallic porous materials: pore formation, mechanical properties, and their applications," The International Journal of Advanced Manufacturing Technology, vol. 95, p. 2641–2659, 2018. L. Stanev, M. Kolev, B. Drenchev and L. Drenchev, "Open-Cell Metallic Porous Materials Obtained Through Space Holders—Part I: Production Methods. A Review," Journal of Manufacturing Science and Engineering, vol. 139, p. 050801, 2017. W. Smarsly, N. Zheng, C. Buchheim, C. Nindel, C. Silvestro, D. Sporer, M. Tuffs, K. Schreiber, C. Langlade-Bomba, O. Andersen, H. Goehler and G. M. N.J. Simms, "Advanced High Temperature Turbine Seals Materials and Designs," Materials Science Forum 21, pp. 492-493, 2005. Haynes, "HASTELLOY® X alloy," 2024. [Online]. Available: https://www.haynesintl.com/wp-content/uploads/2023/06/x-brochure.pdf. [Accessed 11 3 2024]. I. Unver, H. Gulsoy and B. Aydemir, "Ni-625 Superalloy Foam Processed by Powder Space-Holder Technique,," Journal of Materials Engineering and Performance, vol. 22, pp. 3735-3741, 2013. Y.-S. Lee and J.-H. Sung, "Microstructure and Mechanical Properties of Hastelloy X Fabricated Using Directed Energy Deposition," Metals, vol. 13, p. 885, 2023. S. Zhonggang, J. Shuwei, G. Yanhua, L. Yichen, C. Lili and X. Fei, "Microstructure evolution and mechanical properties of Hastelloy X alloy produced by Selective Laser Melting.," High Temperature Materials and Processes, vol. 39, pp. 124-135, 2020. D. Williams and E. Butler, "Grain boundary discontinuous precipitation reactions," International Metals Reviews, vol. 3, pp. 153-183, 1981. J. Spadotto, Dille, J., M. Watanabe and I. Solórzano, "Grain boundary precipitation phenomena in an alloy 33 (Cr-Fe-Ni-N) subjected to direct-aging treatments (700 °C and 900 °C).," Materials Characterization, vol. 140, p. 113–121, 2018. A. Polkowska, S. Lech, P. Bała and W. Polkowski, "Microstructure and mechanical properties of Ni-Fe-Cr-Al wrought alumina forming superalloy heat-treated at 600-1100°C," Materials Characterization, vol. 171, p. 110737, 2021. S. Lech, W. Polkowski, A. Polkowska, G. Cempura and A. Kruk, "Multimodal discontinuous reaction in Ni-Fe-Cr-Al alloy," Scripta Materialia , vol. 194 , p. 113657, 2021 . V. Atrazhev, S. Burlatsky, D. Dmitriev, D. Furrer, N. Kuzminyh, I. Lomaev, D. Novikov, S. Stolz and P. Reynolds, "The mechanism of grain boundary serration and fan-type structure formation in Ni-based superalloys," Metallurgical and Materials Transactions A, no. 51, p. 3648–3657, 2020. M. L. Montero-Sistiaga, S. Pourbabak, J. V. Humbeeck, D. Schryvers and K. Vanmeensel, "Microstructure and mechanical properties of Hastelloy X produced by HP-SLM (high power selective laser melting)," Materials & Design, vol. 15, p. 107598, 2019. Y. Yin, J. Zhang, S. Pan, Y. Xing, X. Yue and W. Chang, "Room- and elevated-temperature mechanical property of selective laser melting-fabricated Hastelloy X with different heat treatments,," Materials Science and Engineering: A , vol. 886 , p. 145697, 2023 . M. Łazińska, T. Durejko, S. Lipiński, W. Polkowski, T. Czujko and R. Varin, "Porous graded FeAl intermetallic foams fabricated by sintering process using NaCl space holders," Materials Science and Engineering: A , vol. 636 , pp. 407-414, 2015. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 02 Jan, 2025 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 04 Nov, 2024 Reviews received at journal 31 Oct, 2024 Reviewers agreed at journal 30 Oct, 2024 Reviews received at journal 27 Aug, 2024 Reviewers agreed at journal 12 Aug, 2024 Reviewers invited by journal 12 Aug, 2024 Editor assigned by journal 12 Aug, 2024 Editor invited by journal 07 Aug, 2024 Submission checks completed at journal 07 Aug, 2024 First submitted to journal 07 Aug, 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-4872347","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":345596384,"identity":"de0e1c3a-4a1c-49d3-97bb-257cc978f8af","order_by":0,"name":"Aleksandra Bętkowska","email":"","orcid":"","institution":"Krakow Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Aleksandra","middleName":"","lastName":"Bętkowska","suffix":""},{"id":345596385,"identity":"c082a10f-7d34-4563-b846-b84a1776292c","order_by":1,"name":"Marcin Podsiadło","email":"","orcid":"","institution":"Krakow Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Marcin","middleName":"","lastName":"Podsiadło","suffix":""},{"id":345596386,"identity":"ed8bd227-7566-4633-85cc-b180f010118d","order_by":2,"name":"Adelajda Polkowska","email":"","orcid":"","institution":"Krakow Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Adelajda","middleName":"","lastName":"Polkowska","suffix":""},{"id":345596387,"identity":"5b37d10a-1d72-4bb6-88e8-ba5847f5a74e","order_by":3,"name":"Grzegorz Włoch","email":"","orcid":"","institution":"AGH University of Science and Technology","correspondingAuthor":false,"prefix":"","firstName":"Grzegorz","middleName":"","lastName":"Włoch","suffix":""},{"id":345596389,"identity":"f8e3f55c-7afa-41c7-bee7-14eb27735c2f","order_by":4,"name":"Wojciech Polkowski","email":"data:image/png;base64,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","orcid":"","institution":"Krakow Institute of Technology","correspondingAuthor":true,"prefix":"","firstName":"Wojciech","middleName":"","lastName":"Polkowski","suffix":""}],"badges":[],"createdAt":"2024-08-07 06:31:47","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4872347/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4872347/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-024-84321-3","type":"published","date":"2025-01-02T15:57:01+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":63794144,"identity":"c058202f-227f-4e87-a1b8-8ebb9773acee","added_by":"auto","created_at":"2024-09-02 12:07:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":690052,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eA scheme of the fabrication process of Hastelloy-X porous materials by a space holder approach.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-4872347/v1/08199c179e695b9b6c0df7ec.png"},{"id":63794612,"identity":"aad2cad0-b111-4949-a494-6b67302c5cb3","added_by":"auto","created_at":"2024-09-02 12:15:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":3383847,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eThe materials used in the fabrication process: SEM images showing morphology of H-X powders used as the batch materials (a, b) and a macro view of the carbamide particles (c). An exemplary final H-X sinter having a porosity of 50, 60 or 70 % (d).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-4872347/v1/4fb7ab3230c5bf90b3e1b508.png"},{"id":63794141,"identity":"cd93926f-2d52-45a9-afbc-eb95595a371b","added_by":"auto","created_at":"2024-09-02 12:07:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2480071,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eExemplary results of CT inspections taken from the Hastelloy-X sample having a porosity of 70%.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-4872347/v1/7d92253c845ae4a82ed32898.png"},{"id":63794611,"identity":"27931dcc-03bf-42cf-91e9-34dba520ee26","added_by":"auto","created_at":"2024-09-02 12:15:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":4275666,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eSelected results of the SEM/EDS/EBSD analyses of the Hastelloy-X sample having porosity of 50%: a microstructure of the wall (a); SEM/EBSD/EDS analysis of the grain boundary carbide precipitates.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-4872347/v1/fb2be3e463563c70482556ae.png"},{"id":63794140,"identity":"bcc6ea11-4bff-46b4-8b63-31def6b955e6","added_by":"auto","created_at":"2024-09-02 12:07:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":57645,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eCompressive stress-strain curves of porous Hastelloy-X superalloy samples with 50-70 % porosity (bulk alloy sample, marked as 0% was used as reference).\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-4872347/v1/b1d452207472d912a9199ac4.png"},{"id":73093141,"identity":"2e571ac4-e858-4207-83f3-1e42e00bdab9","added_by":"auto","created_at":"2025-01-06 16:06:29","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":13837533,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4872347/v1/6c3da416-c7a5-46ee-bb13-e60501c057fc.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Highly porous Hastelloy-X nickel superalloy produced by a space holder approach: microstructure and mechanical properties","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMetallic porous materials (MPMs) are attractive candidates for both structural and functional applications. A high surface area and open porosity of these materials provide unique properties that are specifically attractive for lightweight automotive or space applications for example as filters, catalysts, thermal management devices, acoustic panels or energy absorption units[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. The manufacturing techniques of MPMs are based on liquid-state, solid-state or gas-liquid processes, while a selection of proper fabrication technology depends on the materials-based and application-oriented requirements. A solid-state space holder approach[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e] appears as attractive alternative to conventional liquid metal foaming techniques, because it allows easily controlling shape and size of pores inside the volume of processed component, under sustainable and cost-effective conditions. Among various reported examples of MPMs (including these produced by the space holder approach) most of them are focused on aluminum, copper or specifically on biomedical titanium-based foams. On the other hand, there is much more limited information regarding processing and properties of high temperature, creep and oxidation resistant MPMs. This is mostly related to arising technological issues associated with high melting points and superior mechanical properties of materials predisposed for high temperature applications. These properties make them less prone to a pressure-less processing and compaction. A good example of that are porous nickel-based superalloys that are predicted to be potentially applicable as abradable seals materials in gas turbine engines. The European Commission-funded ADSEAL project [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e] provided important practical insights into the requirements for materials and structures in abradable seal technology for future devices. During the project, the same metallic alloy in the form of thin walled honeycombs, gradient fibre or hollow sphere structures, was examined in cyclic oxidation resistance and abradability tests. It has been documented that metal alloy hollow sphere structures combine very good oxidation resistance (that was superior or at least not worse than that of honeycombs) with the required abradability. Furthermore, it has been proposed that the functionality of hollow sphere structured might be further improved by reducing the sphere shell thickness and by increasing the sphere diameter.\u003c/p\u003e \u003cp\u003eThese findings led us to initiate new research efforts focused on Hastelloy-X (H-X) nickel superalloy as a novel material in the field of porous high-temperature materials. The H-X is a Ni-Cr-Fe-Mo alloy that possesses a combination of very good oxidation resistance at temperatures as high as up to 1095˚C (exceeding that of Inconel 600, Alloy 625, Alloy 800H), fabricability and high-temperature strength. Moreover, it has also been found that the H-X alloy exhibit exceptionally resistant to stress corrosion cracking in petrochemical applications [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Therefore, these properties predispose the H-X alloy to be applied as a porous material for many hi-tech high temperature functional and structural applications.\u003c/p\u003e \u003cp\u003eIn this work, we present for the first time the results of our research on the fabrication of highly porous H-X alloys (with a volumetric porosity up to 70%). For this purpose, we used a multi-step powder metallurgy-based approach utilizing a space holder concept. The produced materials were subjected to both non-destructive and destructive structural characterization, as well as to room temperature compression tests.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Fabrication of highly porous H-X alloys by space holder approach\u003c/h2\u003e \u003cp\u003eCommercially gas-atomized spherical Hastelloy-X powders (Imphytek Powders, France) with diameters of D10\u0026thinsp;=\u0026thinsp;5.26 \u0026micro;m, D50\u0026thinsp;=\u0026thinsp;11.60 \u0026micro;m, D90\u0026thinsp;=\u0026thinsp;21.30 \u0026micro;m, were used as batch materials. The certified chemical composition of the powders was: Ni-22.1Cr-18.2Fe-9.2Mo-2.1Co (wt%). To fabricate highly porous H-X alloys, we adopted technical solutions previously reported by Unver et al. for porous 625 Ni superalloy [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. A graphical representation of the entire process can be found in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The powders were firstly mixed with a paraffin wax (3 wt %) and then mechanically stirred on a hot plate (90˚C). After that, spherical granules of carbamide (sieved down to a size of \u003cem\u003ed\u003c/em\u003e\u0026thinsp;=\u0026thinsp;2\u0026thinsp;\u0026minus;\u0026thinsp;1 mm), were added and mixed together with wax-impregnated H-X powders. The following volumetric content of carbamide particles were applied: 50, 60 and 70 vol. %. Next, the mixtures underwent cold compaction in a stainless steel die with a diameter of 23 mm under an isostatic pressure of 150 MPa. The cold compacted sinters were then subjected to a three-stage heat-treatment. During the first stage, a slow heating up (at 0.2˚Cmin\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and annealing at 210˚C in air was applied to remove paraffin wax binder and carbamide particles. The second stage (annealing at 600˚C/2h\u0026thinsp;+\u0026thinsp;700˚C/1h) was planned as a pressure-less preliminary sintering in argon flow atmosphere to complete the removal of organics. Finally, high temperature sintering at 1300˚C/2h under vacuum of p\u0026thinsp;=\u0026thinsp;10\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e mbar, was applied to densify the compacts.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Characterization of highly porous H-X alloys\u003c/h2\u003e \u003cp\u003eThe porous H-X alloys were subjected to a structural characterization by using non-destructive and metallographic techniques. The GE V|TOMEX|L-450 computed tomography (CT) device operating under voltage of 150 kV, current of 80 \u0026micro;A, exposure time of 250 ms and voxel size of 18.4 \u0026micro;m was used for the acquisition of X-ray images. Scanned objects were digitally reconstructed using Datos\u0026mdash;x reconstruction and VGStudio Max 2.0 commercial software.\u003c/p\u003e \u003cp\u003eThe microstructural characterization was carried out on cross-sectioned specimens by using light microscopy and scanning electron microscopy. FEI Scios\u0026trade; Field Emission Gun Scanning Electron Microscope (FEG SEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDS) and Electron Backscatter Diffraction system (EBSD), were utilized. Room temperature mechanical properties were examined in static compression tests carried out on in accordance to ASTM E9 standard cylindrical samples having dimensions of Φ\u0026thinsp;=\u0026thinsp;6.7 mm \u0026times; 10 mm. MTS 312.31 (200 kN) universal machine operating at a traverse speed rate of 0.005 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, was applied.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eThe results of SEM inspections have confirmed a high sphericity and particle size of the commercial H-X superalloy powders that were used as the batch materials (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, b). As declared by the producers, the powder showed an average particle size below 15 \u0026micro;m. The powders were found to be free of shape defects (e.g. satellites). Spherical carbamide particles (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec) combined with the applied processing allowed replicating their shape in the final porous samples. Finally, highly porous H-X sinters were successfully produced (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed).\u003c/p\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Macro- and microstructure of porous H-X alloy\u003c/h2\u003e \u003cp\u003eThe results of non-destructive CT analyses (see example in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) revealed homogeneous distribution of porosity within the produced sinters. As revealed by the reconstructed models, even for a porosity as high as 70%, the sinters showed a good integrity and a well-defined porous structure. The shape and size of pores replicate the morphology of used carbamide particles. The results of metallographic analyses (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) revealed: (i) an average grain size of ~\u0026thinsp;120 \u0026micro;m: (ii) an existence of some internal microporosity (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea); and (iii) a presence of skeleton-like precipitates at grain boundaries (GBs). More detailed analyses by simultaneous SEM/EDS/EBSD method (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb-d) allowed recognizing these structural features as Cr-rich M\u003csub\u003e23\u003c/sub\u003eC\u003csub\u003e6\u003c/sub\u003e and Mo-rich M\u003csub\u003e6\u003c/sub\u003eC carbides. The phase identification of GB precipitates is in line with results reported for additively manufactured (AM) H-X alloy [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOn the other hand ,a skeleton-like morphology of the GB carbides points towards a \u003cem\u003ediscontinous precipitation\u003c/em\u003e (DP) as the main governing reaction. The basic feature of this phenomenon is a \u0026ldquo;\u003cem\u003elamellar, transformation product behind a GB advancing into a supersaturated matrix\u003c/em\u003e\u0026rdquo; [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Over the years, the presence of DP-like products in heat treated nickel based superalloys has been documented and widely discussed by many authors [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Furthermore, we have also recently documented a presence of analogous structural features in another Ni-Fe-Cr-based alloy subjected to non-equilibrium oversaturation followed by aging treatments [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e], [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFor the sake of discussing the results of our present work, the model recently proposed by Atrazhev et al. [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] might be adopted. The authors have proposed that GBs mobility is the key factor governing the formation of either GB serration under low GBs mobility conditions or skeleton-type GBs structures (similar to these observed in the present work) when the mobility of grain boundaries is high. It is reasonable to assume that very high temperature applied during the final processing step (T\u0026thinsp;=\u0026thinsp;1300\u0026deg;C\u0026thinsp;=\u0026thinsp;0.96\u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003em\u003c/em\u003e\u003c/sub\u003e), supports both high GBs mobility and diffusion kinetics. Therefore, it is proposed that the formation of skeleton-like M\u003csub\u003e23\u003c/sub\u003eC6/M\u003csub\u003e6\u003c/sub\u003eC products is driven by a local segregation of Cr to GBs areas combined with an effective grain growth (i.e. a migration of GBs) inside the matrix of produced porous sinters and a high chemical affinity of Mo to carbon. Furthermore, organic materials used during the fabrication process (paraffin wax and carbamide) can easily serve as the carbon source supporting the DP reaction.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Room temperature mechanical properties\u003c/h2\u003e \u003cp\u003eCompressive stress-strain curves obtained for H-X samples with porosities of 50, 60 and 70%, are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, while values of quantitative parameters are listed in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The reference Hastelloy-X sample (marked as 0%) produced according to the same powder metallurgy-based procedure as described in the \u003cb\u003eSection 2.1\u003c/b\u003e, but without introducing porosity formers, was used for the sake of comparison.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe results obtained for the reference H-X alloy sample show a good agreement with these recently reported in the literature for the alloy processed by additive manufacturing techniques [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. However, it should be noted, that literature data regarding mechanical response of the AMed H-X alloy shows rather high scattering (Yield strength of 290\u0026ndash;690 MPa; Ultimate Strength of 560\u0026ndash;1060 MPa), as many variants of the processing and/or heat treatment, are applied in various laboratories.\u003c/p\u003e \u003cp\u003eOn the other hand, it is worth noting that values available in the literature regarding mechanical properties of the Hastelloy-X alloy are mostly limited to those produce in tensile tests on additively manufactured specimens, and are rather highly scattered.\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\u003eThe results of compression tests carried out on porous Hastelloy-X superalloy samples with 50\u0026ndash;70% porosity (bulk alloy sample, marked as 0% was used as reference) and a comparison to reported literature data.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\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 \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003ePorosity [%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eYield Strength (0.2) [MPa]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eUltimate Strength [MPa]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eTotal strain [%]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c8\" namest=\"c5\"\u003e \u003cp\u003eCompressive stress [MPa]\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eat 5% strain\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eat 10% strain\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eat 25% strain\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eat 50% strain\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e251\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e847\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e445\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e583\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003en.a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003en.a\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003en.a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1965\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e116\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e195\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e511\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003en.a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1144\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e163\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e375\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003en.a\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1043\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e75\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e115\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003e280\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe results obtained for porous Hastelloy-X samples show typical effects of introducing a high volumetric content of porosity. With increasing porosity content in the Hastelloy-X alloy, the obtained curves became more and more \u0026ldquo;flattened\u0026rdquo;, i.e. a noticeable decrease of compressive strain was observed over the wide strain range. After reaching yielding point at a low level of ~\u0026thinsp;100 MPa, the porous samples underwent a plastic deformation in two stages: (i) under a near linear-like course associated with a densification of sinters through deforming and breaking individual walls; and (ii) in non-linear regime characterized by a more intensive strain hardening. This destruction mechanism has been reported for example in the case of highly porous Fe-Al intermetallics [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. It is found that a general strain hardening coefficient (expressed as a local slope of the compressive curve) decreases with the increment in the volumetric content of pores. This effect seems to be reasonable, as more pores in the sample volume, means a smaller load bearing cross-section area. Analogous mechanical behavior (and a similar shape of compressive curves and stress/strain values) has been previously reported by Unver et al. for porous 625 Ni superalloy [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions and future remarks","content":"\u003cp\u003eIn this work, we show for the first time, the results of fabrication and characterization of highly porous (50\u0026ndash;70 vol.%) Hastelloy-X alloy. Based on the results of non-destructive and destructive structural characterization, compression tests and reported literature the following conclusions are written down:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe developed powder metallurgy-based space holder approach appears as feasible and sustainable method for producing highly porous Hastelloy-X alloy. The method involves non-toxic, cheap and easily available substances as binders and space holders, while the processing involves cold-compaction and pressure-less sintering steps.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eEven for a porosity as high as 70%, the sinters show a good integrity and a well-defined porous structure. The shape and size of pores replicates the morphology of used carbamide particles.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe walls\u0026rsquo; microstructure consists of an fcc matrix having the average grain size of ~\u0026thinsp;120 \u0026micro;m: (ii) and a presence of skeleton-like precipitates at grain boundaries. The precipitates were recognized as M\u003csub\u003e23\u003c/sub\u003eC6/M\u003csub\u003e6\u003c/sub\u003eC products of the discontinuous precipitation reaction. Their impact on mechanical properties will be further evaluated.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe highly porous Hastelloy-X alloy show prominently different mechanical behavior in compression tests, as compared to the bulk counterpart. Destruction of porous sinters takes place in two stages: densification of sinters trough deforming and breaking individual walls; and a more intensive strain hardening of desified speciemens. A general strain hardening coefficient (expressed as a local slope of the compressive curve) decreases with the increment in the volumetric content of pores.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eFuture planned works will be focused on examining application oriented performance properties. In this regards, mechanical strenght, oxidation behavior and wear resistance of highly porous Hastelloy-X alloys at high temperarures, will be experimentally investigated.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eA.B. made a processing of powders; M.P. carried out cold compaction and heat treatment; A.P. carried out structural characterization; W.P. has acquired the funding and designed the process; A.B. prepared the first draft of the paper; W.P. and G.W. edited and review the manuscript, G.W. supervised the experiments, wrote ans stuctured the manuscript. All authors reviewed the manuscript.\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA financial support from the National Science Centre, Poland, under Grant no. UMO-2021/41/B/ST5/03525 (OPUS 21 call), is gratefully acknowledged. A support of Mr. Sławomir Czarniewicz from Łukasiewicz - Institute of Aviation (Warsaw, Poland) in conducting compression tests, is appreciated.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eRelated research data is available on demand from the corresponding author (at [email protected]).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eL.-Y. Gao, H.-K. Yang, X. Chen, W.-D. Tang, X.-M. Huang and Z.-Q. Liu, \u0026quot;The development of porous metallic materials: a short review of fabrication, characteristics, and applications,\u0026quot; \u003cem\u003ePhysica Scripta, \u003c/em\u003evol. 98, p. 122001, 2023. \u003c/li\u003e\n\u003cli\u003eB. Zhao, A. K. Gain, W. Ding, L. Zhang, X. Li and Y. Fu, \u0026quot;A review on metallic porous materials: pore formation, mechanical properties, and their applications,\u0026quot; \u003cem\u003eThe International Journal of Advanced Manufacturing Technology, \u003c/em\u003evol. 95, p. 2641\u0026ndash;2659, 2018. \u003c/li\u003e\n\u003cli\u003eL. Stanev, M. Kolev, B. Drenchev and L. Drenchev, \u0026quot;Open-Cell Metallic Porous Materials Obtained Through Space Holders\u0026mdash;Part I: Production Methods. A Review,\u0026quot; \u003cem\u003eJournal of Manufacturing Science and Engineering, \u003c/em\u003evol. 139, p. 050801, 2017. \u003c/li\u003e\n\u003cli\u003eW. Smarsly, N. Zheng, C. Buchheim, C. Nindel, C. Silvestro, D. Sporer, M. Tuffs, K. Schreiber, C. Langlade-Bomba, O. Andersen, H. Goehler and G. M. N.J. Simms, \u0026quot;Advanced High Temperature Turbine Seals Materials and Designs,\u0026quot; \u003cem\u003eMaterials Science Forum 21, \u003c/em\u003epp. 492-493, 2005. \u003c/li\u003e\n\u003cli\u003eHaynes, \u0026quot;HASTELLOY\u0026reg; X alloy,\u0026quot; 2024. [Online]. Available: https://www.haynesintl.com/wp-content/uploads/2023/06/x-brochure.pdf. [Accessed 11 3 2024].\u003c/li\u003e\n\u003cli\u003eI. Unver, H. Gulsoy and B. Aydemir, \u0026quot;Ni-625 Superalloy Foam Processed by Powder Space-Holder Technique,,\u0026quot; \u003cem\u003eJournal of Materials Engineering and Performance, \u003c/em\u003evol. 22, pp. 3735-3741, 2013. \u003c/li\u003e\n\u003cli\u003eY.-S. Lee and J.-H. Sung, \u0026quot;Microstructure and Mechanical Properties of Hastelloy X Fabricated Using Directed Energy Deposition,\u0026quot; \u003cem\u003eMetals, \u003c/em\u003evol. 13, p. 885, 2023. \u003c/li\u003e\n\u003cli\u003eS. Zhonggang, J. Shuwei, G. Yanhua, L. Yichen, C. Lili and X. Fei, \u0026quot;Microstructure evolution and mechanical properties of Hastelloy X alloy produced by Selective Laser Melting.,\u0026quot; \u003cem\u003eHigh Temperature Materials and Processes, \u003c/em\u003evol. 39, pp. 124-135, 2020. \u003c/li\u003e\n\u003cli\u003eD. Williams and E. Butler, \u0026quot;Grain boundary discontinuous precipitation reactions,\u0026quot; \u003cem\u003eInternational Metals Reviews, \u003c/em\u003evol. 3, pp. 153-183, 1981. \u003c/li\u003e\n\u003cli\u003eJ. Spadotto, Dille, J., M. Watanabe and I. Sol\u0026oacute;rzano, \u0026quot;Grain boundary precipitation phenomena in an alloy 33 (Cr-Fe-Ni-N) subjected to direct-aging treatments (700 \u0026deg;C and 900 \u0026deg;C).,\u0026quot; \u003cem\u003eMaterials Characterization, \u003c/em\u003evol. 140, p. 113\u0026ndash;121, 2018. \u003c/li\u003e\n\u003cli\u003eA. Polkowska, S. Lech, P. Bała and W. 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Vanmeensel, \u0026quot;Microstructure and mechanical properties of Hastelloy X produced by HP-SLM (high power selective laser melting),\u0026quot; \u003cem\u003eMaterials \u0026amp; Design, \u003c/em\u003evol. 15, p. 107598, 2019. \u003c/li\u003e\n\u003cli\u003eY. Yin, J. Zhang, S. Pan, Y. Xing, X. Yue and W. Chang, \u0026quot;Room- and elevated-temperature mechanical property of selective laser melting-fabricated Hastelloy X with different heat treatments,,\u0026quot; \u003cem\u003eMaterials Science and Engineering: A , \u003c/em\u003evol. 886 , p. 145697, 2023 . \u003c/li\u003e\n\u003cli\u003eM. Łazińska, T. Durejko, S. Lipiński, W. Polkowski, T. Czujko and R. Varin, \u0026quot;Porous graded FeAl intermetallic foams fabricated by sintering process using NaCl space holders,\u0026quot; \u003cem\u003eMaterials Science and Engineering: A , \u003c/em\u003evol. 636 , pp. 407-414, 2015. \u003c/li\u003e\n\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":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Hastelloy-X, nickel superalloys, metallic porous materials, space holder technique, X-ray computed tomography","lastPublishedDoi":"10.21203/rs.3.rs-4872347/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4872347/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eHighly porous nickel-based superalloys appears as attractive candidates to be applied e.g. as seals in gas turbine engines instead of honeycomb structures. Among various methods of producing open-porous materials, a space holder approach provides number of benefits regarding economic and ecological aspects of production. In this work, the pioneering results of microstructure and mechanical properties analyses of highly porous Hastelloy-X nickel superalloy produced by the space holder approach, are presented. The materials were fabricated by using spherical fine Hastelloy-X powders and carbamide particles as batch materials. Multi-step powder metallurgy and thermomechanical processing was applied to produce open porous samples having a total volumetric porosity of 50, 60 and 70%. The produced materials were subjected to non-destructive (X-ray computed tomography) and metallographic inspections. Mechanical properties of the porous Hastelloy-X samples were examined in static room temperature compression tests, to discuss the effect of obtained porosity on compressive response.\u003c/p\u003e","manuscriptTitle":"Highly porous Hastelloy-X nickel superalloy produced by a space holder approach: microstructure and mechanical properties","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-02 12:07:09","doi":"10.21203/rs.3.rs-4872347/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-11-04T05:50:29+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-10-31T11:46:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"240434753589707880978417006468900717805","date":"2024-10-30T09:20:33+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-08-27T08:02:45+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"42894197817895122378409314771357416615","date":"2024-08-13T01:46:47+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-13T00:20:19+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-13T00:11:16+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-08-07T06:48:55+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-07T06:42:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-08-07T06:30:32+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"81352e8e-b60c-4f3c-a9fb-9415e54baa3c","owner":[],"postedDate":"September 2nd, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-01-06T15:58:57+00:00","versionOfRecord":{"articleIdentity":"rs-4872347","link":"https://doi.org/10.1038/s41598-024-84321-3","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2025-01-02 15:57:01","publishedOnDateReadable":"January 2nd, 2025"},"versionCreatedAt":"2024-09-02 12:07:09","video":"","vorDoi":"10.1038/s41598-024-84321-3","vorDoiUrl":"https://doi.org/10.1038/s41598-024-84321-3","workflowStages":[]},"version":"v1","identity":"rs-4872347","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4872347","identity":"rs-4872347","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}