Refractory High-entropy Alloys Research Articles

CALPHAD-aided design for superior mechanical behavior in Ti40Zr20Hf40-xCrx eutectic refractory high-entropy alloys

TiZrHf-based refractory high entropy alloys (RHEAs) are becoming the focus in advanced metal materials owing to the excellent mechanical properties under the condition of medium and high temperatures. Nevertheless, the strength of TiZrHf-based RHEAs at medium temperatures has hindered the further application. This work proposed a novel approach to improve the mechanical properties of TiZrHf-based RHEAs. An innovative series of Ti40Zr20Hf40-xCrx (x = 19, 24 and 29, denoted by HfCr19, HfCr24 and HfCr29, respectively) eutectic refractory high entropy alloys (ERHEAs) were designed and prepared. The designed Ti40Zr20Hf40-xCrx alloys can form lamellar eutectic structure including BCC/HCP phase and Laves precipitating phase in solidification with the decrease of Hf/Cr ratio. The microstructure of HfCr19 and HfCr24 alloys was composed of BCC, HCP and Laves phase, while the HfCr29 alloy consisted of BCC and Laves phase. The formation of HCP phase in the Ti40Zr20Hf40-xCrx alloy were attributed to the lattice of Ti0.5Zr0.5 phase reconstruction during the rapid cooling, which promoted the formation of isomers in the alloy. Hence, the part of BCC phase was transformed into HCP phase in the HfCr19 and HfCr24 alloys, and the lamellar eutectic structure consisted of BCC/HCP phase and Laves phase. In addition, compared with the near-eutectic HfCr19 and HfCr29 alloys, the HfCr24 alloy with a complete lamellar eutectic structure has higher compressive strength at room temperature, which can reach 1648.7 MPa. In addition, the compressive strength (1261.7 MPa) can still be achieved at 600 °C. This work successfully prepared a high-strength TiZrHf-based ERHEA, and the compression mechanical properties at room temperature and middle-high temperature were studied and analyzed.

Synthesis of complex concentrated silicide coatings via reactive melt-infiltration: Exploring interfacial phenomena between Si-B melt and MoNbTaW high-entropy alloy

High-entropy silicide (HES) coatings are a promising solution to the problem of poor oxidation resistance of metallic refractory alloys. However, their practical development is still in the early stages. For the first time, this study evaluates the feasibility of synthesizing complex concentrated silicide coatings using a reactive melt infiltration approach. For this purpose, a sessile drop experiment was carried out in which a binary eutectic silicon‑boron alloy (Si8B at.%) was subjected to contact heating with the MoNbTaW refractory high entropy alloy (RHEA) substrate at temperatures up to 1450 °C. After the high-temperature test, the solidified couple was characterized by X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, electron backscatter diffraction systems, and microhardness techniques. The coating had an average thickness of 75 μm and was composed of different layers, including a primary MSi2 layer, a transition MSi2 + M5Si3 silicide layer, and M5Si3 silicides (M = metal). Additionally, borosilicides and boride particles were dispersed throughout the layers. However, future research should prioritize refining process parameters to eliminate porosity and improve the integrity of the coating layer.

Effect of annealing temperature on the microstructure and mechanical properties of Al0.2CrNbTiV lightweight refractory high-entropy alloy

The DSC analysis and heat treatment of the newly proposed Al0.2CrNbTiV lightweight refractory high-entropy alloy prepared by vacuum arc melting was investigated, and the evolutions of the microstructure and mechanical properties of the alloy after homogenization annealing at 650 °C, 850 °C and 1050 °C for 12 h were analyzed. The results show that the Al0.2CrNbTiV high-entropy alloy could maintain stable BCC solid solution structure from room temperature to 800 °C. The alloy annealed at 650 °C exhibited simple BCC structure with coarse equiaxed grains; after annealing at 850 °C, fine acicular and irregular block-like C14 Laves phases were uniformly precipitated in the grain and grain boundaries, meanwhile, the C14 Laves phase get coarser with the annealing temperature increased to 1050 °C. With the increase of annealing temperature, the microhardness of the Al0.2CrNbTiV alloy increased first and then decreased, reaching the maximum value of 692 HV after annealing at 850 °C. Due to the high dislocation density and the formation of kink bands, the alloy annealed at 650 °C showed a good combination of plastic and strength, with the work hardening ability strengthened simultaneously, the compressive yield strength could be up to 1454 MPa, with strain >50 %. Due to the precipitation of the hard and brittle C14 Laves phase, the load-bearing capacity of the alloy was reduced after annealing at 850 °C and 1050 °C. However, the wear resistance of the alloy also improved with the presence of the hard phase. The friction coefficient of Al0.2CrNbTiV alloy annealed at 650 °C is 0.67, with the abrasive wear acting as the main wear mechanism, and the alloy after annealing at 850 °C shew the best wear resistance, with the friction coefficient of 0.63, and delamination wear mechanism.

Effect of Al and Nb on microstructure and mechanical properties of novel Al-Mo-Nb-Ti-V-Zr lightweight refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) are a new family of metallic alloys which have been intensively studied in the last decade. Indeed, RHEAs likely have the potential of overcoming the high-temperature performances of conventional Ni-based superalloys. Nevertheless, up to date, their high density and poor room temperature ductility still compromise their industrial uptake. In this study, novel RHEAs compositions are investigated to provide insights on the development of strong and ductile lightweight RHEAs. Specifically, AlxMoNbyTiVZr alloys (with x = 0, 0.25, 0.5, 0.75, 1 and y = 1, 2) have been produced and characterized. These alloys have been designed to probe the possibility of reducing the density of RHEAs by increasing the Al content. At the same time, the content of Nb was also increased in the attempt to counterbalance the embrittlement effect related to Al addition.

Machine learning-assisted design of refractory high-entropy alloys with targeted yield strength and fracture strain

In order to improve the traditional “trial and error” material design method, machine learning-yield strength and machine learning-fracture strain models are incorporated into one system to predict yield strength and fracture strain in refractory high-entropy alloys (RHEAs) under compression. The ML-yield strength model and ML-fracture strain model achieve excellent predictions (R2 = 0.942, RMSE=0.35) and (R2 = 0.892, RMSE=0.41) in the testing set, respectively. Based on the machine learning model, Nb0.22Ta0.22Ti0.24V0.23W0.09, Nb0.24Ta0.22Ti0.26V0.04W0.24, Nb0.26Ta0.24Ti0.21V0.24W0.05, and Nb0.18Ta0.26Ti0.22V0.21W0.13 RHEAs in the Nb-Ta-Ti-V-W RHEA system were screened and synthesized. The yield strength (1915 MPa, 1983 MPa) of the Nb0.22Ta0.22Ti0.24V0.23W0.09 and Nb0.24Ta0.22Ti0.26V0.04W0.24 RHEAs are higher than that (1689 MPa) of the NbTaTiVW RHEA. The unfractured Nb0.18Ta0.26Ti0.22V0.21W0.13 and Nb0.26Ta0.24Ti0.21V0.24W0.05 RHEAs under compression exhibit superior performance than the fracture strain (16.6 %) of the NbTaTiVW RHEA. The mixing enthalpy of RHEAs is negatively correlated with the yield strength, whereas a negative relationship exists between electronegativity difference and fracture strain through the SHAP analysis. Decreasing the mixing enthalpy and increasing the electronegativity difference promote the formation of the precipitated phase. The electron probe microanalysis reveals that the differences in mechanical properties (yield strength and fracture strain) in the NbTaTiVW RHEAs primarily stem from the fraction of the precipitated phase.

High-temperature mechanical behavior and microstructural destabilization evolution of the lightweight refractory high-entropy alloy prepared by laser additive manufacturing

Lightweight refractory high-entropy alloy (LRHEA) has lower density and higher specific strength, which makes it very promising for future applications in rocket engine components. Their high-temperature resistance to softening and high microstructure stability in a wide temperature range are also essential to sustaining their performance due to being used in harsh, high-temperature situations. In this work, the high-temperature tensile behavior of Al0.3NbTi3VZr1.5 LRHEA fabricated by laser additive manufacturing has been systematically investigated from 400 °C to 800 °C, and the relationship between the deformed microstructure and the mechanical behavior was revealed, and analyzing the reasons for the thermodynamic instability of the solid solution in LRHEA are discussed in detail. It is found that the weakening of the high entropy effect at intermediate temperatures reduces the stability of the solid solution, and the accelerated diffusion between elements further promotes the solid solution destabilization, mainly manifested as the Laves phase and the local chemical fluctuation (LCFs). The high dislocation density introduced by the additive manufacturing technology leads to the irregular connection of the Laves phases within the grains, and it can be improved by grain growth. The high negative mixing enthalpy competes with the high-entropy effect, and temperature is the decisive factor. In this situation, the atomic clusters were promoted and induced spinodal decomposition for generating LCFs. The formation of LCFs exhibits solute competition with the emergence of the C14 Laves phase, and the phase proportions fluctuate at different temperatures. This work provides new thinking and attention for developing lightweight, heat-resistant materials.

A review on recent progress of refractory high entropy alloys: From fundamental research to engineering applications

Refractory high-entropy alloys (RHEAs) have emerged as frontier materials for high-temperature structural applications due to their exceptional mechanical properties and thermal stability. This review aims to provide a comprehensive and in-depth summary of the latest research progress in the field of RHEAs. By systematically analyzing 253 publications since 2022, this review presents a panoramic overview of the current research status in the field of RHEAs, covering aspects from alloy design, microstructure, processing techniques, mechanical behavior, to physicochemical properties. Key strengthening and toughening mechanisms, such as solid solution strengthening, precipitation strengthening, and grain boundary strengthening, are extensively analyzed. The high-temperature mechanical performance, oxidation resistance, and adaptability of RHEAs in extreme environments including corrosion and irradiation, are critically reviewed, and the potential application value of these research findings in aerospace, nuclear energy, biomedical, and other fields is discussed. Simultaneously, the multidisciplinary characteristics of RHEAs research has revealed the trend of the field evolving from fundamental research to practical applications. Furthermore, with the aid of advanced characterization techniques and computational methods, the review elucidates the controlling effects of chemical short-range order, defect structures, and other factors on the performance evolution of RHEAs, highlighting the importance of multi-principal element synergistic effects. Based on summarizing the key scientific issues and technological bottlenecks faced by RHEAs, the article provides a forward-looking perspective on future research directions, emphasizing development strategies that integrate computation and experiments, and accelerate the transformation of fundamental research into engineering applications, thus providing insights and guidance for developing a new generation of high-performance RHEAs.

A yield strength prediction framework for refractory high-entropy alloys based on machine learning

Machine learning has been widely applied to materials research with the development of artificial intelligence. Here, a new framework mainly based on the LightGBM algorithm was proposed, which predicted the yield strength of refractory high-entropy alloys (RHEAs) in various temperatures. The features of T, D·B, μ, Smix, Gmix and r were recognized as the optimal feature set by several feature screening methods. The framework displayed good prediction results with a coefficient of determination (R2) of 0.9605 and a root mean square error (RMSE) of 111.99 MPa in the test set. A series of RHEA samples validated the generalization of this framework. SHAP with pearson correlation constant (PCC) and maximal information coefficient (MIC) interpreted the framework and analyzed the intrinsic mechanism of features on yield strength, discovering a novel μ-D·B-Gmix design strategy for obtaining RHEAs with enhanced yield strength. Both TiTaNbHfNi0.25 and TiTaNbHfNi0.5 alloys were fabricated as the experimental verification for this framework which showed 1230 and 1311 MPa yield strength with the predicted errors of 6.3 % and 3.7 %. The validations above demonstrated the excellent performance of the present framework and the effectiveness of such a strategy.

Achieving excellent strength-ductility balance in the lightweight refractory high-entropy alloy by incorporating aluminum

Refractory high-entropy alloys (RHEAs) have garnered widespread attention as a novel class of alloys with promising applications in high-temperature environments. However, their limited ductility has constrained their engineering application. In this study, a Ti40Zr40Nb10Ta10 base alloy with enhanced ductility was chosen, and various amounts of Al were added to investigate the microstructure and mechanical behavior of these alloys. The addition of Al aimed to trigger the cocktail effect while simultaneously enhancing the specific strength of alloys. Scanning electron microscopy and transmission electron microscopy were employed to investigate the phase composition and microstructure, revealing that the addition of Al promoted the formation of a B2 structure. With the incorporation of Al, both the yield strength and hardness of alloys increased. When the Al content reached to 8 at.%, the alloy achieved an optimal strength-ductility balance (yield strength 1030 MPa, elongation 13.4 %). The strengthening contribution was calculated and indicated that solid solution strengthening and precipitation strengthening are the two main methods. While the excellent ductility is contributed by the cross-slip of dislocations, the presence of kink bands accommodated dislocation slip and reduced stress concentration, enhancing the elongation. This study reports a system of RHEAs with excellent room-temperature strength-ductility balance, shedding light on its potential engineering applications.

Attaining exceptional wear resistance in an in-situ ceramic phase reinforced NbMoWTa refractory high entropy alloy composite by Spark plasma sintering

The refractory high-entropy alloys (RHEAs) exhibit great potential as structural components for aerospace equipment. However, their lack of wear resistance and increased coefficient of friction at room temperature (RT) impose limitations on their practical applications. Therefore, further enhancements are required to improve their friction and wear properties under RT. In this context, the development of NbMoWTa(h-BN)x RHEA ceramic composites in this work offers a viable solution to address this issue. Experimental results demonstrate that the addition of h-BN leads to the in-situ generation of (Nb,Ta)N/(Nb,Ta)2N and (Nb,Ta)B2 ceramic phases, significantly enhancing the hardness and wear resistance of the composites. The wear rate of NbMoWTa(h-BN)0.5 reaching as low as 1.32 × 10−8 mm3/Nm, which is four orders of magnitude lower than that of the RHEA. The NbMoWTa RHEA exhibits significant adhesive wear, which can be effectively mitigated in composites through the uniform dispersion of ceramic phase particles with lower mean free path. The abrasive particles primarily interact with the hard strengthening phase, effectively inhibiting plastic deformation in their vicinity. Consequently, the reduced mean free path between the ceramic phases limits the likelihood of metal matrix removal. Subsequently, aided by the presence of ceramic phases, the spontaneous formation of protective third bodies further inhibit surface material removal and ultimately ensures exceptional wear resistance.

High-throughput screening of MoNbTaW-based Refractory High-Entropy Alloys through Direct Energy Deposition in-situ alloying

Refractory High-Entropy Alloys (RHEAs) have been presented as attractive materials for high-temperature applications, such as combustion engines for the aerospace sector, due to the possible service temperature increase, which can result in superior yield of the combustion itself. This work presents the combination of CALPHAD simulations with the in-situ alloying of the MoNbTaW system by Direct Energy Deposition (DED) for a high-throughput screening of RHEAs. CALPHAD simulations show that the addition of V and Ti allows a single-phase BCC structure to be kept. For the DED setup available, MoNbTaW single-track deposits were optimised through Response Surface Methodology. Afterwards, in-situ alloying with V was conducted. Microstructural characterisation was assessed at room temperature through SEM/EDS, and the mechanical characterisation was performed at room temperature (RT), 350 °C and 700 °C, through nanoindentation tests. The microstructural characterisation shows that alloying up to 40 at.% V to the base system allows to keep a single-phase structure; however, some segregations are observed. Mechanical characterisation revealed that V alloying of 22 % promotes a 93 % increase in hardness at RT and a 150 % increase at 700 °C. This study contributes to increasing the practical knowledge of RHEAs, thus accelerating the application of these alloys in aerospace applications.

Deformation mechanisms of additively manufactured Hf10Nb12Ti40V38 refractory high-entropy alloy: Dislocation channels and kink bands

To date, the reported refractory high-entropy alloys (RHEAs) with strength and ductility synergy all have up to 1 GPa strain hardening rate (SHR). Keeping the SHR of more than 1 GPa in the RHEAs can postpone the point of plastic instability, thus prolonging the uniform tensile ductility (UTD). In this paper, Hf10Nb12Ti40V38 RHEA with high strength and ductility is successfully manufactured by laser-directed energy deposition (L-DED). The yield strength of the alloy is about 1011 MPa with the tensile ductility of ∼12.6 %. Interestingly, the SHR of the alloy is very low, far below 1 GPa. The stress-strain curve of the alloy does not only show no premature dropout, but also exhibits a flat plateau-type curve. For this interesting phenomenon, the deformation mechanisms of Hf10Nb12Ti40V38 alloy are systematically investigated by investigating the deformation microstructures. The results indicate that the dislocation modes are diversifying, but the limited number of tangles generated by the intersection of dislocation channels are the main source of SHR. Various dislocation configurations such as single dislocation channel and cross-slip are stimulated sequentially, and although they have no positive effect on the SHR of the alloy, they allow alloy to accommodate plastic deformation. Combined with the kink bands as a strain softening mechanism, it improves the ductility while decreasing the SHR. Multiple dislocation channels interactions and kink bands are clearly identified as the two deformation mechanisms governing strain hardening and strain softening. The competition between two mechanisms generates the phenomenon mentioned above. This study does not only shed new insights on the unique deformation behavior of Hf10Nb12Ti40V38 RHEA but also complement the current general perception about the relationship between SHR and ductility of RHEAs.

A systematic study on wear behavior of TiCrNbTaWx refractory high-entropy alloy: Inducing amorphization to achieve anti-wear

Refractory high-entropy alloys (RHEAs) possess exceptional properties at elevated temperatures. However, their limited wear resistance at room temperature (RT) hinders their widespread application. In this work, based on compositional modulation and surface engineering strategies, two TiCrNbTaWx (x = 0, 0.5) RHEAs were prepared using spark plasma sintering. The results show that the addition of W can effectively improve the hardness (10.3 GPa) and deformation resistance of the BCC matrix, alleviating the severe adhesion and the fracture behavior. In addition, the friction-induced oxidized amorphous layer with high hardness (12.3 GPa) can further improve the anti-wear properties. Because of these, the TiCrNbTaW0.5 alloy has a low wear rate of 9 × 10−5 mm−3·N−1·m−1, which is 67 % lower than the pristine TiCrNbTa alloy. In conclusion, our design strategy provides a new idea for designing RT wear-resistant RHEAs.

First-principles calculations to investigate phase stability, elastic and thermodynamic properties of TiMoNbX (X=Cr, Ta, Cr and Ta) refractory high entropy alloys

This work uses first-principles calculations to investigate the phase stability, thermophysical and mechanical properties of refractory high entropy alloys (RHEAs) at finite temperatures. On the basis of plane wave quasi-potential and density functional theory, construct the structure model of a solid solution. The TiMoNbX (X = Cr, Ta, Cr and Ta) RHEAs have been determined to preserve a single body-centered cubic solid solution structure by calculations and the equilibrium lattice parameters and elastic modulus are consistent with experimental data obtained by laser cladding, which is combined with TC4 (Ti-6Al-4V) substrate. Using the quasi-harmonic Debye-Grüneisen model, the thermophysical characteristics of three RHEAs are investigated. The Voigt-Reuss-Hill scheme is used for calculating the Young's modulus (E), bulk modulus (B), shear modulus (G), and Poisson's ratio (ν), which indicates that all three RHEAs are ductile materials. Additionally, the modulus and hardness of materials decrease as temperature rises, whereas the properties of TiMoNbX RHEAs are predicted, as the nanoindentation hardness values at room temperature are comparable to, and slightly higher than the calculated values.

Chemical short-range order increases the phonon heat conductivity in a refractory high-entropy alloy

We study the effects of the chemical short-range order (SRO) on the thermal conductivity of the refractory high-entropy alloy HfNbTaTiZr using atomistic simulation. Samples with different degrees of chemical SRO are prepared by a Monte Carlo scheme. With increasing SRO, a tendency of forming HfTi and TiZr clusters is found. The phonon density of states is determined from the velocity auto-correlation function and chemical SRO modifies the high-frequency part of the phonon density of states. Lattice heat conductivity is calculated by non-equilibrium molecular dynamics simulations. The heat conductivity of the random alloy is lower than that of the segregated binary alloys. Phonon scattering by SRO precipitates might be expected to reduce scattering times and, therefore, decrease thermal conductivity. We find that, in contrast, due to the increase of the conductivity alongside SRO cluster percolation pathways, SRO increases the lattice heat conductivity by around 12 %. This is expected to be a general result, extending to other HEAs.

Origins of strength stabilities at elevated temperatures in additively manufactured refractory high entropy alloy

While refractory high-entropy alloys (RHEAs) show promising potential for extreme applications, those directly fabricated via additive manufacturing methods have been hindered by their inferior mechanical properties, particularly at high temperatures. In this study, we successfully produced a Hf-Nb-Ti-V RHEA using directed energy deposition (DED) technique, achieving satisfactory tensile properties across a wide temperature range. This was accomplished by inducing severe lattice distortions in the fabricated RHEA, which can be traced back to the local chemical fluctuations present in the newly fabricated RHEA and the significant atomic radius mismatch. Due to strong solute pinning, these factors contribute to the superior yield strength of the DED-fabricated RHEA across a wide temperature range. Furthermore, the elastic constants in the fabricated RHEA show a negligible temperature dependence, revealed by first-principles calculations, ensuring satisfactory strengths even at high temperatures. This alloy design strategy, which involves introducing significant lattice distortion and maintaining the temperature-low sensitivity of the elastic moduli, opens up new possibilities for directly fabricating RHEAs with superior high-temperature properties.

Strong and ductile bicontinuous Cu-RHEA composites formed by liquid metal dealloying

Inter-granular etching often leads to ‘weakened grain boundaries’, which deteriorate the ductility of (nano-) composites or porous materials fabricated by liquid metal dealloying (LMD). In this paper, we report that inter-granular etching can be suppressed by the addition of Zr in Ti-Ta precursor, while it is dealloyed in liquid Cu. This procedure removes grain-boundary-like defects in obtained bicontinuous Cu-Ta composites, and improve their ductility in tension. By replacing Ta with a refractory high entropy alloy (RHEA), i.e. MoNbTaVW, we also obtain high-quality bicontinuous Cu-RHEA composites with low density, high tensile strength and excellent ductility. The suppression of inter-granular etching, which is essential to the mechanical stability of LMD-made materials, might arise from the decrease of grain boundary energy of Ti-Ta precursor alloys induced by Zr addition. New materials developed in this study may enable novel applications by enlarging the property space and overcoming the poor malleability and machinability of conventional materials of this type (such as Cu-W composites).

A single-phase Nb25Ti35V5Zr35 refractory high-entropy alloy with excellent strength-ductility synergy

Refractory high entropy alloys (RHEAs) are promising candidates for high-temperature structural materials due to their excellent high-temperature performance. However, the room-temperature brittleness of most alloys results in poor plastic formability, thereby limiting their engineering applications. In this study, Nb25Ti35V5Zr35 alloy with cold-rollability was developed, and its phase stability, tensile mechanical properties, elastic properties, strengthening mechanism, and deformation mechanism were investigated by combining experiments, CALPHAD, DFT and molecular statics (MS) methods. The results demonstrate that Nb25Ti35V5Zr35 alloy possesses good phase stability, with both the as-cast and annealed states maintaining a single-phase BCC structure without undergoing phase transformation. The tensile yield strength of the alloy reaches its peak at 1152 MPa in the cold-rolled condition, while its annealed state exhibits an optimal balance of strength and plasticity, with a yield strength of 836 MPa and an elongation of 22.45 %, respectively. Solid solution strengthening is the primary source of its high strength, with V and Zr playing key roles in this strengthening mechanism. Dislocation slip plays a dominant role in plastic deformation. Additionally, compared to traditional BCC alloys, the significance of edge dislocations in the deformation process is notably enhanced, with the {112} plane serving as the preferred slip plane for dislocations. DFT results indicate that the alloy exhibits significant anisotropy in both Young's modulus and shear modulus. This work contributes to understanding the material properties of the Nb25Ti35V5Zr35 alloy and provides a reference for the design of new ductile RHEAs.

A novel method for reducing the brazing temperature of C/C composite with TiZrHfTa/Ni composite interlayers

C/C composite was successfully brazed with TiZrHfTa/Ni composite interlayers at lower temperature far below the melting point of TiZrHfTa refractory high entropy alloy. The influence of brazing parameters on the joint morphology, indentation fraction toughness of the reaction layer, shear strength at room temperature and at 1000 °C, and fracture behavior was investigated. The results show that all of the C/C composite joints contained two main phases: an equimolar (Ti-Zr-Hf-Ta)C hard phase and a near-pure Ni binder phase. The maximum indentation fracture toughness of the obtained joint reaction layer material was 15.52 ± 0.64 MPa·m1/2. This was proved to be beneficial to improve the strength and toughness of the joint. Meanwhile, the maximum shear strengths of C/C-TiZrHfTa/Ni-C/C joint at room temperature and at 1000 °C reached 33.24 ± 1.85 MPa and 21.70 ± 2.12 MPa, respectively. Abundant slip lines, in-situ dimples, and tearing ridges resulting from Ni plastic deformation suggest a predominantly ductile fracture mode in the joint. This work introduces an innovative method, which can not only ensure the service of C/C composite in high temperature environment, but also effectively reduce the joining temperature.

Ablation-resistant yttrium-modified high-entropy refractory metal silicide (NbMoTaW)Si2 coating for oxidizing environments up to 2100 °C

Refractory high-entropy alloys (RHEAs) are pivotal in ultra-high temperature applications, such as rocket nozzles, aerospace engines, and leading edges of hypersonic vehicles due to their exceptional mechanical ability to withstand severe thermal environments (in excess of 2000 °C). However, the selection of materials that satisfy the stringent criteria required for effective ablation resistance remains notably restricted. Here, a novel yttrium-modified high-entropy refractory metal silicide (Y-HERMS) coated on a refractory high-entropy NbMoTaW alloy is developed via pack cementation process. The developed Y-HERMS coating with sluggish diffusion effect demonstrates extraordinary ablation resistance, maintaining near-zero damage at sustained temperatures up to 2100 °C for a duration of 180 s, surpassing state-of-the-art high-performance silicide coatings. Such exceptional ultra-high ablation performance is primarily ascribed to the in-situ development of a high viscosity Si-Y-O oxide layer with increased thermal stability and the presence of high-melting Y(Nb0.5Ta0.5)O4 oxides as skeleton structure. Theoretical results elucidate that the Y-HERMS promotes the formation of SiO2, which impedes the diffusion of O into metal silicide layer, synergistically contributing to the superior ablation resistance. These findings highlight the potential of utilizing high-entropy materials with excellent ablation resistance in extreme thermal environments.