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Refractory high‐entropy alloys (RHEAs) show potential for use in extreme environments, such as advanced nuclear reactors, owing to their high melting temperature, and often outstanding combinations of mechanical properties, corrosion resistance, and irradiation‐damage tolerance. This study evaluates the fracture toughness of a TiZrNbHfTa RHEA across different scales and microstructures, with a focus on the impact of He2+‐ion irradiation. Micro‐ and millimeter‐scale specimens with nanocrystalline (NC) microstructures are compared to existing ASTM standard sized coarse‐grained (CG) specimen data, with critical dimensions spanning over three orders of magnitude, from 10 μm to 12 mm. The ASTM standard sized CG specimens exhibit a fracture toughness 41‐fold greater than their NC microscale counterparts (210–5.1 MPa m1/2), while NC millimeter‐scale specimens show a 7.5‐fold higher fracture toughness than NC microscale specimens (38.1–5.1 MPa m1/2). He2+‐ion irradiation leads to a 27% decrease in fracture toughness in the NC microscale specimens. The results highlight the impact of sample dimensional scale, microstructure, and ion irradiation on the fracture toughness of the RHEA, indicating a need for thorough examination of such factors when investigating the mechanical properties of these materials. [ABSTRACT FROM AUTHOR]

The oxidation of six NbTi-i refractory medium- and high-entropy alloys (NbTi + Ta, NbTi + CrTa, NbTi + AlTa, NbTi + AlMo, NbTi + AlMoTa and NbTi + AlCrMo) were investigated at 1000 °C for 20 h. According to our observation, increased Cr content promoted the formation of protective CrNbO4 and Cr2O3 oxides in NbTi + CrTa and NbTi + AlCrMo, enhancing oxidation resistance. The addition of Al resulted in the formation of AlTi-rich oxide in NbTi + AlTa. Ta addition resulted in the formation of complex oxides (MoTiTa8O25 and TiTaO4) and decreased oxidation resistance. Meanwhile, Mo's low oxygen solubility could be beneficial for oxidation resistance while protective Cr2O3/CrNbO4 layers were formed. In NbTi + Ta, NbTi + AlTa and NbTi + CrTa, a considerable quantity of Ti-rich oxide was observed at the interdendritic region. In NbTi + AlCrMo, the enrichment of Cr and Ti at the interdendritic region could fasten the rate of oxidation. Compared to the recent research, NbTi + AlCrMo alloy is a light-weight oxidation-resistant alloy (weight gain of 1.29 mg/cm2 at 1000 °C for 20 h and low density (7.2 g/cm3)). [ABSTRACT FROM AUTHOR]

Abstract: The refractory complex concentrated alloy (RCCA) 5Al–5Cr–5Ge–1Hf–6Mo–33Nb–19Si–20Ti–5Sn–1W (at.%) was studied in the as-cast and heat-treated conditions. The partitioning of solutes in the as-cast and heat-treated microstructures and relationships between solutes, between solutes and the parameters VEC and Δχ, and between these parameters, most of which are reported for the first time for metallic UHTMs, were shown to be important for the properties of the stable phases A15–Nb3X and the D8m βNb5Si3. The nano-hardness and Young's modulus of the A15–Nb3X and the D8m βNb5Si3 of the heat-treated alloy were measured using nanoindentation and changes in these properties per solute addition were discussed. The aforementioned relationships, the VEC versus Δχ maps and the VEC, Δχ, time, or VEC, Δχ, Young's modulus or VEC, Δχ, nano-hardness diagrams of the phases in the as-cast and heat-treated alloy, and the properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. The significance of the new data and the synergy and entanglement of solutes and phases for the design of metallic ultra-high temperature materials were discussed. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEA) are considered as potential candidates for new-generation energy-related high-temperature applications. However, the poor high-temperature oxidation resistance of RHEAs, resulting in phenomena such as significant weight gain, scale spallation, pesting, and even complete oxidation, limits their applications. In this study, the oxidation behavior of AlxHfNbTiVY0.05 (x = 0.75; 1; 1.25) high-entropy alloys was investigated at 700–900 °C. The isothermal oxidation tests showed that the oxidation resistance of AlxHfNbTiVY0.05 RHEA is strongly influenced by temperature and time. In addition, accelerated oxidation, known as pesting, was observed to occur at 700 °C for all alloys; while, partial spallation was observed at 800 °C for the Al1 and Al1.25 alloys. Detailed analyses of oxidation kinetics have been carried out for the oxidation test series at 900 °C. The mechanism behind disintegration was investigated and attributed to accelerated internal oxidation followed by the formation of voluminous Nb2O5, TiNb2O7, and fast-growing AlNbO4, and is also thought to be related to the partial evaporation of V2O5. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) typically exhibit excellent high-temperature strength but limited ductility. In this study, a comprehensive machine learning strategy with integrated material knowledge is proposed to predict the elongation of TiZrNbVAl RHEAs. By referring to the ductility theories, a set of cost-effective material features is developed with various mathematical forms of thermodynamic parameters. These features are proven to effectively incorporate material knowledge into ML modeling. They also offer potential alternatives to those obtained from costly first-principles calculations. Based on Pearson correlation coefficients, the linear relationships between pairwise features were compared, and the seven key features with the greatest impact on the model were selected for ML modeling. Regression tasks were performed to predict the ductility of TiZrNbVAl, and the CatBoost gradient boosting algorithm exhibiting the best performance was eventually selected. The established optimized model achieves high predictive accuracies exceeding 0.8. These key features were further analyzed using interpretable ML methods to elucidate their influences on various ductility mechanisms. According to the ML results, different compositions of TiZrNbVAl with excellent tensile properties were prepared. The experimental results indicate that Ti44Zr24Nb17V5Al10 and Ti44Zr26Nb8V13Al9 both exhibited ultimate tensile strengths of approximately 1180 MPa and elongations higher than 21%. They verified that the ML strategy proposed in this study is an effective approach for predicting the properties of RHEAs. It is a potential method that can replace costly first-principles calculations. Thermodynamic parameters have been shown to effectively predict alloy ductility to a certain extent. [ABSTRACT FROM AUTHOR]

Abstract: The refractory complex concentrated alloy (RCCA) 5Al–5Cr–5Ge–1Hf–6Mo–33Nb–19Si–20Ti–5Sn–1W (at.%) was studied in the as-cast and heat-treated conditions. The partitioning of solutes in the as-cast and heat-treated microstructures and relationships between solutes, between solutes and the parameters VEC and Δχ, and between these parameters, most of which are reported for the first time for metallic UHTMs, were shown to be important for the properties of the stable phases A15–Nb3X and the D8m βNb5Si3. The nano-hardness and Young’s modulus of the A15–Nb3X and the D8m βNb5Si3 of the heat-treated alloy were measured using nanoindentation and changes in these properties per solute addition were discussed. The aforementioned relationships, the VEC versus Δχ maps and the VEC, Δχ, time, or VEC, Δχ, Young’s modulus or VEC, Δχ, nano-hardness diagrams of the phases in the as-cast and heat-treated alloy, and the properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. The significance of the new data and the synergy and entanglement of solutes and phases for the design of metallic ultra-high temperature materials were discussed.

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.

This study aims to investigate the phase composition, microstructure, and mechanical properties of three refractory high-entropy alloys (RHEAs): Al7(NbTiZr)93, Al7(NbTiZrMo)93, and Al7(NbTiZrTa)93. The as-cast Al7(NbTiZr)93 alloy exhibits a single-phase BCC structure, with a yield strength (YS) of 1050 MPa at room temperature. The addition of Mo and Ta leads to the formation of Al7(NbTiZrMo)93 and Al7(NbTiZrTa)93 alloys with BCC-B2 and B2-FCC dual-phase structures, whose YS values reach 1741 MPa and 2350 MPa, respectively. At 800 °C, trace amounts of ordered B2 phase are generated in the Al7(NbTiZr)93 matrix, and the YS decreases from 1050 MPa at room temperature to 219 MPa at 800 °C. In the Al7(NbTiZrMo)93 alloy, the 10 nm B2 precipitates grow to several hundred nanometers, and the YS decreases to 431 MPa after compression at 800 °C. In the Al7(NbTiZrTa)93 alloy, the grid B2-FCC structure at room temperature transforms into a disordered BCC structure at 800 °C, yet possessing the YS of 908 MPa and demonstrating acceptable ductility. Moreover, Ta exhibits the better structural and performance matching than Mo in AlNbTiZr-based RHEAs. Therefore, it opens up new prospects for regulating the structure and high-temperature performance of AlNbTiZr-based RHEAs.

Refractory high-entropy alloys (RHEAs) have remarkable properties like high strength and thermal stability at high temperatures. Their practical application is hindered by ambient brittleness and inferior formability. Titanium (Ti) is recognized as a crucial element in improving the ductility and oxidation resistance of RHEAs, but its plasticizing mechanism is still unclear. To address this, a series of body-centered cubic (BCC) single-phase VNbTaTix alloys were designed. The alloys exhibited exceptional ductility and cold-rolling formability at room temperature. Remarkably, the tensile fracture elongation of VNbTaTi alloy was about 26.9%, which surpassed most RHEAs. The major plastic mechanisms of the alloys are a/2-type dislocation and {1 1 2}-type twinning. Solid solution strengthening was identified as the primary strengthening mechanism. Although Ti reduced the solid solution strengthening effect, it contributed to weight reduction and enhanced local lattice distortion. This study not only designed several RHEAs with ductility but also provided insights into the effect of Ti content on their phase stability, lattice distortion, and strength mechanisms. These findings have significant implications for designing new RHEAs with ambient ductility.

Refractory high‐entropy alloys (RHEAs) inspire the development of novel high‐temperature structural materials due to their outstanding resistance to softening and phase stability at elevated temperatures. However, they struggle to simultaneously achieve high‐temperature strength and room‐temperature ductility, while exhibiting insufficient room‐temperature strain hardening capability. Heterogeneous structure strengthening possesses a unique plastic self‐coordinated ability, which can effectively maintain strain hardening rate to achieve an excellent combination of strength and ductility. Benefiting from slow atomic diffusion, severe lattice distortion, and broad compositional design space, RHEAs with heterogeneous structures can be prepared from both chemical composition and interface structure perspectives. Chemical composition heterogeneity primarily focuses on fluctuations of alloying elements at the nanoscale, along with the formation of heterogeneous precipitates and unique lamellar eutectic structures. While, interface structure heterogeneity manifests in the activation of phase transformation and twin boundaries within grains, along with the formation of grains of vastly different sizes. The trend in RHEAs development is toward structural‐functional integration. Heterogeneous structures can also optimize functional properties, such as irradiation resistance, biomedical properties, and high‐temperature softening resistance of RHEAs. Finally, a brief outlook is provided on the future development direction of heterogeneous structure RHEAs. [ABSTRACT FROM AUTHOR]

High-temperature creep refers to the slow and continuous plastic deformation of materials under the effects of high temperatures and mechanical stress over extended periods, which can lead to the degradation or even failure of the components' functionality. AlxCr0.2NbTiV (x = 0.2, 0.5, or 0.8) refractory high-entropy alloys were fabricated by arc melting. The effects of Al content on the microstructure of AlxCr0.2NbTiV alloys were studied using X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction. The microhardness, compression properties, and nanoindentation creep properties of AlxCr0.2NbTiV alloys were also tested. The results show that the AlxCr0.2NbTiV series exhibits a BCC single-phase structure. As the Al content increases, the lattice constant of the alloys gradually decreases, and the intensity of the (110) crystal plane diffraction peak increases. Adding aluminum enhances the effect of solution strengthening; however, due to grain coarsening, the microhardness and room temperature compressive strength of the alloy are only slightly improved. Additionally, because the effect of solution strengthening is diminished at high temperatures, the compressive strength of the alloy at 1000 °C is significantly reduced. The creep mechanism of the alloys is predominantly governed by dislocation creep. Moreover, increasing the Al content helps to reduce the sensitivity of the alloy to the loading rate during the creep process. At a loading rate of 2.5 mN/s, the Al0.8Cr0.2NbTiV alloy exhibits the lowest creep strain rate sensitivity index (m), which is 0.0758. [ABSTRACT FROM AUTHOR]

Refractory high‐entropy alloys (RHEAs) are a new class of metallic alloys which have been extensively studied in the past decade due to their excellent high‐temperature performances. However, the design of new lightweight and ductile RHEAs is a challenging task, since an extensive exploration of the immense compositional space of multicomponent systems is practically impossible. Aiming to reduce the experimental effort, several research groups have proposed different predictive criteria to design new high‐performing HEAs. Nevertheless, the criteria proposed so far are often based on a limited amount of data and, generally, do not differentiate between refractory and nonrefractory HEAs. To overcome these limitations, herein, a comprehensive database of properties of 265 RHEAs reported in the open literature from 2010 to 2022 is developed. Such a database is used to assess the validity of predictive empirical criteria and new guidelines for the prediction of solid solution stability in RHEAs are proposed. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys, which involve the mixing of four or more refractory metal elements in an equiatomic or near-equiatomic ratio, hold significant potential for various applications in high-temperature materials fields. This is mainly due to their stable phase structure and excellent high-temperature properties. While considerable interest has been in these alloys, most of them have been developed using melting casting technology. However, powder metallurgy has emerged as a promising alternative for further advancement in this field. It has the potential to expand the application areas and enhance the properties of these alloys. This article introduces to various techniques for fabricating pre-alloyed refractory high-entropy powders and their densification. Additionally, it reviews the methods for regulating the microstructure and properties of powder metallurgy refractory high-entropy alloys.

Cobalt-free refractory high-entropy alloys (RHEAs) are strong contenders for structural materials in nuclear reactors because they do not exhibit cobalt activity under irradiation. The mechanical properties and thermal stability of RHEAs are primarily attributed to the solid solution phase, which is essentially the body-centered cubic (BCC) phase. The BCC phase formation rules thus became the basic criterion in the compositional design of RHEAs. In this paper, the BCC phase formation rules in cobalt-free RHEAs were determined via the calculation of six semiempirical parameters, namely, the entropy of mixing, enthalpy of mixing, atomic size difference, Ω-parameter, d-orbital energy level and valance electron concentration. The mixing enthalpy and atomic size differences are more effective than other semiempirical parameters for predicting BCC phase stability in cobalt-free RHEAs. The presence of aluminum is found to cause a notable alteration in the range of phase stability in cobalt-free RHEAs.

Refractory high-entropy alloys (RHEAs) are strong candidates for use in high-temperature engineering applications. As such, the thermodynamic properties as a function of temperature for a variety of RHEA systems need to be studied. In the present work, thermodynamic quantities such as entropy, enthalpy, heat capacity at constant volume, and linear thermal expansion are calculated for three quaternary and three quinary single-phase, BCC RHEAs: AlMoNbV, NbTaTiV, NbTaTiZr, AlNbTaTiV, HfNbTaTiZr, and MoNbTaVW. First-principle calculations based on density functional theory are used for the calculations, and special quasirandom structures (SQSs) are used to represent the random solid solution nature of the RHEAs. A code for the finite temperature thermodynamic properties using the Debye-Grüneisen model is written and employed. For the first time, the finite temperature thermodynamic properties of all 24 atomic configuration permutations of a quaternary RHEA are calculated. At most, 1.7% difference is found between the resulting properties as a function of atomic configuration, indicating that the atomic configuration of the SQS has little effect on the calculated thermodynamic properties. The behavior of thermodynamic properties among the RHEAs studied is discussed based on valence electron concentration and atomic size. Among the quaternary RHEAs studied, namely AlMoNbV, NbTaTiZr, and NbTaTiV, it is found that the presence of Zr contributes to higher entropy. Additionally, at lower temperatures, Zr contributes to higher heat capacity and thermal expansion compared to the alloys without Zr, possibly due to its valence electron concentration. At higher temperatures, Al contributes to higher heat capacity and thermal expansion, possibly due its ductility. Among the quinary systems, the presence of Mo, W, and/or V causes the RHEA to have a lower thermal expansion than the other systems studied. Finally, when comparing the systems with the NbTaTi core, the addition of Al increases thermal expansion, while the removal of Zr lowers the thermal expansion.

Refractory high-entropy alloys (RHEAs) have attracted extensive attention due to their excellent mechanical properties. However, most RHEAs have high density and exhibit poor ductility at room temperature, which greatly limits their applications. In this work, a series of lightweight Ti3Zr1.5NbVx (x = 0, 1 and 2, respectively) RHEAs with high strength and good ductility were designed and prepared using vacuum arc melting. The effects of V content on the microstructure, mechanical properties and deformation mechanism of the as-cast Ti3Zr1.5NbVx RHEAs were investigated in details. The results showed that all the Ti3Zr1.5NbVx RHEAs exhibit a single body-centered cubic (BCC) phase and their densities are less than 6 g/cm3. With the increase of V content, the grain size of the Ti3Zr1.5NbVx RHEAs decreases from 436.1 to 81.2 μm, and the hardness increases from 199.7 to 297.5 HV. The Ti3Zr1.5NbV2 RHEA possesses optimal tensile mechanical properties with the yield strength of 974.0 MPa, fracture elongation of 6.3%, and a specific yield strength of 165.7 MPa cm3/g, better than most previously reported RHEAs. More importantly, the Ti3Zr1.5NbV2 RHEA also exhibits excellent elevated-temperature mechanical properties with yield strength of 770.9 MPa at 600 °C and 243.6 MPa at 800 °C. The deformation mechanism of the Ti3Zr1.5NbV2 RHEA is governed by dislocation slip, including planar slip bands, dislocation loops, and high density dislocation walls. The high strength of the Ti3Zr1.5NbV2 RHEA is mainly attributed to the solid solution strengthening effect, in which Zr and V elements play an important role.

The oxidation of six NbTi-i refractory medium- and high-entropy alloys (NbTi + Ta, NbTi + CrTa, NbTi + AlTa, NbTi + AlMo, NbTi + AlMoTa and NbTi + AlCrMo) were investigated at 1000 °C for 20 h. According to our observation, increased Cr content promoted the formation of protective CrNbO4 and Cr2O3 oxides in NbTi + CrTa and NbTi + AlCrMo, enhancing oxidation resistance. The addition of Al resulted in the formation of AlTi-rich oxide in NbTi + AlTa. Ta addition resulted in the formation of complex oxides (MoTiTa8O25 and TiTaO4) and decreased oxidation resistance. Meanwhile, Mo’s low oxygen solubility could be beneficial for oxidation resistance while protective Cr2O3/CrNbO4 layers were formed. In NbTi + Ta, NbTi + AlTa and NbTi + CrTa, a considerable quantity of Ti-rich oxide was observed at the interdendritic region. In NbTi + AlCrMo, the enrichment of Cr and Ti at the interdendritic region could fasten the rate of oxidation. Compared to the recent research, NbTi + AlCrMo alloy is a light-weight oxidation-resistant alloy (weight gain of 1.29 mg/cm2 at 1000 °C for 20 h and low density (7.2 g/cm3)).

Refractory high-entropy alloys (RHEAs) typically exhibit excellent high-temperature strength but limited ductility. In this study, a comprehensive machine learning strategy with integrated material knowledge is proposed to predict the elongation of TiZrNbVAl RHEAs. By referring to the ductility theories, a set of cost-effective material features is developed with various mathematical forms of thermodynamic parameters. These features are proven to effectively incorporate material knowledge into ML modeling. They also offer potential alternatives to those obtained from costly first-principles calculations. Based on Pearson correlation coefficients, the linear relationships between pairwise features were compared, and the seven key features with the greatest impact on the model were selected for ML modeling. Regression tasks were performed to predict the ductility of TiZrNbVAl, and the CatBoost gradient boosting algorithm exhibiting the best performance was eventually selected. The established optimized model achieves high predictive accuracies exceeding 0.8. These key features were further analyzed using interpretable ML methods to elucidate their influences on various ductility mechanisms. According to the ML results, different compositions of TiZrNbVAl with excellent tensile properties were prepared. The experimental results indicate that Ti44Zr24Nb17V5Al10 and Ti44Zr26Nb8V13Al9 both exhibited ultimate tensile strengths of approximately 1180 MPa and elongations higher than 21%. They verified that the ML strategy proposed in this study is an effective approach for predicting the properties of RHEAs. It is a potential method that can replace costly first-principles calculations. Thermodynamic parameters have been shown to effectively predict alloy ductility to a certain extent.

Along with the globalization of environmental problems and the rapid development of the field of nuclear technologies, the severe irradiation damage of materials has become a big issue, restricting the development of advanced nuclear reactor systems. Refractory high-entropy alloys (RHEAs) have the characteristics of a complex composition, a short-range order, and lattice distortion and possess a high phase stability, outstanding mechanical properties, and excellent irradiation resistance at elevated temperatures; thus, they are expected to be promising candidates for advanced nuclear reactors. This review summarizes the design, preparation, and irradiation resistance of irradiation-tolerant RHEAs. It encompasses a comprehensive analysis of various aspects, including the evolution of defects, changes in microstructure, and the degradation in properties. Furthermore, the challenges and insufficiently researched areas regarding these alloys are identified and discussed. Building on this foundation, the review also provides a forward-looking perspective, outlining potential avenues for future research. [ABSTRACT FROM AUTHOR]

We compiled, assessed, and ranked refractory high-entropy alloys (RHEAs) from the existing literature to identify promising coating materials for hot-forging dies. The selection methodology was rigorously guided by decision science principles, seamlessly integrating multiple attribute decision making (MADM), principal component analysis (PCA), and hierarchical clustering (HC). By employing a combination of twelve diverse MADM methods, we successfully ranked a total of 22 RHEAs. This analytical technique unveiled the top five RHEAs: Ti20-Zr20-Hf20-Nb20-Cr20, Al20.4-Mo10.5-Nb22.4-Ta10.1-Ti17.8-Zr18.8, Ti20-Zr20-Hf20-Nb20-V20, Al11.3-Nb22.3-Ta13.1-Ti27.9-V4.5-Zr20.9, and Al7.9-Hf12.8-Nb23-Ta16.8-Ti18.9-Zr20.6 pertinent for generating data on other significant properties, including wear resistance, fatigue (both thermal and mechanical), bonding compatibility with the substrate die material, oxidation resistance, potential reactions with the workpiece, cost-effectiveness, fabricability, and more. The three highest-ranked RHEAs share key characteristics, including a body-centered cubic (BCC) crystal structure, thermal conductivity below ~70 W/mK, and impressive yield strength at ambient and elevated temperatures, surpassing 1100 MPa. Moreover, they exhibit a remarkable ~73% similarity among themselves. The decision science-driven analyses yield sound metallurgical insights and provide valuable guidelines for developing RHEA coatings tailored for hot-forging dies. The strategy for designing RHEA-based coating materials for hot-forging dies should focus on compositions featuring a substantial presence of refractory metals while maintaining a BCC crystal structure. This combination is likely to deliver the desired blend of thermal and mechanical properties, rendering these coatings exceptionally well-suited for the demanding requirements of hot-forging operations. [ABSTRACT FROM AUTHOR]

The NbTiAlZrHfTaMoW refractory high-entropy alloy (RHEA) system with the structure of the B2 matrix (antiphase domains) and antiphase domain boundaries was firstly developed. We conducted the mechanical properties of the RHEAs at 298 K, 1023 K, 1123 K, and 1223 K, as well as typical deformation characteristics. The RHEAs with low density (7.41~7.51 g/cm3) have excellent compressive-specific yield strength ( σ YS / ρ) at 1023 K (~131 MPa·cm3/g) and 1123 K (~104.2 MPa·cm3/g), respectively, which are far superior to most typical RHEAs. And, they still keep appropriate plastic deformability at room temperature (ε > 0.35). The superior specific yield strengths are mainly attributed to the solid solution strengthening induced by the Zr element. The formation of the dislocation slip bands with [111](10 1 _ ) and [111](11 2 _ ) directions and their interaction provide considerable plastic deformation capability. Meanwhile, dynamic recrystallization and dislocation annihilation accelerate the continuous softening after yielding at 1123 K. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) are strong candidates for use in high-temperature engineering applications. As such, the thermodynamic properties as a function of temperature for a variety of RHEA systems need to be studied. In the present work, thermodynamic quantities such as entropy, enthalpy, heat capacity at constant volume, and linear thermal expansion are calculated for three quaternary and three quinary single-phase, BCC RHEAs: AlMoNbV, NbTaTiV, NbTaTiZr, AlNbTaTiV, HfNbTaTiZr, and MoNbTaVW. First-principle calculations based on density functional theory are used for the calculations, and special quasirandom structures (SQSs) are used to represent the random solid solution nature of the RHEAs. A code for the finite temperature thermodynamic properties using the Debye-Grüneisen model is written and employed. For the first time, the finite temperature thermodynamic properties of all 24 atomic configuration permutations of a quaternary RHEA are calculated. At most, 1.7% difference is found between the resulting properties as a function of atomic configuration, indicating that the atomic configuration of the SQS has little effect on the calculated thermodynamic properties. The behavior of thermodynamic properties among the RHEAs studied is discussed based on valence electron concentration and atomic size. Among the quaternary RHEAs studied, namely AlMoNbV, NbTaTiZr, and NbTaTiV, it is found that the presence of Zr contributes to higher entropy. Additionally, at lower temperatures, Zr contributes to higher heat capacity and thermal expansion compared to the alloys without Zr, possibly due to its valence electron concentration. At higher temperatures, Al contributes to higher heat capacity and thermal expansion, possibly due its ductility. Among the quinary systems, the presence of Mo, W, and/or V causes the RHEA to have a lower thermal expansion than the other systems studied. Finally, when comparing the systems with the NbTaTi core, the addition of Al increases thermal expansion, while the removal of Zr lowers the thermal expansion. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (HEAs) are a new class of metallic materials based on group 4–6 elements of the periodic table with possible additions of Al, Si, Re, C, or B. Some single-phase refractory HEAs can maintain high strength up to 1600°C, while multiphase compositions have more attractive specific properties at temperatures up to 1200°C. Here we examine the structure and mechanical properties of refractory HEAs Nb30Mo30Co20Hf20, Nb30Mo30Co20Zr20, and Nb30Mo30Co20Ti20 (at %). The alloys consisted of an intermetallic B2 matrix and particles of a disordered bcc phase, as well as a minor volume fraction of additional bcc (Nb30Mo30Co20Hf20 and Nb30Mo30Co20Zr20) or fcc (Nb30Mo30Co20Ti20) phases. When tested for uniaxial compression, Nb30Mo30Co20Ti20 alloy showed a higher yield strength in the temperature range of 22–1000°C than Nb30Mo30Co20Hf20 and Nb30Mo30Co20Zr20 alloys. Nb30Mo30Co20Zr20 alloy did not fail at temperatures of 22–800°C to a given 50% strain, while Nb30Mo30Co20Ti20 alloy turned out to be brittle. All alloys demonstrated high strain hardening in the temperature range of 22–800°C, and they can compete in terms of specific strength with commercial nickel and cobalt superalloys. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) possess excellent high-temperature mechanical properties, making them have the promising potential for applications at temperatures higher than Ni-base superalloys. However, their development has been limited by their poor oxidation resistance, which demands an urgent and effective improvement. In particular, the oxides of RHEAs generally suffer from severe pest phenomenon between 500 °C and 800 °C. In this study, the low-density, strong and ductile BCC Mo0.3Nb1.3TiZr RHEA was designed and prepared, and the isothermal oxidation mechanism was studied at mainly 800 °C.To effectively improve the oxidation resistance of the alloy, dual coating layers with inner-layer aluminide and outer-layer silica protection were designed and investigated. Firstly, pack cementation was used to prepare aluminide coating on RHEA. Secondly, the sol-gel method was used to deposit silica films over the aluminide coating. With single aluminide coating deposited on the base metal, the weight gain was reduced from 92.2 mg/cm2 of uncoated specimen to 4.5 mg/cm2 at 800 °C for 24 h. With the dual coating layers, no pest phenomenon was observed between 700 °C and 900 °C and the weight gain dropped sharply from 88.4 mg/cm2 of aluminized specimen to 2.8 mg/cm2 at 800 °C for 48 h. The aluminide coating and SiO2 film successfully adhered to the surface of the RHEA. In addition, the SiO2 film reduced the consumption of aluminide coating. Cracking, accelerated oxygen penetration, and pest phenomenon were not observed. This dual coating layers are very promising in solving the poor oxidation resistance of RHEAs.

High-temperature creep refers to the slow and continuous plastic deformation of materials under the effects of high temperatures and mechanical stress over extended periods, which can lead to the degradation or even failure of the components’ functionality. AlxCr0.2NbTiV (x = 0.2, 0.5, or 0.8) refractory high-entropy alloys were fabricated by arc melting. The effects of Al content on the microstructure of AlxCr0.2NbTiV alloys were studied using X-ray diffraction, scanning electron microscopy, and electron backscatter diffraction. The microhardness, compression properties, and nanoindentation creep properties of AlxCr0.2NbTiV alloys were also tested. The results show that the AlxCr0.2NbTiV series exhibits a BCC single-phase structure. As the Al content increases, the lattice constant of the alloys gradually decreases, and the intensity of the (110) crystal plane diffraction peak increases. Adding aluminum enhances the effect of solution strengthening; however, due to grain coarsening, the microhardness and room temperature compressive strength of the alloy are only slightly improved. Additionally, because the effect of solution strengthening is diminished at high temperatures, the compressive strength of the alloy at 1000 °C is significantly reduced. The creep mechanism of the alloys is predominantly governed by dislocation creep. Moreover, increasing the Al content helps to reduce the sensitivity of the alloy to the loading rate during the creep process. At a loading rate of 2.5 mN/s, the Al0.8Cr0.2NbTiV alloy exhibits the lowest creep strain rate sensitivity index (m), which is 0.0758.

Herein, the oxidation behaviour of refractory (HfCo)100−x(NbMo)x (x = 0; 10; 25; 40; 75; 100 (at.%)) high-entropy alloys with a bcc+B2 structure subjected to cyclic oxidation at 1000 °C was studied. The single-phase B2-ordered HfCo alloy demonstrated the best spallation resistance and retained a pristine form after 100 h. The oxidation kinetics of the HfCo alloy was near-parabolic, accompanied by the formation of external HfO2 or CoO layers after 1 or 100 h, respectively. Additions of (NbMo)x deteriorated the spallation resistance (x ≤ 25 at.%) or led to complete disintegration (x > 25 at.%). Among the (NbMo)-containing alloys, the (HfCo)90(NbMo)10 alloy with the dual-phase bcc+B2 structure showed the most promising oxidation resistance. This alloy withstood cyclic oxidation up to 15 h with a mass gain close to the HfCo alloy and survived 100 h without changes in geometry of the specimen. Unlike the HfCo alloy, in the (HfCo)90(NbMo)10 alloy, the external CoO layer was found already after 1 h. The effect of chemical and phase compositions on the formation of certain oxides was discussed. Comparison with the other refractory high-entropy alloys was also presented. [ABSTRACT FROM AUTHOR]

Herein, the development of refractory complex concentrated alloys in the Al–Cr–Mo–Ta–Ti alloy system is reported. Alloys with modified Al and Ta concentrations are designed using CALPHAD tools and produced via arc melting and characterized in both as‐cast and annealed forms. Properties of the alloys, nature of the microstructures, and phase transformation behavior are described via X‐ray diffraction, microstructural characterization, microhardness, and differential scanning calorimetry. Two alloys, namely, Al0.25CrMoTa0.8Ti and Al0.75CrMoTa0.8Ti, are represented by a body‐centered‐cubic matrix phase after annealing, along with a secondary Cr–Ta Laves phase of the C15 and C14 polytypes, respectively. In as‐cast and annealed forms, the Al0.75CrMoTa0.45Ti alloy comprises a single‐bcc phase. Microhardness of the Laves phase containing alloys demonstrates susceptibility to cracking, whereas the Al0.75CrMoTa0.45Ti alloy displays high specific hardness, signs of ductility as evidenced by slip traces near indentations, and minimal scatter of hardness values. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) contain alloying elements with a high melting point, promising high-temperature applications due to their unique properties. In this work, laser cladding is used to prepare RHEAS based on NbMoTaTiNi. At the same time as laser cladding, the ultrasonic field is used, and then the microstructural characteristics, grain size, residual stress, wear, and hardness of the coating are evaluated. The results show that the coating is biphasic and includes the γ (Ni) and NbMoTaTiNi phase. The NbMoTaTiNi phase had a uniform distribution throughout the coating when the ultrasonic field was applied, so that when the ultrasonic field was not used, the NbMoTaTiNi powder, in addition to spreading uniformly, had the un-melting of large particles. This caused an increase in the residual tension of the coating. The conversion of columnar grains to the equiaxed, and the reduction in structural defects, were other characteristics of using the ultrasonic field. The formation of equiaxed grains with zigzag grain boundaries reduced the friction coefficient, wear volume loss, and the wear rate of the coating applied with ultrasonic. [ABSTRACT FROM AUTHOR]

[Display omitted] The role of short-range ordering (SRO) in the dislocation kinetics in refractory high-entropy alloys (RHEAs) remains controversial. On one hand, it was shown by simulations that the mobility of edge dislocations was enhanced while that of screw dislocations was reduced, leading to the conclusion that screw dislocations should be dominant. On the other hand, experiments exclusively showed the dominance of edge dislocations. Here, we investigate the impact of SRO in the grain interior and grain boundary on dislocation nucleation and propagation in a BCC MoTaTiWZr RHEA, using a combination of the density-functional theory calculations, Monte Carlo method, and molecular dynamic simulation. Our results show that this RHEA is energetically favorable to undergo SRO, thus forming a pseudo-composite microstructure. This microstructure consists of three categories of clusters: high energy clusters (HECs), medium energy clusters (MECs), and low energy clusters (LECs), with the HECs in grain boundaries acting as weak fillers to induce dislocation nucleation while the MECs/LECs serving as a strong matrix to stabilize the weak HECs. Importantly, SRO is found to enhance the energy barriers for both edge and screw dislocation motion and make the mobility of edge dislocations comparable to or even lower than screw dislocations, contributing to the dominance of edge dislocations in the BCC RHEA. Our work highlights the importance of SRO in influencing the dislocation activity of RHEAs and presents a fascinating route for designing RHEAs to achieve superior mechanical properties. [ABSTRACT FROM AUTHOR]

Over 150 refractory high-entropy alloys (RHEAs) have been proposed in the last decade. Early alloys such as MoNbTaW and MoNbTaVW still show an unparalleled yield strength of approximately 400 MPa at 1600°C. However, RHEAs with even elevated high-temperature strength are necessary in aerospace vehicles and nuclear reactors to cope with advanced technology in the future. Here, solid-solution strengthening calculation and melting point prediction are combined to design single-phase RHEA for attaining ultrahigh strength at 1600°C. The results show that Hf0.5MoNbTaW and HfMoNbTaW alloys after fully homogeneous treatment at 2100°C for 2 h reveal a homogenous body-centered cubic phase. HfMoNbTaW alloy exhibits a yield strength of 571 MPa at 1600°C, much higher than that of MoNbTaVW (477 MPa). It is found that a plateau of strength occurs from 800°C to 1200°C, which is important for raising the strength level of RHEAs at high temperatures. This strengthening mechanism is explained with the change of deformation mode from screw to edge dislocations, which contributes an edge-dislocation-induced strength. A similar alloy design strategy could be applied to develop more RHEAs with an ultrahigh strength level.

WVTaTiCrx (x = 0, 0.25, 0.5, 0.75, 1) refractory high-entropy alloy coatings were prepared on a 42-CrMo steel plate using laser cladding. The purpose of this work is to investigate the effect of the Cr content on the microstructure and properties of the WVTaTiCrx coating. The morphologies and phase compositions of five coatings with different Cr contents were comparatively observed. In addition, the hardness and high-temperature oxidation resistance of the coatings were also analyzed. As a result, with the increase in Cr, the coating grains were more refined. All the coating is mainly composed of the BCC solid-solution phase, which promotes the precipitation of the Laves phase with the increase in Cr. The addition of Cr greatly improves the hardness, high-temperature oxidation resistance and corrosion resistance of the coating. The WVTaTiCr (Cr1) exhibited superior mechanical properties, especially in terms of its exceptional hardness, high-temperature oxidation resistance and outstanding corrosion resistance. The average hardness of the WVTaTiCr alloy coating reaches 627.36 HV. After 50 h of high-temperature oxidation, the oxide weight of WVTaTiCr increases by 5.12 mg/cm2, and the oxidation rate is 0.1 mg/(cm2·h). In 3.5 wt% NaCl solution, the corrosion potential of WVTaTiCr is −0.3198 V, and the corrosion rate is 0.161 mm/a. [ABSTRACT FROM AUTHOR]

We consider a new method for the preparation of refractory high-entropy alloys (HEAs) AlTiZrVNb, which consists in the joint aluminothermic reduction of metals from their oxides. It was determined that V and Nb undergo reduction to a greater extent, and about 90% of their amounts transfer into the metal phase. Metals Ti and Zr undergo reduction to a lesser extent, with 76 and 50% of their amounts transitioning into the metal phase, respectively. The obtained alloy has a multiphase structure consisting of C14 Laves phases, Zr5Al3-type phases, and a B2-type ordered phase, which plays the role of a matrix. The microhardness of the alloy is 6.37 GPa, which is similar to the values of refractory HEAs. The structure of the obtained alloy has a similar structure throughout its bulk, namely, coarse-grained with a number of pores, partly filled with non-metallic inclusions of aluminum oxide. [ABSTRACT FROM AUTHOR]

A TiTaNbZrHf refractory high-entropy alloy (RHEA) was synthesized through a cold isostatic pressing and a pressure-less sintering process in a hydrogen atmosphere using a powder mixture of metal hydride prepared either by mechanical alloying (MA) or by rotating mixing. This study investigates how differences in powder particle sizes impact the RHEA's microstructure and mechanical properties. HCP (a = b = 3.198 Å, c = 5.061 Å) and BCC2 (a = b = c = 3.40 Å) phases were observed in the microstructure of coarse powder TiTaNbZrHf RHEAs at 1400 °C. In contrast, fine powder RHEAs were found to possess two-phase structures of HCP and BCC1 (a = b = c = 3.36 Å) with a higher hardness of 431 HV, compression strength of 1620 MPa, and a plasticity of >20%. [ABSTRACT FROM AUTHOR]

Despite widespread attentions on the HfNbTaTiZr refractory high-entropy alloy (RHEA) owing to various exceptional properties, insights into nanostructure-property relations are severely hindered by the lack of reliable interatomic potentials in atomistic simulations. In this paper, we propose a supervised machine learning framework powered by the distributed breeder genetic algorithm (DBGA) to train a charge transfer ionic potential (CTIP) for the multi-component Hf/Nb/Ta/Ti/Zr/O system. By combining the DBGA with extensive ab initio training data and a carefully designed objective function, a robust and extrapolative potential with massive internal parameters is effectively optimized. A cross-validation of the CTIP is rigorously performed to verify its accuracy in predicting energetic, structural and mechanical properties of metals, intermetallic compounds, alloys and oxides. Using the developed potential, we further study the atomic structure and its evolution in RHEA to reveal the deformation and oxidation mechanisms in response to mechanical loadings and oxygen atmospheres. The simulation results not only provide valuable atomistic information hardly accessible through experiments and thermodynamic calculations, but also display promising prospects for large-scale reactive simulations of the Hf/Nb/Ta/Ti/Zr/O complex-component system.

The NbTiAlZrHfTaMoW refractory high-entropy alloy (RHEA) system with the structure of the B2 matrix (antiphase domains) and antiphase domain boundaries was firstly developed. We conducted the mechanical properties of the RHEAs at 298 K, 1023 K, 1123 K, and 1223 K, as well as typical deformation characteristics. The RHEAs with low density (7.41~7.51 g/cm3) have excellent compressive-specific yield strength (σYS/ρ) at 1023 K (~131 MPa·cm3/g) and 1123 K (~104.2 MPa·cm3/g), respectively, which are far superior to most typical RHEAs. And, they still keep appropriate plastic deformability at room temperature (ε > 0.35). The superior specific yield strengths are mainly attributed to the solid solution strengthening induced by the Zr element. The formation of the dislocation slip bands with [111](101_) and [111](112_) directions and their interaction provide considerable plastic deformation capability. Meanwhile, dynamic recrystallization and dislocation annihilation accelerate the continuous softening after yielding at 1123 K.

Along with the globalization of environmental problems and the rapid development of the field of nuclear technologies, the severe irradiation damage of materials has become a big issue, restricting the development of advanced nuclear reactor systems. Refractory high-entropy alloys (RHEAs) have the characteristics of a complex composition, a short-range order, and lattice distortion and possess a high phase stability, outstanding mechanical properties, and excellent irradiation resistance at elevated temperatures; thus, they are expected to be promising candidates for advanced nuclear reactors. This review summarizes the design, preparation, and irradiation resistance of irradiation-tolerant RHEAs. It encompasses a comprehensive analysis of various aspects, including the evolution of defects, changes in microstructure, and the degradation in properties. Furthermore, the challenges and insufficiently researched areas regarding these alloys are identified and discussed. Building on this foundation, the review also provides a forward-looking perspective, outlining potential avenues for future research.

In order to improve the high-temperature oxidation resistance of refractory high-entropy alloys (RHEAs), we used micro-arc oxidation (MAO) technology to prepare ceramic coatings on AlTiCrVZr alloy, and the effects of voltage on the microstructure and high-temperature oxidation resistance of the coatings were studied. In this paper, the MAO voltage was adjusted to 360 V, 390 V, 420 V, and 450 V. The microstructure, elements distribution, chemical composition, and surface roughness of the coatings were studied by scanning electron microscopy (SEM), energy dispersive (EDS), X-ray photoelectron spectroscopy (XPS), and white-light interferometry. The matrix alloy and MAO-coated samples were oxidized at 800 °C for 5 h and 20 h to study their high-temperature oxidation resistance. The results showed that as the voltage increased, the MAO coating gradually became smooth and dense, the surface roughness decreased, and the coating thickness increased. The substrate elements and solute ions in the electrolyte participated in the coating formation reaction, and the coating composition was dominated by Al2O3, TiO2, Cr2O3, V2O5, ZrO2, and SiO2. Compared with the substrate alloy, the high-temperature oxidation resistance of the MAO-coated samples prepared at different voltages was improved after oxidation at 800 °C, and the coating prepared at 420 V showed the best high-temperature oxidation resistance after oxidation for 20 h. In short, MAO coatings can prevent the diffusion of O elements into the substrate and the volatilization of V2O5, which improves the high-temperature oxidation resistance of AlTiCrVZr RHEAs. [ABSTRACT FROM AUTHOR]

Over 150 refractory high-entropy alloys (RHEAs) have been proposed in the last decade. Early alloys such as MoNbTaW and MoNbTaVW still show an unparalleled yield strength of approximately 400 MPa at 1600°C. However, RHEAs with even elevated high-temperature strength are necessary in aerospace vehicles and nuclear reactors to cope with advanced technology in the future. Here, solid-solution strengthening calculation and melting point prediction are combined to design single-phase RHEA for attaining ultrahigh strength at 1600°C. The results show that Hf0.5MoNbTaW and HfMoNbTaW alloys after fully homogeneous treatment at 2100°C for 2 h reveal a homogenous body-centered cubic phase. HfMoNbTaW alloy exhibits a yield strength of 571 MPa at 1600°C, much higher than that of MoNbTaVW (477 MPa). It is found that a plateau of strength occurs from 800°C to 1200°C, which is important for raising the strength level of RHEAs at high temperatures. This strengthening mechanism is explained with the change of deformation mode from screw to edge dislocations, which contributes an edge-dislocation-induced strength. A similar alloy design strategy could be applied to develop more RHEAs with an ultrahigh strength level. [ABSTRACT FROM AUTHOR]

• A novel MoNbTaTiC 0.5 eutectic refractory high-entropy alloy was fabricated. • The morphology changed from hypoeutectic to eutectic and finally to hypereutectic. • The alloy shows a good combination of strength and room-temperature plasticity. In this letter, MoNbTaTiC x (MNTTC x) alloys with a good combination of room-temperature fracture strain and high-temperature (1473 K) strength were successfully prepared by vacuum arc melting. The alloy is composed of the BCC matrix phase and the FCC carbide phase. With the increase of C content, the morphology changed from hypoeutectic to eutectic to hypereutectic finally. The eutectic structures ensure the good comprehensive mechanical properties of the fully eutectic MNTTC 0.5 alloy by second-phase strengthening and grain refining, which has a fracture strain of 13.4 % at room temperature and a yield strength of 496 MPa at 1473 K, surpassing many of the reported refractory high-entropy alloys. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) are susceptible to phase transition at elevated temperatures, inducing deviation in the phase constitution from the intended design hence affecting the performance significantly. Accurate prediction of phase constitution and modulation of phase stability over a broad temperature range is crucial to develop RHEAs with superior properties. In this study, thermodynamic and first-principles calculations were integrated to investigate the temperature-dependent phase constitution in the CrTaVW alloy system. The effect of V and W atoms occupation sites on the stability of TaCr 2 Laves phase was evaluated. Both V or W atoms tend to occupy Cr sites in TaCr 2 , and V exhibits stronger alloying ability than W due to the reduced electronic density at the Fermi level. Moreover, with increasing the doping content, the V and W exhibit opposite influencing trends on the formation and structural stability of TaCr 2 Laves phase. The Gibbs free energies of the Laves phase and solid solutions with variable compositions were obtained, enabling predictions for the critical temperatures of the phase transitions. The predicted behaviors of phase transitions were validated by experiments. This study provides guidance for modulating phase constitution and achieving desirable phase stability in RHEAs. • In TaCr 2 V and W tend to occupy Cr sites and V has higher alloying tendency. • V and W have opposite effects on formation and structural stability of TaCr 2. • Temperature ranges for single phase and precipitation in CrTaVW are predicted. • Phase transformation behavior is verified by experimental studies. [ABSTRACT FROM AUTHOR]

A crack-free refractory high-entropy alloy (RHEA), composed of (Cr 25 Mo 25 Nb 25 V 25) 99 C 1 was synthesized in-situ from blended elemental powders using laser directed energy deposition (DED). This study revealed that a minor addition of carbon effectively enhances the grain-boundary cohesion of the DEDed RHEA, thereby mitigating microcrack propagation along the grain boundaries during the additive manufacturing process. Notably, the DEDed RHEA exhibited exceptional high-temperature mechanical properties, highlighted by a yield strength of 787 MPa at 1000 °C along with sustained strain-hardening capacity, surpassing other additively manufactured high-entropy alloys reported to date. The remarkable high-temperature strength of the DEDed RHEA can be attributed to the potent pinning effect exerted on dislocations within both dendritic and interdendritic regions at elevated temperature. This effect stems from severe lattice distortion, local chemical fluctuations (LCFs), solute atom pinning, and dislocation interactions. The method employed to inhibit microcrack formation, along with the elucidation of the high-temperature strengthening mechanism, opens avenues for the fabrication of RHEAs tailored for high-temperature structural applications. • A minor addition of carbon can effectively inhibit microcrack propagation in the DEDed RHEA. • The DEDed RHEA exhibits exceptional high-temperature yield strength of 787 MPa at 1000 °C. • The high-temperature strengthening mechanism stems from severe lattice distortion, local chemical fluctuations, solute atom pinning and dislocation interactions. [ABSTRACT FROM AUTHOR]

In this investigation, we focused on the lightweight Nb–Zr–Ti–V alloys as representative model alloys to explore improving mechanical properties of refractory high entropy alloys (RHEAs) through phase engineering. The alloys were deliberately designed with different element concentrations and varied cooling rates after homogenization treatment, resulting in formation of multiple secondary phases. Microstructures of prepared alloys were reported, revealing the decomposition of the BCC matrix into dual-BCC phases with increasing V content. We attribute this phenomenon to the substantial atomic size misfit between V and Zr. Additionally, a metastable FCC phase and an intermetallic Laves phase emerged at intermediate temperatures in Nb 0.75 ZrTiV 0.75 , influenced by the nonequilibrium condition during cooling and the enthalpic effect. Compression tests were performed to access mechanical characteristics of different phases. The dual-BCC phases, featuring granular or multilayer morphology, enhanced ductility of the alloy by facilitating dislocation movement at BCC interfaces. In contrast, the FCC and Laves phases, characterized by a needle-like morphology, increased strength of the alloy owing to their intrinsic hardness. Based on the current discovery and understanding on secondary phases in the Nb–Zr–Ti–V alloys, our future objective is to obtain a modulated microstructure with an appropriate combination of different phases to optimize strength and ductility simultaneously. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pure B particles in the RHEA powder allowed the in-situ precipitation of borides. • Borides limited the grain growth and led to a more homogeneous strain distribution. • Cracking susceptibility of the RHEA was reduced, improving LPBF processability. The equiatomic MoNbTaTiZr biocompatible and refractory high-entropy alloy (RHEA) was additively manufactured by laser powder bed fusion (LPBF). The possibility to reduce the cracking susceptibility of the RHEA during LPBF was investigated. By the addition of pure B to the RHEA powder feedstock, the in-situ precipitation of boride particles was promoted. These precipitates limited the grain growth, thus avoiding the formation of cracks. [ABSTRACT FROM AUTHOR]

Refractory high-entropy alloys (RHEAs) have emerged as promising candidates for high-temperature applications owing to their distinctive mechanical properties. However, their limited tensile ductility and high density pose hurdles for further engineering utilization. In this study, we introduce a novel RHEA composition, Ti 45 Nb 30 Cr 15 V 10 (at.%), distinguished by its reduced density (∼6.2 g/cm3), achieved through the assistance of CALPHAD (CALculation of PHAse Diagrams) modeling. The as-cast RHEA exhibits a single body-centered cubic (BCC) structure, demonstrating a tensile yield strength of ∼904 MPa along with a total elongation of ∼9.7 %. By employing cold rolling followed by recrystallized annealing (designated as the CRRA sample), we achieve notable enhancements in both strength and ductility. Specifically, the CRRA sample manifests an impressive tensile yield strength of ∼1010 MPa and a total elongation of ∼20.6 % at room temperature. Remarkably, even at an elevated temperature of 700 °C, the CRRA sample maintains notable strength, displaying a yield strength of ∼695 MPa. The observed dislocation behavior, transitioning from dual-system slip to multi-system slip, along with the intricate interactions of dislocation substructures (e.g., loops, tangles, networks, bands, and cells) and kink bands, are identified as the principal deformation mechanisms contributing to the exceptional tensile properties of the CRRA sample at room temperature. At 700 °C, the presence of wavy-slipping dislocations, induced by the strong pinning effect of component fluctuations, predominantly contributes to the strength of the CRRA sample. However, the emergence of Cr 2 (Ti, Nb, V) Laves phase at grain boundaries results in premature fracture. The extraordinary tensile properties exhibited by the CRRA sample at both room and elevated temperatures underscore its potential for advanced engineering applications. [ABSTRACT FROM AUTHOR]