Room Temperature Ductility Research Articles

Effect of thermomechanical processing on mechanical properties and the microstructure of binary Al-Ce alloy

A new generation of Al alloys based on the Al-Ce system, with microstructure and properties weakly sensitive to elevated temperatures, is being developed for applications such as thermal engine blocks and supersonic aircraft fuselage components. Cast Al-Ce binary alloys with 4, 10 and 16 wt% Ce were produced by slab casting and further processed by extrusion. Extrusion produces a highly refined microstructure resulting in a twofold increase in strength (tensile strength of 228 MPa at room temperature). We show that the Al-10Ce is highly stable (more than 99 % strength retention after 300 °C 100 h exposure), far superior to high temperature grade Al alloy AA2618. The extruded Al-10Ce also shows a room temperature ductility of more than 20 %, with a fracture toughness of 55.42 kJ/m2 that does not degrade after high temperature exposure (97.6 % toughness retention after 300 °C 100h exposure). Results from testing at different temperatures are also reported.

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.

Accelerated discovery of refractory high-entropy alloys for strength-ductility co-optimization: An exploration in NbTaZrHfMo system by machine learning

High temperature (HT) strength and room temperature (RT) ductility trade-offs are always unavoidable in the design of refractory high entropy alloys (RHEAs). Exploring the vast chemistry space to find optimally strong and ductile RHEAs remains a highly challenging task. Herein, we formulate a machine learning-based design strategy fusing uncertainty estimation and clustering analysis to explore a special alloy system (NbTaZrHfMo) for collaborative optimization of HT strength and RT ductility. Four non-equimolar alloys with a superior combination of HT strength, RT ductility and HT specific yield strength were discovered and synthetized. The influence of elements on mechanical properties are analyzed and an optimal composition range is identified based on model prediction. This work provides a general design approach enabling the concurrent optimization of conflicting properties with a small data-trained machine learning model, thereby producing a recipe to accelerate the discovery of desired RHEAs and other materials within a vast search space.

Atomic-scale compositional complexity ductilizes eutectic phase towards creep-resistant Al-Ce alloys with improved fracture toughness

Hierarchical microstructures spanning from micro-sized eutectic structure to nano-sized precipitates are promisingly engineered in lightweight Al alloys to improve the high-temperature creep resistance that is increasingly required for rapid industrial development. However, the intrinsically-brittle eutectic phase is ready to fracture upon applied loading, which, dramatically reducing room-temperature ductility and fracture toughness, greatly hampers practical applications of the creep-resistant Al alloys. Here, through the combination of Sc microalloying with sub-rapid solidification, we observe the ductilization of Al11Ce3 eutectic phase in cast heat-resistant Al-Ce-Sc alloys due to the formation of atomic-scale compositional complexity. High-concentration Sc atoms are frozen within the Al11Ce3 intermetallic phase by the sub-rapid solidification, which then assemble into unusual atomic-scale compositional dipoles with the Sc atoms enriched at one pole and the Al atoms at the opposite during subsequent heat treatment. The dispersed Sc-Al compositional dipoles induce local lattice distortions that stimulate dislocation activities, as temporally and spatially visualized by in-situ neutron diffraction tensile test and microstructural characterizations. The unexpected plastic deformation triggered in Al11Ce3 improves the deformation compatibility between the eutectic phases, enabling the sub-rapidly-solidified Al-Ce-Sc alloy to reach a room-temperature tensile elongation 3 times and fracture toughness over 8 times of its counterpart derived from traditional solidification. In addition, the sub-rapidly-solidified Al-Ce-Sc alloy exhibits an excellent creep resistance at 300 °C, achieving a tensile creep stress threshold of ∼ 70 MPa. These findings provide new perspectives on the design of ductile intermetallic phases and the development of creep-resistant Al alloys with application-level ductility.

Enhanced ductility of PM Ti-Nb-Ta alloys via reducing solution oxygen by YH2 addition

To reduce the solution oxygen content in the matrix and enhance the ductility of Ti alloys prepared by powder metallurgy, YH2 was added as an additive to the Ti-Nb-Ta alloy in this work. By adding 1.5 wt%YH2, the compressive ductility increased 49 %, when the compression yield strength decreased slightly at room temperature, but the compression yield strength increased 41 % at 800°C compared with Ti-Nb-Ta without YH2 addition. With increase of YH2 content, the solution oxygen content in both α and β phases decreased significantly, while the proportion of ductile β phase increased. Addition of YH2 led to in situ formation of Y2O3 particles, reducing the solution oxygen content and changing the distribution state of oxygen. It is revealed that under a same deformation degree at 800°C, the dislocation density in the (Ti-5 Nb-15 Ta)-1.5 Y alloy sample is much higher than in the Ti-5 Nb-15 Ta alloy, which is attributed to a strong hindering effect of Y2O3 particles on dislocation movement. Thus, the strength of (Ti-5 Nb-15 Ta)-1.5 Y alloy was enhanced under high temperature. This study provided a new method for enhancing room temperature ductility and high-temperature strength of powder metallurgy Ti alloys.

Structure and mechanical properties of a refractory Al7.5(NbTiZr)92.5 medium-entropy alloy subjected to long-term annealing and oxidation

Recently, Al-containing refractory high/medium-entropy alloys (RH/MEAs) have attracted special attention from materials scientists worldwide. Some of these alloys demonstrated an excellent combination of high-temperature strength and room-temperature tensile ductility provided by the formation of a structure consisting of body-centred cubic (bcc) matrix and B2 nanodomains/particles. However, oxidation resistance and phase stability, and, more importantly, the resulting mechanical properties of the Al-containing RH/MEAs, which are critical characteristics for high-temperature structural materials, remain unexplored. Herein, these aspects were extensively investigated for the Al7.5(NbTiZr)92.5 (in at%) alloy having an initial bcc + B2 structure. The alloy was subjected to long-term (500 h) annealing or oxidation tests at 600, 700, and 800 °C followed by the evaluation of mechanical properties during uniaxial tension at 22 and 600 °C. Microstructural studies of the specimens exposed to long-term annealing showed the formation of Zr5Al3 particles that precipitated along the grain boundaries and within grain interiors. The volume fraction of these particles was the highest (∼16 %) after annealing at 600 °C and the lowest (∼0.5 %) after annealing at 800 °C. Oxidation tests revealed that the alloy was prone to pesting phenomena, which intensified with the temperature. The alloy could sustain pesting only up to 10 h at 600 °C, while it experienced a complete disintegration into powder after 5 h at 800 °C. The poor oxidation resistance originated from the formation of volumetric, non-protective (Al,Nb,Ti,Zr)O4 oxide. While insignificantly affecting the strength, long-term annealing or oxidation deteriorated the ductility of the Al7.5(NbTiZr)92.5 alloy both at 22 and 600 °C. The Zr5Al3 phase embrittled the alloy after long-term annealing at 600 or 700 °C, but marginally reduced the ductility in the 800°C-annealed specimens due to the low volume fraction and relatively homogeneous distribution of this phase. The oxidation for 1 h decreased the room-temperature ductility owing to the premature macroscopic localisation of plastic deformation induced by synergistic effects from the volumetric (Al,Nb,Ti,Zr)O4 oxide. This study highlights the necessity for the examination of a set of properties of promising RH/MEAs to critically assess perspectives of these alloys to find applications as new high-temperature materials in aircraft and aerospace sectors.

A novel NbTaV0.5Hf0.1 complex concentrated alloy with superior room-temperature tensile plasticity and elevated-temperature strength

Refractory complex concentrated alloys (RCCAs) encounter difficulties in balancing room-temperature ductility and high-temperature strength, which restricts their applications. Here, a novel NbTaV0.5Hf0.1 RCCA was prepared through vacuum arc melting for the first time. The as-cast alloy exhibited a stable body-centered cubic phase structure with a small amount of micro-scale dispersed Hf-rich oxides. As a result, the designed alloy exhibits a room-temperature tensile fracture strain of 14.9 %, and superior compressive yield strength values under elevated temperatures (472 MPa under 1200 ℃, 358 MPa under 1400 ℃, respectively). A synergy between room-temperature tensile ductility and exceptional high-temperature strength was achieved in RCCAs.

Effects of Er addition on the hot deformation behavior and microstructure evolution of AZ31 alloy

AZ31 magnesium alloys typically have poor room temperature ductility after initial deformation. To address this disadvantage, rare earth element Er is introduced to regulate the structure, weaken texture, and promote non-basal slip to achieve plasticization. In this study, the deformation behavior and microstructure evolution during the hot compression process of Er added AZ31 alloy were systematically investigated. Hot compression stress-strain curves showed that the addition of Er reduced the deformation resistance. The constitutive equation and processing map were established. By combining the processing map with the microstructure analysis and texture evolution, an optimal processing area was determined to be 350–400 °C at a strain rate of 0.01 s−1. The influence of Er addition on the twinning behavior, recrystallization mechanism, and slip system of the AZ31 alloy were discussed. During the deformation process, the twinning behavior violates the maximum SF criterion, which is related to severe local stress. Continuous dynamic recrystallization, discontinuous dynamic recrystallization, and twinning induced recrystallization are all detected in the alloy. Through the in-grain misorientation axis and transmission electron microscopy, it can be confirmed that the addition of Er promoted the occurrence of pyramidal <c+a> slip. This is related to the modification of stacking fault energy by Er and the hindrance effect of the second phase containing Er on the basal dislocation.

Excellent specific strength-ductility synergy in novel complex concentrated alloy after suction casting

Lightweight alloys are known to improve the fuel efficiency of the structural components due to high strength-to-weight ratio, however, they lack formability at room temperature. This major limitation of poor formability is most of the time overcome by post-fabrication processing and treatments thereby increasing their cost exponentially. We present a novel Ti50V16Zr16Nb10Al5Mo3 (all in at. %) complex concentrated alloy (Ti-CCA) designed based on the combination of valence electron concentration theory and the high entropy approach. The optimal selection of constituent elements has led to a density of 5.63 gm/cc for Ti-CCA after suction casting (SC). SC Ti-CCA displayed exceptional room temperature strength (UTS ∼ 1.25 GPa) and ductility (ε ∼ 35 %) with a yield strength (YS) of ∼ 1.1 GPa (Specific YS = 191 MPa/gm/cc) without any post-processing treatments. The exceptional YS in Ti-CCA is attributed to hetero grain size microstructure, whereas enormous strength-ductility synergy is due to the concurrent occurrence of slip and deformation band formation in the early stages of deformation followed by prolonged necking event due to delayed void nucleation and growth. The proposed philosophy of Ti-CCA design overcomes the conventional notion of strength-ductility trade-off in such alloy systems by retaining their inherent characteristics.

Substantially improved room-temperature tensile ductility in lightweight refractory Ti-V-Zr-Nb medium entropy alloys by tuning Ti and V content

Lightweight high/medium-entropy alloys (H/MEAs) possess attractive properties such as high strength-to-weight ratios, however, their limited room-temperature tensile ductility hinders their widespread engineering implementation, for instance in aerospace structural components. This work achieved a transformative improvement of room-temperature tensile ductility in Ti-V-Zr-Nb MEAs with densities of 5.4–6.5 g/cm3, via ingenious composition modulation. Through the systematic co-adjustment of Ti and V contents, an intrinsic ductility mechanism was unveiled, manifested by a transition from predominant intergranular brittle fracture to pervasive ductile dimpled rupture. Notably, the modulated deformation mechanisms evolved from solitary slip toward collaborative multiple slip modes, without significantly compromising strength. Compared to equimolar Ti-V-Zr-Nb, a (Ti1.5V)3ZrNb composition demonstrated an impressive 360% improvement in elongation while sustaining a high yield strength of around 800 MPa. Increasing Ti and V not only purified the grain boundaries by reducing detrimental phases, but also tailored the deformation dislocation configurations. These insights expanded the applicability of lightweight HEAs to areas demanding combined high strength and ductility.

Considering sustainability when searching for new high entropy alloys

Alloying elements bring with them a level of undesirable societal impact. The present paper examines this impact, in terms of primary production, for 340 newly proposed high entropy compositions. Three areas of sustainability are considered: economic viability, environmental impact and human well-being. The high entropy alloys are considered in two classes: body centred cubic and related alloys with potential for high temperature application and face centred cubic and related Cantor-type alloys. The former are compared to Ni-based superalloys and the latter to advanced steels. It is seen that with improved performance of Ni-based superalloys there has been an increase in undesirable impact across many of the indicators we employ. However, some recently proposed high temperature high entropy alloys – especially Yeh-type High Entropy Superalloys - display notably lower levels of undesirable impact than Ni-based superalloy compositions. Cantor-type face centred cubic alloys, however, do not compare so favourably when stacked up against the best steels, for room temperature strength and ductility. We show that, in general, there is good scope for high entropy alloys to be designed that target lower levels of undesirable impact.

Effect of forging routes on the microstructure and mechanical properties of newly-developed Ti-44Al-5.5Nb-0.5W-0.5Cr-0.3Si-0.1C alloys

In this study, the effects of forging routes on the microstructure and tensile properties of Ti-44Al-5.5 Nb-0.5 W-0.5Cr-0.3Si-0.1 C (at%) alloys was comparatively investigated. Results showed that the bare ingot forged at 1400 ℃ (BH) exhibited refined lamellar colonies with narrow lamellar spacing. In contrast, dynamic recrystallized γ grains and lamellar structure with thick lamellar spacing formed in the canned sample deformed at 1320 ℃ (CA). In addition, the heat-treated BH-HT sample showed fully recrystallized texture, while the lamellar structure in the CA-HT presented a preferred orientation, i.e., lamellar interfaces were parallel to rolling direction (RD). The mechanical properties of the CA-HT sample test results showed superior room-temperature ductility of 1.68 % and tensile strength of 515 MPa at 950 ℃ due to the lamellar orientation alignment to tensile direction. According to the temperature drop calculation results, both BH and CA forging routes exhibited rapid cooling rates. The initial microstructure at the pre-heating temperature was maintained until a hot deformation sequence was initiated. Therefore, the BH route could be performed in the α+β region, while the CA route could be carried out in the α-forging condition with the 1101̅12 γ texture.

Microstructure, Compressive Properties and Oxidation Behaviors of the Nb-Si-Ti-Cr-Al-Ta-Hf Alloy with Minor Holmium Addition

In the present research, the Nb-Si-Ti-Cr-Al-Ta-Hf alloys with different Ho addition were prepared. Their microstructure, compressive properties and oxidation behaviors were investigated preliminarily. The results exhibit that the Nb-Si-Ti-Cr-Al-Ta-Hf alloy has coarse microstructure which is mainly composed of Nb solid solution, Nb5Si3 and Ti5Si3 phases. The minor Ho addition could refine the microstructure and suppress the precipitation of Ti5Si3 phase. Moreover, the Ho addition also leads to the formation of Ho2Hf2O7, which prefers to precipitate along the Nbss/Nb5Si3 phase interface. Compared with the Nb-Si-Ti-Cr-Al-Ta-Hf alloy, the minor Ho addition improves the room-temperature and high-temperature compressive properties of the alloy. Its room-temperature compressive strength and ductility obtain the maximum value of 1825 MPa and 16.5% when the Ho content is 0.1 at.%. Moreover, its best compressive strength at 873 K, 1273 K and 1473 K is 1495 MPa, 765 MPa and 380 MPa, respectively, when the Ho addition is 0.1 at.%. The oxidation behavior of the Nb-Si-Ti-Cr-Al-Ta-Hf alloy is diversified with the Ho addition. The oxidation rate of the alloy with 0.1 at.% Ho addition is the lowest while the alloy with 0.2 at.% Ho addition is the highest. Therefore, the 0.1 at.% Ho would be the appropriate content for the Nb-Si-Ti-Cr-Al-Ta-Hf alloy.

Effects of Hf and C on microstructure and mechanical properties of Re0.1HfxTa1.6W0.4(TaC)y refractory medium-entropy alloy

The trade-off between high-temperature strength and room-temperature ductility represents a critical concern for practical application of refractory high-entropy alloys (RHEAs). Here, a set of five Re0.1HfxTa1.6W0.4(TaC)y refractory medium-entropy alloys (RMEAs) were fabricated using the vacuum arc melting technique, and the effect of Hf and C on the microstructures and mechanical properties of these alloys was investigated. These RMEAs exhibit excellent high-temperature strength and favorable plasticity at room-temperature. The microstructures of the Re0.1HfxTa1.6W0.4(TaC)y alloys are hypoeutectic or eutectic structures, consisting of BCC solid solution and primary and secondary carbides. The carbides can be classified into two types: M2C and MC. Hf and C have an important influence on the grain size, carbide type and spatial configuration of the alloy, and in turn, influence the mechanical properties of the alloy. For instance, the content of MC-type carbides is proportional to the addition of Hf element, whereby a higher proportion of Hf results in reduced plasticity at room-temperature and enhanced softening resistance at elevated temperatures in the alloy. The carbides and BCC matrix exhibit synergistic strengthening behavior. The ultimate compressive strength of the five Re0.1HfxTa1.6W0.4(TaC)y alloys at 1450 °C is >950 MPa, while maintaining a fracture strain exceeding 11% at room-temperature. Notably, the Re0.1Hf0.1Ta1.6W0.4(TaC)0.25 alloy demonstrates a compressive strength of 1110 MPa at 1450 °C, which is 2.3 times greater than that of the NbMoTaW alloy at 1400 °C. Additionally, the fracture strain of the Re0.1Hf0.1Ta1.6W0.4(TaC)0.25 alloy at room-temperature is 16.9%, which is 8 times higher than that of the NbMoTaW alloy. The Re0.1HfxTa1.6W0.4(TaC)y alloy exhibit excellent mechanical properties at both room and high temperatures, making it a potential candidate for applications in fields that require high strength at ultra-high temperatures.

The mechanism for annealing-induced ductile to brittle transition in a high-temperature titanium alloy and its mitigation

By investigating the relation between microstructure evolution and tensile responses of a BTi6431S alloy at room temperature and 700 °C, this study finds an annealing-induced ductile-to-brittle transition phenomenon in the near-α titanium alloy. The high strength of the as-received BTi6431S alloy is rooted in the high dislocation density and collective dislocation behavior in the α phase. However, after annealing at 650 °C, the alloy exhibits brittle fracture at room temperature. Using multiscale characterization methods, we attribute this phenomenon to the presence of brittle precipitates and the change of dislocation structures near the surface of BTi6431S titanium alloy. Silicides are generated in the vicinity of the surface region due to the relatively high dislocation density on the surface after the rolling process. The room temperature ductility can be restored by removing the near-surface area. When the alloy is tested at 700 °C, most of the near-surface brittle silicides and dislocations can be eliminated, because the deformation is mainly affected by dislocation behavior and dynamic recrystallization at high temperature. The findings in this study can advance the understanding of alloying effects on the mechanical behavior of high-temperature titanium alloy. The results suggest that inhibiting silicide formation and controlling the environmental temperature during service are potential approaches for maintaining the mechanical performance of this near-α titanium alloy.

Evaluation of Microstructure and Mechanical Properties of TiAl/HI-TEMP 820/ SCM440H Materials Manufactured through Vacuum Brazing according to Process Temperature

The TiAl alloy is attracting attention as a lightweight and heat-resistant material, because of its high specific strength, excellent high-temperature formability, and fatigue strength. However, its applications are limited by its high unit price and low room temperature ductility. To overcome this issue, dissimilarly bonded materials have been extensively employed. This involves joining a brittle metal to a low-cost metal that possesses excellent plasticity, using various dissimilar bonding techniques. In this study, TiAl/HI-TEMP 820/SCM440H materials were fabricated using a vacuum brazing process under different temperature conditions. After the brazing process, the microstructure of the interfacial area revealed seven distinct layers resulting from chemical reactions between the base metals and the filler metal. These reaction layers consisted of a Ni solid solution, intermetallic compounds (Ti&lt;sub&gt;3&lt;/sub&gt;Al, TiNi&lt;sub&gt;2&lt;/sub&gt;Al, Ti&lt;sub&gt;2&lt;/sub&gt;Ni, FeNi), and borides (CrB, TiB&lt;sub&gt;2&lt;/sub&gt;, FeB). To analyze the effect of brazing temperature on the relationship between the microstructure and mechanical properties at the interface of TiAl/HI-TEMP 820/SCM440H materials, conventional uniaxial tests and nanoindentation tests were performed. The measured nanohardness exhibited a significantly large distribution for each reaction layer, with the highest hardness values observed in the intermetallic compounds and borides layers. Additionally, room temperature tensile tests confirmed that fractures initiated in the highhardness and brittle intermetallic compounds and borides layers.

Predicting and understanding the ductility of BCC high entropy alloys via knowledge-integrated machine learning

Despite the fact that body-centered cubic high entropy alloys (BCC HEAs) exhibit exceptional strengthening mechanisms over a wide temperature range, their application is often hindered by poor room temperature ductility. Consequently, the accurate prediction of BCC HEAs′ ductility becomes critical in alloy design but is challenged by the intricate nature of these alloy systems. In this study, a comprehensive machine learning (ML) strategy with integrated material knowledge is proposed to address this problem. By referring to plasticity theories, thermodynamics as well as atomic properties, we develop a set of cost-effective material descriptors with various mathematical forms such as averaging, mismatching, maximum-minimum ranging and averaging as binary pairs for HEAs. These descriptors are proved to effectively incorporate material knowledge into ML modelling. They also offer potential alternatives to those obtained from costly first principles calculations. Classification and regression tasks are conducted to predict the ductility of BCC HEAs, and Gradient boosting algorithm exhibiting the best performance for both tasks is eventually selected. The established optimized models achieve high predictive accuracies exceeding 0.85. Through feature engineering, five key features related to valence electron concentration (VEC) and atomic size are screened out. These features lay the foundation for explicit formulae developed via Symbolic Classification, enabling the straightforward numerical prediction of BCC HEAs′ ductility. These key features are further analyzed by an interpretable ML method and compared against experimental and computational plasticity factors, elucidating their influences on various plasticity mechanisms. An in-depth exploration of why the proposed ML models outperform conventional plasticity criteria for HEAs is also presented. This study highlights an effective knowledge-integrated ML approach in predicting material properties resulting from complex mechanisms, with a particular focus on HEAs that often exhibit sparse data.

Influence of Various Heat Treatments on Microstructures and Mechanical Properties of GH4099 Superalloy Produced by Laser Powder Bed Fusion.

The microstructures and mechanical properties of a γ'-strengthened nickel-based superalloy, GH4099, produced by laser powder bed fusion, at room temperature and 900 °C are investigated, followed by three various heat treatments. The as-built (AB) alloy consists of cellular/dendrite substructures within columnar grains aligning in <100> crystal orientation. No γ' phase is observed in the AB sample due to the relatively low content of Al +Ti. Following the standard solid solution treatment, the molten pool boundaries and cellular/dendrite substructures disappear, whilst the columnar grains remain. The transformation of columnar grains to equiaxed grains occurs through the primary solid solution treatment due to the recovery and recrystallization process. After aging at 850 °C for 480 min, the carbides in the three samples distributed at grain boundaries and within grains and the spherical γ' phase whose size is about 43 nm ± 16 nm develop in the standard solid solution + aging and primary solid solution + aging samples (SA and PA samples) while the bimodal size of cubic (181 nm ± 85 nm) and spherical (43 nm ± 16 nm) γ' precipitates is presented in the primary solid solution + secondary solid solution + aging sample (PSA samples). The uniaxial tensile tests are carried out at room temperature (RT) and 900 °C. The AB sample has the best RT ductility (~51% of elongation and ~67% of area reduction). Following the three heat treatments, the samples all acquire excellent RT tensile properties (>750 MPa of yield strengths and >32% of elongations). However, clear ductility dips and intergranular fracture modes occur during the 900 °C tensile tests, which could be related to carbide distribution and a change in the deformation mechanism.

Correlations to improve high-temperature strength and room temperature ductility of refractory complex concentrated alloys

Correlations were explored between mechanical, thermodynamic and physical properties of refractory complex concentrated alloys (RCCAs). Experimentally measured yield strengths (σy) and ductility were taken from the open literature and were compared against liquidus, solidus and solvus temperatures, elastic properties (Young’s, shear and bulk moduli), density (ρ), surface energy (γ) and valence electron concentration (VEC). If not publicly available, the thermodynamic properties were calculated using CALPHAD while the other properties listed above were estimated using a rule-of-mixtures average of the constituent element properties. This analysis emphasized tensile ductility. Based on the identified correlations, useful criteria for selecting possibly ductile RCCA compositions with good high-temperature strength were proposed, a few ductile and strong RCCAs were made and properties of some of them were reported in this paper. Additionally, multivariate linear regression (MLR) was used to identify new insights from the high dimensional space of the present study by modeling the influence of composition and the input thermodynamic and physical properties on the high-temperature strength and room temperature ductility. Equal concentrations of Mo and Nb in RCCAs were found to give a good balance of strength and ductility. The MLR analysis identified over 50 promising RCCAs for intended high-temperature applications, pending experimental confirmation.

Origin of age softening in the refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) are emerging materials with potential for use under extreme conditions. As a newly developed material system, a comprehensive understanding of their long-term stability under potential service temperatures remains to be established. This study examined a titanium-vanadium-niobium-tantalum alloy, a promising RHEA known for its superior high-temperature strength and room-temperature ductility. Using a combination of advanced analytical microscopies, Calculation of Phase Diagrams (CALPHAD) software, and nanoindentation, we investigated the evolution of its microstructure and mechanical properties upon aging at 700°C. Trace interstitials such as oxygen and nitrogen, initially contributing to solid solution strengthening, promote phase segregation during thermal aging. As a result of the depletion of solute interstitials within the metal matrix, a progressive softening is observed in the alloy as a function of aging time. This study, therefore, underscores the need for a better control of impurities in future development and application of RHEAs.