Ductile Refractory High-entropy Alloys Research Articles

Kinking induced plasticity in a novel refractory high-entropy alloy

A novel Ti41V27Hf13Nb13Zr6 refractory high-entropy alloy (RHEA) with an excellent fracture elongation of 30 % at a high strength level of 986 MPa was successfully fabricated. We discovered that kinking induced plasticity, involving successive stages of dislocation arrangement, pre-kinking, and the coalescence of multiple kink bands. The soft character of kinking originated from the degraded dislocation density within the kink bands. As such, the interplay between strain hardening caused by dislocation accumulation and the softening effect due to kink bands led to a plateau-type engineering stress-strain curve to large strains. These findings provide a novel design pathway to achieve ductile RHEAs via deformation kinking.

Strong yet ductile refractory high entropy alloy fabricated via additive manufacturing

Refractory high-entropy alloys (RHEA), particularly those with a body-centered cubic lattice structure, are garnering increased interest due to their potential industrial applications. However, their strength-ductility trade-off at room temperature presents a challenge that requires resolution. In this study, we fabricated a ductile Ti42Hf21Nb21V16 RHEA for additive manufacturing using a directed energy deposition (DED) technique, with a focused laser serving as the energy source. The additively manufactured RHEA demonstrated an exceptional strength-ductility synergy, boasting a gigapascal yield strength and a substantial tensile strain until failure (∼22.5%). Compared to its as-cast state, the tensile yield strength increased by 32%, and ductility improved slightly by 2%, suggesting a potential solution to the enduring strength-ductility trade-off dilemma. The enhanced yield strength can be attributed to solidification-enabled interstitial atoms resulting from the low-content nitrogen and oxygen atmosphere applied, while the high ductility is linked to the modified dislocation motion mechanism facilitated by the decomposition of the body-centered cubic matrix. This finding opens up possibilities for in-situ tailoring of microstructure and compositions to achieve superior mechanical performance in alloys through additive manufacturing processes.

High‐Throughput Design of Refractory High‐Entropy Alloys: Critical Assessment of Empirical Criteria and Proposal of Novel Guidelines for Prediction of Solid Solution Stability

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.

Development of a Ti30Hf20Nb20Ta10V10Mo7W3 refractory high entropy alloy with excellent mechanical properties and wear resistance

Refractory high entropy alloys (RHEAs) have attracted a great deal of attention for the competitive properties at elevated temperatures, whereas the ambient-temperature brittleness has been still a bottleneck for practical applications. Herein, we report on a newly-designed Ti30Hf20Nb20Ta10V10Mo7W3 RHEA which exhibits an excellent combination of mechanical properties and wear resistance in a wide temperature range. The present RHEA possesses a yield strength of 1225 ± 17 MPa together with a fracture strain of > 45 % at room temperature, showing an excellent strength-ductility synergy. The alloy also maintains considerable yield strength and ductility at elevated temperatures, outperforming numerous ductile RHEAs. In addition, the coefficient of friction and wear rate decrease with increasing temperature from room temperature to 800 ℃. Interestingly, the alloy exhibits a wear rate as low as (5.50 ± 0.11) × 10−6 mm3 N−1 m−1 at 800 ℃, which is related to the formation of continues and dense oxide layer.

Additive manufacturing of ductile refractory high-entropy alloys via phase engineering

Refractory high-entropy alloys (RHEAs) are developed for superior elevated-temperature strength and softening resistance. Additive manufacturing of RHEAs is attracting extensive attention because of their commendable potential in many applications. Yet, most additively manufactured RHEAs are brittle and their tensile properties at room temperature have rarely been reported. Here, tensile ductile RHEAs are fabricated by laser melting deposition for the first time. In the model TiZrHfNbx (x = 0.6, 0.8, 1.0, in molar ratios) RHEAs, the as-printed alloys have an equiaxed-grained microstructure without any special process control or additional treatment. Increasing Nb contents stabilizes the body-centered cubic phase and suppresses the ω phase formation. Such phase engineering strategy transforms the mechanical performance from brittle to ductile fracture. As a result, the as-printed TiZrHfNb variant exhibits a room temperature tensile yield strength of ∼1034 MPa and ductility of ∼18.5%. The solid-solution strengthening contributes to the high tensile yield strength, the local chemical fluctuations promote dislocation interactions and give rise to the large ductility. This work offers a new avenue for additive manufacturing of ductile RHEAs, which is conducive to their industrial application.

Effect of Mo on the morphology, microstructure and mechanical properties of NbTa0.5TiMox refractory high entropy alloy fabricated by laser powder bed fusion using elemental mixed powders

The production of refractory metal pre-alloy powder is difficult and the related production cost is high. In this paper, NbTa0.5TiMox series refractory high entropy alloy was prepared by laser powder bed fusion (L-PBF) with elemental mixed powders. The effects of Mo content on the morphology, microstructure, hardness, density, and compression properties of the NbTa0.5TiMox alloy were studied. All NbTa0.5TiMox alloys fabricated by L-PBF present a single-phase BCC. Typical dendrites are formed within the grains and the elements are enriched in the dendrite trunk and inter-dendrite region from inside to outside depends on the melting point of the different materials. It was observed that the Mo content improves significantly the hardness and compressive strength of the alloy by solution strengthening. NbTa0.5TiMo1.0 alloy has the highest yield strength (1609 MPa), but with reduced ductility, while NbTa0.5TiMox (x ≤ 0.5) alloy has excellent ductility (e > 80%) at room temperature. The increase in Mo content leads to the increase of cracks, pores and non-penetration defects, resulting in the loss of sustained strength & ductility and decrease in density. The results provide a basis for the design and preparation of ductile refractory high entropy alloys by L-PBF of elemental mixed powders.

Strong and ductile refractory high-entropy alloys with super formability

Refractory high-entropy alloys (RHEAs) are considered promising candidate materials for high-temperature structural applications that currently use superalloys. However, for most of the reported RHEAs, their poor ductility and negligible cold-workability at room temperature have hindered their use. Here, we report a new class of non-equiatomic NbTaTi-based RHEAs that can be cold-rolled to a reduction of over 90% from the as-cast state without surface treatment and/or intermediate annealing. This excellent cold-workability is facilitated by activation of a high-density of dislocations and deformation twins as well as by high diffusivity paths, such that these RHEAs can be rendered homogeneous at lower annealing temperatures for much shorter time durations. In addition, we report that the RHEAs retain their high strength at elevated temperatures and exhibit considerable ductility at cryogenic conditions, evading the traditional strength–ductility trade-off. This class of super-formable RHEAs provides a novel design pathway to fabricate high-temperature structural materials via an energy- and time-saving method.

Enhanced strength and ductility of laser powder bed fused NbMoTaW refractory high-entropy alloy via carbon microalloying

NbMoTaW refractory high-entropy alloy (RHEA) shows outstanding mechanical properties at ultra-high temperatures, while room-temperature brittleness limits its application. In this study, carbon (C) microalloying was employed to improve the room-temperature brittleness resistance of NbMoTaW RHEA fabricated using laser powder bed fusion (LPBF). The addition of 0.5-at% C leads to good formability and remarkable microalloying effects. Nano-NbC precipitates distributed at grain boundaries and dislocations effectively refined the grain size. The yield strength, ultimate compression strength, and ductility were successfully improved up to 1725 MPa, 1782 MPa, and 7.0%, respectively. Nano-NbC carbide precipitates in the form of diffusion transformation during multiple reheating was verified by thermodynamics, kinetics calculations, and microstructure characterization. C microalloying endowed NbMoTaW RHEA with remarkable enhancements of strength and ductility, which suggested a strategy for designing strong and ductile RHEA through LPBF.

Design of oxygen-doped TiZrHfNbTa refractory high entropy alloys with enhanced strength and ductility

Refractory high entropy alloys (RHEAs) are considered promising materials for high-temperature applications due to their thermal stability and high-temperature mechanical properties. However, most RHEAs have high density (>10 g/cm3) and exhibit limited ductility at low temperatures and softening at high temperatures. In this study, we show that oxygen-doping can be used as a new alloy design strategy for tailoring the mechanical behavior of the TiZrHfNbTa alloy: a novel low-density (7.98 g/cm3) ductile RHEA. Even though the material is a single-phase BCC with some oxides at room temperature, secondary BCC and HCP nano-lamellar structures start to form above 600 °C in addition to the nano-twins which are shown to be stable up to 1000 °C. This alloy shows superior strength and compressive ductility due to the nanoengineered microstructure. The present study sheds light on tailoring the strength-ductility balance in RHEAs by controlling the microstructure of novel RHEAs at the nanoscale via oxygen-doping.

An ambient ductile TiHfVNbTa refractory high-entropy alloy: Cold rolling, mechanical properties, lattice distortion, and first-principles prediction

Refractory high-entropy alloys (RHEAs) have promising applications in industry due to their outstanding high-temperature properties, but most of them are brittle and poorly plastic formability at ambient temperature. Developing new ambient ductile RHEAs is a challenge in current materials research. In this work, an ambient ductility TiHfVNbTa alloy was prepared, and its microstructure, mechanical properties, phase stability, lattice distortion, and basic physical properties were investigated by combing experiments and density functional theory (DFT) calculations. The obtained results show that TiHfVNbTa alloy has excellent cold-rolled formability with >84% thickness reduction, no crack generation, and ultimate tensile strain up to >12% at ambient temperature. Besides, the tensile yield strength (hardness) of the alloy increases from 997 MPa (363 HV) in the annealed state to 1340 MPa (453 HV) in the 84% cold-rolled state, displaying work-hardening behavior. The TiHfVNbTa alloy has a single-phase solid solution body-centered cubic (BCC) structure, and no structural transformation occurs during rolling and tension, implying its good phase stability. Notably, the TiHfVNbTa alloy exhibits a higher specific yield stress and ductility compared to most RHEAs. DFT results reveal that the high strength is mainly contributed by severe lattice distortion. Hf and V elements are the major sources of lattice distortion. In addition, the physical properties such as metallic bonding, anisotropy and electronic structure are also analyzed based DFT calculations. The obtained results not only help to understand the material properties of TiHfVNbTa alloy, but also provide design guidelines for novel ductile RHEAs.

A ductile Nb40Ti25Al15V10Ta5Hf3W2 refractory high entropy alloy with high specific strength for high-temperature applications

Refractory high entropy alloys (RHEAs) with good high-temperature softening resistance have been revealed as promising candidates for high-temperature structural materials. In this work, a low-density ductile RHEA, Nb40Ti25Al15V10Ta5Hf3W2, has been developed. The RHEA with a BCC matrix and B2 nanoprecipitates exhibits excellent specific yield strength. The compressive specific yield strength (σ0.2/ρ) at 1073 K is as high as 83.2 MPa g−1 cm3. The different deformation behaviors during compression at 1073 K and 1273 K are also identified. The dislocation-dominated deformation provides the initial strain hardening capability, and then microcracks and dynamic recovery accelerate the transition from strain hardening to softening at 1073 K. While the diffusion-controlled dislocation annihilation and continuous dynamic recrystallization (DRX) are the dominant reasons for persistent strain softening at 1273 K. Our work not only reports a promising RHEA with excellent high-temperature properties, but also promotes the development of RHEAs for high-temperature applications.

Designing TiVNbTaSi refractory high-entropy alloys with ambient tensile ductility

Adding oxidation-resistant elements (e. g. Al, Cr, Si) to refractory high-entropy alloys (RHEAs) improves their high-temperature oxidation resistance while commonly degrades the room-temperature ductility, which greatly challenges the further applications of RHEAs. To tackle this trade-off, here in this study, we first identified the tensile-ductile TiVNbTa, alloyed it with Si, and then ductilized the initially brittle TiVNbTaSi0.1 via a simple hot rolling process. Hot processing effectively refines the microstructures of TiVNbTaSi0.1 and induces high-density nano-silicides, which contributes to the unprecedented mechanical properties of ∼1250MPa yield strength and ∼8% tensile ductility. Compared with reported ductile RHEAs, TiVNbTaSi0.1 shows superior oxidation resistance and both TiVNbTa and TiVNbTaSi0.1 demonstrate improved phase stabilities at temperatures >500 °C. These exceptional performances of TiVNbTaSi0.1 highlight its great potential for high-temperature applications.

Deformation and tribological behavior of ductile refractory high-entropy alloys

Refractory high-entropy alloys have attracted widespread attention as potential structural materials for high temperature applications. However, there is limited understanding of their high temperature deformation behavior and tribological response. Here, the temperature dependence of strain rate sensitivity and wear behavior was evaluated for two refractory high-entropy alloys, namely HfNbTiZr and HfNbTaTiZr. HfNbTaTiZr showed higher hardness, strength, and lower strain rate sensitivity compared to HfNbTiZr. The extrinsic strain rate sensitivity of both alloys obtained from micro-pillar compression was higher compared to the intrinsic values from nanoindentation due to the larger surface to volume ratio in micro-pillars leading to annihilation of dislocations and their easier movement. The decrease in strain rate sensitivity with increase in temperature was attributed to activation of both edge and screw dislocations as well as phonon drag controlled screw dislocation motion at high temperatures. Wear rate for both alloys increased with increase in sliding cycle frequency due to frictional heating. Wear rate also increased with increase in temperature from 298 K to 423 K and then decreased with further increase in temperature to 573 K. The underlying mechanism changed from abrasive wear at room temperature to extensive ploughing with fine wear debris at intermediate temperatures and oxidative wear at the higher temperatures.

Spinodal-modulated solid solution delivers a strong and ductile refractory high-entropy alloy.

Body-centered-cubic (BCC) refractory high-entropy alloys (RHEAs) are being actively pursued due to their potential to outperform existing superalloys at elevated temperatures. One bottleneck problem, however, is that these RHEAs lack tensile ductility and, hence, processability at room temperature. The strategy previously invoked to sustain ductility in high-strength HEAs is to manage dislocation movements via incorporating dispersed obstacles; this, however, may also have embrittlement ramifications. Here, a new strategy is demonstrated to achieve ductile BCC HfNbTiV, via decomposing the BCC arrangement (β phase) into a β(BCC1) + β*(BCC2) arrangement via spinodal decomposition, producing chemical composition modulations and, more importantly, elastic strain on a length scale of a few tens of nanometers. The periodically spaced β*, with large lattice distortion, is particularly potent in heightening the ruggedness of the terrain for the passage of dislocations. This makes the motion of dislocations sluggish, causing a traffic jam and cross-slip, facilitating dislocation interactions, multiplication, and accumulation. Wavy dislocations form walls that entangle with slip bands, promoting strain hardening and delocalizing plastic strain. A simultaneous combination of high yield strength (1.1 GPa) and tensile strain to failure (28%) is achieved; these values are among the best reported so far for refractory high-entropy alloys.

On the brittle-to-ductile transition of the as-cast TiVNbTa refractory high-entropy alloy

The fracture properties of as-cast TiVNbTa, a refractory high-entropy alloy (RHEA), were investigated using four-point bending tests from −139 °C to 20 °C under a strain-rate of 10−3s−1. From those tests and fractography, the conditional fracture toughness values and the brittle-to-ductile transition temperature were obtained. The brittle-to-ductile transition temperature was between −47 °C to −27 °C, which gives an estimated activation energy value of 0.52±0.09 eV for the alloy. This study provides a preliminary understanding of the nature of dislocation motion in a relatively ductile RHEA.

On the Brittle-to-Ductile Transition of the As-Cast TiVNbTa Refractory High-Entropy Alloy

The fracture properties of as-cast TiVNbTa, a refractory high-entropy alloy (RHEA), were investigated using four-point bending tests from -139°C to 20°C under a strain-rate of 10-3 s-1. From those tests and fractography, the conditional fracture toughness values and the brittle-to-ductile transition temperature were obtained. The brittle-to-ductile transition temperature was between -47°C to -27°C, which gives an estimated activation energy value of 0.52±0.09 eV for the alloy. This study provides a preliminary understanding of the nature of dislocation motion in a relatively ductile RHEA.

Alloying effect on the oxidation behavior of a ductile Al0.5Cr0.25Nb0.5Ta0.5Ti1.5 refractory high-entropy alloy

Refractory high-entropy alloys (RHEAs) are widely studied because of their promising potential for ultrahigh-temperature applications. One key challenge towards the development of RHEAs as high-temperature structural materials is to concurrently achieve optimum oxidation resistance and mechanical properties. Here in this work, the effect of alloying on the oxidation behavior of ductile RHEAs was studied. Specifically, a ductile RHEA, Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, was alloyed with Al and Zr aiming to improve its oxidation resistance. The two modified RHEAs, Al0.75Cr0.25Nb0.5Ta0.5Ti1.5 and Al0.5Cr0.25Nb0.5Ta0.5Ti1.5Zr0.01, indeed show enhanced oxidation resistance at 800 °C and 1,100 °C, compared with Al0.5Cr0.25Nb0.5Ta0.5Ti1.5. In addition, all three RHEAs studied here show an excellent oxidation resistance at 800 °C compared with other RHEAs, although there is still a large space to further improve their performance at 1,100 °C. Internal oxidation and even nitridation are still present after oxidation exposure, indicating further efforts are required to form protective oxide scales on the surface of ductile RHEAs. Nevertheless, the work is expected to shed some light on future directions of improving the oxidation of ductile RHEAs, via the alloying route.

Forming protective alumina scale for ductile refractory high-entropy alloys via aluminizing

Refractory high-entropy alloys (RHEAs), or multi-principal-element refractory alloys, have been intensively studied in recent years, due to their attractive potential for ultrahigh-temperature structural applications. One formidable challenge facing the materials development for RHEAs though, is to simultaneously achieve good oxidation resistance and good mechanical properties, which unfortunately often impose contradictory microstructural requirements. So far, there exist no ductile RHEAs that possess reasonable oxidation resistance, and the formation of protective oxide scales such as alumina in them is never seen. Here we report, for the first time, a strategy to form protective alumina scale in ductile RHEAs upon high temperature exposure, using a tailor-designed two-step pack aluminizing process. Very importantly, the oxidation resistance of ductile RHEAs improves tremendously, as evidenced from harsh cyclic oxidation tests, thus providing a huge impetus to push forward the further development of RHEAs, towards ultrahigh-temperature applications.

Ultra-high strain-rate strengthening in ductile refractory high entropy alloys upon dynamic loading

The mechanical properties and strain-rate strengthening mechanism of single-phase TiZrHfNbTaMox (x = 0.25, 0.5, and 0.75) refractory high entropy alloys (RHEAs) upon dynamic loading are investigated. Dynamic compression with varying strain rates of 2100–6000 s−1 at room temperature are performed by the split-Hopkinson pressure bar (SHPB), and the dynamic yielding strength is about 1.3 times of the quasi-static one. Positive strain-rate sensitivity (SRS) on the yielding strength is available, and the work-hardening is weakened but the yielding strength is improved with increasing strain rates due to the adiabatic heating upon dynamic loadings and the dislocation movement resistance, respectively. Considering the adiabatic temperature rise converted by plastic deformation work at high strain rates, the flow behavior is characterized by employing the modified Johnson-Cook model. A semi-empirical Zerilli-Armstrong constitutive model is proposed to predict yielding strengths at various strain rate.

Microstructures and mechanical properties of ductile NbTaTiV refractory high entropy alloy prepared by powder metallurgy

Refractory high-entropy alloys (RHEAs) are promising high-temperature structural materials due to their high melting point and extraordinarily high yield strength. However, their industrial application is greatly restricted due to their limited room-temperature ductility. In the present investigation, a ductile and strong single-phase NbTaTiV RHEA was synthesized by powder metallurgy method. Effects of the sintering temperature on the phase formation, microstructural evolution and the mechanical properties of the NbTaTiV RHEA were characterized. The results show that the NbTaTiV RHEA sintered at 1700 °C has an equiaxed single bcc phase microstructure, no obvious porosity and compositional segregation can be observed. The alloy exhibits a relatively high hardness of 510 HV, yield strength of 1.37 GPa, and compressive fracture strength of 2.19 GPa with a high fracture strain of 23% at room temperature. Typical strain softening and steady state flow occur during compressive deformation at high temperatures. During deformation at 1000 °C, the alloy still exhibits high yield strength of 437 MPa with a compression strain over than 40%. The outstanding mechanical properties is mainly attributed to the homogeneous and fine microstructures, and solid solution strengthening effect. It can be concluded that the powder metallurgy is a promising way for preparing ductile RHEAs with outstanding comprehensive mechanical properties.