Body-centered Cubic High-entropy Alloys Research Articles

Strengthening by {110} and {112} edge dislocations in BCC high entropy alloys

Abstract Mechanical tests and microscopy studies on body-centered cubic (BCC) high entropy alloys reveal transitions from screw to edge dislocation slip and from {110} to {112} slip plane activity. Here, a strengthening theory for BCC edge dislocation slip on {112} planes is thus developed that parallels a recent theory for {110} slip. Using the atomistic dislocation pressure fields for four BCC elements (Nb, Ta, Mo, W) as proxies to span the range of likely alloy cores, theory predicts that the zero temperature yield strength for {112} slip is slightly lower (0%–20%) than that for {110} slip but that the associated energy barrier is slightly (0%–20%) higher. This leads to cancelling effects, and hence very similar strengths, at finite temperatures and strain rates. Full atomistic results on selected dilute binary alloys show some shifts in these trends, but with similar magnitudes and cancelling effects. The close strengths of {110} and {112} slip modes indicate that subtle aspects beyond the scope of the theory will determine which slip system controls the observed strengthening. This closeness in strength cements the use of the {110} edge theory for guiding alloy design independent of actual slip system.

Achieving excellent uniform tensile ductility and strength in dislocation-cell-structured high-entropy alloys

Body-centered-cubic (BCC) high-entropy alloys (HEAs) encounter significant challenges in obtaining a high uniform tensile ductility (UTD). A dense dislocation-cell (DC) structure is produced in a heterogeneously grained HEA under tensile deformation, resulting from the anchored dislocation motion by grain interior elemental segregation. This fluctuation in elemental concentration is facilitated by thermomechanical processing. The activation of multiple-slip mechanisms, prompted by strain incompatibility among grains of varying sizes, significantly propels this process forward. This novel DC structure simultaneously increased the UTD (by 349.1 %) and yield strength (σ0.2, by 29.0 %) for a stable BCC HEA. Specifically, the single-phase alloy achieved a record-high UTD of 7.5 % and an σ0.2 of > 1,200 MPa, outperforming the counterparts of all the single-phase BCC HEAs. We employed a combination of transmission electron microscopy, in-situ scanning electron microscopy tensile testing coupled with an electron backscatter diffraction technology to investigate underlying strengthening mechanisms and identified that the serious stress concentration as a result of prevalent planar slip caused premature failure and localized strain of common BCC HEAs. At the initial stage of deformation, the DC structure promoted the activation of multiple slip systems and facilitated the extension of a plastic flow across the sample volume, effectively weakening stress concentration and premature failure. The extended plasticity zone and intensified dislocation interactions contributed to the increased UTD and σ0.2. These findings offer valuable inspiration for tailoring alloy properties via microstructure strategies and promoting their adoption in advanced manufacturing.

Understanding the tensile ductility of a novel low-activation BCC high-entropy alloy deformed at intermediate temperature

Low-activation body-centered cubic (BCC) high-entropy alloys (HEAs) often possess poor tensile ductility. Here we report a novel low-activation VCrFeMn0.33 BCC HEA that exhibits good tensile ductility at 650 °C and above. However, it shows poor tensile ductility at 500 °C and below, indicating that the ductile-to-brittle transition temperature (DBTT) of the current HEA falls between 500 °C and 650 °C. At temperatures above DBTT, the current BCC HEA deforms via dislocation glide, similar to the tensile deformation observed in conventional ductile BCC alloys, which is rarely reported for non-refractory BCC HEAs.

Machine learning assisted design of BCC high entropy alloys for room temperature hydrogen storage

Body-centered cubic (BCC) alloy systems can theoretically store double amounts of hydrogen compared with commercial metal hydrides at room temperature, and BCC high entropy alloys (HEAs) have shown the potential to reach this theoretic limit. However, the high thermodynamic stability of the dihydrides formed during hydrogen storage results in high operating temperatures. Here, by employing multi-objective Bayesian optimization-aided density functional theory calculations, we discovered 8 new HEA candidates for hydrogen storage, including the VNbCrMoMn HEA that can store 2.83 wt% hydrogen at room temperature and atmospheric pressure, vastly exceeding the hydrogen capacities of 1.38 wt% and 1.91 wt% for commercial LaNi5H6 and TiFeH2. Such a high performance of VNbCrMoMn is ascribed to the optimized hydrogen absorption thermodynamics, which is achieved under the guidance of interpretable machine learning which revealed that the thermodynamics of the first and second stages of absorption are largely determined by the bulk modulus and the number of states in the d-band, respectively.

Microstructure evolution and strengthening mechanism of CoCrFeMnNi HEA/Zr-3 brazed joints reinforced by fine-grained BCC HEA and HCP Zr

In the pursuit of manufacturing intricate components for the nuclear industry, we developed a novel Zr63.2Cu36.8 (wt.%) alloy via vacuum melting for brazing applications involving equiatomic high-entropy alloys (HEA) of CoCrFeMnNi and zircaloy (Zr-3). We systematically investigated the influence of various brazing parameters on microstructure evolution and shear properties. Furthermore, we established a comprehensive understanding of the relationship between the lattice structure of interfacial products, residual stress, and fracture behavior in HEA/Zr-Cu/Zr-3 joints. Our findings revealed that under specific conditions (1010 °C for 10 min), the reaction products in HEA/Zr-Cu/Zr-3 joints consisted of lamellar HEAP/lamellar Zr(Cr,Mn)2, granular (Zr,Cu)/Zr2(Cu,Ni,Co,Fe), bulk Zr(Cr,Mn)2, and Zrss. With increasing temperature and prolonged holding time, the layered HEAP and Zr(Cr,Mn)2 phases adjacent to the HEA substrates thickened, while the relative amounts of Zr2(Cu,Ni,Co,Fe) decreased, with a remarkable increase in ductile Zrss. Growth kinetics analysis of the reaction layer and EBSD analysis indicated that the HEAP phases exhibited a lower growth rate compared to the Zr(Cr,Mn)2 layer during brazing, and both phases exhibited random grain orientations. Particularly noteworthy was the precipitation of (Zr,Cu) within the layered Zr(Cr,Mn)2, which increased and coarsened with higher temperatures and extended durations. Finite element analysis and TEM analysis revealed higher residual stresses at the non-coherent Zr(Cr,Mn)2/HEAP interface with a lattice mismatch of 40.6%. The body-centered cubic (BCC) structural HEAP, composed of fine grains, effectively mitigated the concentrated residual stresses due to its superior plasticity. Moreover, micro-nanoscale close-packed hexagonal (HCP) precipitates (Zr,Cu) were distributed within the brittle Zr(Cr,Mn)2 phases, contributing to the overall strength improvement of the joints. Consequently, high-quality HEA/Zr-3 joints were achieved, featuring a maximum strength of 172.1 MPa, equivalent to approximately 62.6% of the yield strength of Zr-3. These results highlight the potential of Zr63.2Cu36.8 (wt.%) alloys in advanced brazing applications.

Accelerated design of low-activation high entropy alloys with desired phase and property by machine learning

Low-activation high-entropy alloys (HEAs) have been regarded as novel candidate structural materials for fusion reactors due to their excellent mechanical and radiation resistant properties. Nevertheless, the potential vast composition space brings a prominent challenge in the design of low-activation HEAs. Herein, a new strategy based on machine learning (ML) was proposed to accelerate the exploitation of low-activation HEAs with desired phase and property. Two optimized classification models with accuracy of > 85% were developed to identify single-phase body-centered-cubic (BCC) solid-solution (SS) HEAs. One regression model with correlation coefficient (R) of > 0.9 was constructed to predict the hardness of HEAs. The phase and hardness prediction models were combined in accordance with an integrated design strategy to identify the low-activation HEAs with desired phase and property from 284,634 candidates. With this design strategy, a new desired single-phase BCC Fe35Cr30V20Mn10Ti5 low-activation HEA with 555.9 ± 15.3 HV was designed and fabricated via only two experimental iterations. Besides, two new phase selection rules with accuracy of > 95% were given to efficiently screen SS and BCC HEAs, respectively. Our research work not only provides new ideas to search for low-activation HEAs, but also can be extended to the integrated design of structure and performance of other advanced materials.

Atomistic simulations of compressive response and deformation mechanisms of body-centered-cubic AlCrFeCoNi high-entropy alloys

Atomistic simulations are performed to study the compressive response and deformation mechanisms of the body-centered-cubic (BCC) AlCrFeCoNi high-entropy alloys (HEAs). Our computations show that the BCC HEA mainly experiences three typical deformation stages, i.e. linear elastic, plastic yielding, and yield flow regimes, and the predicted Young's modulus and yield strength agree well with available experiments. The plastic deformation mechanism of BCC HEA is dominated by the dislocation nucleation and slip accompanied by shear bands but depends little on the phase transformation of the HEAs. Compared with the face-centered-cubic counterpart, the BCC HEA is harder and more brittle due to the less dislocation activities in the alloy. In addition, the dependences of mechanical properties and deformation mechanisms on influential factors including temperature, strain rate, and Cr concentration are also analyzed in detail. These findings may deepen our understanding of the underlying deformation mechanisms in BCC HEAs.

Atomistic evaluation of tension-compression asymmetry in nanoscale body-centered-cubic AlCrFeCoNi high-entropy alloy

Abstract The tension and compression of face-centered-cubic high-entropy alloy (HEA) nanowires are significantly asymmetric, but the tension-compression asymmetry in nanoscale body-centered-cubic (BCC) HEAs is still unclear. In this study, the tension-compression asymmetry of the BCC AlCrFeCoNi HEA nanowire is investigated using molecular dynamics simulations. The results show a significant asymmetry in both the yield and flow stresses, with BCC HEA nanowire stronger under compression than under tension. The strength asymmetry originates from the completely different deformation mechanisms in tension and compression. In compression, atomic amorphization dominates plastic deformation and contributes to the strengthening, while in tension, deformation twinning prevails and weakens the HEA nanowire. The tension-compression asymmetry exhibits a clear trend of increasing with the increasing nanowire cross-sectional edge length and decreasing temperature. In particular, the compressive strengths along the [001] and [111] crystallographic orientations are stronger than the tensile counterparts, while the [110] crystallographic orientation shows the exactly opposite trend. The dependences of tension-compression asymmetry on the cross-sectional edge length, crystallographic orientation, and temperature are explained in terms of the deformation behavior of HEA nanowire as well as its variations caused by the change in these influential factors. These findings may deepen our understanding of the tension-compression asymmetry of the BCC HEA nanowires.

Mechanical and corrosion properties of biomedical (TiZr)90-xNbxTa5Mo5 medium entropy alloys

High- or medium-entropy alloys with single phase body centered cubic (BCC) structure have attracted significant scientific interests recently, due to their great potentials for biomedical applications. However, BCC high entropy alloys generally do not have good tensile ductility at room temperature. Herein, we utilize the valence electron concentration (VEC) theory to design three (TiZr)90-xNbxTa5Mo5 (x = 5,10,15) biomedical medium entropy alloys (BioMEAs). The BioMEAs showed a single phase BCC structure, a low elastic modulus of 79.2 ± 1.6–82.9 ± 1.2 GPa, a good tensile ductility of 24.4%, and a high specific yield strength of 141.2–137.4 10−3 MPa∙m3/kg. The good tensile ductility of the alloys was found attributed to the cross-slip of mixed dislocations and the formation of kink bands, avoiding the excessive entanglement of dislocations and relieving stress concentration. The increase of Nb content (5–15 at.% Nb) contributed little to the yield strength of the BioMEAs. In addition, the (TiZr)90-xNbxTa5Mo5 alloys showed excellent corrosive resistance in modified Fusayama saliva (MFS) solution, in which passive films composed of a variety of oxides spontaneously developed on the exposed surfaces. Our work has cast new light to develop lightweight and economical BioMEAs with excellent comprehensive properties.

In situ analysis of the Portevin-Le Chatelier effect from low to high-entropy alloy in equal HfNbTaTiZr system

The HfNbTaTiZr system is designed from low-entropy to high-entropy by unary, binary, ternary, quaternary, and quinary equal-molar element alloys. At first, the effect of mixing entropy on elevated temperature tensile behaviors ranging from 400 °C to 800 °C and strain rate 1 × 10−4 s–1 in body-centered-cubic (BCC) alloys is systematically discussed, including dynamic strain aging, serration behavior and in-situ deformation behavior of Portevin-Le Chatelier (PLC) band analysis. The results show that in BCC high-entropy alloys (HEAs), with the increase of the alloy elements, the range of the transition temperature of PLC bands expands, and the degree of dynamic strain aging at high temperature also enhances. The theoretical model was verified with the critical strain of unary, binary, and ternary alloys, and it predicts the PLC effect under the specific temperature and strain rate. Apply Applied to quinary alloys, the difference in serration phenomena between face-centered-cubic (FCC) and BCC HEA are investigated, showing that the high melting point of Nb and Ta elements lead to the slower self-adjusting velocity in HfNbTaTiZr alloy. Digital image correlation analysis is in-situ to analyze different types of PLC bands at various strain rates from 1 × 10–2 to 1 × 10−3 s–1 and elevated temperature from 200 °C to 600 °C. The BCC single-phase HEA reveals the transmission properties of the PLC bands and it is consistent with the serration behavior in the tensile stress-strain curve.

Study on martensitic transformation twinning in ductile metastable body-centered-cubic high entropy alloys

Martensitic transformation twinning including {111}α’’ type I, ‘‘{351}’’<2‾11 >α’’ type II and {021}α’’ twins, and extrinsic {111}α’’ stacking faults were investigated in a deformed body-centered-cubic (BCC) high entropy alloy (HEA). The interaction between martensitic twinning and local chemical environment variation made great contributions to strain accommodation upon straining, implying a novel route to develop ductile BCC HEAs.

In-situ synthesis of a FeCoCrNiCu/FeCoCrNiAl composite high entropy alloy coating by laser cladding

FeCoCrNiCu is a face-centered cubic (FCC) high entropy alloy (HEA) with good dislocation storage capacity, ductility, and toughness. FeCoCrNiAl is a body-centered cubic (BCC) HEA whose phase is hard and brittle with high strength and low toughness. In this study, a FeCoCrNiCu/FeCoCrNiAl composite HEA coating was prepared by laser cladding technology. After preparation, the microstructure and mechanical properties of the composite HEA were evaluated. A new two-phase (FCC + BCC) HEA was generated in situ between FCC and BCC HEA, achieving a good transition from hard to soft. The composite coating has the best wear resistance, the friction coefficient is about 0.3, the wear rate is 4.2 × 10−5 mm3/ N·m, and the wear mechanism is abrasive wear. While maintaining high hardness and good wear resistance, the composite coating has good compression properties. The FeCoCrNiCu cladding layer reduced the energy of the FeCoCrNiAl cladding layer by storing dislocations, which reduced its tendency to crack during compression, thus extending the service life of the coating surface.

High Hardness and Wear Resistance in AlCrFeNiV High-Entropy Alloy Induced by Dual-Phase Body-Centered Cubic Coupling Effects.

High-entropy alloys (HEAs) with high hardness are promising materials for advanced industrial manufacturing. In this study, the AlCrFeNiV HEA was designed and successfully prepared using a plasma instantaneous process. The hardness test showed that AlCrFeNiV had a high hardness of 1076 ± 15 HV, which was much higher than those reported in the literature. The microstructure of AlCrFeNiV was composed of two different types of body-centered cubic (BCC) structures, BCC1 (Al, Cr, Fe, and Ni) and BCC2 (enriched V and Cr). A mixture of different BCC systems produced solution strengthening, which was responsible for the superior hardness. Moreover, the reciprocating sliding wear behavior of HEA against Al2O3 balls under dry and lubricated conditions at ambient temperature was investigated. The wear rates of AlCrFeNiV against Al2O3 under dry wear and lubrication were 17.2 × 10−5 mm3 N−1·m−1 and 12.4 × 10−5 mm3N−1·m−1, respectively, which were of the same order of magnitude as the wear rates of BCC HEAs. Regardless of the dry wear or wear with lubrication, the wear mechanism of the HEA was abrasive and delamination wear.

Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate

Realizing high work hardening and thus elevated strength–ductility synergy are prerequisites for the practical usage of body-centered-cubic high entropy alloys (BCC-HEAs). In this study, we report a novel dynamic strengthening mechanism, martensitic twinning transformation mechanism in a metastable refractory element-based BCC-HEA (TiZrHf)87Ta13 (at.%) that can profoundly enhance the work hardening capability, leading to a large uniform ductility and high strength simultaneously. Different from conventional transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) strengthening mechanisms, the martensitic twinning transformation strengthening mechanism combines the best characteristics of both TRIP and TWIP strengthening mechanisms, which greatly alleviates the strength-ductility trade-off that ubiquitously observed in BCC structural alloys. Microstructure characterization, carried out using X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) shows that, upon straining, α” (orthorhombic) martensite transformation, self-accommodation (SA) α” twinning and mechanical α” twinning were activated sequentially. Transmission electron microscopy (TEM) analyses reveal that continuous twinning activation is inherited from nucleating mechanical {351}α” type I twins within SA ‘‘{351}’’<2¯11>α” type II twinned α” variants on {351}α” twinning plane by twinning transformation through simple shear, thereby accommodating the excessive plastic strain through the twinning shear while concurrently refining the grain structure. Consequently, consistent high work hardening rates of 2–12.5 GPa were achieved during the entire plastic deformation, leading to a high tensile strength of 1.3 GPa and uniform elongation of 24%. Alloy development guidelines for activating such martensitic twinning transformation strengthening mechanism were proposed, which could be important in developing new BCC-HEAs with optimal mechanical performance.

Precipitation enhancing strain hardening capability of ductile AlTiVZrNb high-entropy alloys upon dynamic loading

Strain softening upon dynamic loading is a roadblock of most body-centered cubic (BCC) structured high entropy alloys (HEAs), which can easily lead to dynamic plastic instability. Here, the precipitation was applied in Al–Ti–V–Zr–Nb system BCC HEAs for enhancing strain hardening capability upon dynamic loading. The α2 precipitates (D019 structure, P63/mmc space group) with contents of ∼25% are developed with burgers orientation relationship (BOR) interface and homogenously dispersed in the BCC matrix. The α2 precipitates can effectively reduce the slip length, activate more slip systems, and impose high density geometrically necessary dislocations near interface for hardening. Owing to the BOR, dislocations can slip transfer across phase boundaries offering the co-deformation for plasticity. Compared with single BCC counterpart, the strain hardening rate is changed from negative to positive and the fracture feature transforms from cleavage to dimples. Consequently, a ∼60% increasement in the product of strength and ductility is achieved upon dynamic loading.

Solid-State Hydrogen Storage Properties of Ti–V–Nb–Cr High-Entropy Alloys and the Associated Effects of Transitional Metals (M = Mn, Fe, Ni)

Recently, high-entropy alloys (HEAs) designed by the concepts of unique entropy-stabilized mechanisms, started to attract widespread interests for their hydrogen storage properties. HEAs with body-centered cubic (BCC) structures present a high potential for hydrogen storage due to the high hydrogen-to-metal ratio (up to H/M = 2) and vastness of compositions. Although many studies reported rapid absorption kinetics, the investigation of hydrogen desorption is missing, especially in BCC HEAs. We have investigated the crystal structure, microstructure and hydrogen storage performance of a series of HEAs in the Ti–V–Nb–Cr system. Three types of TiVCrNb HEAs (Ti4V3NbCr2, Ti3V3Nb2Cr2, Ti2V3Nb3Cr2) with close atomic radii and different valence electron concentrations (VECs) were designed with single BCC phase by CALPHAD method. The three alloys with fast hydrogen absorption kinetics reach the H/M ratio up to 2. Particularly, Ti4V3NbCr2 alloy shows the hydrogen storage capacity of 3.7 wt%, higher than other HEAs ever reported. The dehydrogenation activation energy of HEAs’ hydride has been proved to decrease with decreasing VEC, which may be due to the weakening of alloy atom and H atom. Moreover, Ti4V3NbCr2M (M = Mn, Fe, Ni) alloys were also synthesized to destabilize hydrides. The addition of Mn, Fe and Ni lead to precipitation of Laves phase, however, the kinetics did not improve further because of their own excellent hydrogen absorption. With increasing the content of Laves phase, there appear more pathways for hydrogen desorption so that the hydrides are more easily dissociated, which may provide new insights into how to achieve hydrogen desorption in BCC HEAs at room temperature.

Microstructural optimization of FexCrNiAl0.5Ti0.5 high entropy alloys toward high ductility

It remains challenging to develop low-cost body-centered cubic (BCC) high-entropy alloys (HEAs) with superior mechanical properties. In this work, we investigated the microstructure and mechanical properties of BCC FexCrNiAl0.5Ti0.5 HEAs containing L21 nanocrystals. With increasing the Fe content from Fe2 to Fe4 and Fe6, the formation of a big blocky L21 phase and a σ phase can be fully suppressed, and the size and fraction of the L21 crystals also decrease. Fe4 and Fe6 HEAs exhibit an optimized dual-phase microstructure with high-density L21 nanocrystals homogeneously distributed in the BCC matrix. Fe4 and Fe6 HEAs show high strength and do not fracture even at a large compressive strain of 70%, exhibiting the best combination of strength and fracture strain among all the reported BCC HEAs with nanocrystals. The superior mechanical properties of Fe4 and Fe6 HEAs are attributed to the optimized microstructure. These findings promote the development of low-cost HEAs with superior mechanical properties.

Strength can be controlled by edge dislocations in refractory high-entropy alloys

Energy efficiency is motivating the search for new high-temperature (high-T) metals. Some new body-centered-cubic (BCC) random multicomponent “high-entropy alloys (HEAs)” based on refractory elements (Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr) possess exceptional strengths at high temperatures but the physical origins of this outstanding behavior are not known. Here we show, using integrated in-situ neutron-diffraction (ND), high-resolution transmission electron microscopy (HRTEM), and recent theory, that the high strength and strength retention of a NbTaTiV alloy and a high-strength/low-density CrMoNbV alloy are attributable to edge dislocations. This finding is surprising because plastic flows in BCC elemental metals and dilute alloys are generally controlled by screw dislocations. We use the insight and theory to perform a computationally-guided search over 107 BCC HEAs and identify over 106 possible ultra-strong high-T alloy compositions for future exploration.

Local chemical fluctuation mediated ductility in body-centered-cubic high-entropy alloys

High-entropy alloys (HEAs) open up a new horizon for discovering un-explored mechanical properties and deformation mechanisms. Local chemical fluctuations (LCFs) in HEAs were found to have significant influences on their mechanical performance, however, the underlying origins remain unclear. In this work, direct dynamic observation of the interaction between LCFs and dislocations was captured by in situ transmission electron microscopy in a ductile body-centered-cubic (BCC) HfNbTiZr HEA under loading. The observed dislocation pinning induced by LCFs contributes to the increment not only in the strength but also in the ductility due to strongly promoted dislocation interaction. The observed local double cross-slips caused by the LCFs distribute dislocations onto various atomic planes homogenously, which is also beneficial for ductilization in HfNbTiZr. Our findings not only shed light on the understanding of deformation mechanisms of HEAs, but also provide a new perspective to design ductile BCC HEAs.

Deformation Mechanisms and Remarkable Strain Hardening in Single-Crystalline High-Entropy-Alloy Micropillars/Nanopillars.

There have been very limited studies on plastic deformation mechanisms in single-crystalline high-entropy alloys (HEAs) with body-centered cubic (BCC) phases. We performed in situ uniaxial compression on single-crystalline BCC AlCrFeCoNi micropillars/nanopillars with three orientations (including [100], [110], and [111]) and diameters of 270-1583 nm, inside a scanning electron microscope. The experimental results showed the significant size effects on yield/flow stress and the remarkable strain hardening in these HEA micropillars/nanopillars. Especially, HEA micropillars/nanopillars with ⟨100⟩ orientation exhibited higher strain hardening exponents than BCC pure metals and Al0.7CrCoFeNi counterparts. A combination of transmission electron microscopy observations and large-scale atomistic simulations revealed that dislocation slip, reaction, tangling and accumulation, and solid solution effects are responsible for the observed size effects on yield/flow stress and remarkable strain hardening, but these dislocation mechanisms are dependent on nanopillar orientation. Our present study sheds light on the underlying deformation mechanisms in BCC HEA single crystals.