We developed a series of Co-rich Co_xCr₂₅(FeNi)_75−x (x = 35, 45, 55, 65) high entropy alloys with improved strength and/or ductility, derived from lowering the stacking fault energy (SFE) and reducing the fcc phase stability of the equiatomic CoCrFeNi alloy. Thermodynamics and ab initio calculations demonstrated that increasing Co while decreasing Fe and Ni concentrations lower the SFE and reduce the fcc phase stability. The Co₃₅Cr₂₅Fe₂₀Ni₂₀ and Co₄₅Cr₂₅Fe₁₅Ni₁₅ alloys with single fcc phase, exhibit superior tensile properties, contributing to the twinning and fcc → hcp martensitic transformation. The present study offers a guideline for designing high-performance high entropy alloys. GRAPHICAL ABSTRACT IMPACT STATEMENT A series of novel Co-rich non-equiatomic high entropy alloys with enhanced tensile properties were developed by lowering the stacking fault energy and reducing the phase stability of equiatomic CoCrFeNi alloy.

Materials innovation calls for an integrated framework combining physics-based modelling and data-driven informatics. A dislocation-based constitutive model accounting for both transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) was built to interpret the mechanical characteristics of metastable titanium alloys. Particular attention was placed on quantitatively understanding the composition-sensitive phase stability and its influence on the underlying deformation mechanism. For this purpose, a pseudoelastic force balance incorporating thermodynamics and micromechanics was applied to calculate the energy landscapes of β → α″ martensitic transformation, {332}〈113〉 twinning and dislocation slip. Extensive material data were probed, computed and fed to the model. Our results revealed that TRIP and TWIP may operate simultaneously because of the presence of a noticeably overlapped energy domain, and confirmed {332}〈113〉 twinning is an energetically favourable deformation mechanism. The model validation further unveiled that the activation of β → α″ transition remarkably enhances the strain-hardening and plasticity, even though the dynamically formed α″ volume fraction is much less than that of deformation twinning. Our work suggests that the synchronised physical metallurgy and data-driven strategy allows to identify the compositional scenarios for developing high-performance engineering alloys.

The effect of Cr and Ti on the fundamental mechanical properties of V-Cr-Ti alloys has been investigated using the all-electron exact muffin-tin orbitals method in combination with the coherent-potential approximation. The static lattice constant and elastic parameters have been calculated for the body-centered-cubic V${}_{1-x-y}$Cr${}_x$Ti${}_y$ (0 ≤ x,y ≤ 0.1) random solid solution as a function of composition. Our theoretical predictions are in good agreement with the available experimental data. Alloys along the equicomposition region are found to exhibit the largest shear and Young's modulus as a result of the opposite alloying effects obtained for the two cubic shear elastic constants. The classical solid-solution hardening (SSH) model predicts larger strengthening effect in V${}_{1-y}$Ti${}_y$ than in V${}_{1-x}$Cr${}_x$. By considering a phenomenological expression for the ductile-brittle transition temperature (DBTT) in terms of Peierls stress and SSH, it is shown that the present theoretical results can account for the variations of DBTT with composition.

Recently developed high-entropy alloys (HEAs) consisting of multiple principal elements represent a new field of metallurgy and have demonstrated appealing properties for a wide range of applications. Usingab initioalloy theory, we reveal the alloying effect on the elastic properties and the ideal tensile strength (ITS) in the [001] direction of four body-centered cubic (bcc) refractory HEAs based on Zr, V, Ti, Nb and Hf. We find that these HEAs show high elastic anisotropy and large positive Cauchy pressure, suggesting good extrinsic ductility. Starting from ZrNbHf, it is found that the ITS decreases with equimolar Ti addition. On the other hand, if both Ti and V are added to ZrNbHf, the ITS is enhanced by about 42%. An even more captivating effect is the ITS increase by about 170%, if Ti and V are substituted for Hf. The alloying effect on the ITS is explained by thed-band filling. An intrinsic brittle-to-ductile transition is found in terms of the failure mode under uniaxial tension. These investigations suggest that intrinsically ductile HEAs with high ideal strength can be achieved by controlling the proportion of group four elements to group five elements.

Metal hydrogen storage materials have been the focus of intensive research in the field of hydrogen-based economy. An outstanding question is that the number of hydrogen atoms accommodated in metal hydrides is generally much below the number of interstices, which limits their hydrogen storage capacities. Unlike traditional FCC metal hydrides where hydrogen can only occupy tetrahedral interstices, this study demonstrates that hydrogen can also occupy octahedral interstices in FCC high entropy alloy (HEA) hydrides, which leads to the violation of the Switendick criterion. For Ti₂₅V₂₅Nb₂₅Ta₂₅ and Ti₂₅V₂₅Nb₂₅Zr₂₅ HEAs, nearly 20% and 17.5% of octahedral interstices can be occupied by hydrogen, respectively. The anomalous hydrogen occupation mainly originates from the intrinsic electron delocalization between hydrogen atoms in HEA hydrides, which presents a sharp contrast to traditional metal hydrides. Such electron delocalization decreases repulsive interactions between hydrogens and promotes the electron localization at octahedral interstices. Additionally, this study reveals that hydrogen occupation at octahedral interstices enhances the structural disordering and decreases the thermal stability of HEA hydrides, which will be beneficial to reduce the dehydrogenation temperature. The presented results may provide a new strategy for the design of high-density storage materials.