An initio study of influence of substitution of Sc with Al on intrinsic mechanical properties of hexagonal high-entropy alloys Hf0.25Ti0.25Zr0.25Sc0.25−xAlx (x ≤ 15%)

Abstract

The intrinsic mechanical properties of novel hexagonal high entropy alloys (HEAs) Hf0.25Ti0.25Zr0.25Sc0.25−xAlx (x ≤ 15%) have been studied using first-principles theory based on special quasi-random structure, and influence of Al content is stressed. The derived formation enthalpy and elastic constants indicate the thermodynamical and mechanical stability of the studied alloys. With substitution of Al for Sc, bulk modulus, shear modulus and Young's modulus tend to decrease, while HEAs become more ductile. The estimated elastic anisotropies of alloys show an ascending trend with increasing Al content, and the shear and tensile Young's moduli along several typical directions have been obtained for the studied HCP crystal. Furthermore, the studied ideal strength of HCP HEAs suggest that the ideal tensile strength (ITS) of HEAs occurs in [112̅0] direction, and decreases from 5.34 GPa to 3.80 GPa with increasing Al content for x goes from 0.00 to 0.15, whereas corresponding critical tensile strain increases from 0.07 to 0.08. The ideal shear strength (ISS) of HEAs takes place in (101̅0)< 112̅0 > shear system. With increasing Al content for x goes from 0.00 to 0.15, the ISS decrease from 3.71 GPa to 2.46 GPa, the corresponding critical shear strains increase also from 0.10 to 0.11. So HEAs are more ductile as increasing Al concentration at expense of both ITS and ISS. It is worth noting that the initial slopes of tensile and shear stress–strain are in good agreement with the tensile and shear modulus from elastic parameter, also exhibiting descending trend with Al content. The critical resolved shear stresses (CRSS) estimated from the ITS for HEAs is smaller than the corresponding ISS, suggesting that slip is preference to cleavage. The shearability and half-width of the dislocation are also evaluated and discussed. Furthermore, the detailed electronic structure in bond length evolution is further studied, from feature of the Al-M atom pairs due to addition of Al element, the mechanism for enhancement of ductility for HfTiZrSc1−xAlx is uncovered, all of these provide more details for more adequate understanding deeply the intrinsic mechanism of mechanical properties of alloys HfTiZrSc1−xAlx.