Effects of short range ordering on the generalized stacking fault energy and deformation mechanisms in FCC multiprincipal element alloys

Abstract

Equiatomic FCC CrCoNi, VCoNi and CrCoNiFeMn exhibit distinctly different deformation mechanisms where twinning is prevalent in CrCoNi and CrCoNiFeMn, but not in VCoNi. Extant density-functional theory (DFT) calculations show that all 3 alloys possess similarly low/negative intrinsic stacking fault energies (SFEs) and thus high twinnability in random-solute states, which obviously contradicts experimental observations. Here, the mean and local generalized SFE-lines (γ-lines) are systematically determined using DFT in all 3 alloys with special quasirandom structure (SQS) and Monte Carlo (MC) simulation optimized structures. Linear elasticity with DFT-based γ-lines predicts wide core separations and twinnabilities higher than elemental FCC metals in all 3 alloys with SQS. During MC simulations, chemical short range ordering (SRO) is formed, with VCoNi exhibiting the strongest SRO, followed by CrCoNi and CrCoNiFeMn. SRO always raises the mean intrinsic SFE relative to that in base SQS configurations. The average increase scales linearly with the magnitude of the SRO, resulting in moderate, large and drastic increases in CrCoNiFeMn, CrCoNi and VCoNi, respectively. Furthermore, SRO has strong effects on the intrinsic and extrinsic SFEs, but weaker effects on the unstable SFEs, leading to disparate dislocation and twinning behaviour. In SRO-saturated states, CrCoNi and CrCoNiFeMn retain high twinnability comparable to Ag, while VCoNi has narrow core separations similar to Al and twinnability lower than all elemental FCC metals. The DFT-based γ-lines and mechanics-based prediction not only reveal SRO effects on GSFE, dislocation and twinning, but also shed light on the physical origin of the different deformation mechanisms in these alloys.