Effect of lattice distortion and chemical short-range order on the phase transformation behavior of high entropy alloys under high strain rates

Phase transformation offers a promising strategy to overcome the long-standing strength-toughness trade-off in materials by accommodating plastic deformation through strain redistribution. The FCC high entropy alloy (HEA) CoCuFeNiPd has received attention for its excellent mechanical properties due to its intense chemical short-range order (SRO)and severe lattice distortion effect (LD). In this study, the effect of SRO and LD, as well as strain rate, on the mechanical responses and phase transformation behavior of CoCuFeNiPd HEA is investigated via a combination of molecular dynamics (MD) and Monte Carlo (MC) simulations. This study demonstrates that the deformation mechanism in CoCuFeNiPd HEA transitions from dislocation slip dominance at 1 × 10⁸/s to FCC-BCC-HCP phase transformation dominance at 1 × 10¹⁰/s. During the initial deformation stage, yield behavior is controlled by BCC structure nucleation. LD effects substantially reduce the nucleation barrier, promoting premature BCC formation and accelerating the yielding process. The SRO effect induces the phase transformations that predominantly occur in regions where Cu-Fe-Pd clusters aggregate, which promotes the rapid development of dislocations and maintains a high flow stress. In addition, the twinning substructures of BCC martensite by specific atom shear movements are observed under the strain rate of 10¹⁰/s, which maintains the high strength, and the subsequent HCP phase transformation provides the continuous plastic deformation. This study provides important insights into the stress-induced phase transformation mechanism under extreme strain rates.