Phase transition in medium entropy alloy CoCrNi under quasi-isentropic compression

Under high strain-rate loading, prominent increases in pressure usually triggers phase transition (PT), but the concomitant temperature rise may also cause melting. Quasi-isentropic (QI) compression provides a strategy to explore solid-state phase transition by reducing the temperature rise while retaining high pressure. Using large-scale molecular dynamics simulations, we investigate PTs in single crystal CoCrNi medium entropy alloys (MEAs) under QI compression. With the applied strain rates ranging from 1 0 8 s −1 to 1 0 11 s −1 , the strain-rate dependence and anisotropy of yield stress and solid-state PT path are revealed by comparing the mechanical responses along three compressed crystallographic orientations ([100], [1 1 ¯ 0], and [111]). Positive strain-rate sensitiveness is found in the yield stress along the [1 1 ¯ 0] and [111] directions, while insensitiveness along the [100] direction. Various PTs occur alongside massive plastic deformation in the post-yield regime. As the strain rate rises, face-centered-cubic (FCC) to body-centered-cubic (BCC) PT overrides the stacking fault-induced hexagonal-close-packed (HCP) phase formation and dominates the plasticity for the [100] loading. By contrast, crystalline PTs give way to amorphization for [1 1 ¯ 0] and [111] loading at high strain rates. Chemical short-range order hinders dislocation slip and promotes dislocation interactions, which further facilitate early formation of the BCC phase, suggesting a potential strategy to tailor polymorphism in MEAs. • Phase transition (PT) in CoCrNi medium entropy alloy is studied by MD simulations. medium entropy alloy under quasi-isentropic compression. • The effects of loading orientation and strain rate on PT path are investigated. • The role of CSRO in incipient plasticity and BCC phase expansion is revealed. • The competition between crystalline PTs and amorphization is discussed.