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

ZC Xie and WR Jian and SZ Xu and IJ Beyerlein and XQ Zhang and XH Yao and R Zhang, INTERNATIONAL JOURNAL OF PLASTICITY, 157, 103389 (2022).

DOI: 10.1016/j.ijplas.2022.103389

Under high strain-rate loading, prominent increases in pressure usually triggers phase tran-sition (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 simula-tions, we investigate PTs in single crystal CoCrNi medium entropy alloys (MEAs) under QI compression. With the applied strain rates ranging from 108 s-1 to 1011 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, 110, and 111). Positive strain-rate sensitiveness is found in the yield stress along the 110 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 110 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.

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