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 $10^8$ s⁻¹ to $10^{11}$ 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$\overline{1}$0], and [111]). Positive strain-rate sensitiveness is found in the yield stress along the [1$\overline{1}$0] and [111] directions, while insensitiveness along the [100] direction. Various PTs occur alongside massive plastic deformation in the post-yield regime. As the strain rate rises, face-centered-cubic (FCC) to body-centered-cubic (BCC) PT overrides the stacking fault-induced hexagonal-close-packed (HCP) phase formation and dominates the plasticity for the [100] loading. By contrast, crystalline PTs give way to amorphization for [1$\overline{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.

在高应变率载荷下，压力的显着增加通常会引发相变 (PT)，但伴随的温度升高也可能导致熔化。准等熵 (QI) 压缩提供了一种通过在保持高压的同时降低温升来探索固态相变的策略。使用大规模分子动力学模拟，我们研究了 QI 压缩下单晶 CoCrNi 中熵合金 (MEA) 中的 PT。施加的应变率范围为$10^8$s ^-1至$10^{11}$s ^-1，通过比较沿三个压缩晶体取向的机械响应（[100]，[1$\overline{1}$0]和[111]）。在沿 [1] 的屈服应力中发现正应变率敏感性$\overline{1}$0]和[111]方向，而沿[100]方向不敏感。在屈服后状态下，各种 PT 与大量塑性变形同时发生。随着应变率的增加，面心立方 (FCC) 到体心立方 (BCC) PT 覆盖了堆垛层错诱导的六方密堆积 (HCP) 相形成，并主导了 [100] 加载的可塑性. 相比之下，结晶 PT 让位于非晶化 [1$\overline{1}$0]和[111]在高应变率下加载。化学短程有序阻碍位错滑移并促进位错相互作用，这进一步促进了 BCC 相的早期形成，这表明了一种在 MEA 中定制多态性的潜在策略。