Shock-induced deformation and spallation in CoCrFeMnNi high-entropy alloys at high strain-rates

Shock-induced deformation and fracture in single-crystalline and nanocrystalline CoCrFeMnNi high-entropy alloys (HEAs) under different shock intensities are investigated systematically using large-scale molecular dynamics simulations. The strong anisotropy in the single-crystalline HEA and how the reversibility of HCP-structured atoms influences the void nucleation and growth are revealed. Specifically, shock-induced FCC to HCP structural transformations are largely reversible in the case of the [001] loading direction, leading to homogeneous void nucleation; while they are largely irreversible in the cases of the [110] and [111] loading directions, resulting in void nucleation in the intersection of HCP layers. In nanocrystalline HEA, the deformations in the grain interior are similar to those in the single crystals, while the grain boundaries serve as void nucleation sites and thus weaken the spall strength. The SRO effects on the shock response in single-crystalline and nanocrystalline CoCrFeMnNi HEA are revealed, and the chemical heterogeneity is analyzed. Our results show that SRO can only slightly increase the shear stress and the spall strength, but it can cause a reduction of ductility during the spallation. The differences in SRO effects between single-crystalline and nanocrystalline models are attributed to the grain boundaries, where Cr and Mn are favorable, while Ni is favored inside the grains in the SRO model. Moreover, the comparison between CoCrFeMnNi HEA and its subsystem as well as singleelement metal indicates Mn seems to be the most influential elements in CoCrFeMnNi HEA, resulting in significant changes in strength. A relation between the spall strength and strain rate is proposed to describe the simulation results and reported experimental data. Our comprehensive and systematical studies provide deep insights into the deformation and fracture in CoCrFeMnNi HEA subjected to shock loading, which deepens the understanding on the dynamic deformations of CoCrFeMnNi HEA and benefits the rational design of new HEAs/MEAs with enhanced performance.