Shock-Induced Deformation and Spallation in CoCrFeMnNi High-Entropy Alloys at High Strain-Rates

Shock-induced microstructural evolution, deformation mechanism, and spallation failure in single-crystalline and nanocrystalline CoCrFeMnNi high-entropy alloys (HEAs) under different shock intensities are investigated systematically using large-scale molecular dynamics simulations. The simulation results show that dislocation slip, deformation twinning and cavitation are the primary deformation mechanisms due to the low average stacking fault energy of the CoCrFeMnNi HEA. Interestingly, the single-crystalline HEA exhibits a strong anisotropy in the wave profile, microstructural evolution, and deformation mechanisms under shockwave loading, can be attributed to the differences in elastic modulus, elastic/plastic transition and activated slip systems among the three orientations. More specifically, four slip systems are triggered in the [001] and [110] loading conditions, while only three slip systems are activated in the [111] case. The identified HCP structure in the [001] case is largely reversible while it becomes almost irreversible in the [110] and [111] cases due to the apt nucleation of voids at the intersection of stacking faults, in contrast to the homogenous void nucleation in the [001] case. Moreover, spall strength is higher in [001] than in [110] and [111] cases due to the reversible defects. 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. However, the chemical short-range ordering (SRO) reduces the ductility by accelerating the void nucleation and growth although it only slightly increases the spall strength, which dues to the increase of Cr and Mn in grain boundaries. A relation between the spall strength and strain rate is proposed to describe the simulation results and reported experimental data. Our work not only demonstrates the high spall strength but also provides insights into nanoscale deformation and damage mechanisms of CoCrFeMnNi HEA under dynamic loading, which will benefit the design and application of HEAs under shock loading in general.