Effect of Ni concentration on the spallation behavior of CoCrFeMnNi high entropy alloy via atomistic simulation

We employ non-equilibrium molecular dynamics simulations to investigate the spallation behavior of CoCrFeMnNi high-entropy alloys (HEAs) with varying Ni concentrations. Our findings reveal a non-monotonic relationship between Ni concentration and spall strength at low shock velocity (1.0 km/s), arising from the interplay between lattice distortion (LD) effects and shock-induced phase transitions. At Ni concentration below 10%, the alloy experiences an inverse phase transformation from BCC to FCC during unloading. In this case, voids tend to nucleate homogeneously within the FCC matrix, leading to higher spall strength. In contrast, higher Ni concentration (>10%) induce FCC-to-HCP phase transformations under shock, and the residual dislocations and stacking faults during unloading facilitates heterogeneous void nucleation, thereby reducing spall strength. However, this phase-transformation-based trend in spall strength does not fully align with our simulation results. To clarify this discrepancy, we adopt a binary alloy model and systematically vary the atomic size ratio to investigate the influence of LD. The simulations reveal that spall strength decreases monotonically with increasing LD. Therefore, the introduction of smaller Ni atoms reduces LD, leading to an enhancement in spall strength. When the Ni concentration exceeds 70%, further changes in LD become insignificant, and the spall strength remains nearly constant. At higher piston velocity (3.0 km/s), the HEA matrix undergoes shock melting or release melting, leading to the formation of a disordered phase. Under such conditions, the influence of Ni concentration on spall strength becomes negligible.