Cluster impact into high-entropy alloys: Deformation, hardness changes, and subgrain formation

The impact of nanoclusters on a high-entropy alloy (HEA) of the same material is studied using molecular dynamics simulation. Both a bcc (HfNbTaTiZr) and an fcc (FeNiCrCoCu) HEA are investigated. The system sizes are large enough (almost 30 millions atoms) to contain the plasticity formed by the nanocluster (30 nm diameter) at 1 km/s speed. Both the cluster and the target are initially single-crystalline. Bombardment induces a crater and extensive dislocation plasticity in both materials. The projectile is strongly deformed by the impact and the prevalence of easy slip directions render its shape strongly anisotropic. We analyze the hardness in the impact region using simulated nanoindentation. The projectile has lost hardness due to the intense defect formation; however, the target material under the crater has hardened by the impact, in particular for the bcc HEA. In addition, the impact leads to subgrain formation both in the projectile and in the target material under the crater. The grains have diameters of typically 8 nm, i.e., one quarter of the projectile. Subgrain formation is more pronounced for the bcc HEA; in this material, we studied the velocity dependence and found that the grain number increases with projectile velocity. Multiple impacts would lead to surface restructuring and enhanced surface hardness.