Dualistic effect of ultrasonic shot peening on the mechanical properties of high- and medium-entropy alloys in annealed and cold-rolled states

In this study, we processed medium-entropy and high-entropy alloys by ultrasonic shot peening (USSP) treatment. This study reveals the formation of a unique submicron-scale layered structure at the surface and a deeper layer with the hierarchical structure. The results indicate that the surfaces of both annealed and cold-rolled samples were significantly hardened after USSP. The maximum hardening was found in the Cantor and Fe60 alloys in the annealed state. Mechanical strength changed in a non-uniform way. It increases after USSP of the annealed samples due to the generation of defects such as submicron-scale surface grains, sub-grains, twins and dislocations but decreases after USSP of the cold-rolled samples despite their higher surface hardness. This reduction was attributed to the migration of defects toward grain boundaries due to enhanced atomic mobility during USSP.