Insights into the microstructural evolution and wear behavior of underwater friction stir processed CoCrFeNiMn high-entropy alloy

The equiatomic CoCrFeNiMn high-entropy alloy (HEA) has demonstrated excellent strength-ductility-toughness synergy over a wide range of temperatures, especially at cryogenic temperatures, but poor wear resistance. Herein, we explored the feasibility of constructing ultrafine-grained (UFG) surface layer on the CoCrFeNiMn HEA by underwater friction stir processing (FSP) and evaluated its dry sliding wear behavior. During air-FSP of CoCrFeNiMn HEA, continuous and discontinuous dynamic recrystallization (CDRX and DDRX) compete with each other responsible for grain refinement. Water-FSP promotes the occurrence of DDRX and inhibits the growth of recrystallized grains, leading to the formation of an UFG surface layer with average grain size of 1.14 +/- 0.78 mu m. The surface hardness of base material is about 179 HV, which is increased to 216 HV for the air-FSPed sample and 275 HV for the water-FSPed sample. Nonetheless, the three samples exhibit similar wear behavior, mainly adhesive wear and oxidation wear. This is mainly attributed to a combination of the formation of oxides on the worn surfaces during sliding and the significantly reduced strain-hardening effect with decreasing grain size. This work provides a fundamental understanding of the microstructural evolution of CoCrFeNiMn HEA during FSP as well as its sliding wear behavior before and after FSP.