Effects of nitrogen alloying and friction stir processing on the microstructures and mechanical properties of CoCrFeMnNi high-entropy alloys

Equiatomic CoCrFeMnNi high-entropy alloys (HEAs) with and without nitrogen alloying were produced by vacuum induction melting and then subjected to friction stir processing. The effects of nitrogen alloying and friction stir processing on the microstructures and mechanical properties of these alloys were systematically investigated. The results suggested that nitrogen alloying effectively increases the yield strength and ultimate tensile strength of CoCrFeMnNi HEAs in the as-cast condition. Friction stir processing considerably broke down the coarse dendritic structures of the as-cast HEAs and caused grain refinement and composition homogenization. More significantly, nitrogen alloying promoted grain refinement by increasing the nucleation rate and retarding grain growth during dynamic recrystallization over the course of friction stir processing. The yield strength, ultimate tensile strength, and uniform elongation of CoCrFeMnNi HEAs treated by nitrogen alloying and friction stir processing reached 493 MPa, 832 MPa, and 32.6%, respectively. These improvements could be attributed to solid solution strengthening through nitrogen alloying and grain boundary strengthening by friction stir processing. This study provides an alternative technical route to enhance the mechanical properties of CoCrFeMnNi HEAs.