Influences of nitrogen alloying on microstructural evolution and tensile properties of CoCrFeMnNi high-entropy alloy treated by cold-rolling and subsequent annealing

The influences of nitrogen alloying on the microstructural evolution and tensile properties of CoCrFeMnNi high-entropy alloys (HEAs), which were subjected to cold rolling and subsequent annealing at 773-1173 K, were systematically investigated. The results show cold rolling-induced microbands in the nitrogen-alloyed HEAs instead of the deformation twins and shear bands found in nitrogen-free HEAs. During annealing at 773-873 K, the cold-rolled nitrogen-free HEAs experience a partial annihilation of the deformation twins, while the cold-rolled nitrogen-alloyed HEAs exhibited higher microstructural stability. When the annealing temperature exceeds 973 K, a large number of Cr2N precipitates form in the recrystallized regions of the nitrogen-alloyed HEAs, and the tensile strength of the cold-rolled nitrogen-free HEAs decreases with increasing annealing temperature. However, the tensile strength of the nitrogen-alloyed HEAs annealed at 773 and 873 K experiences an abnormal increase compared to that of the cold-rolled sample. The best combination of strength and ductility are achieved in the nitrogen-alloyed HEAs treated by cold-rolling and annealing at 973 K. When the annealing temperature increases to 1073-1173 K, the nitrogen-alloyed and -free HEAs exhibit completely recrystallized structures with relatively low strength and excellent ductility. The influences of nitrogen alloying on the microstructural evolution are related to the interaction between nitrogen atoms and dislocations, and the multi-mechanism (including nitrogen solid-solution strengthening, precipitation strengthening, and grain refinement strengthening) accounts for the improved tensile properties of the nitrogen-alloyed HEAs treated by cold-rolling and subsequent annealing at 973 K.