Recrystallization tuning to optimize mechanical properties in heavily rolled CoCrFeNi medium entropy alloy

The microstructure, crystallographic texture and tensile properties have been investigated in a CoCrFeNi medium entropy alloy after cold rolling to 85% thickness reduction and in several annealed conditions. It is found that the microstructure of the rolled sample is highly heterogeneous, with extended regions having orientations either along the (110)//ND fiber or along the (111)//ND fiber and layers of mixed orientations. The heterogeneity is enhanced by profuse shear bands. During annealing the shear bands and layers of mixed orientations act as preferential nucleation sites for recrystallized grains. Recrystallization is accompanied by annealing twinning, which leads to texture weakening. Analysis of mechanical properties in the CoCrFeNi alloy demonstrates that the strengthening effect of heavy rolling is retained in partially recrystallized conditions. Good combinations of high mechanical strength and fairly high ductility are obtained in partially recrystallized samples produced by annealing at either 550 degrees C or 570 degrees C. These combinations are attributed to fine-scale bimodal microstructures, 38-50% of which contain recrystallized grains with an average size of 1-1.2 mu m, while deformation structures of nanometer to submicrometer scale and with high dislocation densities are present in non-recrystallized regions.