Recrystallization kinetics and mechanical properties of cold-rolled and annealed CoCrFeNi multi-principal element alloy

The microstructure and mechanical properties have been investigated in a CoCrFeNi alloy cold-rolled to 80 % thickness reduction and subsequently annealed at 600 degrees C. It is observed that the as-rolled microstructure comprises extended regions of different dominant crystallographic orientations along with layers of mixed orientations. Shear bands are also present in this microstructure, with the susceptibility to shear banding varying significantly from region to region. Shear bands are most pronounced in extended regions containing narrow deformation twins, and are a crucial source of recrystallization nuclei. Analysis of the recrystallization kinetics indicates that the Avrami exponent is similar to 1.6 for the first 30 min at 600 degrees C and that it decreases during further annealing. Tensile test data provide evidence that the sample annealed for 8 min, with a recrystallized fraction (f(RX)) of 13 % and an average recrystallized grain size of 0.8 mu m, does not show any significant improvement in ductility compared to that in the as-rolled condition. However, the ductility is considerably improved in the sample annealed for 15 min, where f(RX) is 43 % and the average recrystallized grain size is 1.1 mu m. This sample demonstrates a yield strength of 850 MPa and a total elongation to failure of 25 %. The data obtained in this work and in previous publications on partially recrystallized CoCrFeNi indicate that for samples annealed after 80-85 % deformation optimized combinations of strength and ductility are obtained when the recrystallized fraction is in the range 30 % < f(RX) <= 50 %.