Coupled effects of recrystallization and precipitation on microstructure and mechanical properties of CoFeMnNi-5Cr-5Al-1C alloy

Microstructural evolution and changes in mechanical properties during annealing have been studied in a CoFeMnNi-5Cr-5Al-1C alloy cold rolled to a thickness reduction of 90%. The rolled microstructure is found to contain twinned regions and shear bands, layers of mixed orientations with high frequencies of high-angle boundaries, as well as extended regions dominated by low-angle boundaries. The rolled sample demonstrates a yield strength of 1319 MPa and a uniform elongation of 2% at room temperature. Shear bands and layers of mixed orientations are observed to be preferential sites for nucleation of recrystallized grains and for precipitation of B2 and carbide particles during annealing at 800 °C. Samples annealed for 30 min and for 1 h show good combinations of a comparatively high yield strength (∼800 MPa) and significantly improved uniform elongation (14% and 17%, respectively). In these samples, the fraction of the recrystallized microstructure in the face-centered cubic (FCC) phase is 55% and 60%, while the total volume fraction of particles is 11% and 13%, respectively. Particles are estimated to contribute ∼20% to the yield strength, whereas the partially recrystallized FCC microstructure contributes ∼80%. The recrystallized fraction and the total fraction of particles both increase with increasing annealing duration. However, the material is not fully recrystallized even after annealing for 5 h at 800 °C. The effects of recrystallization and precipitation on mechanical properties of the investigated alloy are compared with those of several other FCC-based MPEAs.