Deformation mechanism and mechanical properties of a CoCrFeNi high-entropy alloy via room-temperature rolling, cryorolling, and asymmetric cryorolling

Abstract

The deformation mechanism and mechanical properties of CoCrFeNi high-entropy alloys (HEA) processed by room-temperature rolling (RTR), cryorolling (CR) and asymmetric cryorolling (ACR) were studied. As the rolling reduction increased from 20 % to 80 %, a transition in the dislocation and twinning deformation mechanisms occurred. The cryogenic environment in rolling process can promote the activation of the twins in the HEA at a lower strain, improving the deformation efficiency. The introduction of differential speed ratio led to the finer micro-shear bands with a more uniform distribution, so that the grains were refined to a greater extent. In the ACR HEA after large deformation, the stacking faults accumulated inside the deformation twins further refined the twins along the direction of long axis. This increased the strength of ACR HEA together with Lomer-Cottrell locks and FCC/HCP boundaries, reaching the maximum value compared with CR and RTR. The ultimate tensile strength exceeded 1450 MPa, and the yield strength of ACR HEA increased by nearly 1200 MPa compared to the initial material, while it still maintained the same elongation as RTR and CR. ACR showed the best strength-ductility combination during the whole rolling deformation process. The microstructure evolution and the strengthening contribution mechanism of different rolling were discussed and calculated, and the specific mechanism of how the introduced differential speed ratio further improves the strength-ductility synergistic strengthening in CR was clarified.