Twinning- and transformation-induced high cryogenic strength and ductility of the CoCrFeNi high-entropy alloy: Experiment and MD simulation

The CoCrFeNi high-entropy alloy (HEA) processed by asymmetric cryorolling (ACR) followed by short-term annealing exhibited an exceptional strength-ductility synergy under cryogenic conditions. Compared to room temperature, the tensile strength increased from 880 to 1281 MPa, and the fracture elongation increased from 50.1 % to 89.8 %. To reveal the underlying mechanism, molecular dynamics (MD) simulations were employed to investigate the deformation processes at room and cryogenic temperatures. The simulations revealed distinct microstructural evolution pathways involving intrinsic stacking faults (ISFs), extrinsic stacking faults (ESFs), twins, and HCP phases. The combined experimental and simulation results demonstrate that twinning and HCP-phase formation are the dominant deformation mechanisms responsible for the enhanced strength and ductility at cryogenic temperatures. Under cryogenic conditions, twinning and HCP-phase formation initiated at lower strain levels. Compared with tensile deformation at room temperature, the higher flow stress and the cryogenic environment during cryogenic tensile deformation led to denser twin systems, more extensive Lomer-Cottrell (L–C) lock networks, and greater HCP-phase formation, resulting in exceptional strain-hardening capacity and ultrahigh ductility. These findings provide a comprehensive understanding of the microstructural evolution and strengthening mechanisms in HEAs under extreme conditions, offering a promising processing route for producing materials for aerospace applications with high strength and ductility.