Annealing hardening in cryo-rolled high-entropy alloys by belated deformation twinning

Deformed alloys commonly soften after annealing because of the decrease in defects. This work reports an annealing hardening that occurs in a cryo-rolled CoCrFeNi high-entropy alloy. This abnormal behavior is related to continuous twinning in nanoscale, which would have occurred in cryo-rolling but was belated until annealing was performed. This is because the stress accumulated in cryo-rolling was not enough to activate the reaction between dislocations and stacking faults, so as to require additional thermal activation (by annealing) for supplying the energy requirement for twinning. Due to the extra strengthening from stable twin boundaries, the annealed samples exhibit an ultrahigh yield strength close to 1.4 GPa (~6 times higher than the solid-solution counterpart), even after annealing for 24 h. These results confirm a possibility for introducing numerous stable nanotwins in bulk FCC metals to improve their mechanical properties.