Deformation induced hcp nano-lamella and its size effect on the strengthening in a CoCrNi medium-entropy alloy

• Hcp nano-lamellae were induced by cryogenic dynamic deformation in a CoCrNi MEA. • Size effect of hcp nano-lamellae in the MEA was studied by MD for the first time. • Sample with small-spaced hcp lamellae is even stronger than pure hard hcp phase. • Strength increases with increasing/decreasing width/spacing of hcp nano-lamellae. • More interfaces provide stronger resistance for dislocation slip and transmission. Deformation-induced hcp nano-lamellae with various widths and interspacings were observed in the CoCrNi medium-entropy alloy (MEA) under high strain rate and cryogenic temperature in the present study. Higher hardness was found in the cryogenic-deformed samples compared to the room temperature-deformed samples without hcp phase. Then, size effects of embedded hcp nano-lamellae on the tensile behaviors in the fcc CoCrNi MEA were investigated by molecular dynamics simulations. The overall strengthening was found to have two components: phase strengthening and extra interface strengthening, and the interface strengthening was observed to be always stronger than the phase strengthening. Both overall strengthening and interface strengthening were found to increase with increasing width and decreasing interspacing of embedded hcp nano-lamellae. The samples with small spaced hcp nano-lamellae are even stronger than the pure hard hcp phase due to the extra interface strengthening. The samples with larger width of embedded hcp nano-lamellae can provide stronger resistance for dislocation slip and transmission. Nanotwins were observed to be formed in the embedded hcp nano-lamellae. Higher density of phase boundaries and newly formed twin boundaries can provide more barriers for dislocation glide in the other slip systems, resulting in higher strength for samples with smaller interspacing.