Nanotwin-assisted dynamic recrystallization achieves high strength and ductility in nanocrystalline CrMnFeCoNi high entropy alloy

Abstract

Nanocrystalline (NC) high entropy alloys (HEAs) possess superior strength but often exhibit poor ductility due to suppressed dislocation activity and limited strain hardening. Dynamic recrystallization (DRX) has been recognized as an effective mechanism to improve plasticity and strength in coarse-grained materials. However, its activation in NC HEAs at room temperature is typically constrained by insufficient thermal activation and the requirement for extremely high local strains. In this work, a high density of pre-existing nanotwins was introduced into NC CrMnFeCoNi HEA to facilitate DRX through a nanotwin-assisted dynamic recrystallization (ntDRX) mechanism during deformation. The alloy, fabricated by magnetron sputtering, exhibits a single-phase face-centered cubic (FCC) structure with columnar grains of approximately 60 nm. Micropillar compression tests demonstrate a high yield strength of 2.3 GPa and a compressive strain exceeding 40%. During deformation, in addition to dislocation slip and grain boundary (GB) activities, DRX is activated within shear bands, leading to the formation of equiaxed nanograins. These newly formed grains increase interface density, impede dislocation motion, and elevate the flow stress to 2.75 GPa. Additionally, the newly formed GBs of equiaxed grains facilitate GB-mediated deformation at large strains, improving plasticity and suppressing shear localization. Furthermore, tensile experiments reveal two interconnected ntDRX mechanisms. One involves dislocation accumulation at twin boundaries (TBs) leading to subgrain formation through dislocation rearrangement and its evolution into high-angle grain boundaries (HAGBs), while the other involves the direct transformation of coherent TBs into HAGBs through dislocation interactions. This study advances the understanding of room-temperature DRX in NC HEAs and highlights nanotwin engineering as a promising strategy for optimizing mechanical performance.