Strong yet strain-hardenable equiatomic CoCrFeMnNi high-entropy alloys by dynamic heterostructuring

Abstract

Heterostructuring gives solutions to achieve better mechanical performance of metallic materials. For instance, partial static recrystallization by short-term annealing induces bimodal grain size distribution. Distinct from the well-studied ‘static heterostructuring’ by cold rolling followed by annealing, this study introduces ‘dynamic heterostructuring’ by hot rolling. Dynamic recrystallization (DRX) during the hot rolling as a softening mechanism leaves room for dislocation accumulation. With the expectations of enhanced strain hardening of the as-rolled materials with the aid of the DRX, we monitored tensile responses of the as-hot-rolled equiatomic CoCrFeMnNi high-entropy alloys (HEAs). Among the hot rolling temperatures from 800 ℃ to 900 ℃ with a thickness reduction ratio of 78.6% for the HEAs, the hot rolling at 850 °C results in a bimodal grain size distribution, i.e., partial DRX. It comprises relatively fine DRXed grains and coarse unrecrystallized grains: the latter is harder than the DRXed ones with relatively low dislocation density. The microstructural heterogeneity results in uniform elongation of ∼13% due to the accumulation of dislocations at the grain boundaries inside the soft DRXed grains. This endows the as-hot-rolled single-phase HEAs with strain hardenability without the aid of phase transformation, deformation twinning, or precipitates. In addition, the hot-rolled HEAs with high dislocation density have doubled yield stresses of those of conventionally cold-rolled and annealed counterparts. Considering the industrial benefits of the simplified thermo-mechanical control processes, this work extends the academic and practical value of DRX by dynamic heterostructuring through annealing-free hot rolling.