Impact of uneven distribution of grain characteristics on yield strength and martensitic transformation of as-hot-rolled medium-entropy alloys

Abstract

As-hot-rolled medium-entropy alloys (MEAs) with unevenly distributed grain sizes of face-centered cubic grains exhibit better yield strength without uniform elongation loss compared to cold-rolled and annealed ones. Successive operation of dynamic recrystallization (DRX) during several hot rolling passes leads to a wide range of grain sizes from submicrons to tens of micrometers due to the grain growth after nucleation: early recrystallized grains are coarser than recently recrystallized ones. Not only the grain size but internal dislocation density of the recently recrystallized grain is low. During the tensile deformation of the hot-rolled MEAs at –196 °C, dislocation pile-ups in the relatively soft and fine DRX grains enhance yield stress and hetero-deformation-induced strain hardening. Thanks to the enhanced yield stress of the as-hot-rolled MEAs, stress-induced martensitic transformation easily occurs. Notably, partially DRXed MEAs hot-rolled at 800 °C have lower yield stress than fully DRXed ones, hot-rolled at 900 and 1000 °C. This is attributed to the softening effect of the stress-induced body-centered cubic martensitic transformation in unrecrystallized coarse grains prior to the yielding, which lowers the yield stress of the partially DRXed ones. After yielding, the martensitic transformation facilitates strain hardening and early necking is precluded. This study presents a fresh outlook on the uneven distribution of grain sizes by hot rolling beneficial to mechanical responses of uniform elongation of ∼45% despite the as-rolled states with an advantage of simplified thermo-mechanical processes.