Unique deformation behavior and microstructure evolution in high temperature processing of HfNbTaTiZr refractory high entropy alloy

Hot-deformation behaviors of an equiatomic HfNbTaTiZr refractory high entropy alloy having BCC structure were investigated. Uniaxial compression was carried out at different temperatures ranging from 1000°C to 1200°C and various strain rates from 10-4 s-1 to 10-2 s-1. Stress-strain curves indicated distinctive sharp drops at yielding followed by continuous decreases of flow stress. The sharp drop of the stress was explained from a viewpoint of dislocation pinning possibly by any solute atom(s). Flow stress analysis was carried out by using Arrhenius type power law relationship. The apparent activation energy (Q) for hot-deformation estimated from the flow stress data was 124 ~ 113 kJmol-1 over the entire range of strain. The flow stress analysis indicated high strain rate sensitivity (m) of the flow stress (m ~ 0.35). The relatively high m value and dynamically recrystallized (DRX) microstructures composed of coarse unrecrystallized regions and fine DRX grains with necklace morphologies indicated a possibility of grain boundary sliding (GBS) in the fine-grained DRX regions. The fine DRX grain sizes indicated inhibited grain growth after DRX, possibly because of the sluggish diffusion in the HfNbTaTiZr alloy. Preferred orientations of and parallel to the compression axis in the unrecrystallized coarse regions indicated normal dislocation slips in the BCC crystal, while weak and near-random texture observed in the fine-grained DRX regions supported the occurrence of GBS.