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

Abstract

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 where BCC single phase was stable. Stress-strain curves indicated distinctive sharp drops at yielding followed by continuous decreases of flow stress. Interrupted compression tests indicated that the sharp drop of the stress was explained from a viewpoint of dislocation unlocking possibly by solute atom(s) atmosphere or short-range ordering. 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 258–232 kJ mol−1 over the entire range of strain. The flow stress analysis indicated high strain rate sensitivity (m) of the flow stress (m ∼ 0.33). 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, which might be due to sluggish diffusion in the HfNbTaTiZr alloy. Preferred orientations of <001> and <111> 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.