Deformation behavior of HfNbTaTiZr high entropy alloy singe crystals and polycrystals

Microstructure and deformation behavior of equiatomic HfNbTaTiZr high entropy alloy were investigated using single crystals and polycrystals of the alloy. It was found that the microstructure of the alloys depended strongly on annealing temperature. At 773–1073 K, phase separation into two bcc phases took place. In addition, the hexagonal close-packed (hcp) phase was precipitated at 773–973 K. The ω phase which is typical in β-Ti alloys was also observed at 673 K. We found for the first time that 1/2<111> screw dislocations played an important role in the deformation behavior, similar to body centered cubic (bcc) metals such as β-type Ti alloys. Moreover, a yield stress anomaly was found to appear in the solutionized alloys. The yield stress decreased rapidly with increasing deformation temperature up to 673 K, while the stress increased at 873 K. Further increase in temperature resulted in a decrease in yield stress. The strain-rate sensitivity of the deformation behavior and Vickers hardness after the heat treatment suggest that the dynamic precipitation of the hcp phase is responsible for the yield stress anomaly.