Deformation behavior of HfNbTaTiZr high entropy alloy singe crystals and polycrystals

Deformation behavior of equiatomic HfNbTaTiZr high entropy alloy single crystals and polycrystals were investigated. The single crystalline specimens could be obtained from the coarse-grained polycrystals annealed just below the melting point. 1/2<111> screw dislocations played an important role in the deformation behavior, similar to body centered cubic (bcc) metals. Moreover, a yield stress anomaly was found to appear in the alloys solutionized at 1473 K. The yield stress decreased rapidly with increasing temperature up to 673 K, while the stress increased at 873 K. Further increase in temperature resulted in a decrease in yield stress. It is also noted that the microstructure of the alloys depended strongly on annealing temperature. At 773–1073 K, phase separation into two bcc phase took place, while the hexagonal close-packed (hcp) phase was precipitated at 773–973 K. The ω phase which is typical in β-titanium alloys was also observed at 673 K. The strain-rate sensitivity of the deformation behavior and micro-Vickers hardness after the heat treatment suggest that the dynamic precipitation of the hcp phase is responsible for the yield stress anomaly.