Mechanical Instability and Tensile Properties of TiZrHfNbTa High Entropy Alloy at Cryogenic Temperatures

Abstract

The equiatomic TiZrHfNbTa alloy is one of the few refractory high entropy alloys that exhibit tensile ductility at room temperature, the deformation of which is only dominated by dislocation slip. Here, we observed the activation of {112} nano-twinning accompanied by the deformation induced body-centered cubic structure (BCC) to non-closed packed hexagonal ω phase transformation along with the dislocation slip during tensile deformation at cryogenic temperatures, which indicates the intrinsic mechanical instability of the single-phase BCC TiZrHfNbTa solid solution. The alloy maintains a high tensile elongation of 20.8% while the yield strength increases significantly up to 1,549 MPa as the temperature is decreased from 277 K to 77 K, without obvious ductile to brittle transition. This exceptional combination of high strength and high ductility at cryogenic temperatures can be interpreted by considering the synergistic effect of screw dislocation glide, {112} mechanical twinning and BCC→ω phase transformation. These results provide new insights on our understanding of the refractory TiZrHfNbTa-based alloys and extend their application to cryogenic temperatures at the extreme service conditions like aerospace, marine shipbuilding and natural gas industries, albeit they are promising for high-temperature application.