Temperature-dependent tensile behavior of the HfNbTaTiZr multi-principal element alloy

HfNbTaTiZr is a refractory multi-principal element alloy (RMPEA) that exhibits high strength combined with good ductility at room temperature. An insufficient understanding of RMPEA tensile behavior, however, limits their ability to be thermo-mechanically processed and optimized for engineering applications. In this investigation, uniaxial tensile tests were conducted at room temperature, 800 °C, and 1200 °C under vacuum on sheet materials fabricated by hot and cold rolling combined with annealing treatments. At testing temperatures of 25 °C and 1200 °C, the HfNbTaTiZr alloy exhibited high tensile ductility (16.0% and 121.0%, respectively). At 800 °C, the HfNbTaTiZr alloy exhibited significantly decreased ductility (2.0%) compared to room temperature, and coincided with a transition from ductile to intergranular fracture. SEM and TEM studies revealed precipitation reactions at grain boundaries following thermal exposure and during tensile testing at 800 °C, resulting in tantalum and niobium-rich BCC precipitates, and finer-scale hafnium and zirconium-rich precipitates. After annealing at 800 °C for 100 h, nanoindentation revealed soft and stiff tantalum and niobium-rich BCC precipitates located at grain boundaries. Thermodynamic calculations corroborated the existence of two BCC phases and predicted an HCP phase to be stable between 703–804 °C, which was consistent with TEM observations. At 1200 °C, high tensile ductility resulted from the combined effects of dynamic recovery and partial dynamic recrystallization. These experimental and computational results underscore a need to understand the high temperature phase equilibria and complex deformation behaviors of refractory MPEs so that their microstructure and mechanical performance can be better controlled across the elevated temperature ranges of interest.