PNNL

Abstract

The tensile creep behavior of a vacuum arc-melted Nb45Ta25Ti15Hf15 refractory high entropy alloy was investigated in this study over a constant true stress range of 50-300 MPa and a temperature of 1173 K. Creep tests were carried out in both high vacuum (5×10-6 torr) and ultrahigh purity Ar gas to examine the environmental effect. The samples in vacuum exhibited power law behavior with a stress exponent of 4.1 and exceptional tensile creep ductility, whereas samples in Ar suffered significant embrittlement due to HfO2 formation at grain boundaries, which was exacerbated at low applied stresses where extended exposure to residual O2 gas resulted in more extensive brittle intergranular fracture. Phase decomposition occurred after long-term thermal exposure, where a second Hf-rich BCC phase formed predominantly at grain boundaries but did not cause embrittlement. Compared to equiatomic TaNbHfZrTi (Senkov alloy) and FCC MPEAs, Nb45Ta25Ti15Hf15 has superior creep resistance, especially at high applied stresses, while maintaining excellent creep ductility. Transmission electron microscopy revealed that creep deformation in Nb45Ta25Ti15Hf15 at 1173 K is controlled by kink collisions from screw dislocations that results in dipole drag for lower strain rates and jog drag for higher strain rates.