Microstructure, mechanical properties and oxidation behavior of NbTaTi and NbTaZr refractory alloys

Abstract

This study reports the microstructure, phase composition, mechanical properties and oxidation behavior of NbTaTi and NbTaZr ternary equiatomic alloys. These ternary components, together with earlier reported NbTiZr and MoNbTi, are commonly observed in refractory high entropy alloys (RHEAs) with a single-phase BCC or multi-phase crystal structures. Therefore, the reported information about these ternary alloys is thought to be useful for the development of advanced RHEAs. While NbTiZr and MoNbTi are single-phase BCC structures, NbTaTi and NbTaZr are multi-phase alloys. NbTaTi consists of a BCC matrix and discontinuous, Ti-rich precipitates with BCC and FCC crystal structures. NbTaZr consists of a co-continuous spinodal mixture of two BCC phases, one of which is rich in Ta and Nb and another is rich in Zr, and it also contains a Zr-rich HCP phase in the form of coarse grain-boundary particles. NbTaTi has compression yield stress of 724 MPa at 25 °C, 268 MPa at 800 °C and 129 MPa at 1200 °C. NbTaZr is stronger, with the compression yield stress of 1027 MPa at 25 °C, 530 MPa at 800 °C and 183 MPa at 1200 °C. Both alloys show rapid oxidation with formation of complex oxides during holding at 1200 °C.