High-temperature mechanical properties and oxidation behavior of Hf-27Ta and Hf-21Ta-21X (X is Nb, Mo or W) alloys

Abstract The microstructure, mechanical properties (at 25 °C, 1000 °C, 1200 °C, 1400 °C and 1600 °C) and oxidation behavior (at 1600 °C) of a binary Hf-27%Ta alloy and three ternary alloys, Hf-21%Mo-21%Ta, Hf-21%Nb-21%Ta and Hf-21%Ta-21%W (compositions are in atomic percent) are reported. The alloys were produced by vacuum arc melting, hot isostatically pressed under a high-purity argon pressure of 207 MPa at 1400 °C for 3 h and slow cooled to room temperature. The Hf-27Ta alloy had a microstructure consisting of coarse primary HCP particles and a monotectoidally transformed matrix consisting of a nano-lamellar mixture of the Ta-rich BCC and Hf-rich HCP phases. The Hf-21Mo-21Ta alloy consisted of a BCC matrix phase and HCP precipitates. Hf-21Nb-21Ta had a microstructure consisting of a BCC matrix phase, spherical nano-precipitates inside the matrix grains and two-phase BCC lamellar regions at grain boundaries, with no HCP phase detected. The Hf-21Ta-21W alloy had a three-phase structure consisting of a Hf-rich HCP matrix, W-rich cubic Laves phase and monotectoidally transformed regions consisting of nano-lamellae of Ta-rich BCC and Hf-rich HCP phases. The alloys had limited ductility at room temperature but they all were well workable at T ≥ 1000 °C. Among the studied alloys, the binary Hf-27Ta was the strongest at room temperature and the Hf-21Mo-21Ta was the strongest at high temperatures. The oxidation kinetics of Hf-27Ta, Hf-21Mo-21Ta and Hf-21 W-21Ta were similar, with 30–40 mg/cm2 mass gain per 1 h holding at 1600 °C. The Hf-21Nb-21Ta alloy exhibited much higher (~230 mg/cm2/h) mass gains at 1600 °C. The ternary alloys all showed evidence of the formation of gaseous and/or liquid species in the oxide scales.