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

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 twophase 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.