Ultra-high strength of Ir–Hf–Nb ternary alloys with an fcc/L12 microstructure at 1950 °C

This paper investigates the strength response of Ir-xHf-yNb alloys (x = 3, 4, 5, 8, 9, 10 at.%, and y = 5, 6, 7, 11, 13, 15 at.%) with a dual-phase fcc/L12 microstructure at room and high temperatures. The experimental strength at 1950 °C was compared with that obtained by the rule of mixture. The results showed that in the fcc/L12 structure the fcc phase always shows higher strength than the L12 phase, at both room and high temperatures. A dual-phase fcc/L12 microstructure with different fcc to L12 fraction ratio formed when Hf and Nb contents in the Ir-xHf-yNb ternary alloys were larger than 3 at.% and 5 at.%, respectively, and the pure L12 microstructure was obtained at 10 at.% Hf and 15 at.% Nb. The Ir-5Hf-7Nb alloy, possessing an fcc-dominant microstructure in which the optimization fcc fraction is about 78%, has an outstanding 0.2% yield compressive strength of 270 MPa even at 1950 °C. This ultra-high strength is attributed to strong interface hardening by the large lattice misfit between the fcc and L12 phases. The failure mode of the Ir–Hf–Nb ternary alloys is by debonding of the grain boundary.