Ultra-high strength of Ir-Hf-Nb ternary alloys with an fcc/L1 ₂ 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/L1 ₂ 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/L1 ₂ structure the fcc phase always shows higher strength than the L1 ₂ phase, at both room and high temperatures. A dual-phase fcc/L1 ₂ microstructure with different fcc to L1 ₂ 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 L1 ₂ 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 L1 ₂ phases. The failure mode of the Ir-Hf-Nb ternary alloys is by debonding of the grain boundary.