Solid solution hardening and precipitation hardening of α₂-Ti₃Al in Ti-Al-Nb alloys

Microstructure and mechanical properties of Ti-Al-Nb-based alloys were investigated as they are important in the development of new generation high temperature Ti alloys due to good oxidation resistance. The alloy compositions Ti-15Al-2Nb (to obtain α₂ precipitates) and Ti-15Al-2Nb-0.5Si (at%) (to obtain solid-solution hardening by Si) were selected. Heat treatment at 1000°C after forging and rolling at 900°C transformed the β phase to α phase and α′ martensite during cooling. By heat treatment at 900°C, a single α phase was obtained in both alloys. The α₂ phase was formed by aging treatment at 600 and 700°C of Ti-15Al-2Nb and at 600, 700, and 800°C of Ti-15Al-2Nb-0.5Si. Compressive strength was investigated for samples heat treated at 900°C with a single α phase and those aged at 600, 700, 800°C with α₂ precipitates. The solid solution hardening effect of Si was found at all test temperatures. Precipitation hardening effect by α₂ precipitates was also observed in both alloys and the effect improved with Si addition. Peaks of precipitation hardening were obtained at 300°C in both alloys; the precipitation hardening effect decreased with increase of test temperature above 300°C. It is suggested that a shearing mechanism occurs up to 300°C, which changes to a bypass mechanism above 450°C.