Thermal stability of Ti-44Al-4Nb-4Zr-0.2Si-1B alloy

A near lamellar γ−TiAl alloy Ti-44Al-4Nb-4Zr-0.2Si-1B was ingot cast and exposed at 700 °C for up to 10,000 h. The combined addition of Nb and Zr rather than Nb alone has a positive effect on stabilization of α2 lamellae. Compared to alloy Ti-44Al-8Nb-1B, less amount of α2 dissolved and less amount of β (B2 + ω) formed in the alloy after the same exposure scheme. However, the exposure-induced embrittlement is still severe in this alloy. The ductility after 10,000-h exposure is only 1/4 of the original value. The exposure-induced embrittlement is attributed to the profound changes in prior β regions: the precipitation of fine D88-ω particles in the B82-ω matrix; the prevalent formation of γ grains inside the prior β regions; and the widespread precipitation of silicide particles at cell interfaces and β grain boundaries. The tensile strengths remain essentially unchanged while fatigue limit increased significantly. The mechanism behind the microstructural changes and mechanical properties is discussed in detail.