Thermal stability of Ti-44Al-8Nb-1B alloy

A fully lamellar γ-TiAl alloy Ti-44Al-8Nb-1B was exposed to the temperature of 700 °C in air for up to 5000 h. The changes in microstructure were investigated using scanning and transmission electron microscopies. The fine-grained lamellar microstructure produced by ingot-casting and hot-isostatic pressing is found to be thermodynamically unstable. Metastable α2 lamellae readily decompose and three types of phase transformation occur during the exposure. The first is α2 → γ, occurring with the parallel decomposition of coarse α2 laths. The second is α2 → B2(ω), occurring along single α2 laths. The third is α2 + γ → B2(ω), occurring on decomposing α2 + γ lamellae packets. The B2(ω) formed is found to be not very different from their parent phase(s) in composition. A widespread precipitation of B2(ω) therefore proceeds, causing segregation both at colony boundaries and inside lamellar colonies. The volume fraction of B2(ω) is almost doubled: from 3% in the as-cast condition to 5.8% after 5000-h exposure.