Microstructural stability of a cast Ti–45.2Al–2W–0.6Si–0.7B alloy at temperatures 973–1073 K

Microstructural stability of a cast intermetallic Ti–45.2Al–2W–0.6Si–0.7B (at.%) alloy after ageing in the temperature range from 973 to 1073 K up to 10,000 h was studied. The microstructure of the alloy consists of lamellar and TiAl-rich regions. Lamellar regions consist of fine lamellae of γ(TiAl) and α2(Ti3Al) phases, fine B2 and Ti5Si3 precipitates on the lamellar interfaces. Fine α2 lamellae are partially decomposed before ageing. Small volume fraction of coarse B2 particles and few ribbon-like borides were found within lamellar regions. TiAl-rich regions contain coarse α2 lamellae, rod-like B2 particles, small volume fraction of coarse Ti5Si3 precipitates and ribbon-like borides. The as-received microstructure of the alloy is unstable during long-term ageing at temperatures ranging from 973 to 1073 K. The α2 phase in the lamellar and TiAl-rich regions transforms to the γ phase and fine needle-like B2 precipitates. The microstructural instabilities lead to softening of the alloy. The microhardness decrease is measured to be faster in the lamellar regions compared to that in the TiAl-rich regions.