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

Abstract

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 α 2(Ti 3Al) phases, fine B2 and Ti 5Si 3 precipitates on the lamellar interfaces. Fine α 2 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 α 2 lamellae, rod-like B2 particles, small volume fraction of coarse Ti 5Si 3 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 α 2 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.