The effect of heat treatments on the microstructural stability of the intermetallic Ti–46.5Al–2W–0.5Si

The effect of heat treatment on the microstructure of a γ-based TiAl alloy containing W and Si has been studied. The starting microstructure of a cast and heat-treated commercial γ-based TiAl alloy component has been interpreted in terms of the phase changes between an α-Ti3Al structure at high temperatures, an α3+γ decomposition at intermediate temperatures, and the change to a B2+γ microstructure at lower temperatures. The fine lamellar microstructure produced after fairly rapid cooling from high temperatures consists mostly of α2+γ lamellae but transforms surprisingly rapidly at lower temperatures (950–1000°C) into a globular structure composed of a γ matrix and equiaxed particles of B2 phase, occasionally of remnant α2 phase or of Ti5Si3 silicide. Material strength, as assessed by microhardness, falls rapidly as the initial α2+γ lamellar microstructure transforms to the globular one becoming an essentially γ+B2 mixture.