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 gamma -based TiAl alloy containing W and Si has been studied. The starring microstructure of a cast and heat-treated commercial gamma -based TiAl alloy component has been interpreted in terms of the phase changes between an a-Ti,Al structure at high temperatures, an alpha (3) + gamma decomposition at intermediate temperatures, and the change to a B2 + gamma microstructure at lower temperatures. The fine lamellar microstructure produced after fairly rapid cooling from high temperatures consists mostly of alpha (2) + gamma lamellae but transforms surprisingly rapidly at lower temperatures (950-1000 degreesC) into a globular structure composed of a gamma matrix and equiaxed particles of B2 phase, occasionally of remnant alpha (2) phase or of Ti5Si3 silicide. Material strength, as assessed by microhardness, fails rapidly as the initial alpha (2) + gamma lamellar microstructure transforms to the globular one becoming an essentially gamma + B2 mixture. (C) 2001 Elsevier Science Ltd. All rights reserved.