Thermal stability of Ti–46Al–5Nb–1W alloy

The TiAl-based alloy Ti-46Al-5Nb-1W has been exposed at the temperature of 700 degreesC for up to 5000 h in air. The decomposition processes of all major phases in this alloy: gamma, alpha(2), beta and omega, have been investigated in detail by transmission electron microscopy and have been correlated with tensile properties. It is found that considerable amounts of beta phase are retained in the as-cast ingot condition and these readily decompose into the omega phase. As the exposure in air proceeds, along with the coarsening of omega particles, a reverse transformation from omega to beta can occur at 700 degreesC. During exposure, alpha(2), lamellae begin to decompose significantly after 3000 and 5000 h exposures. Some break up into small sized alpha(2), + gamma grains through a 'perpendicular decomposition' mode, whilst gamma lamellae remains essentially unchanged. Up to 5000-h exposure times do not produce any significant changes in proof stress and tensile strength in this alloy. A marginal decrease in tensile ductility measured at room temperature is, however, suggested. (C) 2002 Elsevier Science B.V. All rights reserved.