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

Abstract

The TiAl-based alloy Ti–46Al–5Nb–1W has been exposed at the temperature of 700 °C for up to 5000 h in air. The decomposition processes of all major phases in this alloy: γ, α 2, β and ω, have been investigated in detail by transmission electron microscopy and have been correlated with tensile properties. It is found that considerable amounts of β phase are retained in the as-cast ingot condition and these readily decompose into the ω phase. As the exposure in air proceeds, along with the coarsening of ω particles, a reverse transformation from ω to β can occur at 700 °C. During exposure, α 2 lamellae begin to decompose significantly after 3000 and 5000 h exposures. Some break up into small sized α 2+γ grains through a ‘perpendicular decomposition’ mode, whilst γ 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.