Microstructural stability of γ-based TiAl intermetallics containing β phase

High temperature microstructural changes are examined for two Ti-Al alloys, containing W or Nb-Mn additions. Changes of mechanical properties on annealing are related to the stability of the α/γ lamellae, which depends sensitively on whether β phase is stabilised at any temperature. W has a strong effect, first stabilising β phase, then inducing a β to γ change at lower temperatures. Its diffusivity is slow and any changes take place slowly. Mn and Nb have much milder effects and structural changes are similar to binary TiAl alloys. Property changes can still be rapid when fine scale α/γ lamellae are annealed, due to easy small-scale diffusion.