Formation of α phase in the massive and feathery γ-TiAl alloys during aging in the single α field

Precipitation of α phase in massive and feathery microstructures was studied during aging of a Ti-48 pct Al-2 pct W-0.5 pct Si alloy in the single α field. It was found that the α phase mainly precipitates along the γ-plate interfaces as laths in the feathery structure, while it nucleates at various sites in the massive structure in the form of idiomorphs and especially of plates. The γm → α reaction proceeds by the growth of pre-existing α precipitates and chiefly by the development of new α plates. The α plates are likely to originate from the splitting of unit dislocations into Shockley partials and to grow by the diffusional ledge mechanism, which shows both diffusional and shear character. During aging, the stacking faults (SFs) in the massive γ domains evolve into SF-shaped α precipitates through a transition γprime phase