Engineering the alpha two phase morphology in gamma titanium aluminide based alloys

Conventionally processed, fully lamellar, TiAl based alloys ordinarily possess α₂ laths having a thickness on the order of hundreds of nanometers. Polycrystalline samples, having compositions between Ti-42Al and Ti-46Al, can be solution heat treated in the a phase field, and then rapidly quenched, to produce a microstructure that consists entirely of supersaturated α ₂ phase. Upon aging the supersaturated solid solution, in the α ₂ + γ phase field, a fully lamellar structure results. In contrast to the conventionally processed fully lamellar TiAl based alloys, the laths resulting from the aging treatment have a thickness on the order of tens of nanometers. Additionally, this nanoscale microstructure consists of rigidly alternating laths of γ and α₂. A transformation mechanism from the supersaturated α₂ to the ultra-fine lamellar structure, via nucleation and glide of dislocation loops is proposed. Consideration is also given to the question of whether or not these ultra-fine lamellar structures can aid slip transmission between the two phases.; The gettering of interstitial contaminants, by the α₂ laths in fully lamellar TiAl based alloys, has been shown to be responsible for the enhanced slip activity observed in the γ laths of these alloys. However, absorption of the impurities from the γ, limits the number of active slip systems that can operate in the α₂ phase. The result is that the α₂, an already brittle phase, becomes even less likely to undergo plastic deformation. The brittle α ₂ laths constrain the γ phase, and the overall ductility of the fully lamellar material is reduced. By suitably engineering the sequence of solid state phase transformations leading to microstructural development in TiAl based alloys, it is possible to employ the gettering benefits provided by the α₂ phase, while eliminating the constraints imposed by it on the γ phase. The addition of small amounts of the ternary alloying element, Ta, makes it possible to subject the material to a γ solution heat treatment. Upon cooling from the single phase γ region, multiple variants of the α₂ phase precipitate directly from the γ phase allowing for the formation of discrete α₂ precipitates in the matrix.