Transformations in α<SUB>2</SUB>+γ titanium aluminide alloys containing molybdenum: Part I. Solidification behavior

The solidification structures of 12 alloys in the Ti-Al-Mo system with Al contents ranging from 44 to 50 at. pct and Mo contents ranging from 2 to 6 at. pct have been characterized metallographically and, for composition gradients, by electron probe microanalysis. All alloys solidify dendritically through sequences that fall into four distinct categories: alloys that solidify completely into the β phase field, alloys that solidify first into an L+β phase field and finally into an α+β phase field through an L+β+γ region, alloys that solidify first into a L+β phase field and finally form α+β+γ structures, and alloys that solidify into a L+α phase field. Postsolidification transformations occur as consequence of transitions from the high-temperature β, α+β, and α+γ phase fields to low-temperature α+β+γ or β+γ region. A variety of phase distributions result, such as Widmanstätten α2+B2 structures from the α→α+β reaction followed by ordering, lamellar α2+γ structures from the α→α+γ reaction followed by ordering of the α phase, eutectoid B2+γ structures from the α→B2+γ reaction, and platelike γ structures from the β→β+γ reaction. The former are observed in Al lean alloys, while the latter are present in Mo rich alloys