Stress-induced phase transformations in a hot-deformed two-phase (α2+γ) TiAl alloy

Abstract

Stress-induced γ→α2, α2→γ and γ→9R phase transformations in a hot-deformed Ti–45 at% Al–10 at% Nb alloy have been investigated using high-resolution transmission electron microscopy. The γ→α2 phase transformation is an interface-related process. The interfacial superdislocations emitted from the misoriented semicoherent α2–γ interface react with each other or with the moving dislocations in the γ phase, resulting in the formation of the α2 phase. The nucleation of the α2→γ phase transformation takes place either at the α2–γ interfaces or at the stacking faults on the basal plane of α2 phase, and the growth of γ plate is accomplished by the moving of a/6〈1 0 1 0〉 Shockley partials on alternate basal plane (0 0 0 1)α2. The 9R structure was usually found to form at incoherent twin or pseudotwin boundaries. During deformation the interfacial Shockley partial dislocations of these incoherent twin and pseudotwin boundaries may glide on (1 1 1)γ planes into the matrix, resulting in the formation of 9R structure. The interfaces (including α2–γ and γ–γ interfaces) as well as the crystallographic orientation relationship between the as-received or stress-induced α2, γ and 9R phase have been analysed. The mechanisms for the stress-induced γ→α2, α2→γ and γ→9R phase transformations were also discussed. © 1998 Chapman & Hall