TEM study on lamellar microstructure and α₂/γ interfacial structure in a hot-deformed two-phase γ-TiAl-based alloy

The hot deformation behavior of the lamellar microstructure in a Ti-45Al-8Nb-2.5Mn-0.05B alloy has been studied by transmission electron microscopy (TEM). Numerous subgrain boundaries and two types of deformation twins were frequently observed in the hot-deformation-induced lamellae. The occurrence of nonequilibrium semi-coherent α₂/γ interfaces is an important microstructural feature of hot-deformed lamellar microstructure in a similar Ti-45Al-10Nb-2.5Mn-0.05B alloy. The distinctiveness of these α₂/γ interfaces is the misorientation from the conventional α₂/γ orientation relationship {111}_γ{0001}_α2, 〈11̄0〈_γ〈112̄0〉 _α2, as well as the asymmetry in the sense of interfacial boundary planes deviating from the equilibrium atomic planes (111)_γ or (0001)_α2. Numerous interfacial ledges containing 1/3[111] Frank partial dislocations exist in this nonequilibrium semi-coherent α₂/γ interface. The analyses point out that, in the hot-deformation-induced bent lamellae, the 1/3[111] Frank partial dislocations in the ledges of α₂/γ interfaces can be formed by reactions between the matrix and interfacial dislocations. This type of semi-coherent α₂/γ interface is formed in order to accommodate the relative rotation of α₂ and γ plates resulting from their heterogeneous deformation behavior. In the sharply bent lamellae, there even occur largely misoriented, non-coherent α₂/γ interfaces with the {111}_γ plane no longer being parallel to the {0001}_α2 plane, resulting from the lamellae bending/kinking during heavy deformation. At these nonequilibrium, non-coherent α₂/γ interfaces with high interfacial strain energy, the T(Q) deformation twins, being inclined to the lamellar interface, are observed to be preferentially formed. The process is favored by the localized stress field of the interfacial misfit dislocations.