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

Abstract

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 α 2/ 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 α 2/ γ interfaces is the misorientation from the conventional α 2/ 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 α 2/ 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 α 2/ γ interfaces can be formed by reactions between the matrix and interfacial dislocations. This type of semi-coherent α 2/ γ interface is formed in order to accommodate the relative rotation of α 2 and γ plates resulting from their heterogeneous deformation behavior. In the sharply bent lamellae, there even occur largely misoriented, non-coherent α 2/ 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 α 2/ 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.