TEM STUDY ON LAMELLAR MICROSTRUCTURE AND ALPHA(2) GAMMA INTERFACIAL STRUCTURE IN A HOT-DEFORMED 2-PHASE GAMMA-TIAL-BASED ALLOY/

The hot deformation behavior of the lamellar microstructure in a Ti-45 Al-8Nb-2.5Mn-0.05B alloy has been studied by transmission electron mic roscopy (TEM). Numerous subgrain boundaries and two types of deformati on twins were frequently observed in the hot-deformation-induced lamel lae. The occurrence of nonequilibrium semi-coherent alpha(2)/gamma int erfaces is an important microstructural feature of hot-deformed lamell ar microstructure in a similar Ti-45Al-10Nb-2.5Mn-0.05B alloy. The dis tinctiveness of these alpha(2)/gamma interfaces is the misorientation from the conventional alpha(2)/gamma orientation relationship{111}(y) parallel to {0001}(alpha 2), <1 (1) over bar 0<(gamma) parallel to <11 (2) over bar 0>(alpha 2), as well as the asymmetry in the sense of in terfacial boundary planes deviating from the equilibrium atomic planes (111)(gamma) or (0001)(alpha 2). Numerous interfacial ledges containi ng 1/3[111] Frank partial dislocations exist in this nonequilibrium se mi-coherent alpha(2)/gamma interface. The analyses point out that, in the hot-deformation-induced bent lamellae, the 1/3[111] Frank partial dislocations in the ledges of alpha(2)/gamma interfaces can be formed by reactions between the matrix and interfacial dislocations. This typ e of semi-coherent alpha(2)/gamma interface is formed in order to acco mmodate the relative rotation of alpha(2) and gamma plates resulting f rom their heterogeneous deformation behavior. In the sharply bent lame llae, there even occur largely misoriented, non-coherent alpha(2)/gamm a interfaces with the {111}(gamma) plane no longer being parallel to t he {0001}(alpha 2) plane, resulting from the lamellae bending/kinking during heavy deformation. At these nonequilibrium, non-coherent alpha( 2)/gamma interfaces with high interfacial strain energy, the T(Q) defo rmation twins, being inclined to the lamellar interface, are observed to be preferentially formed. The process is favored by the localized s tress field of the interfacial misfit dislocations. (C) 1998 Elsevier Science S.A. All rights reserved.