The local chemical composition, and the atomic structures of TiAl/TiAl lamellar boundaries in binary and ternary polysynthetically twinned materials, were investigated by using a transmission electron microscope that was equipped with a Schottky-type field emission gun. Ordered twin boundaries, on which 2 neighboring TiAl lamellae were related by a 180 rotation about the (111) lamellar plane normal, were atomically coherent and the local chemical composition at the boundary was essentially the same as that of the bulk TiAl lamellae. On the other hand, lattice mismatch and compositional modulation were observed on pseudo-twin (60 rotation) and 120 rotational boundaries. These 2 types of boundary in binary, Mn-containing and Nb-bearing polysynthetically twinned crystals were enriched with Ti; as compared with the bulk of TiAl lamellae. Those in Cr-, Mo- and Ta-containing crystals were enriched not only with Ti but also with the corresponding ternary element. The occurrence of such compositional variations at pseudo-twin and 120 rotational boundaries was explained in terms of the relaxation of mismatch strains at these boundaries.

H.Inui, K.Kishida, M.Kobayashi, M.Yamaguchi, M.Kawasaki, K.Ibe: Philosophical Magazine A, 1996, 74[2], 451-64