Compositional variations at TiAl-TiAl lamellar boundaries in binary and some ternary polysynthetically twinned Ti-Al

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Abstract
Local chemical compositions as well as their atomistic structures of TiAl-TiAl lamellar boundaries in binary and some ternary polysynthetically twinned (PST) Ti-Al have been investigated using a transmission electron microscope equipped with a Schottky-type field emission gun. Ordered twin boundaries, on which two neighbouring TiAl lamellae are related by a 180° rotation about the (111) lamellar plane normal, are atomistically coherent and the local chemical composition at the boundaries is virtually the same as that of the bulk TiAl lamellae. On the other hand, lattice mismatches and compositional modulations are observed on pseudo-twin (60° rotation) and 120°-rotational boundaries. These two types of boundary in binary, Mn-bearing and Nb-bearing PST crystals are enriched with Ti with respect to the bulk of TiAl lamellae while those in Cr-, Mo- and Ta-bearing crystals are 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 is discussed in terms of the relaxation of mismatch strains at these boundaries.