The structure of antiphase domain boundaries in massively transformed gamma-TiAl studied by high-resolution electron microscopy

Antiphase domain boundaries (APDBs) in the gamma-TiAl phase, which are formed by the alpha --> gamma massive transformation in a Ti-48at.% Al alloy, have been investigated using high-resolution electron microscopy. These APDBs were previously examined in detail by Zhang and Loretto using an electron microdiffraction method, and it was found that thin rotated domain (RD) walls are formed at the boundaries. In this work, the detailed structure of such a thin RD wall, in relation to that of the matrix, is clarified on an atomic scale. The interesting findings are firstly an antiphase relationship between the regions on either side of the thin RD and secondly a local lattice distortion in the thin RD wall region. From these facts, complex antiphase-boundary (APE) fringe contrasts in the massively transformed gamma-TiAl phase can be explained, which has been described by the additional displacement of Delta R to R = 1/2 [101] by Li and Loretto. The formation process of thin RD walls during the massive transformation is discussed in terms of the APE energy of the gamma-TiAl.