ORDERED DOMAINS IN TIAL COEXISTING WITH TI3AL IN THE LAMELLAR STRUCTURE OF TI-RICH TIAL COMPOUNDS

The morphology and distribution of ordered domains and their relative orientation in the TiAl phase coexisting with the Ti3Al phase in the lamellar structure of Ti-rich TiAl compounds have been studied by transmission electron microscopy. Six different types of ordered domain exist in the TiAl phase because of the orientation relationship between the TiAl and Ti3Al phases, (111)TiAl//(0001)Ti2Al and [110]TiAl//[1120]Ti3Al. The [110] direction in all the types of TiAl ordered domain is exactly parallel to the [1120BAR] direction of Ti3Al while [011BAR] directions are rotated by 0.2-0.3-degrees on the (111) lamellar planes with respect to the [1120BAR] direction of Ti3Al. The TiAl ordered domains exist not only across the lamellar boundaries but also within each of TiAl lamellae. The lamellar domain boundaries, that is the boundaries between the two neighbouring TiAl lamellae, are flat and parallel to both (111)TiAl and (0001)Ti3Al, while the domain boundaries existing within each of TiAl lamellae do not have any preference for certain crystallographic planes. All five possible orientation relationships have been found to occur between two neighbouring domains separated by the lamellar domain boundaries while only two orientation relationships in which the stacking sequence of (111)TiAl is maintained occur for the domain boundaries in TiAl lamellae.