Precipitation behavior in Ag-modified L12–Al3Ti and L10–TiAl(Ag)

A detailed transmission electron microscopy (TEM) study has been made of the crystallographic characteristics of precipitation of L10–TiAl and D023–Al11Ti5 in Ag-modified L12–Al3Ti as well as the precipitation of L12–Al3Ti in L10–TiAl(Ag) matrix in Ti–Al–Ag ternary system. TEM observations revealed that L10–TiAl precipitates form a kinked plate surrounded by four {225} planes in L12 matrix. D023–Al11Ti5 precipitates form multi-domain structures which can be considered as twin related structures with the tetragonal axis of twin I parallel to [100] direction and that of twin II parallel to [010] direction of the L12 matrix. On comparison plate-like L12–Al3Ti precipitates form a kinked plate surrounded by four {112} planes in L10 matrix. The particular habit plane, orientation relationship and twin plane variants were carefully determined. It is found that these crystallographic and morphological features can be explained excellently by adopting the linear elastic theory developed primarily by Khachaturyan. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.