Crystallography and morphology of D023-Al11Ti5 precipitation in Ag-modified L12-Al3Ti

A detailed transmission electron microscopy (TEM) study has been made of the crystallographic characteristics of precipitation of D023-Al11Ti5 in Ag-modified Al3Ti with an L12-ordered structure. TEM observations revealed that plate-like D023-Al11Ti5 precipitates form multi-domain structures which can be considered as a 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 matrix. The particular habit plane, orientation relationship and twin plane variants were carefully determined. A quantitative X-ray microanalysis for determining the chemical compositions of the precipitate and matrix has been done by using an analytical electron microscope. 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.