TEM characterization of the precipitation reaction in Ti-48Al-10Nb alloy

A detailed transmission electron microscopy (TEM) study has been made of the morphology and the crystallographic characteristics of precipitation of the γ1-Ti4Nb3Al9 in the γ-TiAl matrix in Ti-Al-Nb system. It is revealed that the γ1-Ti4Nb3Al9 precipitates, which are formed by a normal quench at 1473 K and ageing treatment at 1073 K, take needle-like morphology with a growing axis parallel to [0 0 1] direction of the matrix. The needle-like precipitates act as barrier grid to inhibit dislocation moving on the slipping plane {1 1 1} of the γ-TiAl matrix, which helps to improve the strength of the alloy. Selected electron microscopy analysis shows that the orientation relationship between the γ1 phase and the γ-TiAl matrix is [0 0 1]γ1//[0 0 1]γ, (1 0 0)γ1//(1 1 0)γ and (010)γ1//(1¯10)γ. The needle-like morphology of the precipitated phase is discussed with the coherency stress across the precipitate/matrix interface that is considered to be the main factor controlling the precipitate morphology.