Morphological stability of γ precipitates in an elastically constrained Ni-Al-Ti alloy

The morphological changes of the coherent gamma precipitate particles in the gamma' matrix of a Ni-Al-Ti alloy were investigated by means of the transmission electron microscopy (TEM). The energetically stable shape of the gamma precipitate was analyzed on the basis of the microelasticity theory. As a result, the gamma particles having an Al (disordered fcc) crystal structure precipitate coherently in the gamma' matrix(Ll(2)) at 973 K and 1073 K. At the early stage of ageing, the shape of the gamma particles is a small sphere and randomly distributed in the gamma' matrix. With the progress of ageing, gamma particles gradually become to be aligned along the elastically soft directions of <100>, and the particle shape changes to a plate-like ellipsoid on the {100} plane. The theoretical analysis realized that such morphological changes are mainly controlled by the elastic strain energy.