Interfacial dislocation mechanism for diffusional phase transformations exhibiting martensitic crystallography: formation of TiAl + Ti 3Al lamellae

Abstract

The displacive–diffusive transformation, which gives rise to lamellar γ-TiAl+ α 2-Ti 3Al microstructures, has been analysed theoretically. Using the formulation due to Hirth and Pond, the shear produced and extent of diffusional flux associated with interfacial defect motion can be quantified in terms of the Burgers vectors, step heights of the defects and the chemical compositions of the adjacent crystals. Moreover, this formulation also enables the effects of chemical composition, misfit at the interface and ordering to be investigated systematically. It is shown that for the defects observed on such interfaces in recent high-resolution transmission electron microscopy studies, their motion would lead to the conservation of sites and, correspondingly, to equal and opposite fluxes of Ti and Al atoms. Consequently, although this transformation is diffusional, it exhibits crystallographic features consistent with martensitic processes.