Interfacial dislocation mechanism for diffusional phase transformations exhibiting martensitic crystallography: Formation of TiAl+Ti(3)Allamellae

The displacive-diffusive transformation, which gives rise to lamellar gamma -TiAl+alpha(2)-Ti3Al microstructures, has been analysed theoretically. Usin g the formulation due to Hirth and Pond, the shear produced and extent of d iffusional flux associated with interfacial defect motion can be quantified in terms of the Burgers vectors, step heights of the defects and the chemi cal compositions of the adjacent crystals. Moreover, this formulation also enables the effects of chemical composition, misfit at the interface and or dering 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 a nd, correspondingly, to equal and opposite fluxes of Ti and Al atoms. Conse quently, although this transformation is diffusional, it exhibits crystallo graphic features consistent with martensitic processes. (C) 2000 Acta Metal lurgica Inc. Published by Elsevier Science Ltd. All rights reserved.