Crystallography of phase transformations

Abstract

Microstructural and crystallographic features which are associated with transformation products are generalized on the basis of factors such as interfacial surface energy, strain energy and diffusion. In the case of diffusion-controlled Widmanstätten structures, surface energy factors such as a discrete orientation relationship, a low index habit plane and a tendency for a pseudomorphic or coherent crystal structure are dominant. Factors associated with tolerable strain energy, such as stress relief by recrystallization or concurrent plastic deformation, play only minor roles in diffusion-controlled transformations. On the other hand, it is proposed that in the case of diffusionless martensitic structures the strain energy factors, such as spontaneous deformation by slip and twinning, dominate to modify the factors established by interfacial surface energy. The strain energy factors lead to modified or pseudo-habits and pseudo-orientation relationships in much the same way as surface energy may lead to a pseudomorphic crystal structure. The consequence of spontaneous deformation by slip is illustrated for martensitic structures in iron-base alloys. It is proposed that the (111)A habit and Kurdjumow-Sachs orientation relationship are primitive characteristics of the γ to α transformation in iron-base alloys and that spontaneous slip modifies these characteristics to those observed for bainitic and martensitic structures. The suggestion is advanced that the (225)A habit arises because of rotation of the interface away from (111)A by slip in the austenite whereas the (259)A habit is associated with the dominance of slip in the martensite. Slip in each phase can account for lattice rotations that explain variables of the observed orientation relationships. A generalization that the habit plane tends to be one with indices of the type {ithhl} in a variety of martensitic structures involving cubic phases is explained by spontaneous deformation. On the basis of this viewpoint, shear transformations and martensitic structures are to be regarded as being accompanied by shear rather than generated by shear.