The atomic structures of the inter-phase boundaries which enclosed body-centered cubic lath-like precipitates that formed in the face-centered cubic matrix of a 45wt%Cr alloy were studied by means of conventional and high-resolution transmission electron microscopy. Growth ledges were observed on the broad faces of the laths. The growth ledge terrace, with a macroscopic habit plane of roughly (112)fcc||(231¯)bcc, contained a regular array of structural ledges whose terraces were made up of (111)fcc||(110)bcc planes. A structural ledge had an effective Burgers vector that corresponded to an a/12[12¯1]fcc transformation dislocation in the face-centered to body-centered transformation. The side facet, and presumably the growth ledge riser, of the body-centered cubic lath contained 2 distinct types of lattice dislocation which accommodated transformation strains. One type comprised glissile dislocations which existed on every 6 layers of parallel close-packed planes. These perfectly accommodated the shear strains that were caused by the stacking sequence change from face-centered to body-centered cubic. The other type comprised sessile misfit dislocations, some 10nm apart, whose Burgers vector was a/3[111]fcc = a/2[110]bcc. These perfectly accommodated dilatational strains along the direction normal to the parallel close-packed planes. The results demonstrated that the inter-phase boundaries which enclosed the laths were all semi-coherent. The nucleation and migration of growth ledges which were controlled by the diffusion of substitutional solute atoms resulted in the virtual displacement of the transformation dislocations that were associated with the climb of sessile misfit dislocations and the glide of glissile dislocations. Such a growth mode assured the retention of atomic site correspondence across the growing interface.

T.Furuhara, K.Wada, T.Maki: Metallurgical and Materials Transactions A, 1995, 26[8], 1971-8