ATOMIC-STRUCTURE OF INTERPHASE BOUNDARY ENCLOSING BCC PRECIPITATE FORMED IN FCC MATRIX IN A NI-CR ALLOY

The atomic structure of the interphase boundaries enclosing body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane similar to(112)(fcc)/(23(1) over bar)(bcc)) contains a regular array of structural ledges whose terrace is formed by the (111)(fcc)/(110)(bcc) planes. A structural ledge has an effective Burgers vector corresponding to an a/12[1(2) over bar1$](fcc) transformation dislocation in the fcc --> bcc transformation. The side facet (and presumably the growth ledge riser) of the bcc lath contains two distinct types of lattice dislocation accommodating transformation strains. One type is glissile dislocations, which exist on every six layers of parallel close-packed planes. These perfectly accommodate the shear strain caused by the stacking sequence change from fee to bcc. The second set is sessile misfit dislocations (similar to 10 nm apart) whose Burgers vector is a/3[111](fcc) = a/2[110](bcc). These perfectly accommodate the dilatational strain along the direction normal to the parallel close-packed planes. These results demonstrate that the interphase boundaries enclosing the laths are all semicoherent. Nucleation and migration of growth ledges, which are controlled by diffusion of substitutional solute atoms, result in the virtual displacement of transformation dislocations accompanying the climb of sessile misfit dislocations and the glide of glissile dislocations simultaneously. Such a growth mode assures the retention of atomic site correspondence across the growing interface.