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-cente red cubic (bcc) lath-shape precipitates formed in the face-centered cu bic (fcc) matrix of a Ni-45 mass pct Cr alloy was examined by means of conventional and high-resolution transmission electron microscopy (HR TEM). 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 l edges whose terrace is formed by the (111)(fcc)/(110)(bcc) planes. A s tructural 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 a ccommodating transformation strains. One type is glissile dislocations , which exist on every six layers of parallel close-packed planes. The se perfectly accommodate the shear strain caused by the stacking seque nce change from fee to bcc. The second set is sessile misfit dislocati ons (similar to 10 nm apart) whose Burgers vector is a/3[111](fcc) = a /2[110](bcc). These perfectly accommodate the dilatational strain alon g the direction normal to the parallel close-packed planes. These resu lts demonstrate that the interphase boundaries enclosing the laths are all semicoherent. Nucleation and migration of growth ledges, which ar e controlled by diffusion of substitutional solute atoms, result in th e virtual displacement of transformation dislocations accompanying the climb of sessile misfit dislocations and the glide of glissile disloc ations simultaneously. Such a growth mode assures the retention of ato mic site correspondence across the growing interface.