Atomic Structure of Interphase Boundary Enclosing Bee-Precipitate Formed in Fee Matrix in a Ni-Cr Alloy

摘要

The atomic structure of the interphase boundaries enclosing- body-centered cubic (bcc) lath-shape precipitates formed in the face-centered cubic (fee) matrix of a Ni-45 mass pet Cr alloy was examined by means of conventional and high-resolution transmissioirelectron microscopy (HRTEM). Growth ledges were observed on the broad faces of the laths. The growth ledge terrace (with the macroscopic habit plane ~(112)_(fcc)//(23l)_(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[121]_(fcc) transformation dislocation in the fee - > 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 (~10 nm apart) whose Burgers vector is a/3[l 1 l]_(fcc) = a/2[l 10]_(bcc). These perfectly accommodate the dilatational strain along the direction normal to the parallel close-packed planes. These results demonstrate that the inteiphase 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.