COMPUTER MODELING STUDY OF PARTIALLY COHERENT FCC-BCC BOUNDARIES

A computer modeling study has been made of the structure of f.c.c.:b.c.c. boundaries which are expected to be partially coherent. These boundaries are defined by {111}f.c.c∥{110}b.c.c and a rotation about an axis normal to these conjugate habit planes leading to orientation relationships ranging from Nishiyama-Wasserman to beyond Kurdjumov-Sachs. A wide range of ratios of the f.c.c. to the b.c.c. lattice parameter was employed. Regions of good atomic matching exist in the {111}f.c.c∥{110}b.c.c interface for all orientation relationships and lattice parameter ratios investigated. Extending the study of Hall et al., incorporation of structural ledges in the boundary allows the switching of matching layers and results in an increase in the interfacial coherency. The interledge spacing is a function of orientation and lattice parameter ratio and varies from ca. 3 to 30 Å. The structural ledges result in a deviation of the apparent habit plane from {111}γ although on an atomic scale this plane is always retained. The amount of deviation of the apparent habit plane from {111}γ is a function of orientation and lattice parameter ratio. Interfacial misfit dislocations lie midway between coherent regions on the broad faces of the structural ledges. The misfit dislocations consist of a single array whose spacing is a function of the orientation and lattice parameter ratio and ranges from ca. 8 to 40 Å. At least one partially coherent interface, normally incorporating both structural ledges and misfit dislocations, is predicted for each orientation and lattice parameter ratio investigated. In every case, the misfit dislocations are sessile with respect to slip out of the atomic conjugate habit planes, thus making a martensitic f.c.c. → b.c.c. transformation mechanistically impossible for these interfaces. © 1979.