COMPUTER MODELING OF PARTIALLY COHERENT BCC - HCP BOUNDARIES

A modeling study of partially coherent b.c.c.:h.c.p. coherent boundaries, using a computer-aided graphical technique, has been made for parallel pairs of low index planes in the presence of specified lattice orientation relationships. Three well-known lattice orientation relationships between the b.c.c. and h.c.p. phases, each of which was also slightly perturbed in various ways, were utilized. All of the planar interfaces thus formed which were based upon parallel pairs of low index planes can be described by two arrays of parallel misfit dislocations. The possibility of replacing one array of misfit dislocations with an array of structural ledges was then analyzed. In the presence of near-Burgers orientation relationships, the most probable structural ledges were found to have (1100BAR)h.c.p.//(211BAR)b.c.c. terraces with risers 2, 4 or 6 atomic layers high. This type of structural ledge has a Burgers vector of 1/12[111]b.c.c., which lies in the terrace plane, associated with its riser. Thus it can replace a set of a-type misfit dislocations; hence only a single set of c-type misfit dislocations is now necessary on the terrace of structural ledges. This types of structural ledge was also found to step down along the lattice invariant line in order to accommodate simultaneously the misfit normal to the terrace plane. Further, the possibility of structural ledges with (1101)h.c.p.//(1101)b.c.c. terraces was also discerned in the presence of near-Potter orientation relationships.