THE ROLE OF STRUCTURAL LEDGES AT PHASE BOUNDARIES .3. FCC-BCC INTERFACES IN KURDJUMOV-SACHS ORIENTATION

The ledge mode of misfit accommodation is extended to {111} f.c.c. parallel-to {110} b.c.c. interfaces with the Kurdjumov-Sachs (KS) orientation relationship. As with the Nishiyama-Wassermann (NW) relationship the geometric quantities are uniquely related by the misfit ratio r of atomic diameters. A rigid lattice analysis shows that the driving forces for a KS orientation relationship are significantly larger than those for the NW-x and even more so than those for the NW-y relationships. The rigid analysis also predicts that the terrace width which satisfies the periodicity conditions for a stepped interface are well within the terrace widths which allow significant energy gains. The models developed for the NW analysis are modified for KS configurations and employed to sum energies for the terrace patches, tilt misfit dislocations and the riser interface energy. Comparing this energy with that of a planar interphase boundary comprised of only misfit compensating dislocations shows that a stepped interphase boundary is energetically favored at r values near and greater than r(KS) but less than r(NW-y).