DEFECT STRUCTURES IN SINGLE-CRYSTAL TIC

A transmission electron microscopy study is presented of sub-boundary defect structures in single crystal TiC. Sub-boundaries consisting of organized dislocation arrays and hexagonal networks were observed. The Burgers vectors of dislocations in the sub-boundaries were determined to be 1/2[110], and the dislocations were approximately on {111} planes. Dislocation nodes in hexagonal networks provided preferential sites for nucleation of triangular planar defects. These defects were identified as platelet precipitates of TiB2, although it was difficult to distinguish between precipitates and impurity-stabilized stacking faults during the initial stages of the precipitation. The precipitates were bounded by Shockley partial dislocations and resided on {111} habit planes. Segregation of impurities to sub-boundaries resulted in unusual platelet configurations involving beak-like extensions on intersecting {111} planes. In such cases, the beak-like extensions were bounded by two uncommon partial dislocations: 1/6[114] and 1/6[121BAR] on delta-(111). Formation of the extensions involved dislocation climb and was driven by elastic constraint exerted by dislocations of the sub-boundary network. The platelet precipitates were successfully removed by multiple-pass zone refinement, although the substructure was unaffected.