THE EFFECT OF TEMPERATURE ON DISLOCATION-STRUCTURES IN L1(2)-TYPE NI3(SI, TI) SINGLE-CRYSTALS

The morphological feature and the dissociation of dislocations in L1(2)-type Ni3(Si, Ti) single crystals were investigated by means of the transmission electron microscope (TEM) weak-beam method, at temperatures below the peak temperature of the yield stress. At 673 K, dislocations mainly consisted of screw orientation of [1BAR01] and were dissociated into the two 1/2[1BAR01] unit dislocations bounding the anti phase boundary (APB), thus suggesting an operation of the locking mechanism due to micro-cross slip. The APB boundary was not lying perfectly on the cross-slipped (010) plane, and also was not lying perfectly on the (111) plane. At 293 and 77 K, the morphological feature was similar to those observed at higher temperatures but more edge parts of dislocations were observed as the deformation temperature decreased. The superdislocations in the specimen deformed at 77 K were determined to be dissociated into the four Shockley partials bounding the APB and the complex stacking fault (CSF) boundaries. The origin of the fourfold dissociation of a superdislocation was discussed from the points of view of alloying effect and temperature effect. Kink, superkink or edge parts were frequently involved in a screw super-dislocation and discussed in relation to the (K-W) pinning process.