Lattice Rotation at Slip Band Intersections

Abstract

INTERSECTION of a slip band by an active slip band of different slip direction results in the formation of a step in the first band. Slip band intersections in magnesium oxide crystals suggested that the distortion of the intersected band was not merely a shear due to lattice translation; this was supported by the observation that subsequent widening of the intersected band (as described by Gilman and Johnston1) added to it a margin that followed the step. This indicated that there was a change of lattice orientation at the intersection. Imaging X-ray photographs of the Berg–Barrett2,3 type confirmed the change of lattice orientation. Fig. 1 shows a compressed magnesium oxide crystal, afterwards etched to reveal dislocations; the dislocation etch-pits make the bands appear dark. Fig. 2 is an X-ray image of the same crystal; the slip bands appear dark because of the reduction of primary extinction. The slip band intersections are white, and the reflexions from them can be found in more or less displaced positions, showing that there is a macroscopic change of orientation of the lattice at the intersections. Such a change can arise in the way shown in Fig. 3. Since the intersection is full of edge dislocations of both slip directions [110] and [1&1macr;0], and the shear stress is the same in both slip systems, the shear strain in the active slip band may be due to slip and kinking4,5 (= slip on the planes of the intersecting band) in the area of intersection. In Fig. 3 all strain in the intersection is assumed to have been the result of kinking.