FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.5 Generation of defect structures

Dislocations in ice are initiated at Frank-Read sources (Weertman & Weertman 1964). Frank-Read sources occur when a dislocation is pinned at two points within the crystal lattice. The dislocation may be pinned by impurities or it may be a dislocation loop that lies oblique to the basal plane and so is immobile. In the latter case, the section intersecting the basal plane can be considered as a line dislocation pinned at its ends by the immobile sections that are oblique to the basal plane (Fig. 2.5.1). When a small stress is applied to the lattice the pinned dislocation bows. With increasing stress the bow becomes unstable and grows to form the shape illustrated in figure 2.5.1d. The dislocation becomes unstable if the shear stress , where G is the shear modulus of ice, b the Burgers vector, and l the length of the Frank-Read source. With continued stress the loop grows and eventually curls onto itself (Fig. 2.5.1e). Where the loop touches itself, the dislocations have opposite signs and so obliterate each other, leaving a complete dislocation loop and a dislocation line pinned in the original position. The dislocation loop is now able to propagate through the lattice, while the pinned section goes onto initiate further dislocations. Ahmad et al. (1986) observed multiple generations of dislocations from a single Frank-Read source using X-ray diffraction techniques. Their topographs show the development of dislocations in regions of crystals initially free of dislocations. The dislocations were predominantly in screw orientations or 60° to the Burgers vector and on the basal plane. Dislocations in this orientation encounter a Peierls barrier which forces the expanding dislocation loop to take on a hexagonal shape (Ahmad et al. 1992).

Ahmad et al. (1992) proposed a mechanism related to edge dislocations on non-basal planes as a second source for initiating dislocations. If a significant stress is applied to an appropriate prismatic plane, a segment linking two basal planes will begin to glide. The motion of non-basal edge dislocations is high compared to basal edge dislocations. The rapid motion of the non-basal segment generates dislocation loops on many of the basal planes that the segment passes through. Ahmad et al. (1992) have used X-ray topographs to show a series of dislocation loops in basal planes stacked one upon the other that appear to have been generated in this fashion.

Figure 2.5.1: A Frank-Read source for the multiple initiation of dislocation loops. A dislocation is pinned in the basal plane at two ends by either impurities or an immobile non-basal dislocation. If a shear stress is resolved onto the basal plane, the dislocation line becomes unstable and begins to bow. With increasing stress, the line bows back onto itself to produce a new loop that is free to propagate, and a section that remains pinned which may initiate more loops.