Formation of prismatic loops in AlN and GaN under nanoindentation

The formation of prismatic loops in metals has been studied extensively, but the corresponding research related to ceramics can rarely be found in the literature. In this article, the formation of prismatic loops in B4 ceramics under nanoindentation were investigated with molecular dynamics (MD) simulations. The indentation directions are respectively in the normal of the basal plane and the two prismatic planes in AlN and GaN single crystals. Two mechanisms for the formation of prismatic loop in B4 ceramics are found. One is the “lasso”-like mechanism, which is similar to that observed in metals, and the other is “nested-loops” mechanism, which has never been reported in the literature. MD simulations show that the screw segments of the two different shear loops, be adjacent to each other, can intersect with each other and finally produce a prismatic dislocation loop. The pile-up symmetry after unloading, associated with dislocation loops propagation, can be found, which is consistent with experimental results. More detailed analysis of dislocation patterns and directions indicate that plastic deformation is dominated by the shuffle-set dislocations along 13〈112¯0〉, and the glide-set dislocations along 13〈1¯010〉 are asymmetric.

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