The brittle fracture of polycrystalline zinc

In polycrystalline materials, grain boundaries provide an important contribution to the resistance to the propagation of both brittle and ductile cracks. Initially, in this paper, a three-dimensional geometrical model of the brittle fracture of polycrystalline zinc is developed, assuming a single (0001) cleavage plane in each grain. The model predicts that about one-half of the fracture process of the material will be associated with accommodation effects at grain boundaries. In contrast, experimental work over a range of temperatures shows that at low temperatures very little grain boundary failure occurs. There are two reasons for this discrepancy. Firstly, cleavage occurs on (0001) and also on the three variants of the {10-10} planes and secondly, deformation twinning plays a major role in the fracture process. The characteristics of these phenomena have been investigated in detail using focused ion beam microscopy, including subsurface examinations and metallographic techniques. The models were then extended to incorporate these additional mechanisms. Comparisons between the predictions and the experimental observations are discussed and enable information to be deduced about the relative energies of the different fracture mechanisms.