It is possible for zinc crystals to undergo very large plastic strains at temperatures below room temperature. This is not true in polycrystalline zinc, which normally shows a very low ductility at sub?ambient temperatures. Give an explanation for this face based on the mechanical properties of zinc.
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Solution:
Zinc is a metal that deforms very easily by basal slip at a very low level of stress, even at low temperatures. In other words, it deforms readily by easy glide and the work hardening rate is low. At the same time, slip on other slip planes in zinc becomes increasingly more difficult the lower the temperature. In a single crystal of zinc, easy glide is effectively unrestricted because the shears along the slip planes do not end at a free surface, but rather at a grain boundary where the basal plane of the next grain is not normally aligned so as to pass the shear into the next grain. Compounding the problem is the fact that zinc cleaves easily along its basal at low temperatures. Thus, the high stresses that develop at the grain boundaries, due to the inability of the basal slip to be accommodated between grains, leads to brittle cleavage or intergranular fracture.
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