The relationship between plastic flow and the fracture mechanism in magnesium oxide single crystals

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Abstract
Magnesium oxide crystals plastically deformed under a three-point load develop internal slits coplanar with the (110) slip plane and parallel to the [001] bending axis in the tension region. Using etch pit techniques it has been shown that the slits lie along the edge of a (110) slip band and are confined between two adjacent orthogonal (110) slip bands. It is proposed that under certain circumstances edge dislocations moving in one of the (110) slip planes pile into the barrier provided by a wider (110) slip band and coalesce to form a slit nucleus along the lines of intersection. This nucleus lies parallel to the (110) plane and grows by cleavage over this plane until it meets another (110) slip band where it becomes halted. Thus cracks can be nucleated where slip bands intersect but, more important, they can also be stabilized by slip bands. If microcracks form in the early stages of plastic flow their growth is unrestricted and the material is brittle. If the crystal is first plastically deformed to introduce sufficient slip the growth of the microcracks is impeded in certain directions and they develop into narrow slits. The material is then able to deform even further. The correlation between crystal ductility and the appearance of the fracture surface is consistent with this interpretation. For brittle specimens fracture starts from a point, whereas for ductile specimens the fracture appears to start from one of the narrow slits.

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