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.nIf 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 direc¬tions 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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