Plastic deformation in mgo single crystals

Abstract

Slip in MgO takes place by the development of dislocation bands. Like Lüders bands, dislocation bands have undergone a fairly constant shear strain, which in this case is about 0·09 radians. Dislocation bands are nucleated usually at surface defects and widen by a dislocation sprouting (multiplication) mechanism. The constant shear strain, and the observed dislocation density inside a band, indicate a mean free path of 10⁻² cm for a dislocation in MgO at room temperature. When two orthogonal dislocation bands intersect, one band produces kinking in the other at the intersection. The lattice rotation in the kinked portion of the second band is approximately equal to the shear strain of the first band. The non-accommodated kink boundaries around the intersection produce local stresses which oppose the applied shear stress in the kinked band. These stresses may contribute to the Bauschinger effect and to permanent softening upon reversal of deformation. The lattice rotation has the direction observed in micro-asterism. If a narrow dislocation band is intersected and kinked by a greatly widening band, it is embedded in the latter as a birefringent kink band under a biaxial tension due to non-accommodation of the kink.