X-ray topography evidence for energy dissipation at indentation cracks in MgO crystals

Abstract

Previously, the diffraction constrast in X-ray topographs of diamond pyramid microhardness indentations in MgO crystals was matched with optical microscope observations of cracking and the extent of slip bands made visible by etch pitting [ 1 ]. Results from a further X-ray topography study were correlated with scanning electron microscope observations to determine the three-dimensional arrangement of dislocations surrounding such microhardness indentations [2]. X-ray topographic results for macro-indentations are reported here. Measurement of the extent of diffraction contrast at the indentations shows that appreciable internal strain energy release has been achieved by dislocations escaping through {1 1 0} crack surfaces. The results give support to the dislocation model of "hot spots" (very small, high temperature regions) being generated by the collapse of dislo~ cation pile-ups, particularly, during cracking [3]. Figs. la and b show a matched pair of optical and X-ray topographic pictures of macro-indentations put into an MgO (0 0 1) cleavage surface at 15 and 100kgf load, 147 and 981N, respectively, with a 1.59mm Rockwell spherical steel indenter. Stable support of the load was not obtained at either indentation and so the load was removed after the expiration of a contact period of approximately 15 sec when the rate .of indenter penetration had decreased very appreciably. Diamond pyramid microhardness indentations were placed at 0.1 kgf load (0.981N) around the larger 100kgf indentation to probe the local state of deformation [4]. In Fig. lb, the general appearance of tile white and black diffraction contrast at the 15 kgf indentation relates to previous results reported for the extent of cracking and dislocation etch pits, respectively, centred on microhardness inden-