Fig. 2. The dislocation deformation pattern surrounding an aligned diamond pyramid microhardness indentation with indenter edges parallel to <100> put into an (001) MgO crystal surface [31]. The sub-surface dislocation structure leading to observation on the surface of diagonal <110> type cracking is illustrated in the cut-away diagram of Fig. 3. As mentioned, the cracking is on the unfavorable {110} planes compared to normally observed {010) cleavage planes. Fig. 3. Sub-surface dislocation reaction on intersecting <110>{1-10} slip planes in MgO [32]. The same type of crack-forming dislocation reaction mechanism was shown to occur at aligned microhardness impressions put into the (-210) crystal surfaces of body-centered tetragonal ammonium perchlorate (AP) crystals [33]. In this case, greater plastic anisotropy was reflected in the distorted shape of the residual diamond pyramid indentation such that cracking was only initiated in the adjacent top two indentation quadrants of the indented crystal surface in which a pair of intersecting slip systems had produced greater plastic deformation --- thus giving emphasis to the Cottrell mechanism of cleavage cracking being attributed to the local intersecting slip system blockages.
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