HVEM observations of dislocation structures near a crack tip in MgO crystals

Dislocation structures near the tip of a crack in MgO thin plate crystals have been investigated using a high voltage electron microscope (HVEM). In (001) thin plate crystals, a (010) crack is most commonly observed and an array of dislocations is seen along the [100] direction in front of the crack tip. The dislocations in the array are those of (011[ 0 1 ¯ 1 ] or ( 0 1 ¯ 1 )[011] slip systems and their Burgers vectors have large screw components. Dislocation image analyses have been applied to determine the signs of their Burgers vectors, and the screw dislocations on the (011) plane are found to be left handed and those on the ( 0 1 ¯ 1 ) plane are right handed. The effect of crack tip dislocations on fracture behavior is qualitatively examined and clarified to be a mode I shielding type so as to accommodate the tensile stress concentration near the crack tip.