HVEM OBSERVATIONS OF DISLOCATION-STRUCTURES NEAR A CRACK-TIP IN MGO CRYSTALS

Dislocation structures near the tip of a crack in MgO thin plate cryst als have been investigated using a high voltage electron microscope (H VEM). In (001) thin plate crystals, a (010) crack is most commonly obs erved 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) over bar1$] or (0(1) over bar1$)[011] slip systems and thei r Burgers vectors have large screw components. Dislocation image analy ses have been applied to determine the signs of their Burgers vectors, and the screw dislocations on the (011) plane are found to be left ha nded and those on the (0(1) over bar1$) plane are right handed. The ef fect of crack tip dislocations on fracture behavior is qualitatively e xamined and clarified to be a model shielding type so as to accommodat e the tensile stress concentration near the crack tip. (C) 1997 Elsevi er Science S.A.