BURGERS VECTOR OF CRACK-TIP DISLOCATIONS IN MAGNESIUM-OXIDE CRYSTALS

Crack tip dislocations in MgO thin crystals have been investigated usi ng a high voltage electron microscope (HVEM). Particular emphasis is l aid on determining the sign of the Burgers vectors of those dislocatio ns, because it is essentially important for understanding the fracture toughness of materials;. For this determination, image simulation for a dislocation inclined to the foil surface has been performed, and ch aracteristics of image contrast are discussed. Crack tip shielding due to dislocations is also discussed briefly.