High-voltage electron microscopy analyses for crack-tip dislocations and their shielding effect on fracture toughness in MgO and Si crystals

In this study, characteristics of dislocation structures around a crack-tip in both crystals of MgO and Si were investigated using high-voltage electron microscopy (HVEM). The interaction between a crack and dislocations was analyzed to discuss the effect of crack-tip plasticity on fracture toughness. The present paper first demonstrates a dislocation configuration around a crack-tip in a MgO thin crystal, where the plastic zone called 45°-shear type is dominant under the plane stress condition. Then, the slip systems necessary for brittle-to-ductile transition in ionic crystals with the rock-salt structure are discussed based on the temperature dependence of stress-strain relationships and the aspect of crack front observed in NaCl crystals. Second, crack-tip dislocations near the surface of bulk Si crystals are exhibited based on the 3D analyses using HVEM tomography. We focus on a newfound plastic zone being perpendicular to the crack plane, which is different from either 45°-shear type or the hinge type generally well-known. The dislocation loops observed in this plastic zone fundamentally have a crack-tip shielding effect to increase fracture toughness. Still, in addition, they contribute to activating crack-tip dislocations by their localized antishielding field. These analyses reveal a fundamental toughening mechanism for crystalline materials.