Mechanism of the emission of shielding dislocations from a crack tip

点击翻译按钮获取中文摘要

In this research project, the mechanism of dislocation emission from a crack tip in MgO, NaCl and Si crystals have been studied using microscopic techniques of a electron microscope and an atomic force microscope. The results obtained are summarized as in the following.(1) In thick crystals, hinge-type structures of slip bands are formed around a crack tip, while in thin crystals dislocation structures are observed on slip planes intersecting through a crack tip line.(2) Dislocations are emitted from a crack tip through the nucleation of half-loop dislocations, and in thin crystals, the half-loop dislocations are generated at the intersection of a crack tip to free surface as well as at a crack jog and kink.(3) Dislocations are introduced not only ahead of a crack tip, but also formed behind a crack tip.(4) The sign of dislocations emitted ahead of crack tip is opposite to that of dislocations behind a crack tip, when they are lying on the same slip plane.(5) In any cases, dislocations which shield the crack stress field from an applied stress are mostly emitted from a crack tip.(6) In the deformation of NaCl crystals at high temperature, microscopic cross slip takes place on indefinite planes, so that half-loop dislocations can be expanded along a crack tip line by the cross slip to shield all over the crack front, leading to the suppression of crack growth.