Mesoscopic-scale dynamics of materials fracture and workability of silicon crystals

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Mesoscopic-scale dynamics of materials fracture have been investigated based on the theory of crack-dislocation interaction. Particular attention has been paid on fracture and deformation behaviors in silicon single crystals.Since silicon single crystals exhibit a sharp transition from brittle-to-ductile behavior in the narrow temperature range, they have attracted much attention as model crystals to understand the dislocation process for toughening crystalline materials. In this research, the nature of crack tip dislocations and their multiplication processes in silicon crystals have been examined by using high-voltage electron microscopy. The crack tip dislocations observed were characterized by matching their images to those simulated, and it was found that they were shielding type which increases fracture toughness of materials. In addition to this, three-point bending tests were made at high temperatures around 1100K in order to introduce dislocations around a crack tip. And crack … More tip stress fields were visualized by infrared photoelastic method in those silicon crystals deformed at high temperatures. By using this method, it was confirmed that the compressive stress field was formed around the crack tip by the introduction of dislocations, and that the compressive stress fields substantially increases the fracture toughness of silicon crystals. Further, the dislocation distribution was simulated by using the program code developed by Dr. Hartmaier, MPI, and the simulated photoelastic images using the calculated dislocation distribution were in good agreement with the observed images.From the viewpoint of the research on the workability of silicon crystals, the effects of impurity and strain rate on the behavior of brittle-to-ductile transition was investigated. In this study, boron was added, which caused the increase of brittle-to-ductile transition temperature (BDTT). Based on the results of strain rate dependence of BDTT, activation energy for brittle-to-ductile transition was estimated, and the value estimated coincided well with the value of the activation energy of dislocation glide. Those results suggest that the problem of brittle-to-ductile transition and workability of silicon crystals are fundamentally discussed based on the theory of dislocation motion and the interaction between crack and dislocations. Less