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

基于裂纹-位错相互作用理论，研究了材料断裂的细观尺度动力学。对硅单晶的断裂和变形行为给予了特别的关注。由于硅单晶在狭窄的温度范围内表现出从脆性到延性的急剧转变，它们作为模型晶体受到了广泛的关注，以了解晶体材料增韧的位错过程。在本研究中，利用高压电子显微镜研究了硅晶体中裂纹尖端位错的性质及其增殖过程。观察到的裂纹尖端位错与模拟图像相匹配，发现其为屏蔽型，提高了材料的断裂韧性。除此之外，在1100K左右的高温下进行三点弯曲试验，以便在裂纹尖端周围引入位错。利用红外光弹性方法对高温变形的硅晶体进行了裂纹尖端应力场的观察。通过该方法证实，位错的引入在裂纹尖端周围形成了压应力场，压应力场显著提高了硅晶体的断裂韧性。此外，利用Hartmaier博士开发的程序代码对位错分布进行了模拟，计算得到的位错分布模拟光弹性图像与观测图像吻合较好。从硅晶体可加工性研究的角度出发，研究了杂质和应变速率对硅晶体脆性向韧性转变行为的影响。在本研究中，硼的加入使脆性向韧性转变温度（BDTT）升高。根据BDTT的应变速率依赖结果，估计了脆性到韧性转变的活化能，其值与位错滑动的活化能值吻合较好。这些结果表明，基于位错运动理论和裂纹与位错相互作用理论，从根本上讨论了硅晶体的脆-韧转变和可加工性问题。少

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