Microstructure of Crack Tip Plastic Zone and Fracture Toughness In Silicon Crystals

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In order to understand the dislocation process In the plastic zone and its relation to the fracture toughness In silicon crystals, microstructures of plastic zones around crack tips have been investigated using high voltage electron microscopy (HVEM) and atomic force microscopy (AFM). Cracks were Introduced Into (110) silicon wafers at room temperature by Vickers indentation method. The temperature of specimens indented was raised to higher than 823K to activate dislocation sources around a crack tip under the presence of residual stress due to the indentation. AFM study has revealed two types of fine slip bands : one type Is slip bands oblique to the direction of crack propagation, I.e., <110>, and another type is those parallel to this <110> direction. The former is corresponding to a hinge-type plastic zone, and the later is so-called 45゜-shear-type. HVEM study has also revealed the characteristic of dislocation structures developed in the both types of plastic zones. The correspond … More ence between AFM and HVEM results is verified, and the characteristics found in the both plastic zones are discussed. In addition to the HVEM and AFM studies, stress fields around a crack tip in silicon crystals have been investigated by using infrared photoelasticity with the aim of clarifying the shielding effect due to crack tip dislocations on the steep increase of fracture toughness in the brittle-to-ductile transition (BDT). first, compact tension tests were carried out at room temperature to make in-situ observation of elastic behavior of crack tip stress fields. The photoelastic images observed were In good agreement with those simulated for the usual elastic fields around the tip of a mode I crack. Next, to clarify the stress modification due to crack tip plasticity, three point bending tests were also made by using notched specimens at high temperatures around 1000K. After the high temperature test, in spite of the absence of the applied load, residual bright images were observed around the notch. Those images correspond to an internal stress due to dislocations multiplied around the notch, and they have an effect of shielding (accommodating) the stress concentration due to the applied load. The fracture toughness at room temperature was increased by the introduction of the residual stress. Less