一部分がSiC 粒子とAl_2O_3 ウィスカーで強化されたAl 鋳造合金の単調および繰返し荷重下における破壊機構の評価：境界およびウィスカー方位の効果

指導教員: 埼玉大学大学院理工学研究科教授 荒居善雄This dissertation presents the research work done for the degree of Doctor of philosophy. The research project is on Fracture mechanism of an aluminium cast alloy locally reinforced by SiC particles and Al2O3 whiskers under monotonic and cyclic load: boundary and whisker orientation effect. Metal matrix composites (MMCs) have been widely considered as possible substitute of traditional materials (such as metals, plastics, ceramics etc.) for structural applications because of their high strength and stiffness, low density, high temperature properties and excellent wear resistance. These advantages made this material more and more potential and alternative in the engineering application. However, the high productive cost, poor ductility and low fracture toughness of MMCs are the major barriers for their structural application. In this dissertation, fracture mechanisms and corresponding stress distributions in an aluminium cast alloy locally reinforced by SiC particles and Al2O3 whiskers, under monotonic and cyclic load, are investigated experimentally and numerically. The effect of whisker orientation on monotonic and fatigue strength has also been investigated. The material is monotonically and cyclically deformed to failure at room temperature. The fracture origin and the fracture path are investigated on the fracture surfaces. The stress distributions around the boundary between the reinforced part and the unreinforced part are calculated based on an inclusion array model considering the microscopic inhomogeneous effects. A three-dimensional single whisker unit cell model of cylindrical shape whisker in the periodic boundary condition is conducted using finite element method (FEM) to describe the overall behavior of the composite Chapter 1 is an introduction of the research, which describes the back ground, the motivation, proposed concepts, the objectives and the scopes of this research. In this chapter also reviews the past research on the MMC. In chapter 2, Materials fabrication and its microstructure, the experimental set up and the experimental procedures are discussed. Materials are successfully fabricated by squeeze casting method. The polishing surface observations are treated by optical microscope and the fracture surface observations are treated by the scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis. In chapter 3, the experimental results are discussed, which describes the fracture mechanisms, boundary effect and whisker orientation effect. The fracture origin and fracture path are investigated by SEM on the fracture surfaces. The fracture occurs in the reinforced part under both monotonic and cyclic loads. SEM analysis of the fracture surfaces shows that the fatigue fracture is controlled by the fracture of coarse Al2O3 whiskers. The static fracture (monotonic loading) shows that the fracture mechanism is the combination of reinforcing particle fracture and interfacial debonding between reinforcing ceramics and matrix metal. A significant effect of reinforcement orientation on the monotonic strength and fatigue strength are observed experimentally and numerically. With respect to the stress direction the whisker orientation gives significant difference in strength of this material. SEM analysis shows almost all whiskers are transversely debonded when whisker direction is perpendicular to the stress direction and almost all whiskers are broken when the whisker direction is random to the stress direction. In chapter 4, a numerical analysis is discussed. The stress distributions around the boundary between reinforced part and unreinforced part are calculated based on an inclusion array model considering the microscopic inhomogeneous effects. A three-dimensional single whisker unit cell model of cylindrical shape whisker in the periodic boundary condition is conducted using FEM to describe the overall behavior of the composite. The prediction results based on FEM analysis are found to be in reasonable agreement with the experimental observations. In chapter 5, the general conclusions and directions for future investigation are given