Influence of Solidification Microstructure and Distribution of Reinforcement on Fatigue Characteristics of Notched SiC Reinforced AC4B Alloy Composites

摘要

The influences of SiC particle distribution, surface notch size and solidification microstructure of the matrix on the fatigue characteristics of SiC reinforced JIS-AC4B alloy composites were investigated. Al-6.79mass percent Si-2.93 mass percent Cu-0.17 mass percent Mg-0.59 mass percent Fe matrix composites with relatively homogeneously dispersed 11 mum SiC particles were fabricated through a combination of pressure infiltration and a melt stirring casting method. The matrix microstructure consisted of a dendritic alpha phase and eutectic Si with a few volume fractions of Fe intermetallic compound among the dendrites. All specimens contained some gas and shrinkage porosities, and all composite specimens contained SiC particle clusters. Vickers hardness of composites clearly increases due to the dispersion of SiC particles and age hardening. The hardening ability increases with an increasing volume fraction of SiC. Rotating bending fatigue tests were carried out on notch-free and notched specimens that had peak aging. In the notch-free matrix alloy specimen, cracks generated from porosities, whereas cracks generated from the SiC particles/the matrix interfaces in the composite specimens. Thus, the fatigue strength decreased with an increase in the SiC volume fraction. In the notch-introduced matrix alloy specimen, where the stress concentration factor is high, the notch becomes the crack generation site and dominated the fatigue strength. The cracks, however, generate near SiC particles instead of the notch bottom in the composite specimen. Moreover, it was found that the fatigue limit stress is unchanged in composite specimen even when the notch is introduced, although the critical stress for crack generation declines. Microstructural observation revealed that the cracks were spread and diverted m and around the cluster of SiC particles, suggesting that crack propagation resistance was improved in the composite specimen.