Influence of notch size and the solidification microstructure on the rotating bending fatigue characteristics of SiC particle-reinforced AC4B alloy composites

The influence of the surface notch size, the SiC particle distribution and the solidification microstructure of the matrix on the fatigue characteristics were investigated, and the parameter governing fatigue was evaluated for SiC particle-reinforced JISAC4B alloy composites. 10 to 30% vol 11 um SiC particles were relatively homogeneously dispersed in an Al-6.79% mass Si-2.93% mass Cu-0.17% mass Mg-0.59% mass Fe matrix alloy through a combination of pressure infiltration and melt stirring casting technique. The matrix microstructure consists of a dendritic alpha phase and eutectic structure. A few intermetallic Fe compounds were observed in the dendritic alpha phase. Furthermore, all composite specimens contained SiC particle clusters. Vickers hardness of composites increased due to the dispersion of SiC particles and age hardening effect, and the hardening ability increased with an increase in the volume fraction of SiC particles. Rotating bending fatigue tests were carried out on both notch-free and notched specimens that were aged to reach their peak hardness. When the stress concentration factor was low, cracks generated from cast defects in the matrix alloy specimen. On the contrary, they generated in boundaries between the SiC particles and the matrix in the composite specimens. Furthermore, the fatigue strength decreased with an increase in the SiC volume fraction. For the notched matrix specimen, in which the stress concentration factor is high, the notch size governed the fatigue strength. However, the cracks generated from the boundaries near SiC particles instead of the notch base in the notched composite specimen. Moreover, it was found that the fatigue limit stress is unchanged in the composite specimen even when the notch is introduced, although the critical stress for crack generation is reduced. Microstructural observations revealed that the cracks are spread and diverted in and around clusters of SiC particles. This suggests that crack propagation resistance is improved by the SiC particles in the notched composite specimen.