Stress-induced microstructural defects in a 15% SiC aluminium alloy metal-matrix composite

Abstract

The introduction of SiC particles into an alloy matrix system will change the stress field within the matrix. Residual stresses will be introduced when the composites are undergoing temperature changes, cooling down during solidification and also when undergoing thermal treatments, due to the differences in the coefficient of thermal expansion (CTE) between the SiC particles and the alloy matrix. The substantial changes in the stress field surrounding the ceramic particles can affect diffusion processes and precipitation kinetics, and hence may have an effect on the microstructure, upon which the mechanical properties of the composite are strongly dependent. Previous studies [1, 2] have shown that the microstructure of a cast aluminium alloy matrix can be greatly modified by the presence of SiC particles, with a reduction of the primary aluminium grain size. A high dislocation density in the composite materials has been commonly observed in the microstructural studies of different composite systems [3-5], as a result of the reinforcements introduced. This increased dislocation density has been shown to have accelerated the ageing process of the material, but not necessarily to have increased the hardness levels [6]. The influence of the prior strain on precipitation in an aluminium alloy was investigated by Rack [7], who showed that its presence can have a marked influence on precipitation phenomena by changing the kinetics of both the diffusion and nucleation processes. Therefore, an understanding of the defects related to the reinforcement agents in composite materials becomes important with respect to an appreciation of their microstructural response. Thus, the objective of this study was to investigate the effects of SiC particle reinforcement and thermal treatment on the microstructure of the matrix of a cast aluminium alloy A356-15% SiC particle composite, with emphasis on the microstructural defects present in the material. The composite was an A356 aluminium alloy reinforced with 15 vol % /3-SIC particles which was fabricated using a standard sandcasting technique by Alcan. The nominal chemical composition of the composite was 6.99% Si, 0.34% Mg and 0.11% Fe. The thermal treatments employed for the composite were solution-treated at 540°C for 8h, waterquenched and then aged at 155 °C for various times. The microstructures of the composite were then examined using electron microscopy. Fig. 1 shows the microstructure of the as-cast SiC A356 alloy composite. One of the significant