Study on Temperature Dependence of Thermal Expansion Behavior of SiCw/Al Composite by Internal Stress Analysis

Abstract

High speci®c stiffness, strength and low coef®cient of thermal expansion (CTE) make SiC whiskerreinforced aluminum alloy (SiCw=Al) composites an attractive class of candidates for many applications such as engine components, precision optical instruments and space structures [1±3]. An important feature is that the CTE of the SiCw=Al composites is adjustable by changing whisker volume fraction (Vf) and depends on the whisker orientation. The effect of whisker volume fraction and orientation on the CTE of the SiCw=Al composites has been predicted theoretically by using the Eshelby equivalent inclusion theory [4] and the Mori±Tanaka mean ®eld theory [5]. Experimental data were also reported by Ohnuki and Tomota [6], in which a favorable method was used to predict the CTE for any distribution of whisker orientation in SiCw=Al composites. However, it is worth noting that the CTE of the SiCw=Al composites is also affected by heat treatment, and it greatly depends on temperature, which has not been reported enough. It was found by Cao et al. [7] that the CTE of the SiCw=Al composite increases with increasing temperature from 100 8C to 400 8C, and the rate of increase was higher from 300 8C to 400 8C. However, there was no explanation about this result in his report, and so far no further research has been reported about this phenomenon. The previous result was obtained for the composites in the as-fabricated state. Recently, we observed a complicated temperature dependence of the CTE of the SiCw=Al composites for different heat-treated states. The aim of this research is to explain this result by means of internal stress and local plastic deformation analysis. The SiCw=Al composite used here was fabricated by the squeeze-casting route. The reinforcement was TWS-100 â-SiC whisker, and the matrix was commercial 6061 aluminum alloy. The orientation of the whiskers was random. The Vf was 32%. The following three states were realized by changing heat treatment: The ®rst state is the T6 state (solutiontreated at 520 8C for 1.5 h and aged at 150 8C for 9 h), hereafter called the T6-state. The second and third states are the deep-cooling state (T6-treated and then deep-cooled to y76 8C and y196 8C, respectively), hereafter called D1-state and D2-state, respectively. The CTE was measured for these three states using a push rod type dilatometer at a heating rate of 10 8C miny1 from 20 8C to 500 8C. It is known that the CTE of the aluminum alloys increases with increasing temperature, which is also the main reason for the increasing CTE of the SiCw=Al composite with increasing temperature, as shown in Fig. 1. However, a complicated temperature dependence behavior was found in Fig. 1. The rate of increase in the CTE of the T6-state composite is low below 250 8C but obviously is high above this temperature, which is hereafter called the critical temperature. A similar phenomenon was also found in the D1and D2-state composites. However, the critical temperatures are 120 8C and 50 8C, respectively, being much lower than that of the T6-state composite. Because of the large difference of the CTE between the SiC whisker and the aluminum alloy, internal stress will form in the matrix of the SiCw=Al composite during heating and cooling. If one assumes no sliding at the interface and no yielding of the whiskers, the change in average internal stress of the matrix can be explained by