Development of High Strain Rate Superplastic Aluminum-SiC Particulate Composits

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7075 aluminum alloys reinforced with SiC particulates, of which volume fractions are 5,10, and 15 % (MA7075+5SiC,+10SiC,+15SiC composite, respectively), have been developed by using mechanical alloying process. Superplastic properties of these composites have been investigated at the initial strain rates ranging from 10^<-5> s^<-1> to 2x10 s^<-1> at a temperature of 793 K.1. After the pre-straining, SiC particulates are distributed uniformly within matrix, and a extremely fine grain structure, with average grain size less than 1 mu m is developed.2. In log flow stress-log strain rate curves for the three composites, two strain rate regions showing low and high strain rate sensitivity exponent, m, (region 1 and 2, respectively) were recognized, as in usually observed in superplasticity. At very high strain rates from 5x10^<-1> to 2x10 s^<-1> (region 2), values of m were larger than 0.36, and superplastic elongations were larger than 200% in the three composites. In contrast, m values were less than 0.13 at lower strain rates (region 1), where low elongations (< 50%)appeared.3. With increasing of the SiC particulate content, region 2 moved to the range of higher strain rate. Grain boundary sliding took place more remarkably in region 2 rather than in region 1.The threshold stress, sigma_<th>, estimated by using an extrapolation method increased with increasing of the SiC content.4. A double logarithmic plot of the strain rate and the effective stress, sigma_e (=sigma-sigma_<th>), can be approxi- mated by straight lines with a slope of about 0.5 for MA 7075+SiC composites.5. The high strain rate superplasticity of the composites can be explained by the grain boundary sliding model accommodated by dislocation slip. The SiC particulates on grain boundaries can resist the boundary sliding, thus resulting the threshold stress, sigma_<th>.