HIGH-STRAIN RATE SUPERPLASTICITY OF SIC PARTICULATE-REINFORCED 7075-ALUMINUM COMPOSITES

Extremely fine grained 7075 aluminum alloys reinforced with SiC partic ulates, of which volume fractions are 5, 10, and 15% (MA7075+5SiC, +1O SiC, +15SiC composite, respectively), have been developed by using mec hanical alloying process. Superplastic properties of these composites have been investigated at the initial strain rates ranging from 10(-5) s(-1) to 2 x 10 s(-1) at a temperature of 793 K. In log flow stress-l og strain rate curves for the three composites, two strain rate region s showing low and high strain rate sensitivity exponent, m, (region 1 and 2, respectively) were recognized, as in usually observed in superp lasticity. At very high strain rates from 5 x 10(-1) to 2 x 10 s(-1) ( region 2), values of m were larger than 0.36, and superplastic elongat ions were larger than 200% in the three composites. In contrast, m val ues were less than 0.13 at lower strain rates (region 1), where low el ongations (<50%) appeared. With increasing of the SiC particulate cont ent, 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 extrapolatio n method increased with increasing of the SiC content. A double logari thmic plot of the strain rate and the effective stress, sigma(e)(=sigm a-sigma(th)), can be approximated by straight lines with a slope of ab out 0.5 for MA7075+SiC composites, which suggests the superplasticity of the composites to be explained by the grain boundary sliding model accommodated by dislocation slip. The SiC particulates on grain bounda ries can resist the boundary sliding, thus resulting the threshold str ess, sigma(th).