Abstract
Extremely fine grained 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 2×10 s−1 at a temperature of 793 K.
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 5×10−1 to 2×10 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. 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, σth, estimated by using an extrapolation method increased with increasing of the SiC content. A double logarithmic plot of the strain rate and the effective stress, σe(=σ−σth), can be approximated by straight lines with a slope of about 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 boundaries can resist the boundary sliding, thus resulting the threshold stress, σth.
