THE EFFECT OF GRAIN-SIZE AND PRESTRAIN ON THE SUPERPLASTIC BEHAVIOR OF A 7475 ALUMINUM-ALLOY

A superplastic 7475 Al alloy with grain sizes ranging between 9 and 35 mum was tensile-tested at a strain rate of 1 x 10(-4) s-1 and temperatures of 730 K and 790 K. At 790 K, the ductility was found to decrease with an increase in grain size. At 730 K, on the other hand, the ductility was found to increase initially with an increase in grain size and then decrease with increasing grain size when the grain size is larger than 14 mum. The latter decrease in ductility is attributed to the lowered ability for grain boundary sliding with decreasing grain boundary area. The observed low ductility of the as-received material (grain size is 9 mum) at 730 K is due to an inhomogeneous microstructure. Strain distribution measurements showed that the deformation was quasi-uniform at a strain rate of 2 x 10(-4) s-1 and a temperature of 790 K and that necking was diffuse rather than localized. The study of the effect of prestraining at a fast strain rate (Region III) on the subsequent superplastic behavior (Region II) of the alloy has shown that prestraining caused a decrease in the elongation to failure as compared with the non-prestrained samples. This decrease in elongation is postulated to be associated with the growth of cavities formed during prestraining as well as prestrain-enhanced grain growth during deformation in Region II.