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 tempera tures of 730 K and 790 K. At 790 K, the ductility was found to decreas e with an increase in grain size. At 730 K, on the other hand, the duc tility was found to increase initially with an increase in grain size and then decrease with increasing grain size when the grain size is la rger than 14 mum. The latter decrease in ductility is attributed to th e lowered ability for grain boundary sliding with decreasing grain bou ndary area. The observed low ductility of the as-received material (gr ain size is 9 mum) at 730 K is due to an inhomogeneous microstructure. Strain distribution measurements showed that the deformation was quas i-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 subs equent superplastic behavior (Region II) of the alloy has shown that p restraining caused a decrease in the elongation to failure as compared with the non-prestrained samples. This decrease in elongation is post ulated to be associated with the growth of cavities formed during pres training as well as prestrain-enhanced grain growth during deformation in Region II.