THE EFFECT OF STRAIN RATE HISTORY ON THE DUCTILITY IN SUPERPLASTIC AA-5083

Abstract

Rapid oscillations of load during deformation of superplastic materials have been reported by several researchers. In an investigation on AA7475, Ghosh and Raj developed a model to study the oscillations observed within the strain rate range from 10{sup {minus}3}s{sup {minus}1} to 5 {times} 10{sup {minus}3}s{sup {minus}1}. The oscillations in the stress-strain curve were related to changes in the grain size due to refinement by dynamic recrystallization and grain growth during deformation. Using the same alloy Hamilton et al. observed oscillations in stress-strain curves at strain rates of 5 {times} 10{sup {minus}3}s{sup {minus}1} and 10{sup {minus}2}s{sup {minus}1}. These oscillations were attributed to dynamic strain aging. Yang et al. observed serrations in the stress-strain curves of AA7475 tested at temperatures ranging from 20 to 525 C. The amplitude of the load fluctuation was related to the strain-rate sensitivity. The explanation they gave for this was that serrations are caused by continuous shifting of multiple necks from one region to another, and dynamic change in the grain structure. Li and Blackwell studied load oscillations in tensile deformation of Al-Li and Al-Mg alloys at temperatures > 500 C and strain rate < 10{sup {minus}2}s{sup {minus}1}. They also observed that the amplitude of the fluctuations more » was found to very according to the strain rate sensitivity of the material. They concluded that load serrations observed in their study are a closed-loop controlled screw-driven machine effect. They also observed that cross-head speed varied in a cyclic manner. The purpose of this study is to determine the effects of applying non-uniform strain rates on superplastic properties of AA5083 using a motor-driven system operating in an open-loop mode. « less