| The primary focus of research on superplasticity has been on the characterization and modeling under deformation conditions considered to be "steady-state". Many mechanistic models have been proposed to represent superplastic deformation, which determines the kinetics of the steady-state deformation as well as the characteristics of the relationship among flow stress, grain size, diffusion parameters, and other physical parameters. Although the general form of the mechanistic models has been useful in formulating reasonable constitutive relations for use in modeling the superplastic forming (SPF) process, the well documented flow hardening and the increasing interest in variable strain rate processing with SPF suggest that the suitability of relations based on steady-state needs to be examined. Particularly, the transient behavior with strain rate change should be studied to determine if related effects could alter the flow behavior from that predicted with relations based on steady-state assumption. This will not only provide a better constitutive relation which has a better representation for superplastic forming process, it will also provide an opportunity in investigating the mechanism(s) with superplastic deformation, which is a controversy aspect with superplasticity.; In this study, the primary goal is to investigate the transient behavior with superplastic deformation of model material Pb-Sn eutectic alloy by determining the stress corresponding to the non-elastic strain rate during changing of strain rate in deformation, both strain rate increasing as well as strain rate decreasing. A strong transient effect is observed with Pb-Sn eutectic alloy, which is manifested by the difference of relationship between flow stress/non-elastic strain rate in strain rate increasing and decreasing as well as their departure from the steady-state relation. Efforts have been made to give an insight into transient behavior with superplastic deformation by addressing the deformation mechanisms. A transient deformation model has been proposed, which incorporate an internal stress and provides a relatively good representation of the transient deformation behavior with superplastic Pb-Sn eutectic alloy. |
