An internal variable theory of structural superplasticity

Abstract

A new approach for structural superplasticity (SSP) is attempted in this study based on the internal deformation variables. The basic mechanisms of SSP are thought here to consist of interface sliding (IS), i.e. grain or phase boundary sliding (GBS/PBS), and a dislocation glide process accommodating the incompatibilities due to IS. For this purpose, a new constitutive framework for inelastic deformation is first developed by a simple consideration of dislocation kinematics to reveal the existence of "internal strain" and "internal spin" tensors. The evolution relationship of internal strain tensor is then shown to lead into a kinematic relationship between the observable deformation variables. To complete the constitutive structures, the constitutive relationships for each deformation Variable including the strain rate due to IS are also prescribed by kinetics consideration of each process. The theoretical results are then applied to the experimental results on a fine-grained 7475 Al and a Pb-Sn eutectic alloy obtained from load relaxation and tensile tests. (C) 1998 Acta Metallurgica Inc.