AN INTERNAL VARIABLE THEORY OF STRUCTURAL SUPERPLASTICITY

A new approach for structural superplasticity (SSP) is attempted in th is study based on the internal deformation variables. The basic mechan isms of SSP are thought here to consist of interface sliding (IS), i.e . grain or phase boundary sliding (GBS/PBS), and a dislocation glide p rocess accommodating the incompatibilities due to IS. For this purpose , a new constitutive framework for inelastic deformation is first deve loped by a simple consideration of dislocation kinematics to reveal th e 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 vari ables. To complete the constitutive structures, the constitutive relat ionships for each deformation Variable including the strain rate due t o IS are also prescribed by kinetics consideration of each process. Th e theoretical results are then applied to the experimental results on a fine-grained 7475 Al and a Pb-Sn eutectic alloy obtained from load r elaxation and tensile tests. (C) 1998 Acta Metallurgica Inc.