| A new approach to structural superplasticity is attempted in this study based on the internal deformation variables. The basic concept of this theory is that the high temperature deformation, especially superplasticity, of crystalline materials consists mainly of grain boundary sliding (GBS) and grain matrix deformation by dislocation glide process to accommodate the incompatibility caused by GBS. The high temperature deformation behavior of 8090 Al-Li alloy has been studied experimentally within the framework of the new theory proposed. A series of load relaxation and tensile tests at various temperatures ranging from 200℃ to 530℃ have been conducted to obtain the flow curves and the results were systematically analyzed based on the internal variable theory. The flow curve of log σ vs. log ε has been found to be a composite curve consisting of GBS and plastic deformation confirming the kinematic relation of the steady state form, ε=α+g at sufficiently high temperature with α and g representing the strain rates due to plastic deformation and GBS, respectively. The GBS in 8090 Al-Li alloy examined in this study appears to be a Newtonian viscous flow process characterized by the power index value of Mg= 1.0. The internal strength variable (σ*) is found to be inversely proportional to the square root of grain size and the activation energy for grain matrix deformation is obtained as 124.9 kJ/mol in the temperature range from 470℃ to 530℃, which is very similar to the activation energy for self diffusion in pure Al. |
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