Superplastic deformation behavior of 8090 aluminum-lithium alloy

Abstract

The superplastic deformation behavior of crystalline materials has generally been described phenomenologically by a power law relation between the two external variables, stress ({sigma}) and strain rate ({dot {epsilon}}). A new approach for structural superplasticity has been made in this study by taking the dislocating glide mechanism as the major accommodation process for grain boundary sliding (GBS) instead of the generally accepted high temperature diffusion process. For the purpose of generating a proper relationship between {sigma} and {dot {epsilon}}, a series of load relaxation tests has been carried out at temperatures ranging from 470 C to 530 C after obtaining the required grain sizes through a thermomechanical treatment. The load relaxation test provides the flow data in a much wider range of strain rate with minimal microstructural change during the test. The flow curves of 8090 Al-Li alloy have consequently been analyzed systematically based on the internal deformation theory described. From the analysis of load relaxation test results for 8090 Al-Li alloy with d = 10{mu}m, the following important results are obtained. (1) The load relaxation curves at high temperatures consists of the plastic strain rate ({dot {alpha}}) and the grain boundary sliding rate ({dot g}). (2) The plastic flow more » curves of log {sigma}{sup 1} vs. log {dot {alpha}} can precisely be described by Eq. (3) for 8090 Al-Li alloy. (3) The grain boundary sliding appears to be a Newtonian viscous flow process characterized by Eq. (5) with M{sub g} = 1.0. (4) The condition {Sigma}{sub g} {le} {sigma} {le} {sigma}* must be satisfied for the superplastic deformation of crystalline materials. « less