Relationship between properties and microstructure of superplastically formed Al-Li alloy

Shakesheff, Alan John (1990) Relationship between properties and microstructure of superplastically formed Al-Li alloy. University of Southampton, Doctoral Thesis.

Abstract

The effect of superplastic deformation on the microstructure and post-form tensile and fatigue properties of Al-Li-Cu-Mg-Zr (8090) alloy sheet has been studied. During superplastic deformation the sub-grains are consumed at low superplastic strains as the misorientation between grains increases and grain growth occurs. This leads to the transition from an aligned to a larger more equiaxed grain structure and a reduced texture intensity as a result of the more randomly oriented grains produced. No evidence for a recrystallisation process was found. The changes in grain structure and texture are considered therefore to be consistent with strain enhanced grain growth accompanied by grain boundary sliding and rotation. The tensile properties of superplastically formed sheet are influenced by copper content and superplastic strain. Increasing the copper content results in greater quench rate sensitivity which is related to the grain boundary precipitation of T₂ (Al₂CuLi₃) phase which occurs during cooling after forming. The reductions in strength that occur with increasing superplastic strain can be attributed to the presence of the lithium and magnesium depleted surface layer, grain growth and to a lesser extent the change in grain aspect ratio. The fatigue properties of superplastically formed sheet have been shown to be influenced by grain structure. Additional factors such as the increase in surface roughness and the presence of the depleted surface layer may encourage the initiation of fatigue cracks and thereby lead to reduced fatigue performance.

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