Deformation and reconstruction mechanisms in coarse-grained superplastic Al-Mg alloys

This paper concentrates on the superplastic response of fine-grained and coarse-grained Al-Mg alloys under uniaxial tension. To identify the main characteristics of superplastic deformation and to determine the optimum deformation parameters, the microstructure and dislocation substructure of the alloys are analyzed as a function of strain, strain rate and temperature using electron backscatter diffraction and transmission electron microscopy (TEM). Under optimum deformation conditions of temperature and strain rate, these Al-Mg alloys have an elongation to failure in excess of 300%. Dynamic recrystallization is dominant at strain rates in excess of 10(-1) s(-1) and results in a strong coarsening of the microstructure and premature failure. Dynamic recovery prevails at a strain rate of around 10(-2)s(-1), leading to great enhancement of the plasticity of the coarse-grained materials. TEM observations show that subgrain formation proceeds slowly. During initial straining, subgrains are formed primarily along the original grain boundaries. This results in a "core and mantle" microstructure, with dynamic recovery mainly taking place in the mantle region. A uniform substructure is established at a strain of the order of 1. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.