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.