Grain boundary evolution and continuous recrystallization of a superplastic Al—Cu—Zr alloy

Abstract

Two distinct microstructural transformation processes have been observed to enable superplastic response in aluminium alloys, depending upon the composition and thermomechanical history. Continuous recrystallization has been used to describe the transformation leading to a superplastically enabled state in alloys that respond to processing, as does the commercial alloy Supral 2004. Such a transformation is characterized by significant retention of the deformation microstructure and texture. Microtexture analysis methods have been employed to examine the grain–boundary misorientation distribution in as–processed Supral 2004, and its evolution during annealing of this alloy. A bimodal boundary misorientation distribution is observed in as–processed material and is seen to persist through subsequent annealing. Comparison of correlated (nearest neighbour) and uncorrelated (predicted by the texture) grain–boundary misorientation data reveals that the populations of boundaries in the misorientation ranges of 0–15° and 55–62.8° exceed those predicted. A model is presented describing high–angle boundaries as interfaces between symmetric variants of the deformation texture components. The misorientation distribution of lower angle boundaries can be fitted by a probability density function. It is concluded that grain subdivision into deformation bands during severe straining leads to the formation of high–angle grain boundaries as interfaces between symmetric variants of deformation texture components, while lower angle boundaries develop by dislocation reaction within these variants, leading to observed bimodal grain–boundary misorientation distributions. During post–processing static annealing, stable coarsening of deformation–induced features occurs gradually and homogeneously throughout the microstructure without recrystallization involving the formation of new grains by the migration of high–angle grain boundaries. The ability to quantify and model the reactions that occur during thermomechanical processing, and that lead to the microstructures present in Supral 2004, will facilitate processing for similar reactions in a wider family of aluminium alloys for enhanced formability.