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

Two distinct microstructural transformation processes have been observed to enable superplastic response in aluminium alloys, depending upon the compo sition and thermomechanical history. Continuous recrystallization has been used to describe the transformation leading to a superplastically enabled s tate in alloys that respond to processing, as does the commercial alloy Sup ral 2004. Such a transformation is characterized by significant retention o f the deformation microstructure and texture. Microtexture analysis methods have been employed to examine the grain-boundary misorientation distributi on in as-processed Supral 2004? and its evolution during annealing of this alloy. A bimodal boundary misorientation distribution is observed in as-pro cessed material and is seen to persist through subsequent annealing. Compar ison of correlated (nearest neighbour) with uncorrelated (predicted by the texture) grain-boundary misorientation data reveals tl-lat the populations of boundaries in the misorientation ranges of 0-15 degrees and 55-62.8 degr ees exceed those predicted. ii model is presented describing high-angle bou ndaries 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 sub division into deformation bands during severe straining leads to the format ion 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 gr ain-boundary misorientation distributions. During post-processing static an nealing, 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 duri ng thermomechanical processing, and that lead to the microstructures presen t in Supral 2004, will facilitate processing for similar reactions in a wid er family of aluminium alloys for enhanced formability.