Texture, grain boundaries and deformation of superplastic aluminum alloys

Two distinct microstructural transformation processes have been observed to enable superplastic response in aluminum alloys, depending of composition and thermomechanical processing (TMP). Texture: observations, including microtexture data acquired by newly developed electron backcsatter diffraction (EBSD) analysis methods, and mechanical property results are summarized in this work. These data reveal a clear dependence of superplastic response on the mechanism of microstructural transformation that leads to a superplastically enabled condition. Transformation by continuous recrystallization involves recovery-dominated changes that occur gradually and homogeneously throughout the deformation microstructure in the absence of high angle boundary formation and migration. Important features of microstructural evolution include deformation banding leading to grain boundaries that are interfaces between symmetric variants of the main texture component, and the presence of a fine, cellular structure within the bands. Both grain boundary sliding and dislocation creep operate in the superplastic regime and alloys such as Al-5%Ca-5%Zn and Supral 2004 respond in this manner. Alternatively transformation via primary recrystallization processes involves the heterogeneous formation and growth of grains by the migration of high-angle grain boundaries from the deformation zones surrounding coarse precipitate particles. This leads to essentially random grain orientations and a predominance of high-angle boundaries in the microstructure. The mechanical behavior of these materials is governed by grain boundary sliding in the superplastic range and alloys such as 5083 and 7475 behave in this way. The implications of these results for aluminum alloy development and the design of deformation processing procedures for grain refinement and superplasticity will be discussed. Finally, we present a table describing the superplastic behavior of a number of aluminum alloys according to the two patterns mentioned above.