Superplastic Elongation through Deformation Mechanism Transition during High-Temperature Deformation in Thermally Unstable Fine-Grained Aluminum Solid Solution Alloy

In this study, the superplastic behavior on a fine-grained aluminum solid solution alloy consisting of thermally unstable microstructures was investigated. In order to obtain the fine-grained microstructure, friction stir processing (FSP) was applied to a commercial 5083 aluminum alloy. An equiaxial fine-grained microstructure of 7.8 mm was obtained after FSP, but this microstructure was thermally unstable at high temperatures. Commonly, for fine-grained superplasticity to occur (or to continue grain boundary sliding (GBS)), it is necessary to keep the fine-grained microstructure to less than 10 mm during the high-temperature deformation. However, in this study, a large elongation of over 200% was observed at high temperatures in spite of the occurrence of grain growth. From the microstructural observations, it was determined that the fine-grained microstructure was maintained until the early stage of deformation, but the transgranular deformation was observed at a strain of over 100%. The microstructural feature of the abovementioned transgranular deformation is similar to the deformation microstructure of the solute drag creep occurring in "Class I"-type solid solution alloys. This indicates that the deformation mechanism transition from GBS to the solute drag creep occurred during high-temperature deformation. Here, the possibility of occurrence of the superplastic elongation through deformation mechanism transition is discussed as a model of the thermally unstable aluminum solid solution alloy.