Abstract
This paper discusses large superplastic-like elongation through a deformation mechanism transition in fine-grained Al−Mg alloys. To obtain a fine-grained microstructure, we adapted friction stir processing (FSP) for use in Al−Mg alloys. The average grain size after FSP in this study was approximately 7 μm. However, this microstructure was thermally unstable, and grain growth easily occurred above 773 K. High-temperature tensile testing showed large elongation of over 200 % over a wide range of high-temperature tensile conditions. In the initial deformation stage at a strain of 0.1, the dominant deformation mechanism in this alloy, from the viewpoint of the stress exponent and activation energy for deformation, was considered to be grain boundary sliding (GBS) rate-controlled by the interdiffusion of Mg in Al. However, the deformed microstructures were equiaxial until a strain of 0.5, while above a strain of 0.5 they were elongated in the tensile axis direction. This feature of elongation of the microstructure is similar to solute-drag creep deformation. Solute-drag creep is known as one of the deformation mechanisms that exhibit superplastic-like behavior in Class I solid solutions. Moreover, Al−Mg alloys are widely known as typical Class I solid solutions. These microstructural changes were discussed by constructing a deformation mechanism map. From this, it was considered that grain growth during high-temperature deformation resulted in transition of the deformation mechanism from GBS to solute-drag creep.
