Microstructure evolution during superplastic deformation of an Al-coated Mg alloy sheet

Mg alloys have been gaining attention as the lightest metal material utilized for improvement of fuel efficiency of automobiles. However, their disadvantages, i.e., poor corrosion resistance and poor plastic deformability limit their application fields. The authors have fabricated Al-coated Mg alloy sheets by newly-developed hot extrusion process followed by hot rolling process. In the authors' previous study, it was demonstrated that the sheet exhibited superplasticity without peeling-off of the Al coating layer. However, the mechanism of superplastic deformation of the Al-coated Mg alloy sheet has not been clarified yet. Especially, the reason why the pure Al coating was able to be superplastically deformed was not been clearly understood yet. In the present study, therefore, the deformation mechanisms of both the Mg alloy substrate and the Al coating layer have been closely investigated by observing the change of a micro-grid structure drawn on the surface of the sheet based on an electron beam lithography technique, and by carrying out electron backscatter diffraction analysis of the crystal orientations. From the microgrid observation, it has been shown that the dominant deformation mechanism of the Mg alloy substrate is grain boundary sliding, while that of the Al coating layer is intragranular dislocation slip. Additionally, from the crystal orientation investigation, it has been demonstrated that the texture of the Mg alloy substrate is randomized during the superplastic deformation, which is indicative of the occurrence of grain boundary sliding. On the other hand, in the Mg alloy substrate near the Al/Mg interface, it has been observed that twinning occurs to accommodate shear stress generated by the deformation limit of the Mg alloy by the Al coating layer. (C) 2019 Elsevier B.V. All rights reserved.