Magnesium alloys are the lightest commercial alloys and have great potential for weight reduction of automobiles to save the natural resources and to reduce the exhaust. As well as the development of the cast alloys for the power train components, production of the wrought alloys at low cost is attractive owing to the thinning of structural components. In structural applications, the strength of magnesium alloys should be increased along with their ductility to improve the relatively low stiffness, so that the alloys can be formed into complex shapes. Refining grain size is even more effective in increasing the strength of magnesium alloys because of the relatively large Taylor factor in its HCP structure compared to that of the other lightweight metal, aluminum. Distribution of the basal plane orientation can be modified by changing the applied shear loading direction during severe plastic working, and/or by changing the recrystallization behavior by a combination of distributing nano-scale particles and plastic working. Tensile elongation also increases with grain refinement due to the enhanced activity of grain boundary sliding (GBS) and non-basal dislocations. Especially at temperatures over 453 K, it has been extensively demonstrated that the fine-grained alloys exhibit superplasticity at high strain rates and/or low temperatures. Grain refinement of pure magnesium and the alloys has been demonstrated by severe plastic deformation through extrusion and/or rolling. It was found that the grain boundary plasticity in fine-grained pure magnesium is active even at room temperature when the grain size is reduced less than a few micro-meters, and then it exhibits an inverse Hall-Petch relation but large tensile elongation ~200 % due to GBS. On the other hand, GBS in fine-grained commercial alloys is hardly active at room temperature due to the segregation of solute atoms and precipitation on grain boundary. The effect of solute has not been, however, fully understood yet to enhance the mechanical properties in the fine-grained alloys. Kinds of solute, i.e., Al, Li, Y, etc., were investigated for the microstructure modification and enhancing the strength-ductility balance. Concentration of solute atoms inspected with high resolution TEM and EDS correlates with the grain refinement and mechanical properties in tension.

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