Abstract
Nowadays, reduction of greenhouse gases, e.g., CO2 and NOx, has been treated as an important global environmental issue. Since one of the major greenhouse gas sources is exhaust from automobiles, weight reduction of automobiles is strongly desired. Magnesium alloys are being widely investigated all over the world for weight reduction of structural components because of their low density. In case of the structural application, strength of magnesium alloys should be increased together with enhancing ductility, toughness and fatigue resistance. Ductility enhancement is important to form the alloys into complex shapes, improving the relatively low stiffness. Enhancing the fracture toughness and fatigue resistance are also other important issues to raise the reliability in structural applications. In this study, enhancement of strength-ductility balance and strength-toughness balance has been investigated by the modification of the microstructure. Tensile yield stress of pure magnesium and Mg alloys increased with decreasing grain size, expressed as the well known Hall-Petch relations. The yield stress varied with the kind of solute element for a fixed grain size ~ 1mm. The other interesting finding is that the elongation-to-failure also increased with grain refinement owing to the enhanced activity of grain boundary plasticity. The grain boundary plasticity decreased as the grain boundaries were strengthened by segregation of the solute elements. The other important factor in enhancing the ductility is distribution of the basal planes, i.e., basal texture. The basal texture can be modified by changing the applied shear loading direction, e.g., strong basal texture is formed parallel to the extrusion direction by the conventional extrusion, and relatively random texture can be formed by repetitive shear loading with ECAE. Yield stress of magnesium alloys decreases, but strain hardening rate after yielding and uniform elongation increases with texture randomization. Thus a combination of grain refinement and texture randomization is important to enhance the ductility in high-strength Mg alloys. At the same time, it was found that the fracture toughness of the Mg alloys could be also enhanced with grain refinement and texture randomization.
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