Development of Environmental-friendly Wrought Magnesium Alloys

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Magnesium alloys are said to have inferior cold workability because only basal slip could occur at room temperature. Therefore, in the present research, in order to establish principles for improving the cold workability of Mg-Al and Mg-Li alloys, Equal Channel Angular Extrusion (ECAE) was utilized to subject the alloys to severe working in order to: 1) make the interior of the materials experience intense shear strain or refine the grains through dynamic recrystallization that may occur due to the high strain energy; and 2) obtain a texture that is unattainable with conventional working processes in which the basal plane, which is the most active slip plane at room temperature, is inclined to the extrusion direction. Then the effects of these microstructural changes on the tensile properties of ECAE-processed materials were thoroughly investigated.The result shows that in Mg alloys containing small amounts of Al and Zn, which were subjected to ECAE processing at 200-250℃, if the tensi … More le test specimens are extracted such that the tensile direction is parallel to the extrusion direction, a peak intensity in basal pole figure is inclined at 45° to the tensile direction and, therefore, the 0.2% proof stress is below 100MPa, which is about 50% lower than that of conventional hot-extruded or hot-rolled specimens. Furthermore, the elongation of the as-ECAE specimens, which is 35-40%, is nearly 2 times higher. The improved elongation is due to an increase in the uniform elongation. After annealing for 1 hour at temperatures higher than 250℃, the intensity in the basal pole figure decreases and the grains coarsen. Under that microstructural condition, the 0.2% proof stress depends on the Hall-Petch relationship and the strain relaxation that accompanies annealing makes it possible to attain high ductility. The deformation modes are such that in addition to slip deformation, twinning also occurs at the final stage of the deformation. From the above results, it is possible to carry out cold working of magnesium alloys through grain refinement and texture control of the raw materials. It is also possible to suppress decrease in elongation, while remarkably improving the proof stress when the basal plane is aligned in a direction normal to the compression direction, that is, parallel to the tensile direction by carrying out forging after ECAE processing. Consequently, as-forged specimen of AZ31 alloy indicates higher tensile properties than those of T6-treated specimens of 6061 wrought aluminum alloy specified by JIS, and the tensile properties positively depend on the strain rate in the range up to 10^3s^<-1>.In the case of dual phase (hcp α-phase + bcc β-phase) Mg-Li alloys, dynamic recrystallization occurs during tensile deformation at temperatures lower than T_m/2 (T_m = melting temperature) due to high shear strain that accumulates during ECAE processing at temperatures below 50℃, and, therefore, the grain boundaries increase as a result of grain refinement Furthermore, the highly deformable β-phase with bcc structure precipitates along the grain boundaries of recrystallized α-phase. These microstructural developments make grain boundary sliding easy, resulting in the occurrence of high strain rate superplasticity even at temperatures below T_m/2. Less