Development of Environmental-friendly Wrought Magnesium Alloys

据说镁合金具有较差的冷加工性，因为在室温下只会发生基底滑移。因此，在本研究中，为了建立提高Mg-Al和Mg-Li合金冷加工性能的原理，采用等通道角挤压（ECAE）对合金进行剧烈加工，目的是：1)使材料内部经历强烈的剪切应变或通过高应变能可能产生的动态再结晶细化晶粒；2)获得常规加工工艺无法获得的织构，其中基面是室温下最活跃的滑移面，向挤压方向倾斜。然后深入研究了这些微观组织变化对ecae加工材料拉伸性能的影响。结果表明：在200 ~ 250℃条件下，对含少量Al和Zn的镁合金进行ECAE处理，如果拉伸方向与挤压方向平行，提取少量试样，基底极图峰值强度向拉伸方向倾斜45°，则0.2%的抗应力低于100MPa，比常规热挤压或热轧试样的抗应力降低50%左右。此外，as-ECAE试样的伸长率为35-40%，提高了近2倍。伸长率的提高是由于均匀伸长率的提高。在高于250℃的温度下退火1小时后，基极图强度降低，晶粒变粗。在这种显微组织条件下，0.2%的抗应力依赖于Hall-Petch关系，退火过程中的应变松弛使材料获得高延展性成为可能。变形模式是这样的，除了滑移变形，孪晶也发生在变形的最后阶段。从以上结果可以看出，通过对原料进行晶粒细化和织构控制，可以对镁合金进行冷加工。在ECAE加工后进行锻造，将基面对准与压缩方向垂直，即与拉伸方向平行的方向，也可以抑制延伸率的下降，同时显著提高抗应力。结果表明，AZ31合金锻态试样的拉伸性能优于经t6处理的6061变形铝合金试样，且拉伸性能与应变速率呈正相关，应变速率范围为10^3s^<-1>。对于双相（hcp α-相+ bcc β-相）Mg-Li合金，在低于T_m/2 （T_m =熔化温度）的温度下，由于ECAE加工过程中积累了较高的剪切应变，在拉伸变形过程中发生动态再结晶，晶粒细化导致晶界增大，具有bcc组织的高变形β相沿再结晶α-相晶界析出。这些微观组织的发展使得晶界易于滑动，导致即使在低于T_m/2的温度下也会出现高应变速率的超塑性。少

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