Development of Environmental-friendly Wrought Magnesium Alloys

环保型变形镁合金的开发

基本信息

批准号：
11305049
负责人：
KOJIMA Yo
金额：
$ 26.76万
依托单位：
Nagaoka University of Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2001
项目状态：
已结题

项目摘要

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

由于在室温下只能发生基础滑移，因此镁合金的冷加工性能较差，因此，在本研究中，为了建立改善Mg-Al和Mg-Li合金的冷加工性能的原理，采用了等通道角挤压。利用ECAE（ECAE）对合金进行严格的加工，以便：1）使材料内部经历强烈的剪切应变或通过由于高应变能而可能发生的动态再结晶来细化晶粒； 2）获得传统加工工艺无法获得的织构，其中基面（室温下最活跃的滑移面）倾斜于挤压方向然后研究这些微观结构变化对ECAE-拉伸性能的影响。结果表明，在含有少量Al和Zn的镁合金中，在200-250℃进行ECAE加工时，如果提取拉伸试样，如由于拉伸方向与挤压方向平行，基极图峰值强度与拉伸方向倾斜45°，因此0.2%屈服强度低于100MPa，比传统材料降低约50%此外，ECAE 样品的伸长率提高了近 2 倍。在高于250℃的温度下退火1小时后，基础极图的强度降低并且晶粒粗化，在该显微组织条件下，0.2%屈服强度取决于Hall-Petch关系。伴随退火的应变松弛使得可以获得高延展性。变形模式除了滑移变形外，在最终阶段还发生孪生。由以上结果可知，通过对原材料进行晶粒细化和织构控制，可以在提高基面取向时的屈服强度的同时抑制伸长率的降低。在垂直于压缩方向的方向上，即平行于拉伸方向，通过ECAE加工测试后进行锻造，AZ31合金的锻造态样品表现出比T6处理的样品更高的拉伸性能。 JIS 规定的 6061 变形铝合金，拉伸性能正向依赖于最大 10^3s^<-1> 范围内的应变速率。双相 (hcp α 相 + bcc β 相) Mg -Li合金，由于ECAE加工过程中在低于T_m/2（T_m =熔化温度）的温度下积累的高剪切应变，在拉伸变形过程中会发生动态再结晶50℃，因此，晶粒细化导致晶界增加，此外，具有bcc结构的高变形β相沿着再结晶α相的晶界析出，这些微观结构的发展使晶界易于滑动，从而导致晶粒细化。即使在低于 T_m/2 的温度下也会出现高应变率超塑性。