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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

镁合金被认为具有较差的冷加工性能，因为在室温下只会发生基面滑移。因此，在本研究中，为了建立改善镁铝和镁锂合金冷加工性能的原则，采用等通道转角挤压(ECAE)对合金进行严重加工，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发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，在传统加工工艺中，基面是室温下最活跃的滑移面，倾向于挤压方向。结果表明，在含少量Al和Zn的镁合金中，在2 0 0~2 5 0℃条件下进行ECAE处理的镁合金中，如果拉伸Si…提取了较多的LE试件，使拉伸方向与挤压方向平行，基极图上的峰值强度向拉伸方向倾斜45°，从而使0.2%的试验应力低于100 Mpa，比常规的热挤压或热轧试件低约50%。此外，AS-ECAE试件的延伸率为35-40%，几乎高出2倍。延伸率的提高是由于均匀延伸率的增加。在高于2 5 0℃的温度下退火1h后，基极图中的强度降低，晶粒粗化。在该组织条件下，0.2%的验证应力取决于Hall-Petch关系，而伴随着退火的应变松弛使获得高塑性成为可能。变形模式是这样的，除了滑移变形外，孪生还发生在变形的最后阶段。从以上结果可以看出，通过对原料的细化和织构控制，可以实现镁合金的冷加工。还可以通过在ECAE处理之后进行锻造来抑制伸长率的降低，同时显著提高当基面沿垂直于压缩方向的方向、即平行于拉伸方向的方向对准时的验证应力。因此，AZ31合金锻态试件的拉伸性能高于JIS规定的6061变形铝合金T6处理试件的拉伸性能，并且在10^3s^~(-1)~(-1)~gT范围内，拉伸性能与应变速率成正比；对于双相(hcpα相+体心立方β相)镁锂合金，由于在50℃以下等温挤压过程中积累的高剪切应变，在低于T_m/2(T_m=熔化温度)的拉伸变形过程中发生动态再结晶，细化了晶界，并沿再结晶β相晶界析出具有体心立方结构的高度可变形的α相。这些组织的发展使得晶界容易滑动，导致即使在T_m/2以下的温度下也会出现高应变速率超塑性。