| 题名 | Mg-Al-RE系压铸镁合金的组织调控与性能改进 |
| 作者 | 杨强 |
| 学位类别 | 博士 |
| 答辩日期 | 2015-04 |
| 授予单位 | 中国科学院研究生院 |
| 授予地点 | 中国科学院长春应用化学研究所 |
| 导师 | 孟健 ; 刘孝娟 |
| 关键词 | Mg-Al-RE合金 微观组织 力学性能 蠕变性能 腐蚀性能 |
| 中文摘要 | 近年来，随着全球能源日趋紧缺以及人类自然环境恶化，提高燃料效率及减少污染物排放等成为世界关注的热点。镁是目前最轻的金属结构材料，在汽车、航空航天、国防军工、电子通讯等领域内具有重要的应用价值，其中汽车是镁合金最大的消耗市场。Mg-Al-RE系合金具有优异的高温性能，是最有潜力广泛应用到汽车上的镁合金体系之一。但由于其强度还不能满足汽车传动件的要求且成本较高，其应用范围受到极大的限制。因此，提高Mg-Al-RE系合金性能及研究相关强化机制对推动其在汽车上的广泛应用具有重要的价值。 本文首先研究了含廉价稀土元素La的Mg-4Al-4La-0.3Mn（ALa44）合金，结果表明：a）该合金中主要第二相形貌为树枝状，在刚压铸得到的合金中为Al8La3相（体心正交结构，a = 0.44 nm, b = 1.456 nm, c = 1.265 nm）。经过高温热处理或长时间室温时效，Al8La3相会转变为Al11La3相（体心正交结构，a = 0.4432 nm, b = 1.0132 nm, c = 1.3142 nm），这种相变的发生与相的尺寸等有关。另外，在该合金中还存在少量的Al2La相，Al10La2Mn7相和依附在树枝状相上的纳米尺寸的Al8Mn5相。b）第一性原理计算结果表明，在Al?La二元系统中，Al11La3，Al3La，Al2La和AlLa相在温度低于1000 K时是稳定的，而Al4La相在1000 K一下都是不稳定的。因此，在Mg-Al-La系合金中，树枝状相应该标记为Al11La3，并且在1000 K以下，Al11La3相对于Al+Al2La两相平衡都是热力学稳定的。另外，在0 K下不稳定的AlLa3 (Pm-3m)相在温度高于590 K时将比AlLa3（P63/mmc）相稳定，这与实验结果比较吻合。c）压铸ALa44合金在150?200 ?C和50?93 MPa条件下，蠕变激活能落在80?120 kJ/mol范围内，蠕变应力指数约为7，表明依靠位错核心扩散过程控制的位错攀移控制着蠕变，相应的TEM位错观察也验证了这一结论。 其次，研究了含廉价稀土元素Sm的Mg-4Al-(4,6)Sm-0.3Mn（ASm44或ASm46）合金，结果表明：a）Sm比La的晶粒细化效果更好，且Sm含量越高，晶粒越细小，合金强度越高。另外，该体系合金可时效强化，中低温短时时效后，合金的屈服强度明显提高，其根本原因是时效后在Mg基体中析出了大量的纳米尺寸（约3.7 nm）的Al3Sm相。根据经典强化理论，并结合第一性原理计算，揭示了析出强化机制为：Orowan位错绕过机制，相应的TEM位错观察也证明了这一结论是正确的。b）Mg-Al-Sm系合金的应变硬化速率受加工方式、温度、时效和应变速率等多种因素的影响，但是合金中Sm含量对其影响不大。另外，时效对合金的高温抗蠕变性能有明显的影响：时效析出能够明显提高ASm46合金的高温抗蠕变性能，但却会降低ASm44合金的抗蠕变性能，这可能是由于在这两种合金中相应的蠕变机制不一样。 最后，研究了三种改进ALa44性能的方法：一是利用两种固溶度差别较大的稀土元素来代替单一稀土元素，得到Mg-4Al-3La-2Sm-0.3Mn（AE432）合金，结果表明：La和Sm同时添加具有比单一稀土元素更好的晶粒细化效果，合金的平均晶粒尺寸仅为7.5 ?m。在该合金中主要有四种形貌的第二相：块状Al2(Sm-La)相、树枝状Al11(La-Sm)3相、花瓣状Al11(La-Sm)3相和棒槌状Al2(Sm-La)相。另外，该合金在室温和高温下的拉伸性能和高温抗蠕变性能都明显优于ALa44合金。在200 ?C和80?95 MPa条件下的蠕变应力指数呈类“S”形，表明合金在蠕变过程中除了蠕变变形外还发生了长程塑性变形，同时位错攀移的回复过程控制蠕变。二是利用廉价碱土元素Ca来代替一部分稀土元素，得到Mg-4Al-2La-2Ca-0.3Mn（ALaX422）合金，结果表明：La和Ca同时添加具有比单一稀土更好的晶粒细化效果，合金的平均晶粒尺寸仅为7.1 ?m。该合金中主要有两种第二相：粗条状Al11La3相和多孔网状或连续块状C36相，（六方结构，a = 0.596 nm, c = 1.979 nm）。其中，C36相与Mg基体具有确定位向关系：[11 0]C36||[0001]Mg，(0001)C36||(10 0)Mg。经过中低温时效之后，在?-Mg基体中析出了大量的C15相，在靠近晶界处，C15析出相较长，分布密度较高；在晶粒中心，析出相较短，分布密度较低。另外，该合金在室温和高温下的拉伸强度明显高于ALa44合金，但在温度较低时，其断裂延伸率相对较低。蠕变实验结果表明，ALaX422合金的高温抗蠕变性能比传统AE44合金或ALa44合金低，在200 ?C和75?110 MPa时相应的蠕变机制为沿位错核心管道扩散过程控制的位错攀移。同时，位错交滑移及C15析出相等都对合金的高温蠕变行为有一定的影响。三是利用微合金元素B或Sr对ALa44合金进行改性，结果表明微量B和少量Sr都能在一定程度上提高ALa44合金的拉伸性能，同时也能明显改变合金中共晶区的形貌。研究还发现，微量B能够明显提高合金的抗腐蚀性能，尤其是添加量为0.01 wt.%；少量Sr对合金的高温抗蠕变性能有影响，但影响规律较复杂。 本文为开发低成本高性能汽车用压铸稀土镁合金提供了重要数据和设计思路。 |
| 英文摘要 | Recently, with the depletion of energy resource and the deterioration of the environment, worldwide emphasis will be placed on increased fuel efficiency and reduced levels of emissions. Magnesium is the lightest structural metal and also is abundant in the earth?s crust, thus providing an attraction for automotive industry, aerospace, military industry, communication, wherein the automobile is the largest consumption market. As is well known, high pressure die cast (HPDC) Mg-Al-RE based alloys with outstanding high temperature properties, are the most potential to widely using on automobile. However, as the strength cannot satisfy the safety requirements and the cost is very great, it has been impeded their use to powertrain components and even not widely used in automobile industry. Therefore, the investigations dealing with improving the properties of Mg-Al-RE based alloy and studying the corresponding strengthening mechanisms to promote the widely using of magnesium alloys in automobile industry are very important. Firstly, this work paid attentions on the HPDC Mg-4Al-4La-0.3Mn (ALa44) alloy, which contains the cheap rare earth of La. The results indicate that the morphology of the dominant intermetallic phase is dendrite, which is Al8La3 (body-center orthorhombic structure, a = 0.44 nm, b = 1.456 nm, c = 1.265 nm) in the just as-HPDC alloy, but will transformed to Al11La3 (body-center orthorhombic structure, a = 0.4432 nm, b = 1.0132 nm, c = 1.3142 nm）after heat treatment at high temperatures or aging at room temperature for long term. Furthermore, there are also some Al2La, Al10La2Mn7 and nano-scale Al8Mn5 attached on Al11La3 phase. Based on the first-principle calculations, Al11La3, Al3La, Al2La and AlLa are all stable at temperature below 1000 K while Al4La is metastable. Subsequently, the dendrite phase in Mg-Al-La based alloy should be indexed as Al11La3. In addition, it is also stable compared with the Al+Al2La equilibrium. Moreover, AlLa3 (Pm-3m) that is metastable at 0 K will become stable while the temperature is higher than approximately 590 K, which is very well in line with the experimental result. Finally, the creep results for the HPDC ALa44 alloy at 150?200 ?C and 50?93 MPa indicate that the creep activation energy is 80?120 kJ/mol, and the creep stress exponent is approximately 7, demonstrating that dislocation climb controlled by dislocation core diffusion controls creep, which is revealed by the dislocation observations by TEM. Then, the HPDC Mg-4Al-(4,6)Sm-0.3Mn (ASm44/ASm46）alloys containing the cheap rare earth of Sm were studied in this work, and the results demonstrate that Sm has a better effect on grain refinement, and finer grains with more Sm content, also with a higher strength. Additionally, this system can be further hardened by aging. The yield strength of this system is obviously improved by short-term aging at intermediate temperatures, because there are amounts of nano-scale precipitates (approximately 3.7 nm) of Al3Sm phase precipitated in the Mg matrix. Based on the classical strengthening theories, along with the first-principle calculations, the mechanism of the precipitation strengthening is revealed as Orowan dislocation bypassing, which is proved by the corresponding dislocation observations by TEM. Furthermore, it is found that the strain hardening rate of the Mg-Al-Sm based alloys is related to several factors, such as fabrication method, temperature, aging condition and strain rate, but is independent on the content of Sm in the studied alloys. Moreover, the creep resistance of ASm44 and ASm46 alloys can be influenced by aging: the creep resistance of ASm46 alloy is improved by aging while that of ASm44 is decreased, which is possibly contributed to the different creep mechanisms in these two studied alloys. Finally, three methods to further improve the properties of ALa44 alloy were explored and conducted in this work. The first method is instituting the single rare earth in ALa44 alloy by two rare earths with different solid solubilities in Mg, and the HPDC Mg-4Al-3La-2Sm-0.3Mn (AE432) alloy was obtained. The results suggest that the combinative addition of La and Sm has a better effect on grain refinement, with approximately 7.5 ?m for the average grain size of the studied alloy. In addition, there are mainly four intermetallic phases, with blocky Al2(Sm-La), dendrite Al11(La-Sm)3, petal-like Al11(La-Sm)3 and hammer-like Al2(Sm-La). Finally, both the tensile properties at both ambient temperature and high temperatures and the high temperature creep resistance are more excellent than those of the HPDC ALa44 alloy. At 200 ?C and 80?95 MPa, the curve of the creep stress exponents exhibits a similar “S” shape, indicating a long range plastic deformation occurred during creep except the creep deformation, and the recovery of dislocation climb controlled the creep. The second method is instituting a part of rare earth by the much cheaper element of Ca, and the HPDC Mg-4Al-2La-2Ca-0.3Mn (ALaX422) alloy was got. The results indicate that the combinative addition of La and Ca has a better effect on grain refinement, with the average grain size of the studied alloy being approximately 7.1 ?m. There are mainly two intermetallic phases, with the coarse dendrite Al11La3 and net-like or continuous blocky C36 phase (hexagonal structure, a = 0.596 nm, c = 1.979 nm), wherein the orientation relationship between C36 and Mg is [11 0]C36||[0001]Mg, (0001)C36||(10 0)Mg. Furthermore, there precipitated amounts of C15 plates in the ?-Mg matrix after aging at 200 ?C for 100h, with large size and high distribution density near the grain boundary and small size and low distribution density in the grain center. In addition, ALaX422 alloy has much high strength at both ambient temperature and high temperatures, but the elongation to fracture is relatively lower at temperatures below 200 ?C. Finally, ALaX422 alloy have relatively worse high temperature creep resistance compared with ALa44 alloy. At 200 ?C and 75?110 MPa, the corresponding creep mechanism is dislocation climb controled by diffusion along dislocation core. Meanwhile, the dislocation croos-slip and the C15 precipitation can also influence the creep deformation in some extent. The third method is modification of ALa44 alloy by trace B (< 0.03 wt.%) and Sr (< 0.50 wt.%). The results demonstrate that trace B or Sr can further improve the tensile properties in some extent and also can change the morphoogies of the eutectic regions. In addition, trace B, such as 0.01 wt.%, can significantly improve the corrosion resistance of ALa44 alloy, and trace Sr can complicately influence the high temperature, such as 200 ?C, creep resistance of ALa44 alloy. This work also provide important information and design trains for developing HPDC rare earth magnesium alloys with low cost and high strength for widely using in automobile industry. |
| 语种 | 中文 |
| 公开日期 | 2016-05-03 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.ciac.jl.cn/handle/322003/64493]  |
| 专题 | 长春应用化学研究所_长春应用化学研究所知识产出_学位论文 |
| 推荐引用方式 GB/T 7714 | 杨强. Mg-Al-RE系压铸镁合金的组织调控与性能改进[D]. 中国科学院长春应用化学研究所. 中国科学院研究生院. 2015. |
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