| 铜模激冷Mg-Al-Ca合金的微观结构与力学性能 | |
| Alternative Title | Mircrostructure and Mechanical Properties of Chill-cast Mg-Al-Ca Ternary Alloys |
| 史玲玲 | |
| Subtype | 博士 |
| Thesis Advisor | 马恩 |
| 2008-05-26 | |
| Degree Grantor | 中国科学院金属研究所 |
| Place of Conferral | 金属研究所 |
| Degree Discipline | 材料学 |
| Keyword | 镁合金 凝固 共晶 力学性能 金属玻璃 |
| Abstract | 镁合金是质量最轻的结构金属材料，具有高比强度，高比刚度，减震性好，电磁屏蔽和抗辐射能力强，易于铸造和回收以及成本低廉等优势。为了满足广泛应用需求，仍有待于发展高强度与良好塑性相匹配的新型镁合金体系。本工作研究了在Mg-Al-Ca 三元轻质合金体系中，两个成分系列合金的铸态微观结构演化及其力学行为，包括感应熔炼后缓冷和铜模激冷两种条件下合金的凝固组织、冷却速率对晶粒尺寸和相选择的影响等。另外，本工作还研究了块体金属玻璃形成合金Mg61Cu28Gd11缓冷的凝固组织，确定出与玻璃形成相关联的共晶反应；建立了根据金属玻璃的结构预测二元低溶质浓度合金形成金属玻璃成分的模型。利用x-射线、扫描电镜、透射电镜、热分析等技术分析表征铸态样品的微观结构，单向压缩试验评价合金的力学行为。主要内容与结论如下： （1） 铜模激冷Mgx(Al0.44Ca0.56)100-x（86≤x≤95）系列合金的凝固组织为含初生Mg枝晶的亚共晶组织。随着Mg含量的增加，合金中Mg枝晶的体积相对量增加，导致合金的强度下降，但塑性变形能力提高。Mg含量为90 at.％的合金具有最佳的强度与塑性匹配（σf＝445 MPa，εp＝4.5%）。Mg含量低于79 at.％的过共晶组织合金，Mg含量下降导致强度和塑性均下降。共晶成分的Mg82(Al0.44Ca0.56)18合金具有最高的压缩断裂强度，可达609 MPa。 （2） Mg-Al-Ca三元体系沿着Mg与Al2Ca相成分连线的Mgx(Al2Ca)100-x（76≤x≤87）系列合金，主要涉及[L→(Mg,Al)2Ca+Mg]伪二元共晶转变。Mg含量高于81 at.%的合金，铜模激冷的凝固组织为Mg枝晶分布在细化的[(Mg,Al)2Ca+Mg]共晶基体上的复合结构。共晶组织体积相对量最高的合金为Mg79(Al2Ca)21，形成Mg枝晶包围的初生(Mg,Al)2Ca颗粒分布于共晶基体上的复合结构。合金的力学性能取决于凝固组织中初生相和共晶组织的体积相对量。共晶组织体积含量最高的Mg79(Al2Ca)21合金具有最高的断裂强度（σf＝600 MPa）和比断裂强度（3.4105 N•m•kg−1）。 （3） Mg61Cu28Gd11块体金属玻璃（Bulk Metallic glass，BMG）形成合金的缓冷凝固组织仅包含有二元化合物Mg2Cu和三元化合物Mg2CuGd两相，表明合金熔体的凝固主要经历（L→Mg2Cu+Mg2CuGd）伪二元共晶反应。Mg2CuGd三元化合物的晶体结构被确定为b心正交点阵，点阵常数为a=0.42 nm、b=2.63 nm和c=2.00 nm，空间群可能为Bm21b(36)、B2mb(40)和Bmmb(63)三种类型中的某一种。Mg2CuGd相晶体结构的复杂性和（L→Mg2Cu+Mg2CuGd）共晶反应的存在是Mg61Cu28Gd11合金易于形成金属玻璃的主要原因。 （4） 基于金属玻璃中异类原子间的化学交互作用强于同类原子的短程有序结构特征，建立了以溶质原子为中心、溶剂原子位于其第一配位壳层的短程有序团簇结构，同时这些团簇按照配位数为12的二十面体模式进行堆垛的金属玻璃结构模型。利用这一结构模型预测了二元合金体系中低溶质浓度（小于30 at.%）成分范围内因原子的有效密堆而有利于形成金属玻璃的成分。模型预测的成分值与由实验获得的实际玻璃形成范围（Glass Forming Range，GFA）大体上一致，这表明形成金属玻璃的化学成分本征上强烈地依赖于其原子高效密堆的结构特征。对于液相线对称的共晶体系，模型预测的稳定过冷液体成分（可间接地反映热力学上形成金属玻璃的趋势）与相图中的平衡共晶点成分十分吻合；而对于非对称共晶体系，模型预测的易于形成金属玻璃的成分偏离相图给出的共晶点成分。根据理想化的金属玻璃结构特征可较圆满地解释关于相图中共晶点出现几率的“Stockdale/Hume-Rothery困惑”。 |
| Other Abstract | As the most lightweight metallic construction materials, Mg alloys have significant advantages such as high specific strength and stiffness, excellent absorption of vibration, castability and recyclability. For industrial applications, it is of great interest to develop new Mg alloys with good combination of mechanical properties. For Mg-Al-Ca ternary alloys, the microstructure evolution and mechanical properties for the as-cast alloys along two composition lines were investigated. Using the induction-melting and chill-cast techniques, the grain size and phase selection are justified with cooling rate. Additionally, the microstructure after slow solidfication and the correlated eutectic reaction for the Mg61Cu28Gd11 BMG forming alloy were investigated; using an atomic packing model for metallic glasses, we predicted the alloy compositions structurally favorable for the formation of metallic glasses in the solvent rich terminal of binary alloy systems. XRD, SEM, TEM, and DSC were employed to characterize the structures. Compression test was used to investigate the mechanical properties. The main contents and conclusions are in the following: 1. For Mgx(Al0.44Ca0.56)100-x (86≤x≤95) hypoeutectic alloys, Mg dendrites were introduced into the refined eutectic matrix. The reduced volume fraction of Mg dendrites with the decrease of Mg content caused the decreased strength but improved plasticity. Mg90(Al0.44Ca0.56)10 alloy exhibited the best combination of mechanical properties: the compressive σf and εp were measured to be 445 MPa and 4.5%. Mg82(Al0.44Ca0.56)18 eutectic alloy possesed the hightest σf of 609 MPa. 2. A series of Mgx(Al2Ca)100-x (76≤x≤87) ternary alloys along the Mg-Al2Ca composition line of the Mg-Al-Ca ternary system were determined to be mostly correlated with the pseudo binary eutectic reaction [L→Mg+(Mg,Al)2Ca]. In-situ composites with primary Mg dendrites embedded in the eutectic matrix were obtained with 81≤x. Primary (Mg,Al)2Ca particles and Mg dendrites were developed with Mg content less than 79 at.%. Mg79(Al2Ca)21 alloy with the largest volume fraction of eutectic matrix shows the highest compressive σf of 600 MPa and the specific fracture strength reached 3.4×105 N•m•kg-1. 3. The mircrostructure of Mg61Cu28Gd11 BMG forming alloy under slow-cooled condition is only composed of Mg2Cu and Mg2CuGd, indicating it mainly correlates with the (L→Mg2Cu+Mg2CuGd) pseudo eutectic reaction. The structure of Mg2CuGd is determined to be b-centered orthorhombic with lattice parameters of a=0.42 nm, b=2.63 nm and c=2.00 nm. The complex crystal structure of Mg2CuGd and the existence of the (L→Mg2Cu+Mg2CuGd) pseudo eutectic reaction are the primary factors for the easiness of Mg61Cu28Gd11 alloy to form BMG. 4. A solute-centered atomic cluster packing model in the solute lean terminal of binary alloy systems are established. The centered solute atom is surrounded by solvent atoms only and these clusters interpenetrate and tend to have 12 neighboring clusters with icosahedral-like arrangement. The model predicted the alloy compositions structurally favorable for the formation of metallic glasses for the efficient filling of space. The predicted compositions coincide with the experimental GFR. For symmetric eutectic systems, the predicted compositions are consistent with the eutectic compositions in the equilibrium phase diagrams; for asymmetric ones, these compositions are skewed off from the eutectic compositions. Rationales from the structural standpoint are presented to explain the increased frequency of appearance of eutectics at certain composition ratios (the Stockdale / Hume-Rothery puzzle). |
| Pages | 130 |
| Language | 中文 |
| Document Type | 学位论文 |
| Identifier | http://ir.imr.ac.cn/handle/321006/17042 |
| Collection | 中国科学院金属研究所 |
| Recommended Citation GB/T 7714 | 史玲玲. 铜模激冷Mg-Al-Ca合金的微观结构与力学性能[D]. 金属研究所. 中国科学院金属研究所,2008. |
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