| 摘要 | 相对于多晶态镁合金，镁基金属玻璃具有高强度、高比强度、高弹性极限等优异性能。为了满足工程应用的需求，发展具有强玻璃形成能力、可获得大尺寸 (厘米尺度)块体材料、并兼有良好塑韧性的新型玻璃态镁合金具有重要意义。本工作以Mg-TM-RE (TM=Cu, Ag, RE = Y, Nd, Gd)合金为基础，研究了一系列合金的玻璃形成能力、高玻璃形成能力的原因和力学行为。采用合金熔体铜模浇铸，形成不同直径圆棒样品的方法，用于评价合金的玻璃形成能力。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结构与玻璃态性质，单向压缩实验评价代表性合金的力学行为。主要结论如下：
1. Mg-Cu-Gd三元合金的玻璃形成能力 (Glass forming ability, GFA)强烈地依赖于合金的成分变化。在铜模浇铸的条件下，Mg-Cu-Gd三元系形成块体金属玻璃 (Bulk metallic glass, BMG) 临界直径 (Critical diameter, Dc)大于等于8 mm的合金主要位于11 at. %Gd的成分线上。GFA最强的合金定位于Mg61Cu28Gd11，Dc为12 mm，是目前三元Mg合金体系中Dc最大的合金。三元合金的强GFA为发展GFA更高的高阶合金奠定了基础。另外，Mg-Cu-Gd三元BMG的压缩断裂强度大约为1070 MPa。
2. 在Mg61Cu28Gd11三元合金基础上，用Ag元素部分替代Cu，采用“3D法”对Mg-Cu-Ag-Gd四元系的GFA进行系统研究。该四元系在很宽的成分范围都可以形成Dc至少为20 mm的BMG。有7个合金的Dc可达到25 mm，其中GFA最强的合金定位于Mg59.5Cu22.9Ag6.6Gd11，铜模浇铸条件下的Dc为27 mm，为迄今为止Mg基BMG中Dc最大的合金。
3. 根据Angell关于液体“脆度”的概念，对Mg61Cu28Gd11三元和Mg59.5Cu22.9Ag6.6Gd11四元合金的液体脆度进行了研究。通过测量玻璃态的驰豫时间，然后通过VFT (Vogel-Fulcher-Tamman) 公式拟合可以得到合金的脆度参数 D*值。Mg61Cu28Gd11三元合金和Mg59.5Cu22.9Ag6.6Gd11四元合金的脆度参数D*值分别为22.4和25。Mg59.5Cu22.9Ag6.6Gd11四元合金的D*值高于以前所有报道的BMG的脆度值，表明合金超常的GFA是与其强液体性质相关联的。
4. 通过研究合金的“近平衡”凝固组织，建立了所形成BMG合金在Mg-Cu-Gd体系中的部分准平衡相图。这些合金分布于三个成分共轭三角形中。GFA最佳的合金Mg61Cu28Gd11位于Mg2Cu、Mg3CuGd (τ4)和Cu60Mg24Gd16构成的区域内。在非平衡凝固的过程中，Mg61Cu28Gd11形成玻璃的过程与伪二元共晶反应L→Mg2Cu+Mg2CuGd (τ3) 相联系，竞争相为Mg2Cu。在近平衡相图中，Mg61Cu28Gd11成分与伪二元共晶反应L→Mg2Cu+Mg3CuGd (τ4) 相联系。相应地，Mg59.5Cu22.9Ag6.6Gd11四元合金在非平衡凝固的过程中，Ag可大量固溶于Mg2Cu相，从而对于竞争晶体相Mg2Cu起到失稳的作用。同时，Ag元素部分替代Cu还可以降低Mg61Cu28Gd11的固相线和液相线温度，起到稳定过冷液体的作用。
5. 在Mg-Cu-Ag-Gd四元体系中，通过调整AgMg含量，Mg57.2Cu17.1Ag17.1Gd8.6经铜模浇铸，可以得到直径2~7 mm的AgMg树枝晶相作为初生相的镁基金属玻璃内生复合材料，其中枝晶相的体积百分数大约为30 %。Mg59.5Cu22.9Ag6.6Gd11单相BMG的压缩断裂强度为815~990 MPa。与单相BMG相比，Mg57.2Cu17.1Ag17.1Gd8.6复合材料的力学性能并没有得到明显改善，压缩断裂强度为789~1035 MPa，并且二者都没有明显的宏观塑性变形并且断裂强度数据分散，说明复合材料的断裂仍然是由铸态材料中的缺陷所控制。
6. 在Mg58.5Cu30.5Y11三元合金基础上，采用“3D法”，用Nd元素部分替代Y，对Mg-Cu-Y-Nd四元合金GFA进行了系统研究。表明，该四元系中GFA最强的合金为Mg57Cu31Y6.6Nd5.4，Dc为14 mm。相对于Mg-Cu-Y三元合金，Nd元素部分替代Y提高了GFA，说明“3D法”不仅适用于高溶质浓度元素的替代，也适用于低溶质组元的替代。Nd元素部分替代Y对合金的泊松比没有明显的影响。Mg57Cu31Y6.6Nd5.4 四元BMG的压缩断裂强度大约为 1180 MPa, 塑性应变为 1.2 %。这是迄今为止断裂强度最高的Mg基BMG。 |
| 其他摘要 | Formability and Mechanical Properties of Mg-Cu(Ag)-Gd Bulk Metallic Glasses
Qiang Zheng (Materials Physics and Chemistry)
Supervised by Prof. Man-ling Sui, Prof. Evan Ma and Prof. Jian Xu
In contrast to polycrystalline Mg alloys, Mg-based bulk metallic glasses (BMGs) have significant advantages such as high strength, high specific strength and high elastic limit. For engineering application, it is of great interest to develop new Mg-based BMGs with extraordinary glass forming ability, large critical size, and excellent mechanical properties. Based on Mg-TM-RE (TM=Cu, Ag, RE = Y, Nd, Gd) alloys, the glass forming ability (GFA), the reason of the high glass forming ability and mechanical properties for a series of alloys were investigated. The glass−forming ability for these alloys was evaluated using as-cast rod samples fabricated by copper mold casting. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and differential scanning calorimetry were employed to characterize the structures and glass nature. Compression test was used to investigate the mechanical properties for some representative alloys. The following conclusions are drawn:
1. Glass-forming ability of the Mg-Cu-Gd ternary system is strongly composition-dependent. The BMG forming alloys with a critical diameter (Dc) larger than or equal to 8 mm are mainly located on the composition with Gd content fixed at 11 at.%. The best BMG former was located at Mg61Cu28Gd11 with a Dc of 12 mm via copper mold casting. Currently, it is the largest BMG in Mg-based ternary systems. Also, this can offer a basis for developing high order alloys with better GFA. In addition, the compressive fracture strength of Mg61Cu28Gd11 BMG is around 1070 MPa.
2. Using the “3D approach”, research in the Mg-Cu-Ag-Gd quaternary system led to an alloy, Mg59.5Cu22.9Ag6.6Gd11, with a Dc of 27 mm via copper mold casting, which is the largest Mg-based BMG so far. Moreover, BMGs with Dc of at least 20 mm can be obtained in a wide range composition. Among them, seven alloys have a Dc of 25 mm.
3. According to Angell’s concept of fragility parameter, the liquid properties of ternary Mg61Cu28Gd11 and quaternary Mg59.5Cu22.9Ag6.6Gd11 alloys were systematically investigated. The alloy’s fragility parameter can be obtained by measuring the average relaxation time of the glassy alloy and then by fitting the data using the VFT-type (Vogle-Fulcher-Tamman) equation. The fragility parameter D* of the ternary Mg61Cu28Gd11 and quaternary Mg59.5Cu22.9Ag6.6Gd11 alloy is 22.4 and 25, respectively. The fragility parameter of Mg59.5Cu22.9Ag6.6Gd11 alloy is larger than that of all the already reported BMG-forming alloys, indicating its extraordinary GFA is correlated with its strong liquid behavior.
4. Partial quasi-equilibrium phase diagram is established for the BMG forming alloys in the Mg-Cu-Gd system by investigating the near-equilibrium solidification microstructure. These alloys distributed within three composition conjugate triangles. Mg61Cu28Gd11 alloy with the best GFA is located in region consisting of Mg2Cu, Mg3CuGd (τ4) and Cu60Mg24Gd16. During the non-equilibrium solidification, the glass formation process of Mg61Cu28Gd11 correlates with the pseudo-binary eutectic reaction L→Mg2Cu+Mg2CuGd (τ3) together with Mg2Cu as the competing phase. For the quasi-equilibrium phase diagram, Mg61Cu28Gd11 composition correlates with the pseudo-binary eutectic reaction L→Mg2Cu+Mg3CuGd (τ4). Accordingly, during the non-equilibrium solidification process of Mg59.5Cu22.9Ag6.6Gd11 alloy, Ag has a larger solubility in Mg2Cu phase and plays a significant role in frustration the competing phase Mg2Cu. Also, the substitution of Ag for Cu lowered the solidus and liquidus temperatures of Mg61Cu28Gd11 ternary and had the effect on stabilizing the liquid.
5. By adjusting the content of AgMg in the Mg-Cu-Ag-Gd system, the in-situ composites with a dendrite phase AgMg homogeniously dispersed on the BMG matrix were obtained at Mg57.2Cu17.1Ag17.1Gd8.6 as-cast alloy with diameters of 2~7 mm. The volume fraction of primary dendrite phase is about 30 %. Compressive fracture strength for the Mg59.5Cu22.9Ag6.6Gd11 monolithic BMG is 815~990 MPa. In comparasion with the monolithic BMG, the mechanical properties of the Mg57.2Cu17.1Ag17.1Gd8.6 glassy composite have no obvious improvement and the fracture strength remains 789~1035 MPa. Both monolithic BMG and composite have no obvious macroscopic plasticity and the data of fracture strength are scattered, indicating that the fracture is still controlled by the flaws in the as-cast materials.
6. Based on the ternary Mg58.5Cu30.5Y11, “3D pinpointing approach” was used to locate the best glass former in the quaternary Mg-Cu-Y-Nd system by using Nd to substitute for Y. Mg57Cu31Y6.6Nd5.4 alloy is discovered to have a Dc of 14 mm for BMG formation. Compared with the ternary alloy, the addition of Nd plays a significant role in improving the GFA, but has no distinct effect on changing the Poisson’s ratio. This indicates that the “3D approach” can be applied not only for the component with higher solute content, but also for the component with lower solute content. Compressive fracture strength and plastic strain of the Mg57Cu31Y6.6Nd5.4 quaternary BMG is about 1190 MPa and 1.2 %, respectively. This is the highest fracture strength in Mg-based BMGs so far. |
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