| 摘要 | 本文采用静态压缩试验(Quasi-static)对典型的韧性Zr基金属玻璃和脆性Co基金属玻璃进行压缩变形和断裂实验，采用扫描电子显微镜(SEM)技术观察了上述金属玻璃的变形行为和断裂形貌。同时，我们还采用微小冲压实验(Small Punch Test)来研究了韧性的Zr基金属玻璃在多轴载荷下的剪切变形与断裂行为。
实验结果表明：对于Zr47Cu46Al7金属玻璃，厚度的变化将影响其冷却速率的变化，而微观组织又严重地依赖于冷却速率。随着样品厚度的增加或冷却速率降低，金属玻璃复合材料中初晶相的体积分数不断地增加，使其变形与断裂行为也有所变化，由延性断裂向脆性断裂转变。当金属玻璃完全结晶化时，其维氏硬度才有所下降；当复合材料中初晶相的体积分数足够多时(约大于30%)，其压缩强度才有所下降；当有初晶相析出时，塑性变形能力也降低。为了明显区分不同样品的塑性变形能力，采用了一种新的实验方法，即微小冲压实验来进一步评价其韧脆转变。结果表明：对于完全金属玻璃可以形成致密的多重剪切带而形成网格状的结构。复合材料中第二相的体积分数比较高时，微小冲压实验也能较单向压缩更好地区分它们塑性变形能力的差异。
对于Co43Fe20Ta5.5B31.5金属玻璃，可以通过制备过程中控制其冷却速率，使其微观结构随冷却速率的降低表现出了越来越高的有序性，甚至析出树枝状晶体相。具体表现为：随着冷却速率的降低，断裂强度有所降低，但其塑性变形能力却有所提高。同时，试样的破碎程度，（即：破碎系数， ）变得越来越小，由于单位新断裂表面积上吸收的弹性能， 越来越多，断裂后引起的熔融的液体就越来越多，从而对裂纹的扩展产生了有效的延迟作用，表现出了微量的宏观塑性。同时，在裂纹的尖端又诱发了一定的剪切带，使复合材料的局部塑性变形区， 有所增加，在微观上增加了复合材料的断裂韧性， ，进而在宏观上表现出少量的塑性。
微小冲压实验主要是施加多轴载荷方式对金属玻璃的变形与断裂行为进行表征。通过剪切应力和拉伸应力的相互作用，在试样的表面上形成了规则的网格状剪切带。断裂后，观察了剪切带在三维空间上的分布。通过分析可以得出变形断裂过程是一个拉伸正应力, 和剪切应力, τ相互竞争的过程。另一方面，通过微小冲压实验确定了金属玻璃本征断裂因子, ( )决定着材料的力学性能。如果想要提高材料的强度，可以提高材料的抗拉伸强度, ；如果想要提高材料的塑性，可以适当降低材料的抗剪切强度, ；如果想要提高材料的韧性，可以同时提高材料的抗拉伸强度, 和适当降低抗剪切强度, ，使材料的本征断裂因子， 适当减小。 |
| 其他摘要 | This work employed the quasi-static compression test to characterize the deformation and fracture behaviors of bulk metallic glass alloys (BMGs). In the experimental process, we chose the typical tough Zr-based and brittle Co-based BMGs as the model materials. After the deformation and fracture, the patterns were observed by the scanning electron microscope (SEM). Besides, we introduced the small punch test to investigate the deformation and fracture behaviors of tough Zr-based BMGs under the multi-axial loading.
The experimental results show: for the Zr47Cu46Al7 BMGs, the sample thickness plays an important role in the cooling rate, and the cooling rate further controls the microstructure. With the increase in the sample thickness or decrease in the cooling rate, the volume fraction of primary crystal phases will become increasing. Furthermore, the deformation and fracture behavior will change continuously, from the ductility to the brittleness. The detailed illuminations are listed as follows: when the BMGs completely changed into the crystal phases, the Vickers hardness will decline notably; when the volume fraction of primary crystal phases is high enough (about＞30%), the strength will decline; and once the primary crystal phases precipitate, the plasticity will decline greatly. Except for the above two testing methods, the small punch test was introduced to estimate the plastic deformation ability. For the fully amorphous sample, the dense multiple shear bands can be triggered to form a grid pattern. For the composites with high volume fraction of primary crystal phase, the small punch test can successfully distinguish the difference in their intrinsic shear deformation ability and transition of ductility to brittleness.
For the Co43Fe20Ta5.5B31.5 BMGs, the cooling rate was controlled during the casting, and the degree of order in microstructure become increasing with the decrease in the cooling rate, and even some ductile dendritic crystal phases in-situ precipitation. The detailed illuminations are listed as follows: with the decrease in the cooling rate, the fracture strength became low and low, but the plasticity will be improved slightly. Meanwhile, the fragmentation degree of the metallic glass, (that is fragmentation coefficient, ) becomes small, and the fracture strength becomes low, and some plasticity appears. Meanwhile, the receiving energy of unit new fracture surface from the elastic energy, , will enhance, which contributes to the local melting behavior, and further makes a retarding effect on the crack propagation. That leads to an increase in plastic zone size, and fracture toughness, , which contribute the macroscopic plasticity of Co-based metallic glass.
Furthermore, the small punch test, as multi-axial loading mode, was employed to characterize the deformation and fracture behavior of BMGs. The regular grid patterns of shear bands will be formed, due to the interaction of shear stress and normal stress. After fracture, it is successful to observe the distribution of shear bands within three-dimensional space. Furthermore, it gives a convincing proof that the deformation and fracture process is the competitive program between the normal stress, and shear stress, τ, depending on change in the fracture mode factor, ( ). On the other hand, it is convinced that the intrinsic fracture mode factor, ( ) is the decisive factor to control the mechanical properties of materials. If one wants to enhance the strength of material, enhancing the intrinsic resistive tensile strength, is always suitable; if one wants to enhance the plasticity of material, decreasing the intrinsic resistive shear strength, is suitable; and if one wants to enhance the fracture toughness of material, enhancing both the intrinsic resistive tensile and shear strength as well as a small intrinsic fracture mode factor, ( ) is suitable. |
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