IMR OpenIR
Zr基单相块体金属玻璃室温形变机制研究
Alternative TitleDeformation Mechanism of Zr-based Monolithic Bulk Metallic Glass at Room Temperature
郭华
Subtype博士
Thesis Advisor隋曼龄
2008-12-26
Degree Grantor中国科学院金属研究所
Place of Conferral金属研究所
Degree Discipline材料物理与化学
Keyword金属玻璃 塑性形变 剪切带 尺寸效应 原位拉伸 显微结构 透射电镜
Abstract应用具有高强度、高弹性极限且耐蚀的金属玻璃一直是这一领域研究学者追逐的目标,而理解金属玻璃的形变机制,提高室温塑性是金属玻璃走向广泛应用的必经之路。本文以Zr52.5Cu17.9Al10Ni14.6Ti5金属玻璃为研究对象,利用扫描电镜和透射电镜系统地研究了不同载荷下剪切带的扩展情况以及样品的整体形变过程,进而揭示了其室温下的宏观和微观形变机制。 块体金属玻璃的室温塑性形变能力非常有限,塑性形变高度集中于剪切带内,主剪切带一旦形成即迅速扩展导致整个样品瞬间断裂。因而揭示主剪切带在形变过程中内部是否发生变化对理解整个金属玻璃样品的失效十分有帮助。为了获得含有主剪切带的样品,将柱状样品装入保护套内进行压缩试验。由于保护套的存在,最大剪应力面上的剪切只能发生在有限的空间内,于是获得了主剪切带已经开动但裂纹扩展被抑制的样品,此时断裂尚未完全结束。剖开样品后,300 µm的剪切台阶明确标明了剪切发生的位置。利用透射电镜可以观察到主剪切带位置约10 µm宽区域发生了优先离子减薄,这意味着其结构发生了明显变化。如此宽的结构变化区域应该是剪切带和热影响区共同作用的结果,并且这个宽度与基于恒温热源模型估算的热影响区厚度基本吻合。也就是说剪切过程不应该是完全绝热的,剪切带应该在较长时间段内保持某一较高温度。同时恒温热源模型还揭示了不仅热影响区的厚度取决与剪切经历的时间,剪切带本身也应该随着剪切的进行逐渐变厚,而不是始终保持不变。 尽管块体金属玻璃在室温下通常脆断,但通过改变样品几何形状的方法可以在完全非晶态样品中实现大的压缩塑性形变。比如小髙径比样品在压缩过程中可以产生严重的塑性形变,甚至不会断裂。尽管这没有从根本上提高材料的塑性形变能力,但它提供了很好的观察金属玻璃形变的机会。在小髙径比块体样品压缩实验中,对比同一样品在不同形变量时刻的扫描电镜显微照片,发现形变量越大剪切带越多,但剪切带的分布十分不均匀。形变量较大时,剪切带增加的趋势放缓,剪切带的增加主要来自已经存在的剪切带的扩展和分叉。与以往认为的块体金属玻璃室温塑性形变集中于剪切带内不同,位于剪切带之间的区域同样会发生塑性形变。形变后无法利用扫描电镜观察到该区域内剪切带的增加,这说明局部发生了均匀流变。发生均匀形变的区域很小,位于间距小于几十微米的剪切带之间,局部应力状态复杂。但无论是由剪切带分隔的封闭区域还是开放区域都可以发生均匀形变。 上面的观察似乎暗示小尺寸样品的形变模式有可能与块体样品不同,而当样品尺寸接近剪切带萌生厚度10 nm时,金属玻璃如何形变是一个很值得思考的问题。利用聚焦离子束加工技术,在块体金属玻璃中加工获得亚微米尺度的拉伸样品,并在透射电镜中对其进行原位拉伸实验。结果表明,这一尺度下样品表现出明显的尺寸效应,拉伸载荷下塑性可达23-45%,其中均匀伸长量超过10%。此时样品的塑性形变不再依赖剪切带,原子尺度剪切事件发生于整个样品中。形变高度集中后,剪切形变也不是唯一的选择,样品甚至可以通过颈缩这一塑性材料特有的方式断裂,而且整个断裂过程稳定可控。这些都说明小尺寸金属玻璃自身具有大塑性形变能力。 利用选区电子衍射和高分辨像技术,在以上所有形变样品的剪切带或热影响区中都没有观察到由形变或温升导致的晶化。利用新近发展的FEM(Fluctuation electron microscopy)方法也无法分辨均匀形变部分与未形变样品的结构差别,这可能是因为FEM方法对如此小的结构差异不敏感。但剪切带和热影响区部分中程有序度下降,说明原子排列应该趋于更加混乱。 总之,不依靠晶化相和剪切带,Zr52.5Cu17.9Al10Ni14.6Ti5单相金属玻璃在室温下有能力均匀形变,只是发生均匀形变的样品尺寸或局部区域的尺度很小。块体样品加载过程中,均匀形变与剪切带同时开动互相协调,最终导致整个样品的宏观塑性形变。剪切带在萌生后,其厚度随着进一步的剪切形变而逐渐增加,当剪切带的迅速扩展不能被有效抑制时,主剪切带的形成最终将导致材料的灾难性破坏。
Other AbstractApplication of metallic glasses with high strength, large elastic limit, good corrosion resistance, is always the prime target pursuited by researchers in this field, and the only way for using bulk metallic glasses widely is plasticity improvement at ambient temperature by understanding the deformation mechanism. Here, by scanning electron microscope (SEM) and transmission electron microscope (TEM), systematic research was carried out on the propagation of shear bands and deformation processes of the entire sample in a Zr52.5Cu17.9Al10Ni14.6Ti5 metallic glass under various loading conditions, revealing the fundamental deformation mechanisms in microscopic and macroscopic scales at room temperature. At ambient temperature plasticity in monolithic bulk metallic glass is very limited, and the plastic deformation is heavily localized in shear bands, which propogate rapidly after initiation, resulting in the catastrophic fracture instantly. So making clear what happens in primary shear band is very helpful for understanding the fracture of metallic glass. To obtain the sample with primary shear band, the rod sample was first compressed with a cylindrical stopper. There was very limited room for shearing on the plane of maximum shear stress due to the restraint of the stopper, and the unfractured specimen with dominant shear band was achieved. The offsets in about 300 µm clearly pointed out the position where shearing operated on cross-section of the splited sample. In TEM, preferentially ion-milled region with a width of 10 µm could be observed on the position of primary shear band, which revealed that the structure of the shear band had been changed locally. Such region should be caused by the combination of shear band and its heat affected zone, the width of which was approximately consistent with the thickness of heat affected zone estimated based on the isothermal model. This meant that the shear band evolution was not an adiabatic process, and shear band should keep on being a certain high temperature for a long time. Based on the isothermal model, it was found that not only the thickness of heat affected zone depended on the shearing time, but also the shear band thickness increased as shearing rather than keeping constant. Bulk metallic glasses usually display brittle fracture, but the compressive plasticity could be realized in fully amorphous sample by changing its geometry. For instance, heavy deformation could be obtained in specimen with low aspect ratio under compressive loading, even no fracture would occur. Although it is impossible to improve plasticity at the source by this method, this provides an opportunity to observe how metallic glass deforms. In the compressive test on box-shaped bulk sample, by comparing the SEM morphologies of the same specimen at different strains, it was found that the amount of shear bands increased with the strain while the distribution of shear bands was uneven. At large strain, the multiplication of shear bands mainly came from the propagation and branching of the formed shear bands. In contrast with the previous viewpoint that the plastic deformation should concentrate in shear bands at room temperature for metallic glasses, the plastic deformation took place at some regions out of shear bands as well. No shear bands were born at such deformed region, which demonstrated that homogeneous flow happened. The homogeneous flow regions were enclosed or half-enclosed by shear bands with spacing in the tens of microns, where the stress state is complicated. The above investigation seems to hint that the fracture mode of a small-volume sample may be different from the bulk one, and it is worthy of consideration how the metallic glasses deform when the specimen dimension is close to the thickness of shear band on order of 10 nm. Therefore in-situ tensile tests were carried out in TEM on sub-micron scaled samples, which were fabricated from a bulk metallic glass by focused ion beam technology. The result shows that the obvious size effect happened on sub-micron scale and the tensile plasticity reached as high as 23-45%, including homogeneous elongation beyond 10%. The homogeneous plastic deformation in metallic glasses did not depend on shear band any more, and atomic-scaled shear events took place through the entire sample. After the occurrence of localized deformation, shearing was not the only choice, and necking, the typical fracture mode for ductile materials, could happen. Also, the localized deformation presented as a controllable process. The experimental evidence demonstrated that small-volume metallic glass had ability to deform with large plastic strain inherently. In all test samples, heat/deformation induced crystallization in the shear bands or the heat affected zone was not observed by select area diffraction and high resolution TEM. No difference between the homogeneous flow region and the undeformed sample was detected by the new developed fluctuation electron microscopy (FEM), because the structure change was too small for FEM. While the medium-range order in shear band and heat affected zone was reduced, which meant that the atomic arrangement was more disorder. In conclusions, the uniform deformation could occur in Zr52.5Cu17.9Al10Ni14.6Ti5 monolithic metallic glass at room temperature, without any crystallization and shear bands involved. But the homogeneous flow was limited in the small-volume sample or the small restricted region. The homogeneous flow and shear band operated at the same time and accommodated all the plastic deformation in macroscopic scale. When the rapid propagation of shear bands could not be confined effectively, the appearance of a primary shear band would lead to the catastrophic fracture.
Pages105
Language中文
Document Type学位论文
Identifierhttp://ir.imr.ac.cn/handle/321006/17019
Collection中国科学院金属研究所
Recommended Citation
GB/T 7714
郭华. Zr基单相块体金属玻璃室温形变机制研究[D]. 金属研究所. 中国科学院金属研究所,2008.
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