金属玻璃微观结构和变形机制的电子显微学研究

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	金属玻璃微观结构和变形机制的电子显微学研究
	邓静伟
学位类型	博士
导师	隋曼龄
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	金属玻璃中程有序波动电子显微术剪切塑性区剪切带样品制备 Metallic Glass Medium Range-order Fluctuation Electron Microscopy Plastic Zone Shear Band Specimen Preparation
摘要	"金属玻璃因为其独特的力学性能、物理性能和优良的抗腐蚀性能，近年来吸引了大量研究人员的关注。然而，由于非晶材料的原子结构无序排列，金属玻璃的微观结构和变形机制的实验表征仍然非常困难。利用透射电子显微镜（TEM）这一微观结构研究的强有力手段，通过发展适合于非晶材料的独特表征技术，将有望为深入理解以上两个问题提供重大帮助。本论文通过综合利用多种电子显微学的方法研究了几种典型体系金属玻璃微观结构和变形机制的特点，主要针对非晶材料结构的先进表征技术，不同金属玻璃的中程有序结构尺度，金属玻璃变形塑性区和剪切带的原位透射电镜研究等问题展开了探索。此外，针对金属玻璃样品的透射电镜样品制备方法也进行了探讨。主要内容和结论如下： 1、波动电子显微术（FEM）是一种研究非晶样品中程有序结构的有力手段。通过在透射电镜中空锥形光暗场模式下使用不同孔径的物镜光阑，实现了可变分辨率的FEM实验，并以此研究了Zr基和Fe基金属玻璃和属于共价键类型的a-Si3N4非晶的中程有序结构尺度。通过拟合可变分辨率FEM实验的结果，获得了金属玻璃样品相干长度范围为0.81-0.88 nm，共价键类型的a-Si3N4非晶的相干长度为0.69 nm。对所研究的四种非晶材料，FEM实验获得的峰值约化方差值均随实验所用的名义分辨率增加先增大而后降低。当FEM实验的名义分辨率与实验研究的非晶材料的相干长度范围相接近时，FEM实验得到最大的约化方差值。 2、利用扫描透射（STEM）成像模式结合透射电镜下的原位拉伸手段，研究了Zr基金属玻璃的变形破坏过程。实验发现，STEM成像模式不仅可以在更厚的区域内对变形产生的塑性区进行观察，而且能够定量地给出塑性区中剪切台阶的高度。通过系列倾转实验，重构了变形产生的塑性区的三维立体图像，该图像显示金属玻璃薄膜样品在拉伸过程中通过剪切撕裂的方式变形破坏。原位的STEM观察发现，塑性区内部的剪切带采取间歇的方式运动。金属玻璃样品破坏前，剪切带需要经过多次剪切过程，并且每一次剪切发生时在塑性区前端都同时又会形成新的剪切带。此外，实验发现塑性区的长度和样品厚度之间存在明显的线性关系。Zr基金属玻璃原位拉伸实验中观察到的现象可以利用剪切带的协同运动模型进行解释。 3、利用透射电镜、扫描电镜、原子力显微镜等手段研究了离子轰击后的金属玻璃样品中出现的特征花样。高分辨透射电镜观察证实，存在特征衬度的区域没有发生晶化等结构变化。X射线能谱分析结果表明，样品中衬度差异明显的区域间不存在明显的成分变化。通过原子力显微镜实验，确定了离子减薄制样过程会导致样品表面产生厚度起伏，进而形成TEM下观察到的特征衬度。利用离子束与样品表面作用过程的粗化机理解释了特征花样的形成原因。在离子束轰击的条件下，金属玻璃样品的动态粗化过程包括迅速长大和饱和两个阶段，不同体系金属玻璃的特征花样的饱和尺寸与该体系中主元素的原子序数密切相关。"
其他摘要	"Bulk metallic glasses (BMGs) have attracted extensive research interests in recent years, attributed to its unique mechanical, physical properties and excellent corrosion resistance. However, experimental characterizations of microstructure and deformation mechanism of metallic glass materials are still challenging, due to the lack of long range order in the materials. As a powerful tool to reveal microstructure of materials, transmission electron microscopy (TEM) has the potential to resolve above problems by developing new methods suit for amorphous materials. In this thesis, we have studied the microstructure and deformation mechanism in several metallic glass systems by TEM. Our research focuses on advanced characterization methods for amorphous materials, the length of medium range order (MRO) in different BMGs, and in situ observation of deformation plastic zone and shear bands in BMGs. Additionally, TEM sample preparation of metallic glass was also addressed. Major contents and conclusions are shown as following: 1. Fluctuation electron microscopy (FEM) is a powerful method to reveal the MRO in metallic glass materials. By varying the objective aperture size in a hollow-cone dark field imaging mode, variable resolution FEM has been successfully performed in TEM on metallic and covalent bond amorphous materials. Correlation lengths of Λ = 0.69 nm for covalently bonded a-Si3N4 and Λ = 0.81-0.88 nm for BMGs were extracted by fitting the variance values at different resolutions. The experiment reveals that maximum intensity variance was obtained when the nominal resolution of FEM images approaches the correlation length of the studied amorphous materials. 2. In situ tensile experiments with STEM were conducted to investigate the dynamic processes of shearing and fracture in a Zr-based BMG. STEM imaging demonstrates its capabilities to reveal shear bands in much thicker region and surface shear offsets quantitatively in the plastic zone of the specimen. These make the in situ STEM technique a promising approach to study the shear banding and fracture in metallic glasses. Through reconstruction of tilt series STEM images, 3-D image of a typical plastic zone was obtained. It clearly demonstrated that the shearing and ductile tearing process is the major deformation mechanism in thin foil metallic glasses under tension. During the stable plastic flow in BMGs, in situ STEM investigation reveals an intermittent initiation of shear band occurred ahead of plastic zone and the stick-slip shear banding behavior took place in the formed shear bands. The synchronization of each starting of a shear event and the initiation of a new shear band was observed. Additionally, a linear relationship between plastic zone length and sample thickness was revealed by in situ examination of the plastic zone propagation. Lastly, a simultaneous and intermittent shear model in thin foil tension situation was proposed, which well explains these in situ observations. 3. Thickness variation in BMG specimens prepared by ion milling was studied by TEM, scanning electron microscopy (SEM) and atomic force microscopy (AFM). HRTEM and EDS confirm that no structure or chemical composition changes in sample regions with significantly different contrasts. According to the AFM results, these features are outcome of surface roughening induced by ion-sputtering and the consequent thickness variation in TEM specimens. The formation of these thickness patterns was interpreted by a roughening mechanism during ion milling. The dynamic roughening process and various pattern sizes in different BMGs were also measured and discussed with surface roughening and smoothing mechanisms during the ion-milling."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64417
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	邓静伟. 金属玻璃微观结构和变形机制的电子显微学研究[D]. 北京. 中国科学院金属研究所,2012.