Influence of Topological Anisotropy on the Mechanical Properties of Silicate Glasses

虽然玻璃通常被视为各向同性材料，但玻璃在载荷作用下的冻结流动结构可以很容易地用于生产各向异性玻璃部件。这种工艺的一个例子是氧化玻璃纤维的拉伸。从宏观上看，玻璃的各向异性表现为双折射现象。此外，各向异性对玻璃的力学性能也有很大影响。事实上，最近的研究表明，拓扑各向异性，即二氧化硅四面体相互连接方式的方向依赖性，是玻璃纤维强度比相同成分的块状玻璃高一个数量级的主要原因。鉴于这些发现的技术相关性，人们对导致各向异性特性的拓扑变化的性质以及拓扑各向异性如何影响各种机械特性知之甚少。本课题旨在研究硅酸盐玻璃的各向异性是如何发展的，如何在拓扑水平上表征它，以及拓扑各向异性如何影响硅酸盐玻璃的应力应变响应和韧性。为此，我们将宏观尺度的实验研究与透射电子显微镜（TEM）的原位纳米力学测试和原子计算机模拟相结合。具体来说，实验工作的关键目标是生产大块各向异性氧化玻璃，通过散射技术和波动电子显微镜对其结构进行详细表征，并通过宏观和微观拉伸测试、断裂实验和压痕研究确定其（方向相关的）力学性能。此外，利用最近发现的可通过电子辐照控制的超塑性现象，在TEM中通过原位机械淬火使二氧化硅纳米结构（纳米球、纳米纤维）呈现各向异性。原子模拟将侧重于表征拓扑各向异性和研究导致各向异性的机制，以及确定各向异性的方向相关力学性能。在整个项目中，实验和模拟工作是紧密联系在一起的，例如通过模拟电子衍射模式和波动电子显微镜图像生成的MD样品并与实验结果进行比较，或者通过将纳米力学测试的MD模拟与相应的原位实验进行比较。这些知识将用于(a)通过生成专门的拓扑各向异性来专门设计各向异性裂纹扩展，以及(b)在拓扑基础上理解更复杂（多轴）拓扑各向异性和应力场中的裂纹扩展。拟议的研究将从“超强玻璃拓扑工程”优先计划内的合作中受益匪浅，例如拓扑对大块金属玻璃机械性能的影响，最近报道了各向异性对弹性性能的类似影响，或氧化物玻璃的微观力学测试。

Although glasses are generally viewed as isotropic material, freezing-in the flow structure of a glass under load can easily be used to produce anisotropic glass components. One example of such a process is the drawing of oxide glass fibers. Macroscopically, the anisotropic nature of the glass manifests itself in the phenomenon of optical birefringence. Moreover, mechanical properties of glass may be strongly affected by anisotropy. Indeed, it was recently shown that topological anisotropy, i.e. a direction dependence in the way the silica tetrahedra are connected with each other, is the main cause for the one order of magnitude higher strength of glass fibers compared to bulk glasses of the same composition. Given the technical relevance of these findings, relatively little is known about the nature of the topological changes which lead to anisotropic properties, and on how topological anisotropy influences the various mechanical properties.The aim of this proposed research project is to study how anisotropy develops in silicate glasses, how it can be characterized on a topological level, and how topological anisotropy affects the stress-strain response and toughness of silicate glasses. For this purpose, we will combine experimental investigations on the macro scale with in-situ nanomechanical testing in the transmission electron microscope (TEM) and atomistic computer simulations.In detail, the key objectives for the experimental work are the production of bulk anisotropic oxide glasses, the detailed characterization of their structure by scattering techniques and fluctuation electron microscopy and the determination of their (direction-dependent) mechanical properties by macroscopic and microscopic tensile tests, fracture experiments and indentation studies. In addition, silica nanostructures (nanospheres, nanofibers) will be rendered anisotropic by in-situ mechanical quenching in the TEM exploiting the recently discovered phenomenon of superplasticiy that can be controlled via electron irradiation.The atomistic simulations will focus on characterizing the topological anisotropy and studying the mechanisms which lead to anisotropy, as well as on determining the direction dependent mechanical properties as function of anisotropy.Throughout the project, the experimental and simulations efforts are closely linked, e.g. by the simulation of electron diffraction patterns and fluctuation electron microscopy images of MD generated samples and comparison to experimental results, or by the comparison of MD simulations of nanomechanical tests with corresponding in-situ experiments.Such knowledge will be used to (a) specifically engineer anisotropic crack propagation by generating dedicated topological anisotropy and (b) understand, on a topological basis, crack propagation in a more complex (multiaxial) field of topological anisotropy and stress. The proposed research will strongly profit from collaborations within the priority programme “Topological Engineering of Ultra-Strong Glasses”, e.g. on the influence of topology on mechanical properties of bulk metallic glasses, where recently similar effects of anisotropy on elastic properties were reported, or on micromechanical testing of oxide glasses.