Materials World Network: Nanoscale studies of fundamental mechanisms of deformation in amorphous materials

传统的金属合金可以通过成形工艺成形。这种承受“塑性变形”的能力在制造部件中得到了利用，但在使材料抗开裂方面也很重要。金属的晶体结构促进了塑性变形，其中原子平面可以通过称为“位错”的缺陷相互滑动。塑性变形的位错机制已经得到了广泛的研究和很好的理解。相比之下，传统的玻璃（无定形固体）没有晶体结构。虽然玻璃在加热软化后可以流动，但当它们在室温下受到压力时，通常会出现脆裂。然而，如果避免破裂，玻璃是坚固的。目前人们对将玻璃开发成具有可用强度的材料很感兴趣，以便增加它们的工程应用。一个起点是具有玻璃状而非晶体结构的金属（“金属玻璃”）。这些材料确实表现出一定的可塑性，但不幸的是，它们在载荷下的变形并不均匀。相反，塑性变形被局限在非常薄的“剪切带”中。但是，人们逐渐清楚地认识到，当变形集中在带中时，在带之间必然存在明显未变形的物质的大量流动。这为玻璃材料的塑性变形研究开辟了一个全新的领域。本项目旨在通过比较不同类型的玻璃，从金属玻璃到非晶硅，对这种变形的机制有一个基本的了解（本质上是比较金属和共价键的影响）。这项研究涉及使用纳米压痕和微柱变形等技术对机械性能进行敏感测试。这种探针对研究变形机制很有用，可以在很宽的温度范围内使用。它们构成了英国工作的核心，然而，这只是与美国（巴尔的摩约翰霍普金斯大学）和日本（仙台东北大学）合作伙伴的国际合作的一部分。其他实验室的工作将集中在变形材料结构的高分辨率研究和基于“剪切转换区”概念的原子建模上。整体互补的专业知识已经组装，以阐明可塑性的机制，在缺乏晶体结构。这样，就有可能开发出机械性能大大提高的任何种类的玻璃，使其作为具有优异性能的工程材料得到更广泛的开发。

Conventional metallic alloys can be shaped by forming processes. This ability to undergo 'plastic deformation' is exploited in making components, but is also important in making the materials resistant to cracking. Plastic deformation is facilitated by the crystalline structure of the metals within which planes of atoms can slide over each other mediated by defects called 'dislocations'. The dislocation mechanisms of plastic deformation have been very widely studied and are well understood. In contrast, conventional glasses (amorphous solids) do not have a crystalline structure. While glasses can flow when softened by heating, when they are stressed at room temperature they typically show brittle fracture by cracking. Yet, if cracking is avoided, glasses are strong. There is much current interest in developing glasses into materials with usable strength so that their engineering applications could be increased. A starting point is metals with glassy rather than crystalline structure ('metallic glasses'). These materials they do show some plasticity, but unfortunately their deformation under loading is not uniform. The plastic deformation is instead localised in very thin 'shear bands'. But it has gradually become clear that while deformation is concentrated in the bands, there must be substantial flow of the apparently undeformed material between them. This opens up a whole new field of study of plastic deformation of glassy materials. This project aims to develop a fundamental understanding of the mechanisms of such deformation by comparing different types of glass ranging from metallic glasses to amorphous silicon (in essence comparing the effects of metallic and covalent bonding). The research involves sensitive tests of mechanical properties using techniques such as nanoindentation and deformation of micropillars. Such probes are useful for studying deformation mechanisms and can be used over a wide range of temperature. They form the heart of the UK-based work, which is however just part of an international collaboration with partners in the USA (Johns Hopkins University, Baltimore) and Japan (Tohoku University, Sendai). Work on these other laboratories will focus on high-resolution studies of the structures of the deformed materials and on atomistic modelling based on the concept of 'shear transformation zones'. Overall complementary expertise has been assembled to elucidate the mechanisms of plasticity in the absence of crystalline structure. In this way, there is the prospect of developing glasses of whatever kind with greatly improved mechanical properties, enabling their wider exploitation as engineering materials with outstanding performance.