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

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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.