13 January, 2010
Elasticity and Anelasticity in Metallic Glasses: A Peek at Local Energy Landscape
Abstract
Metallic glasses share a large number of glassy characteristics with polymeric, colloidal, molecular or inorganic glasses, but are also different in some aspects. For instance their interatomic potentials are more harmonic. Consequently the glass and crystal have a similar density, and their thermodynamic properties are dominated by potential energy than the kinetic energy. Since the structural unit is an atom, allowing possibilities of gaining microscopic insights into the local energy landscape, by connecting structural details with thermodynamic properties. As an example I discuss the elasticity and anelasticity of metallic glasses, studied by x-ray diffraction and modeling.
Elastic constants are determined by slow experiments, such as measurements of physical deformation or sound velocity. Thus all the high-frequency anelasticevents, up to local atomic displacements, are included in the measured compliance. Slow mechanical deformation induces high density of non-linear, non-afine atomic displacement processes, which can be observed by x-ray scattering. I review old and new observations of such anelasticity induced by elastic or creep deformation of metallic glasses, and also evidence for mechanical yielding as stress-induced glass transition by simulation.
References and suggested reading
- Y. Suzuki and T. Egami, “Shear Deformation of Glassy Metals: Breakdown of Cauchy Relationship and Anelasticity”, J. Non‑Cryst. Solids 75, 361 (1985).
- W. Dmowski and T. Egami, “Structural Anisotropy in Metallic Glasses Induced by Mechanical Deformation”, Adv. Eng. Mater. 10, 1003 (2008).
19 January, 2010
Structural Changes in glasses and Liquids
Abstract
A major difficulty we face in developing the science of glasses is that it is not easy to characterize the atomic structure effectively and accurately. Diffraction experiments, for instance, by themselves provide only limited amounts of information. An effective approach to penetrate this barrier is to study the changes in the diffraction data and model simulation in response to external conditions, such as temperature, stresses, and the combination of stress and temperature. I discuss the results of synchrotron x-ray and neutron diffraction experiments and computer simulations on changes in the structure of metallicglasses, due to temperature change and structural relaxation. Variation in the properties due to temperature change and relaxation often related to the volume change, and are explained by the free-volume theory. However, the actual changes in the structure are greater than suggested by volume change. For instance as a consequence of structural relaxation the physical density increases slightly, typically 0.5 % or so. But the internal atomic rearrangements observed by diffraction measurements are much more extensive, and are consistent with the reduction in both positive and negative local density fluctuations. Thisobservation leads to an interpretation of glass transition in terms of the liquid-like and solid-like local density fluctuations. These experimental results guide us to a more realistic understanding of the atomic structure of metallic glasses and liquids.
References and suggested reading
- D. Srolovitz, T. Egami, and V. Vitek, “Radial Distribution Function and Structural Relaxation in Amorphous Solids”, Phys. Rev. B 24, 6936 (1981).
- T. Egami, S. J. Poon, Z. Zhang and V. Keppens, “Glass Transition in Metallic Glasses: A Microscopic Model of Topological Fluctuations in the Bonding Network”, Phys. Rev. B, 76, 024203 (2007).
20 January, 2010
Experimental Method: NMR and Raman Spectroscopy
Abstract
NMR and NQR spectroscopy gives information on the local structure and dynamics in glasses through the peak shape, Knight shift and the spin-lattice relaxation time, T1. The peak shape tells us the local symmetry through the electric-field gradient (EFG). Even though there is no perfect symmetry in the local structure of a glass EFG tells us the symmetry components of the local environment that characterize the local structure. Knight shift in metallic glasses are sensitive to chemical environment and local volume distribution. T1 gives information about the local diffusional atomic jump. Raman spectroscopy provides information on the atomic dynamics through inelastic scattering of light. In glasses and liquids Raman spectroscopy can determine the vibrational density of states. An important progress made by Raman spectroscopy on glasses is the discovery of the so-called boson peak. In glasses there are significant excess low-energy vibrational modes with short wavelengths, in addition to the travelling long-wave phonons. The Raman peak due to these modes is called boson peak. These modes are sensitive to structural details, and thus provide an excellent tool to probe the local structure. However, the exact nature of these modes is still highly controversial. The experimental set-up, interpretation of the results and some results on glasses and liquids are reviewed.
References and suggested reading
- X.-P. Tang, U. Geyer, R. Busch, W. L. Johnson and Y. Wu, “Diffusion mechanisms in metallic supercooled liquids and glasses”, Nature 402, 160 (1999).
- V. K. Malinovsky and A. P. Sokolov, “The Nature of Boson Peak in Raman Scattering in Glasses”, Solid St. Comm. 57, 757 (1986).