Ripples in the Bottom of the Potential Energy Landscape of Metallic Glass (original) (raw)
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Atomic dynamics in metallic liquids and glasses
Materials transactions
How atoms move in metallic glasses and liquids is an important question in discussing atomic transport, glass formation. structural relaxation and other properties of metallic glasses. While the concept of free-volume has long been used in describing atomic transport, computer simulations and isotope measurements have shown that atomic transport occurs by a much more collective process than assumed in the free-volume theory. We introduce a new approach to describe the atomic dynamics in metallic glasses, in terms of local energy landscapes related to fluctuations in the topology of atomic connectivity. This approach may form the basis for a new paradigm for discussing the structure-properties relationship in metallic glasses.
Journal of Non-Crystalline Solids, 2001
Results from two dierent series of neutron inelastic scattering experiments are presented. In the ®rst series the dependence of the atomic dynamics of a metallic glass, NiPdP, on rapid quenching has been studied. The quenched-in density¯uctuations and especially the free volume lead to an increase of the low energy modes (LEMs) for energies between 2 and 20 meV for this glass. This intensity increases in the low energy region is however, not much more than was found for a rapidly quenched, i.e., highly defective fcc-Al lattice. The change of the calculated vibrational entropy due to these additional LEM is of the order of 3%. In search for anharmonic modes in metallic glasses, a second series of experiments investigated the dependence of the atomic dynamics on temperature using a conventional metal±metal glass, NiZr, and one of the more stable Zr-based glasses, ZrCuAl. Anharmonic eects in the form of a shift of the complete generalised vibrational density of states to lower energies on heating were in fact observed for the ®rst time. However these eects can be attributed to the expansion of the glass, which for a metallic glass obviously proceeds by an increase of the atomic distances rather than by formation of additional free volume, or by anharmonic modes expected from the soft potential model. Ó
Universal nature of the saddle states of structural excitations in metallic glasses
Materials Today Physics, 2021
A widely used schematic picture of the potential energy landscape (PEL) for liquid and glass gives an impression that the pathway of moving from a valley to another through a saddle point is predetermined. However, in reality the pathway is much more stochastic and unpredictable because thermal history is wiped out at the saddle point and the pathway down is randomly chosen. Here we explain this puzzling behavior through the study of local structural evolutions in the β relaxation process by atomistic simulations of structural excitations for metallic glasses. We find that the saddle states in the PEL show universal melt-like features in short-range order and atomic dynamics, independent of thermal history and composition. We propose that the short-lived melting at the saddle point is responsible for wiping out the prior thermal history. This explains why the activation and relaxation stages of the β process are decoupled. The findings highlight the importance of understanding the nature of the saddle states in elucidating the system dynamics, and pose a question on the current view on the system evolution in the PEL.
Emergence of Crystal-like Atomic Dynamics in Glasses at the Nanometer Scale
Physical Review Letters, 2013
The vibrational dynamics of a permanently densified silica glass is compared to the one of an -quartz polycrystal, the silica polymorph of the same density and local structure. The combined use of inelastic x-ray scattering experiments and ab initio numerical calculations provides compelling evidence of a transition, in the glass, from the isotropic elastic response at long wavelengths to a microscopic regime as the wavelength decreases below a characteristic length of a few nanometers, corresponding to about 20 interatomic distances. In the microscopic regime the glass vibrations closely resemble those of the polycrystal, with excitations related to the acoustic and optic modes of the crystal. A coherent description of the experimental results is obtained assuming that the elastic modulus of the glass presents spatial heterogeneities of an average size a $ =2.
Local structure and energetics in a model glass
Journal of Non-Crystalline Solids, 1990
We study the relation between the potential energy of atom and the shape of Voronoi polyhedron around the atom in a single-component metal glass produced by the molecular dynamics method. The energy of the central atoms is the lowest for the 13-atom lcosahedral cluster among the clusters appeared in the glass.
Role of Disorder in the Thermodynamics and Atomic Dynamics of Glasses
Physical Review Letters, 2014
We measured the density of vibrational states (DOS) and the specific heat of various glassy and crystalline polymorphs of SiO 2 . The typical (ambient) glass shows a well-known excess of specific heat relative to the typical crystal (α-quartz). This, however, holds when comparing a lower-density glass to a higherdensity crystal. For glassy and crystalline polymorphs with matched densities, the DOS of the glass appears as the smoothed counterpart of the DOS of the corresponding crystal; it reveals the same number of the excess states relative to the Debye model, the same number of all states in the low-energy region, and it provides the same specific heat. This shows that glasses have higher specific heat than crystals not due to disorder, but because the typical glass has lower density than the typical crystal.
Understanding of the structure and dynamics of liquids and glasses at an atomistic level lags well behind that of crystalline materials, even though they are important in many fields. Metallic liquids and glasses provide an opportunity to make significant advances because of its relative simplicity. We propose a microscopic model based on the concept of topological fluctuations in the bonding network. The predicted glass transition temperature, T g , shows excellent agreement with experimental observations in metallic glasses. To our knowledge this is the first model to predict the glass transition temperature quantitatively from measurable macroscopic variables.
Formation and deformation of metallic glasses: Atomistic theory
Intermetallics, 2006
The free-volume theory has been successful in describing various structural and mechanical properties of metallic glasses, and continues to be used nearly half a century after it was created. However, the validity of the free-volume model at an atomistic level is questionable. We suggest that the apparent success of the free-volume model is that it represents the fictive temperature of the system well, rather than its microscopic reality. We propose an alternative approach based upon the exchange and fluctuation of atomic bonds, described in terms of the atomic level stresses. This approach enables much more quantitative description of the properties of metallic glasses, including glass transition, liquid fragility, glass formation and mechanical deformation.