A molecular dynamics simulation of the effects of excess free volume on the diffusion in metallic glasses (original) (raw)
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Acta Materialia, 2021
The bulk and surface dynamics of Cu50Zr50 metallic glass were studied using classical molecular dynamics (MD) simulations. As the alloy undergoes cooling, it passes through liquid, supercooled, and glassy states. While bulk dynamics showed a marked slowing down prior to glass formation, with increasing activation energy, the slowdown in surface dynamics was relatively subtle. The surface exhibited a lower glass transition temperature than the bulk, and the dynamics preceding the transition were accurately described by a temperature-independent activation energy. Surface dynamics were much faster than bulk at a given temperature in the supercooled state, but surface and bulk dynamics were found to be very similar when compared at their respective glass transition temperatures. The manifestation of dynamical heterogeneity, as characterized by the non-Gaussian parameter and breakdown of the Stokes-Einstein equation, was also similar between bulk and surface for temperatures scaled by their respective glass transition temperatures. Individual atom motion was dominated by a cage and jump mechanism in the glassy state for both the bulk and surface. We utilize this cage and jump mechanisms to separate the activation energy for diffusion into two parts: (i) cage-breaking barrier (Q1), associated with the rearrangement of neighboring atoms to free up space and (ii) the subsequent jump barrier (Q2). It was observed that Q1 dominates Q2 for both bulk and surface diffusion, and the difference in activation energies for bulk and surface diffusion mainly arose from the differences in cage-breaking barrier Q1.
Physical Review B, 2011
In this study, we characterize the mechanical properties of Cu 64 Zr 36 nanoglasses under tensile load by means of large-scale molecular dynamics simulations and compare the deformation behavior to the case of a homogeneous bulk glass. The simulations reveal that interfaces act as precursors for the formation of multiple shear bands. In contrast, a bulk metallic glass under uniaxial tension shows inhomogeneous plastic flow confined in one dominant shear band. The results suggest that controlling the microstructure of a nanoglass can pave the way for tuning the mechanical properties of glassy materials.
Free Volume Evolution in Metallic Glasses Subjected to Mechanical Deformation
MATERIALS TRANSACTIONS, 2007
We define the Turnbull-Cohen free volume as the critical excess of the Voronoi volume of an atom less its core volume. Using molecular dynamics simulation we calculated the free volume change in two model binary metallic glasses undergoing tension and shear deformation. We show that the free volume change is an integral part of the deformation process; and the shear localization manifested as a shear band is directly related to the inhomogeneous distribution of the free volumes. Shear band formation may consist of two stages: the initial free volume production in the amorphous solids and the liquefaction of the regions with accumulated deformation strains. We show, for the first time, the formation of voids and the ''vein'' patterns on fracture surfaces at atomic scales; they are the combined result of the free volume change and loading and sample conditions.
Molecular-Dynamics Study of the Atomic Diffusion in Glasses Induced by an External Perturbation
Europhysics Letters (EPL), 1988
We present a constant-temperature nonequilibrium molecular-dynamics (NEMD) study of the mass transport in an amorphous LennardJones system prepared by rapidly cooling the liquid at constant atmospheric pressure. We calculated mobility averages when an external force is applied to the particles thus bypassing the space-time limitations of equilibrium molecular dynamics (EMD) which preclude from the investigation of mass transport in glasses.
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.
Atomistic free-volume zones and inelastic deformation of metallic glasses
Nature materials, 2010
The amorphous nature of metallic glasses and their mechanical properties make them interesting for structural applications. However, the interplay between the nature of atomic structures in metallic glasses and mechanical properties remains poorly understood. In this study, high-frequency dynamic micropillar tests have been used to probe both atomic clusters and flow defects in metallic glasses. We show that loosely bonded atomistic free-volume zones that are enveloped elastically by tightly bonded atomic clusters show a deformation character similar to supercooled liquids. At room temperature, the effective viscosity of these free-volume zones is of the order of 1 x 10(8) Pa s before the occurrence of shear banding. The confined liquid-like deformation of free-volume zones leads to significant mechanical hysteresis in micropillars under dynamic loading, providing important insight into how atomistic structural features affect the deformation behaviours in metallic glasses in the ap...
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.
Relaxation of shear bands in a Pd40Ni40P20 bulk metallic glass is investigated by a combination of the radiotracer technique and molecular dynamics (MD) computer simulation (in the latter case using a model for Ni80P20), serving for determining for the first time the effective activation enthalpy of Ag diffusion along the shear bands in the Pd40Ni40P20 glass as only 55 kJ/mol. The shear bands are established to relax during annealing below glass transition temperature and the diffusion enhancement evolves with time in a non-monotonous manner. A phenomenological model accounting for an inhomogeneous distribution of excess free volume in shear bands and in the surrounding matrix is proposed. In the MD simulation, glass samples subjected to a constant strain rate are considered. Development of shear bands and the subsequent relaxation of stresses are characterized on a microscopic to mesoscopic scales. Mean-square displacement as well as strain maps indicate that the inhomogeneity, as ...
Surface structure and properties of NiZr model metallic glasses: A molecular dynamics simulation
Journal of Non-Crystalline Solids, 2008
A systematic study of the surface structure and properties of NiZr model metallic glasses is reported using atomistic simulations. It is found that at low temperatures below the glass transition temperatures, the surface retains the amorphous structure and the surface energy c is significantly lower ($50%) than that of the corresponding crystalline alloy constituents. The variation of alloy concentration has little effect on c; but increase in cooling rate and annealing temperature can lead to large decrease in c. At elevated temperatures, c increases with temperature and surface melting occurs at a temperature about 30% below T g. At all temperatures up to T g , the surface remains atomically smooth.
Physical review. B, Condensed matter, 1989
We present a constant-temperature nonequilibrium molecular-dynamics study of the mass transport in an amorphous Lennard-Jones system prepared by rapidly cooling the liquid at constant atmospheric pressure. The phase diagram of the liquid-supercooled liquid glass has been determined and the dependence of the glass stability on the quenching rate investigated. We calculated mobility averages when an external force is applied to the particles, thus bypassing the space-time limitations of equilibrium molecular dynamics which prevent the investigation of mass transport in glasses. The response of the system to the external perturbation is highly nonlinear: at each temperature, T, a linear fit of the mobility, p, versus the external force reveals the existence of a threshold perturbation above which the mobility becomes liquidlike (D =pk~T) 10 cm s ') the system still being amorphous. Below this threshold, the mobility is vanishingly small. Our results show that for large perturbations the mobility is temperature independent whereas the data are consistent with an Arrhenius behavior in the range of smaller perturbations. A qualitative comparison of our results with recent experimental data on diffusion and viscosity of amorphous materials under irradiation is proposed.