Comparative molecular dynamics study of melting and crystallization of Ni and Au nanoclusters

The melting and crystallization hysteresis for nanoclusters of two transition metals (nickel and gold) have been comparatively studied using the molecular dynamics method and the tight-binding potential. It is established that the onset and end temperatures of the corresponding first kind of phase transition (i.e., melting and crystallization temperatures) should be considered for Ni nanoclusters, whereas Au nanoclusters are melted and crystallized at certain temperatures. The size dependence of the melting temperature of Au nanoclusters determined using the computation results is compared with the molecular dynamics data from other researchers and the results of two independent laboratory experiments. It is shown that our molecular dynamics results are in much better agreement with the experimental data than those obtained by other researchers.     

Neutron brillouin scattering in a metallic glass

The dispersion of collective modes in a metallic glass (Mg₇₀Zn₃₀) measured previously at the thermal neutron time-of-flight spectrometer, IN4, of the HFR of the ILL has been extended towards lower momentum transfers, down to the first pseudo-Brillouin zone, for the first time. This extension to momentum transfers not accessible up to now was possible using the high-resolution time-of-flight spectrometer, HET, at the new spallation source, ISIS. In the region of overlap, the two parts of the dispersion determined with different samples of the same metallic glass on different instruments agree very well. Also, the earlier discrepancies with the dispersion determined for this metallic glass from a computer simulation have been almost completely eliminated due to a more recent and more complete investigation.     

Theoretical and practical achievements in the electric melting of glass

New information concerning electric power and temperature fields and their influence on the flow of glass will be shown.

Methods of calculation of resistances between electrodes, physical, mathematical and other types of new modelling techniques will be described. The optimal connections of electrodes with the multiphase sources of power and ways of protection of electrodes against corrosion by glasses will be discussed. Furnaces for melting different types of glasses and new processes using electricity for melting will be mentioned.     

A study of mechanical homogeneity in as-cast bulk metallic glass by nanoindentation

A surface softening effect induced during copper-mould suction casting of bulk metallic glass is investigated as a function of rod diameter and glass fragility index, m, by nanoindentation. A reduction in hardness and reduced modulus at the rod surface is found to be favoured in small diameter castings and in fragile systems, respectively resulting from limited in-situ annealing and from a greater diversity of metastable atomic environments in the potential energy landscape of fragile glasses. Enhanced propensity for shear transformation zone nucleation in the low moduli surface is explained in terms of reduced atomic connectivity arising from a reduction in local co-ordination number and a lowering of the shear modulus. Finally, the structure and mechanical diversity that is possible in as-cast bulk metallic glass rods is explored through a relative quantification of shear modulus and plastic zone size across the whole as-cast state and in a single rod. These findings illustrate the sensitivity of bulk metallic glass to preparation, especially in respect of thermal history, potentially making replication of mechanical data between researchers problematic.     

Role of concentration fluctuations in metallic glass formation

The glass forming composition range in several binary metallic systems is correlated with the composition dependence of S_cc(0), the concentration-concentration fluctuation structure factor at its long wavelength limit. The magnitude of S_cc(0) has been evaluated for the liquid phase from available thermodynamic data. It has been observed that S_cc(0) exhibits a maximum and tends to the ideal value in the glass forming composition range. Significant and systematic negative deviations from ideal values or the tending to zero of S_cc(0) are observed at the stoichiometry corresponding to complexes in the liquid state in the case of compound forming or associated systems. These observations are discussed in terms of the chemical short-range order in the liquid state. It is concluded that while a reasonable degree of order may exist for the compound forming compositions, in the glass forming region itself the liquids are nearly random mixtures of complexes and unassociated component atoms.     

Bulk metallic glass formation: The positive effect of hydrogen

Zr₅₅Cu₃₀Ni₅Al₁₀ alloys with different amounts of hydrogen have been prepared by arc melting under the gaseous mixture of hydrogen and argon. Proper additions of hydrogen have been proved to effectively increase the glass-forming ability (GFA) of this alloy. Positive effect of hydrogen on GFA has been interpreted from the thermodynamic and structural points of view. Proper additions of hydrogen can decrease the liquidus temperature, which leads to more stable glass-forming liquid. Structure analysis by positron annihilation lifetime spectroscopy shows that proper additions of hydrogen can increase the concentration of shortest open volume and decrease the concentration of intermediate and largest open volumes. This leads to formation of a denser random packed structure, and thus increases the GFA of Zr₅₅Cu₃₀Ni₅Al₁₀ alloys.     

Mathematical modelling of sand dissolution in a glass melting channel with controlled glass flow

This work presents the results of a mathematical modelling of sand dissolution in a horizontal glass-melting channel with circulation flows when varying the length of the channel and the time needed for the dissolution of the sand particles. The recently found optimal combination of intensive transversal circulations with forward flow, slowed down by a positive longitudinal temperature gradient near the glass level, has been set in the melting channel. The results of the modelling were projected into the values of the space utilisation for the sand-dissolution process. The high values of space utilisation were repeatedly attained under these conditions in a broad interval of channel lengths and sand-dissolution times. Both the growing length of the channel and the growing sand-dissolution time increased the ratio of the transversal to longitudinal temperature gradient necessary for attaining the maximum space utilisation. The pull rate of the channel grew linearly with the growing ratio of the channel length to the sand-dissolution time. The results provide a theoretical base for the design of glass-melting segments with controlled glass flow, characterised by lower energy consumption and higher melting performance.     

Icosahedral and dense random cluster packing in metallic glass structures

It has recently been shown that metallic glass structures can be idealized as inter-penetrating solute-centered atomic clusters that are packed with essentially periodic symmetry. The present work applies the same methodology to explore whether experimental observations can be matched by inter-connected solute-centered clusters that are organized in space via dense random cluster packing, Bergman icosahedral cluster packing or Mackay icosahedral cluster packing. Idealized partial pair distribution functions are developed where the symmetry of the solute positions in the structure is derived from the cluster-packing symmetry and the solute concentration, which establishes occupation of inter-cluster sites, especially β structural sites enclosed by an octahedron of solute-centered clusters. While each of the three models matches major features of the measured solute-solute partial pair distribution functions, the arrangement of clusters with Mackay icosahedral ordering provides the best fit. However, this model is not able to match an essential feature in solute-lean glasses and does not provide the same overall agreement as does periodic cluster packing for solute-rich glasses. Strong similarities between the structure factors in the Mackay icosahedral and periodic cluster-packing models, along with expected deviations from the idealized solute positions studied here, are likely to hinder an unambiguous distinction between these two models.     

Growth of ZnO bulk crystals: A review

A critical review is proposed of the different techniques of bulk growth of ZnO crystals for their use as a substrate in the homoepitaxial growth of this attractive compound. The crystals are assessed from their structural and electrical properties and from the structural properties and purity of homoepitaxial films grown on them by various techniques such as plasma-assisted molecular beam epitaxy, pulsed laser deposition, magnetron sputtering, chemical vapor deposition, metalorganic chemical vapor epitaxy, liquid phase epitaxy.     

Electron microscopy of bulk metallic glass machining chips

Machined Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass (BMG) chips were characterized using high resolution electron microscopy. Compared to conventional processing techniques, machining produces very high heating/cooling, and deformation rates. It is therefore of interest to compare structural changes in machining chips with those produced by conventional processing. Large (～1 μm) crystalline grain, residual amorphous region, and phase separation in the amorphous–crystalline transition region were detected in bright field TEM images. Three equilibrium phases, Zr₂Cu, ZrAl₂, and Zr₂Ni, which have been identified in samples undergoing conventional annealing, were revealed from selected area electron diffraction patterns of the chips. High magnification TEM micrographs showed nanocrystallites, about 10 nm in size, in the amorphous–crystalline transition region.     

Some recent new applications of glass technology

Currently, at Corning Glass Works we are involved in several new, interesting applications of glass technology, which have the potential of impacting a wide diversity of applications. Three such areas presently under investigation will be reviewed, viz., molded optics, dental restorative materials, and inorganic paper. The common denominator relating these three examples is the fact that each relies on a unique glass composition as the starting material.

The first development is a special area of glass composition capable of being molded directly into optical elements without grinding and polishing. The second involves a new castable micaceous ceramic dental material derived from special glass compositions for application in several phases of restorative dentistry. Finally, the third development relates to a ceramic paper based upon a mica-like crystal, which is precipitated from glass.

The current status of the technology will be discussed for each in the context of the potential applications.     

Electrochemical behavior of a Ti-based bulk metallic glass

In this study, potentiodynamic experiments were conducted with a Ti-based BMG alloy with a nominal composition of Ti_43.3Zr_21.7Ni_7.5Be_27.5 [atomic percent (at.%)], commonly known as LM-010. Electrochemical characterization was performed in a phosphate-buffered saline (PBS) electrolyte at 37 °C with a physiologically relevant dissolved oxygen content. This BMG exhibited passive behavior at the open-circuit potential with a low mean corrosion-penetration rate. A susceptibility to localized corrosion was observed but is not a concern at the open-circuit potentials. The resistance of the LM-010 alloy to localized corrosion was statistically equivalent to, or better than, all of the BMG materials and the 316L stainless steel for which direct statistical comparisons were possible. Microscopic examination revealed that the samples predominantly exhibited many scattered, small pits (diameter ⩽100 μm) in addition to several larger pits. Based upon the pit morphology and comparisons with the literature, it appears that localized corrosion initiated at clusters of inhomogeneities within the amorphous matrix.     

Modeling of heat transfer in the melting of a glass batch

On the basis of appropriate assumptions, a mathematical model has been constructed to predict the temperature distribution and heat transfer in a glass batch blanket and to simulate the effects of individual factors on the conversion process. The solution of the model equations is obtained numerically. The model is used to determine the effect of various parameters describing the melting process on heat transfer and melting time of unpreheated and preheated batch. The numerical results obtained for container glass are presented in graphical and tabular form.     

Estimation of shear-banding resistance in metallic glass containing nano-crystalline particles

Bulk metallic glasses (BMGs) have a variety of excellent properties compared with the majority of conventional crystalline alloys. However, they exhibit limited global plasticity at room temperature because of shear banding. Several methods have been proposed to improve the limited ductility of BMG; one method is the homogeneous distribution of crystalline particles. However, our understanding of the interaction between the crystalline particles and shear bands (SB) is not sufficient. Here, we performed molecular dynamics (MD) simulations of mode II deformation of a notched BMG plate and BMG plates containing one nano-crystalline particle ahead of the notch bottom. To compare the effect of crystalline particle size on the resistance to SB propagation, we used the J-integral. By comparing J-R curves and the deformation behavior of the BMG plates with and without nano-crystalline particles, we found that the resistance to shear banding is efficiently improved by introducing crystalline particles with sufficient size, compared to the SB width.     

Theory of fluidity of liquids, glass transition, and melting

This is a presentation of a rigorous theory of fluidity of liquids, glass transition and melting of solids in the frame of an asymmetric double well potential model. Potential wells are doubled time to time by the local density fluctuations caused by the thermal longitudinal waves. The average frequency of doubling of potential wells is equal to the frequency of the most energetic waves which obey a law similar to Wein’s displacement law in black body radiation. Based on the equilibrium thermodynamic theory of fluctuations and the displacement law, a law of linear pre-diffusion mean-square displacement of particles in a solid is derived: the mean-square displacement of molecules within their potential wells increases linearly with temperature. It is shown that when this is broken-down (where the mean-square displacement at a certain temperature rapidly changes its slope as a function of temperature) glass devitrifies and crystal melts, and all possible solid-liquid transitions of a substance occur at the same critical mean-square displacement: any solid (not only crystals) transforms into liquid when the mean-square displacement, as a fraction of the average intermolecular distance, acquires a certain universal critical value - the same for different substances. It is proved that molecules in a liquid perform specific Brownian motion. The average jump distance is a function of temperature and it is much smaller than the nearest intermolecular distances. At a certain temperature, shown to be the Kauzmann temperature, the average jump distance of Brownian motion becomes equal to zero: the super-cooled liquid undergoes glass transition. The transition was proven to be a phase transition of the fourth order: the free energy of the system and its first, second and third derivatives are all continuous functions, but its fourth derivative with respect to temperature is discontinuous. Molecular mobility, diffusion and viscosity are obtained as functions of temperature.     

Icosahedral ordering in Cu60Zr40 metallic glass: Molecular dynamics simulations

The atomic structure of bulk metallic glasses has long been mysterious for the lack of long range order. Although the solute-centered cluster packing model has recently been proposed to disclose the nanoscale medium-range order (MRO), the atomic packing scheme in systems with large solute concentration remains obscure. In this work, the atomic structure of Cu₆₀Zr₄₀ metallic glass is investigated via molecular dynamics simulations with the Finnis–Sinclair potential. It is found that the fragments of icosahedra are dominant in the model metallic glass. The icosahedra are completely centered by solvent atoms. Extended clusters composed of two icosahedra interpenetrate and form MRO with 3–11 icosahedra. It is suggested that the structure of Cu₆₀Zr₄₀ metallic glass can be described by the aggregation of icosahedra.