11,645 research outputs found
Formation of Two Glass Phases in Binary Cu-Ag Liquid
The glass transition is alternatively described as either a dynamic transition in which there is a dramatic slowing down of the kinetics, or as a thermodynamic phase transition. To examine the physical origin of the glass transition in fragile Cu-Ag liquids, we employed molecular dynamics (MD) simulations on systems in the range of 32,000 to 2,048,000 atoms. Surprisingly, we identified a 1st order freezing transition from liquid (L) to metastable heterogenous solid-like phase, denoted as the G-glass, when a supercooled liquid evolves isothermally below its melting temperature at deep undercooling. In contrast, a more homogenous liquid-like glass, denoted as the L-glass, is achieved when the liquid is quenched continuously to room temperature with a fast cooling rate of ∼10¹¹ K/sec. We report a thermodynamic description of the L-G transition and characterize the correlation length of the heterogenous structure in the G-glass. The shear modulus of the G-glass is significantly higher than the L-glass, suggesting that the first order L-G transition is linked fundamentally to long-range elasticity involving elementary configurational excitations in the G-glass
Intermediate Phases, structural variance and network demixing in chalcogenides: the unusual case of group V sulfides
We review Intermediate Phases (IPs) in chalcogenide glasses and provide a
structural interpretation of these phases. In binary group IV selenides, IPs
reside in the 2.40 < r < 2.54 range, and in binary group V selenides they shift
to a lower r, in the 2.29< r < 2.40 range. Here r represents the mean
coordination number of glasses. In ternary alloys containing equal proportions
of group IV and V selenides, IPs are wider and encompass ranges of respective
binary glasses. These data suggest that the local structural variance
contributing to IP widths largely derives from four isostatic local structures
of varying connectivity r; two include group V based quasi-tetrahedral (r =
2.29) and pyramidal (r = 2.40) units, and the other two are group IV based
corner-sharing (r = 2.40) and edge-sharing (r = 2.67) tetrahedral units.
Remarkably, binary group V (P, As) sulfides exhibit IPs that are shifted to
even a lower r than their selenide counterparts; a result that we trace to
excess Sn chains either partially (As-S) or completely (P-S) demixing from
network backbone, in contrast to excess Sen chains forming part of the backbone
in corresponding selenide glasses. In ternary chalcogenides of Ge with the
group V elements (As, P), IPs of the sulfides are similar to their selenide
counterparts, suggesting that presence of Ge serves to reign in the excess Sn
chain fragments back in the backbone as in their selenide counterparts
The role of local structure in dynamical arrest
Amorphous solids, or glasses, are distinguished from crystalline solids by
their lack of long-range structural order. At the level of two-body structural
correlations, glassformers show no qualitative change upon vitrifying from a
supercooled liquid. Nonetheless the dynamical properties of a glass are so much
slower that it appears to take on the properties of a solid. While many
theories of the glass transition focus on dynamical quantities, a solid's
resistance to flow is often viewed as a consequence of its structure. Here we
address the viewpoint that this remains the case for a glass. Recent
developments using higher-order measures show a clear emergence of structure
upon dynamical arrest in a variety of glass formers and offer the tantalising
hope of a structural mechanism for arrest. However a rigorous fundamental
identification of such a causal link between structure and arrest remains
elusive. We undertake a critical survey of this work in experiments, computer
simulation and theory and discuss what might strengthen the link between
structure and dynamical arrest. We move on to highlight the relationship
between crystallisation and glass-forming ability made possible by this deeper
understanding of the structure of the liquid state, and emphasize the potential
to design materials with optimal glassforming and crystallisation ability, for
applications such as phase-change memory. We then consider aspects of the
phenomenology of glassy systems where structural measures have yet to make a
large impact, such as polyamorphism (the existence of multiple liquid states),
aging (the time-evolution of non-equilibrium materials below their glass
transition) and the response of glassy materials to external fields such as
shear.Comment: 70 page
Preparation of Cu-based bulk metallic glasses by suction casting
A series of Cu-Hf-Ti alloys prepared by rapid solidification of the melt and by copper mould casting were studied in the present work. Alloy ingots were prepared by arc-melting mixtures of pure metals in an argon atmosphere. An indication of the cooling rate obtained was determined using an Al-4.5 wt%Cu alloy. Cooling rates varied from 540 K/s for the centre section of a 4 mm die to 885 K/s for the outside wall section of the 2 mm die. The glass-forming ability, structure and thermal stability of Cu-Hf-Ti glassy alloys were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). Bulk glass formation was observed for the Cu64Hf36, Cu55Hf25Ti20 and Cu56Hf25Ti19 alloys, with critical diameters dc for a fully glassy structure of 1, 4 and 5 mm, respectively. The substitution of Hf by Ti increased the glassforming ability (GFA) and the thermal stability
Superconductivity and short range order in metallic glasses FeNiZr
In amorphous superconductors, superconducting and vortex pinning properties
are strongly linked to the absence of long range order. Consequently,
superconductivity and vortex phases can be studied to probe the underlying
microstructure and order of the material. This is done here from resistance and
local magnetization measurements in the superconducting state of
FeNiZr metallic glasses with . Firstly,
we present typical superconducting properties such as the critical temperature
and fields and their dependence on Fe content in these alloys. Then, the
observations of peculiar clockwise hysteresis loops, wide double-step
transitions and large magnetization fluctuations in glasses containing a large
amount of Fe are analyzed to reveal a change in short range order with Fe
content.Comment: 8 pages, 7 figure
Criteria for formation of metallic glasses: The role of atomic size ratio
We consider metallic alloys of Cu*, Cu, and Cu** in which the atoms differ only in their atomic radii and examine how the size ratio affects the local orders in the alloy systems. These studies use molecular dynamics simulations in which the atomic interactions are modeled with a Sutton–Chen many-body potential. Considering rapid cooling of these binary and ternary alloys from the melt, we find three regimes defined by the magnitude of atomic size ratio lambda (lambda<=1.0): with (i) large size ratios of 0.95<lambda<=1.0, crystallization occurs; (ii) with moderate size ratios of 0.60<=lambda<=0.95, a glass phase forms; and (iii) with small size ratios of lambda<0.60, the alloy phase separates into pure phases and crystallize. From analyzing the structures of these binary and ternary alloys, we find that the liquid phase is characterized by local structures in which bonded atoms have local fivefold symmetry, which becomes more prominent as the glass phase forms. For phases that crystallize this local fivefold symmetry disappears as the long-range order of the crystalline phase dominates. The fivefold symmetry in the glass phase is mainly due to the icosahedral cluster formation. Energetically, the formation of icosahedral cluster is favored at the atomic size ratio of lambda~0.85, which is close to the lambda at which our analyses shows the maximum in the fivefold symmetry and the number of icosahedral clusters. As lambda decreases further, the phase separation is observed. The fivefold symmetry character and the number of icosahedral cluster shows the local minimum at this onset of the phase separation
Phase Transformations in Binary Colloidal Monolayers
Phase transformations can be difficult to characterize at the microscopic
level due to the inability to directly observe individual atomic motions. Model
colloidal systems, by contrast, permit the direct observation of individual
particle dynamics and of collective rearrangements, which allows for real-space
characterization of phase transitions. Here, we study a quasi-two-dimensional,
binary colloidal alloy that exhibits liquid-solid and solid-solid phase
transitions, focusing on the kinetics of a diffusionless transformation between
two crystal phases. Experiments are conducted on a monolayer of magnetic and
nonmagnetic spheres suspended in a thin layer of ferrofluid and exposed to a
tunable magnetic field. A theoretical model of hard spheres with point dipoles
at their centers is used to guide the choice of experimental parameters and
characterize the underlying materials physics. When the applied field is normal
to the fluid layer, a checkerboard crystal forms; when the angle between the
field and the normal is sufficiently large, a striped crystal assembles. As the
field is slowly tilted away from the normal, we find that the transformation
pathway between the two phases depends strongly on crystal orientation, field
strength, and degree of confinement of the monolayer. In some cases, the
pathway occurs by smooth magnetostrictive shear, while in others it involves
the sudden formation of martensitic plates.Comment: 13 pages, 7 figures. Soft Matter Latex template was used. Published
online in Soft Matter, 201
Simulation of Cu-Mg metallic glass: Thermodynamics and Structure
We have obtained effective medium theory (EMT) interatomic potential
parameters suitable for studying Cu-Mg metallic glasses. We present
thermodynamic and structural results from simulations of such glasses over a
range of compositions. We have produced low-temperature configurations by
cooling from the melt at as slow a rate as practical, using constant
temperature and pressure molecular dynamics. During the cooling process we have
carried out thermodynamic analyses based on the temperature dependence of the
enthalpy and its derivative, the specific heat, from which the glass transition
temperature may be determined. We have also carried out structural analyses
using the radial distribution function (RDF) and common neighbor analysis
(CNA). Our analysis suggests that the splitting of the second peak, commonly
associated with metallic glasses, in fact has little to do with the glass
transition itself, but is simply a consequence of the narrowing of peaks
associated with structural features present in the liquid state. In fact the
splitting temperature for the Cu-Cu RDF is well above . The CNA also
highlights a strong similarity between the structure of the intermetallic
alloys and the amorphous alloys of similar composition. We have also
investigated the diffusivity in the supercooled regime. Its temperature
dependence indicates fragile-liquid behavior, typical of binary metallic
glasses. On the other hand, the relatively low specific heat jump of around
indicates apparent strong-liquid behavior, but this can
be explained by the width of the transition due to the high cooling rates.Comment: 12 pages (revtex, two-column), 12 figures, submitted to Phys. Rev.
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