Undercooled Regime Research Articles

Density and excess volume of liquid copper, nickel, iron, and their binary alloys

Abstract The densities of liquid copper, nickel and iron and their binary alloys have been measured over a wide temperature range, including the undercooled regime. A non-contact technique was used, consisting of electromagnetic levitation and optical dilatometry. In all cases the density was a linear function of temperature. On the other hand, the concentration dependence was only linear for Fe –Ni, whereas Fe –Cu and Cu–Ni are characterized by a positive or negative excess volume respectively.

Density and excess volumes of liquid copper, cobalt, iron and their binary and ternary alloys

Abstract The densities of liquid copper, cobalt, and iron, their binary and ternary alloys have been measured over a temperature range including the undercooled regime. A non-contact technique was used, consisting of electromagnetic levitation combined with optical dilatometry. For all samples, the density was a linear function of temperature. The concentration dependence was studied by means of the excess volume which was negligible for Co– Fe and positive for Cu– Fe, Cu– Co, and Cu–Co – Fe. The density of the ternary alloy could be predicted from the excess volumes of the binary phases without the need to introduce any ternary interactions.

Density and surface tension of liquid ternary Ni–Cu–Fe alloys

Abstract Density and surface tension of liquid Ni –Cu –Fe alloys have been measured over a wide temperature range, including the undercooled regime. A non-contact technique was used, consisting of an electromagnetic levitator, an optical densitometer, and an oscillating drop tensiometer. At temperatures above and below the liquidus point, density and surface tension are linear functions of temperature. The concentration dependence of the density is significantly influenced by a third-order (ternary) parameter in the volume, while the surface tensions can be derived from the thermodynamic potentials E G of the binary phases alone.

On the link between polyamorphism and liquid-liquid transition: The case of salty water.

We investigate a LiCl:6D2O water solution in the deep undercooled regime as a function of pressure by neutron diffraction, small angle neutron scattering, and molecular dynamics simulations. We probe the structure of the undercooled liquid and the existence of density fluctuations in the system along isotherms just above the observed first-order-like polymorphic transition in the solid between a high density amorphous (s-HDA) and a very high density amorphous (s-VHDA) form [L. E. Bove et al., Phys. Rev. Lett. 106, 125701 (2011)]. We observe a continuous pressure evolution of the structure factor of the liquid and the absence of density fluctuations when crossing the continuation line of the s-HDA and s-VHDA boundary. These results indicate that no liquid-liquid transition is observed in the system in correspondence with the solid polyamorphism.

Temperature dependence of the solid-liquid interface free energy of Ni and Al from molecular dynamics simulation of nucleation.

The temperature dependence of the solid-liquid interfacial free energy, γ, is investigated for Al and Ni at the undercooled temperature regime based on a recently developed persistent-embryo method. The atomistic description of the nucleus shape is obtained from molecular dynamics simulations. The computed γ shows a linear dependence on the temperature. The values of γ extrapolated to the melting temperature agree well with previous data obtained by the capillary fluctuation method. Using the temperature dependence of γ, we estimate the nucleation free energy barrier in a wide temperature range from the classical nucleation theory. The obtained data agree very well with the results from the brute-force molecular dynamics simulations.

Thermophysical properties and atomic structure of liquid Si-Ge alloys

Abstract The thermophysical properties and atomic structure of liquid Si-Ge alloys were investigated by the molecular dynamic simulations with Stillinger-Weber potential over a broad temperature range, including both superheating and undercooled regime. The maximum undercooling in this work is 611 K for pure Ge. For all the Si100-xGex (x = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100) alloys, the densities exhibit quadratic relationship with temperature. The density of Si at the melting temperature is 2.59 g·cm−3, existing a slight deviation of 0.39% compared with the experiential data. For liquid Ge, the density at the melting temperature is 5.62 g·cm−3, nearly identical with the reported values. In all the cases, the densities of Si-Ge alloys with different compositions are larger than those of ideal solution. Therefore, liquid Si-Ge alloys display negative excess volume, which reaches the minimum at 40% Ge content. The specific heat was obtained by thermodynamic function and energy fluctuation approaches, respectively. Both of the results are in good agreement with experimental data around the liquidus temperatures. Furthermore, the diffusion coefficients were obtained by calculating the mean square displacement. The pair distribution function was applied to analyze the liquid structure. It is found that the peaks occur around the liquidus temperatures, which implies the structure change between normal and undercooled liquid alloys.

Processing of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 Bulk Metallic Glass Alloy by Cu Mould Casting and Spark Plasma Sintering

The present investigation deals with the processing of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 bulk metallic glass (BMG) using different techniques. The maximum plate thickness for the amorphous phase formation has been found to be 1.25 mm using a conventional BMG casting technique (forced infiltration casting). For enhancing the dimension, an alternate processing route was employed to produce the BMG in the form of compacts. In this case, the dimensions are not restricted by the glass forming ability of the alloy. The process involved the preparation of amorphous ribbons using a melt spinner followed by high energy ball milling to crush the ribbons into very fine particles. The powder was then compacted in the form of pellets using spark plasma sintering technique. Density achieved was almost near to theoretical density (99%). The density was found to increase with a decrease in the particle size of glassy powder, which could be achieved by increasing the duration of high energy ball milling. The maximum densification occurred when the holding temperature was within undercooled liquid regime (∆Tx). Further increase of temperature beyond crystallization temperature (Tx) resulted in decreasing the compact density.

Coupling between dynamic slowing down and chemical heterogeneity in a metallic undercooled liquid

We investigate the connection between local structure and dynamics in the liquid and undercooled $\mathrm{A}{\mathrm{l}}_{93}\mathrm{C}{\mathrm{r}}_{7}$ alloy using ab initio molecular dynamics simulations. In the liquid phase, we show the striking effects of Cr alloying on the atomic-scale structure, characterized by a heterogeneous local ordering around each component, due to a strong interplay between chemical short-range order and icosahedral short-range order. In the undercooled phase, this interplay leads to the formation of an icosahedral-based medium-range order (IMRO) referring to Cr atoms. In examining the dynamic properties, we observe that this chemically induced structural heterogeneity gives rise to a substantial decoupling of component diffusion in the liquid phase, far above the liquidus temperature. In the undercooled regime, we find the breakdown of the Stokes-Einstein relation and a pronounced increase of the non-Gaussian parameter, indicating the onset of dynamic heterogeneities. Using the isoconfigurational ensemble method, we evidence that the structural origin of dynamics heterogeneities is clearly related to the formation of IMRO. Finally, we discuss the role played by IMRO in nucleation on the preexisting icosahedral cluster in the liquid phase.

Vitrification and Crystallization of Phase-Separated Metallic Liquid

The liquid–liquid phase separation (LLPS) behavior of Fe50Cu50 melt from 3500 K to 300 K with different rapid quenching is investigated by molecular dynamics (MD) simulation based on the embedded atom method (EAM). The liquid undergoes metastable phase separation by spinodal decomposition in the undercooled regime and subsequently solidifies into three different Fe-rich microstructures: the interconnected-type structure is kept in the glass and crystal at a higher cooling rate, while the Fe-rich droplets are found to crystalize at a lower cooling rate. During the crystallization process, only Fe-rich clusters can act as the solid nuclei. The twinning planes can be observed in the crystal and only the homogeneous atomic stacking shows mirror symmetry along the twinning boundary. Our present work provides atomic-scale understanding of LLPS melt during the cooling process.

Correlation between dynamic slowing down and local icosahedral ordering in undercooled liquid Al80Ni20 alloy.

We use ab initio molecular dynamics simulations to study the correlation between the local ordering and the dynamic properties of liquid Al80Ni20 alloy upon cooling. Our results evidence a huge increase of local icosahedral ordering (ISRO) in the undercooled regime which is more developed around Ni than Al atoms. We show that ISRO has a strong impact on self-diffusion coefficients of both species and is at the origin of their crossover from Arrhenius to non-Arrhenius behavior around a crossover temperature TX = 1000 K, located in the undercooled region. We also clearly identify that this temperature corresponds to the development of dynamic heterogeneities and to the breakdown of the Stokes-Einstein relation. At temperatures below this crossover, we find that the behavior of the diffusion and relaxation dynamics is mostly incompatible with predictions of the mode-coupling theory. Finally, an analysis of the van Hove function indicates that the crossover temperature TX marks the onset of a change in the diffusion mechanism from a normal flow to an activated process with hopping. From these results, the glass-forming ability of the alloy is discussed.

On the reaction coordinate for seeded crystallisation

Small pre-structured seeds introduced into an undercooled fluid are known to increase the crystal nucleation rate by some orders of magnitude, if the structure of the seeds is commensurate with the bulk crystalline phase. The presence of such seeds also alters the crystallisation mechanism by favouring particular structures at the early stages of the nucleation process. Here, we study with computer simulations the effect of small face-centred cubic and body-centred cubic seeds on the crystallisation of a Lennard-Jones liquid in the strongly undercooled regime. We find that seeds with body-centred cubic structure lead to a larger enhancement of the crystallisation rate than face-centred cubic seeds. An analysis of recurrence times reveals that the size of the largest crystalline cluster used as reaction coordinate is affected by pronounced memory effects, which depend on the particular seed structure and point to the importance of structural information in the definition of a good reaction coordinate for crystallisation.

Predicting macroscopic thermal expansion of metastable liquid metals with only one thousand atoms

Results of thermal expansion prediction from atomic scale for metastable liquid metals are reported herein. Three pure liquid metals Ni, Fe, and Cu together with ternary Ni60Fe20Cu20 alloy are used as models. The pair distribution functions were employed to monitor the atomic structure. This indicates that the simulated systems are ordered in atomic short range and disordered in long range. The thermal expansion coefficient was computed as functions of temperature and atom cutoff radius, which tends to maintain a constant when the cutoff radius increases to approximately 15 A. In such a case, slightly more than 1000 atoms are required for liquid Ni, Cu, Fe and Ni60Fe20Cu20 alloy, that is, the macroscopic thermal expansion can be predicted from the volume change of such a tiny cell. Furthermore, the expansion behaviors of the three types of atoms in liquid Ni60Fe20Cu20 alloy are revealed by the calculated partial expansion coefficient. This provides a fundamental method to predict the macroscopic thermal expansion from the atomic scale for liquid alloys, especially in the undercooled regime. thermal expansion, liquid metal, undercooling, metastable state

Local structural excitations in model glasses

Structural excitations of model Lennard-Jones glass systems are investigated using the activation-relaxation technique (ART$n$), which explores the potential energy landscape around a local minimum energy configuration by converging to a nearby saddle-point configuration. Performing ART$n$ results in a distribution of barrier energies that is single-peaked for well-relaxed samples. The present work characterizes such atomic-scale excitations in terms of their local structure and environment. It is found that, at zero applied stress, many of the identified events consist of chainlike excitations that can either be extended or ringlike in their geometry. The location and barrier energy of these saddle-point structures are found to correlate with the type of atom involved, and with spatial regions that have low Kelvin eigenshear moduli and are close to the excess free volume within the configuration. Such correlations are, however, weak and more generally the identified local structural excitations are seen to exist throughout the model glass sample. The work concludes with a discussion within the framework of $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ relaxation processes that are known to occur in the undercooled liquid regime.

A comparative study of dendritic growth within undercooled liquid pure Fe and Fe50Cu50 alloy

The dendritic growth characteristics within undercooled liquid pure Fe and binary Fe50Cu50 alloy were investigated by glass fluxing and electromagnetic levitation methods. The growth velocity of pure Fe dendrite increases with undercooling, which reaches 69ms−1 at the maximum undercooling of 280K (0.15Tm). The microstructure appears as well-defined dendrites under slight undercooling, while it is characterized by a network of grain boundaries without discernible dendrites under substantial undercooling. A glass fluxing method was used to study the dendritic growth within the undercooled liquid Fe50Cu50 alloy. The maximum undercooling achieved was 261K (0.15TL), and the corresponding growth velocity was 15ms−1. A critical undercooling for metastable liquid phase separation of 7K was determined. At larger undercoolings, the sample separates into an Fe-rich zone and a Cu-rich zone. The microstructure of the Fe-rich zone displays well-branched dendrites in the condition of slight undercooling, whereas it transforms into monotectic solidification structures in the highly undercooled regime. Slender dendrites and spheres are characteristics of the structural morphology of the Cu-rich zone at all of the undercoolings. Energy-dispersive spectroscopy analysis shows that the average compositions of phase-separated zones have a large deviation from the original alloy composition, and conspicuous solute trapping has taken place.

Linking high- and low-temperature plasticity in bulk metallic glasses: thermal activation, extreme value statistics and kinetic freezing

At temperatures well below their glass transition, the deformation properties of bulk metallic glasses are characterized by a sharp transition from elasticity to plasticity, a reproducible yield stress and an approximately linear decrease of this stress with increasing temperature. In the present work, it is shown that when the well-known properties of the undercooled liquid regime, in terms of the underlying potential energy landscape, are assumed to be also valid at low temperature, a thermal activation model is able to reproduce the observed onset of macroscopic yield. At these temperatures, the thermal accessibility of the complex potential energy landscape is drastically reduced, and the statistics of extreme value and the phenomenon of kinetic freezing become important, affecting the spatial heterogeneity of the irreversible structural transitions mediating the elastic-to-plastic transition. As the temperature increases and approaches the glass transition temperature, the theory is able to smoothly transit to the high-temperature deformation regime where plasticity is known to be well described by thermally activated viscoplastic models.

Vibrational and structural properties of amorphous n-butanol: A complementary Raman spectroscopy and X-ray diffraction study

Raman spectroscopy and X-ray diffraction experiments were performed in the liquid, undercooled liquid, and glassy states of n-butanol. Clear correlated signatures are obtained below the melting temperature, from both temperature dependences of the low-wavenumber vibrational excitations and the intermediate-range order characterized by a prepeak detected in the different amorphous states. It was found that these features are related to molecular associations via strong hydrogen bonds, which preferentially develop at low temperature, and which are not compatible with the long-range order of the crystal. This study provides information on structural heterogeneities developing in hydrogen-bonded liquids, associated to the undercooled regime and the inherent glass transition. The analysis of the isothermal abortive crystallization, 2 K above the glass transition temperature, has given the opportunity to analyze the early stages of the crystallization and to describe the origin of the frustration responsible for an uncompleted crystallization.

Molecular dynamics prediction of density for metastable liquid noble metals

Abstract The thermophysical properties of metastable liquid noble metals are not readily available due to the great experimental difficulties. Here the densities of liquid Pd, Pt, Ag, and Au are predicted by molecular dynamics method. The pair distribution functions are computed to monitor the atomic structure of these noble metals, which indicate that the systems remain in liquid state in the process of simulation. The calculated densities exhibit nonlinear temperature dependences and prove to have a high accuracy. The density data are obtained in a much broader temperature range, especially in the undercooled regime. Moreover, the molar volumes and the thermal expansion coefficients are also derived from the density predictions.

Phase separation in bulk-type Gd–Zr–Al–Ni metallic glass

Phase separation in bulk-type metallic glass and effect of cooling rate on the phase-separated microstructure have been investigated in Zr35Gd20Al25−xNi20+x (x = 0, 2, 4, 8) alloys. Among the alloys investigated, molten Zr35Gd20Al23Ni22 undergoes metastable liquid phase separation in the undercooled regime and subsequently solidifies into two different Zr-rich and Gd-rich amorphous phases, which exhibit enough GFA enabling formation of 1 mm diameter phase-separating glass. The liquid melt undergoes phase separation by spinodal decomposition when the degree of undercooling is relatively large, while by nucleation and growth mechanism when the degree of undercooling is relatively low. The lower degree of undercooling results in the composition more close to the terminal composition expected from the miscibility gap.

EML - an electromagnetic levitator for the International Space Station

Based on a long and successful evolution of electromagnetic levitators for microgravity applications, including facilities for parabolic flights, sounding rocket missions and Spacelab missions, the Electromagnetic Levitator EML provides unique experiment opportunities onboard ISS. With the application of the electromagnetic levitation principle under microgravity conditions the undercooled regime of electrically conductive materials becomes accessible for an extended time which allows the performance of unique studies of nucleation phenomena or phase formation as well as the measurement of a range of thermophysical properties both above the melting temperature and in the undercooled regime. The EML payload is presently being developed by Astrium Space Transportation under contracts to ESA and DLR. The design of the payload allows flexible experiment scenarios individually targeted towards specific experimental needs and samples including live video control of the running experiments and automatic or interactive process control.

Understanding atomic-scale phase separation of liquid Fe-Cu alloy

Using liquid Fe60Cu40 alloy as a model, the structure of liquid Fe-Cu alloy systems is investigated in the temperature range 1200-2200 K, covering a large metastable undercooled regime, to understand the phase separation of liquid Fe-Cu alloys on the atomic scale. The total pair distribution functions (PDFs) indicate that liquid Fe60Cu40 alloy is ordered in the short range and dis-ordered in the long range. If the atom types are ignored, the total atom number densities and PDFs demonstrate that the atoms are distributed homogenously in the liquid alloy. However, the segregation of Fe and Cu atoms is very obvious with decreasing tem-perature. The partial PDFs and coordination numbers show that the Cu and Fe atoms are not apt to get together on the atomic scale at low temperatures; this will lead to large fluctuations and phase separation in liquid Fe-Cu alloy.