Supercooled Melt Research Articles

Моделирование кристаллизации модифицированного наноразмерными частицами расплава при лазерной обработке поверхности металла

A 2D mathematical model is proposed for the modification of an iron-based alloy with refractory nanosized particles. Numerical simulation of the processes during the modification of the surface layer of the substrate metal using the energy of a laser pulse has been carried out. Within the framework of the proposed model, the processes of heating and melting of metal on a substrate covered with a layer of nanosized refractory particles penetrating into the molten metal, convective heat transfer in the melt, and solidification after the end of the pulse are considered. Metal melting is considered in the Stefan approximation, and when the melt is cooled, the model of heterogeneous nucleation and subsequent crystallization is used. The fluid flow is described by the Navier-Stokes equations in the Boussinesq approximation. The distribution of nanoparticles in the melt is modeled by moving markers. Based on the results of calculations, the mode of pulsed laser action is determined, in which a flow is formed for a homogeneous distribution of particles of the modifying substance in the presence of a surfactant in the metal. The volume of the solid phase formed around the nucleus determines the size of the grain structure in the solidified alloy. The liquidus temperature changes depending on the concentration of dissolved carbon in the melt. In the numerical simulation of the solidification of the surface layer of the metal, it was found that the conditions of nucleation and crystallization differ significantly in the volume of the melt. It is determined that the duration of nucleation in a supercooled melt is several tens of microseconds. The maximum number of crystallization centers occurs in areas where heat removal occurs most rapidly. With the growth of the solid phase in the melt and the release of the latent heat of crystallization, the value of supercooling decreases, the nucleation stops and the number of formed crystallization centers does not change further. The distribution of the dispersion of the crystal structure over the volume of the melted metal is estimated. It was found that as the melt cools, sequential-volume crystallization occurs.

Structuring in Metastable Melt

The dendritic parameter is estimated on the assumption that three-dimensional structure of the solid phase is formed in metastable supercooled melt. The solidification of such melt is considered in the quasi-chemical approximation, on the assumption that the transient state corresponds to an ideal solution. The dimensions of the structural cell are found to depend on the relation between the diffusion rate within the cell and the rate of heat loss. The theoretical estimate of the structural parameter agrees with experimental results.

Research on grain refinement and wear behavior of micro-remelted Ta15 alloy surface by laser cleaning

TA15 alloy has received widespread attention due to its excellent properties. The native oxide film limits its application. Laser cleaning can remove the native oxide film. However, research on the surface modification of materials by laser cleaning is rarely discussed. In this paper, the effects of different energy densities (E) on the improvement of surface morphology were studied. The wear morphology and microstructure were observed in situ. The difference between the wear mechanism between the remelted layer and the base material at different wear depths was analyzed. The results show the native morphological defects are transformed into a homogeneous crater after laser cleaning. With increasing energy, the crater morphology changes to a striation morphology. Tip growth of the supercooled melt and the cumulative of multiple processes results in morphology evolution. Compared to the base material, the coefficient of friction of the remelted layer decreased to 40%. Observing the microstructure in situ, the average grain area of the remelted layer decreased by about 40%. The large-angle grain boundary ratio increased. As E increases, the thickness of the remelted layer increases. Abrasion to 10 μm below the surface, the untreated sample showed furrow, scaly flaking, and pitting spalling. The laser-cleaned sample surface showed abrasions and plastic smearing. Abrasion to 50 μm below the surface, the surface of the remelted layer was fine abrasive particles. The surface of the untreated sample was larger abrasive particles and fatigue delamination.

Redox-structure dependence of molten iron oxides

The atomic structural arrangements of liquid iron oxides affect the thermophysical and thermodynamic properties associated with the steelmaking process and magma flows. Here, the structures of stable and supercooled iron oxide melts have been investigated as a function of oxygen fugacity and temperature, using x-ray diffraction and aerodynamic levitation with laser heating. Total x-ray structure factors and their corresponding pair distribution functions were measured for temperatures ranging from 1973 K in the stable melt, to 1573 K in the deeply supercooled liquid region, over a wide range of oxygen partial pressures. Empirical potential structure refinement yields average Fe–O coordination numbers ranging from ~4.5 to ~5 over the region FeO to Fe2O3, significantly lower than most existing reports. Ferric iron is dominated by FeO4, FeO5 and FeO6 units in the oxygen rich melt. For ferrous iron under reducing conditions FeO4 and FeO5 units dominate, in stark contrast to crystalline FeO.

Structural Features Associated with Twinning in the Growth of Gallium Arsenide Single Crystals by the Czochralski Method

The structural features induced by twinning in tin- and tellurium-doped gallium arsenide crystals during their growth by the Czochralski method with liquid melt sealing by boric anhydride have been considered. It is shown that growth twins and edge facets arise at the crystal periphery in the region of supercooled melt. The dopant concentration near a twin is always higher than in the region free of twins. A change is observed in the dislocation-growth direction in crystal during twinning.

Mathematical modeling of crystallization process from a supercooled binary melt

The article is concerned with the analytical solution to the integro‐differential system of balance and kinetic equations that describe the crystal growth phenomenon in a binary system for various nucleation kinetics. The effect of impurity concentration on the evolutionary behavior of crystals is shown. The nonlinear dynamics of a supercooled binary melt is studied with allowance for the withdrawal mechanism of product crystals from a metastable liquid of the crystallizer.

Diffusion in bulk metallic glasses

Due to the simple structure, bulk metallic glasses are very convenient model systems for studying the mechanism of atom diffusion in disordered media. In this paper, we analyze literature data on diffusion for a set of atoms in the metallic alloy Zr46.75Ti8.25Cu7.5Ni10Be27.5 (Vitreloy 4) above and below the glass transition temperature. We show that the activation energy of diffusion, E, is controlled by the high-frequency shear modulus, G∞, and can be presented in the form E=const×G∞ in the whole temperature range studied. We conclude that the displacements of a group of atoms in metallic alloys occur during the elastic shear deformation of their surroundings. The diffusion mechanism is the same both in the glassy state and in the supercooled melt.

Mechanism of structure formation in a metastable melt and estimation of its spatial scale

An estimate of the dendritic parameter based on the assumption of the formation of the spatial structure of the solid phase in a metastable supercooled melt (MSM) is proposed. The solidification of MSM is considered in the quasichemical approximation, which assumes a transition state corresponding to the ideal solution system. It is shown, that the dimensions of the structural cell depend on the ratio of the diffusion rate within the cell and the rate of heat removal. The theoretical estimate of the structure parameter is consistent with the experimental dependence. Keywords: metastable melt, dendritic parameter, cooling rate, segregation zone, diffusion. chirkov@bmstu.ru

Impact of crystallization method on the strain, defect formation, and thermoluminescence of YAG:Ce crystals

Numerous studies have addressed the excellent properties of Ce-doped Y3Al5O12 garnet (YAG) crystals for utilization in lighting applications. However, the differences in the photoluminescence and defect formation of YAG crystals obtained by different methods have not been elucidated. In this study, the effects of the crystallization path of YAG:Ce on the local structure of its emission site and defect formation were investigated. Ce-doped YAG crystals were prepared in three ways: (1) solid-state reaction, (2) heat-treatment of YAG glass, and (3) crystallization from a supercooled YAG melt by a controlled cooling process from a deep supercooled state. For the sample obtained by the crystallization of a supercooled melt, the photoluminescence peak red-shifted by 3 nm relative to those of the other samples, and the thermoluminescence increased by a factor of 50–5000. As the thermoluminescence intensity has been associated with the defects formed, positron annihilation lifetime spectroscopy was performed to investigate the defects. The positron lifetimes of all crystallized samples exhibited two components, and the crystallized sample obtained by melt-cooling showed the longest lifetime among the samples.

Recalescence dynamics and solidification of a supercooled melt in a finite domain

We study the dynamics of supercooled solidification of a pure material in a finite domain subject to isothermal boundary conditions. At early stages when the liquid can effectively be treated as semi-infinite, we derive asymptotic solutions in the limits of both strong and weak latent-heat release, corresponding to large and small effective Stefan numbers, respectively. In particular, the solutions describing a rapid recalescence followed by a gradual change in the interfacial temperature are derived. Once the finite extent becomes effective, the system relaxes to an intermediate stage. For large Stefan numbers, the intermediate stage is quasi-steady, with the linear temperature profiles in the two phases and the interface temperature close to an equilibrium melting temperature. For Stefan numbers less than unity, the intermediate stage has a traveling-wave temperature profile in the liquid, similar to that in the one-sided problem, and a self-similar profile in the solid, where the temperature is close to the interface temperature through the whole solid except for a thermal boundary layer far from the interface. The model is applied to water, copper and salol, providing estimates for the freezing rates, interface position, and the recalescence and complete-freezing times in these pure systems.

Microscopic Theory of Dynamically Heterogeneous Activated Relaxation as the Origin of Decoupling of Segmental and Chain Relaxation in Supercooled Polymer Melts

We combine our force-based theory of activated segmental relaxation for glass-forming polymer melts, with our new formulation of how nm-scale effectively static structural fluctuations induce dynam...

High Pressure Quenched Glasses: unique structures and properties

Zr-based metallic glasses are prepared by quenching supercooled liquid under pressure. These glasses are stable in ambient conditions after decompression. The High Pressure Quenched glasses have a distinct structure and properties. The pair distribution function shows redistribution of the Zr-Zr interatomic distances and their shift towards smaller values. These glasses exhibit higher density, hardness, elastic modulus, and yield stress. Upon heating at ambient pressure, they show volume expansion and distinct relaxation behavior, reaching an equilibrated state above the glass transition. These experimental results are consistent with an idea of pressure-induced low to high density liquid transition in the supercooled melt.

Dynamic Crystallization of a Haplogranitic Melt: Application to Pegmatites

AbstractBoth equilibrium and dynamic crystallization experiments have been performed on a hydrous haplogranitic melt at 200 MPa to model nucleation and growth mechanisms and simulate pegmatite textures. The equilibrium results provide a reference frame (phase assemblages and compositions, liquidus and solidus temperatures and dependence with the melt H2O concentration) to parametrize the kinetic experiments. The seven H2O-saturated dynamic crystallization experiments followed a specific time-temperature path. After a pre-conditioning step at 800°C, charges were cooled between 3·5 and 7°C/min to 700, 660 and 600°C corresponding to ΔT of 20, 60 and 120°C. Dwell times ranged from 42 up to 1440h. Variable mineral assemblages and textures, and two types of polymineralic assemblages were obtained depending on ΔT and t. For ΔT = 120°C, crystallization is sequential and includes graphic quartz–alkali-feldspar intergrowths characteristic of pegmatite textures. The crystallization sequence reflects nucleation and growth of kinetically-favoured metastable phases and solid solution compositions from the supercooled melt. Early alkali-feldspars are more K-rich than expected at equilibrium and late albites more Na-rich. The K-rich graphic texture progressively evolves to a Na-rich intergrowth texture. Melts also follow a progressive though limited sodic evolution with time. At the interface of growing alkali-feldspars, melts are enriched in SiO2 and depleted in Al2O3, Na2O and, to a lesser extent, K2O. H2O accumulates at the interface reaching concentration levels higher (by 1–2 wt %) than the saturation. Rejection of SiO2 and H2O at the interface controls the effective undercooling in the local melt and promotes rapid textural changes toward larger grain sizes at the front of graphic zones. Textural ripening takes place contemporaneously to sequential crystallization. Growth rates for quartz and alkali-feldspar are tightly grouped, between 7·3 x 10-11 and 1·6 x 10-12 m s-1. Textures from the dynamic crystallization experiments closely resemble natural pegmatites, but layered aplite units have not been reproduced. Our results confirm and strengthen the importance of liquidus undercooling to generate pegmatite textures.

From nucleation and coarsening to coalescence in metastable liquids.

The transition of a metastable liquid (supersaturated solution or supercooled melt) occurring from the intermediate stage (where the crystals nucleate and grow) to the concluding stage (where the larger particles evolve at the expense of the dissolution of smaller particles) is theoretically described, with allowance for various mass transfer mechanisms (reaction on the interface surface, volume diffusion, grain-boundary diffusion, diffusion along the dislocations) arising at the stage of Ostwald ripening (coalescence). The initial distribution function (its 'tail') for the concluding stage (forming as a result of the evolution of a particulate assemblage during the intermediate stage) is taken into account to determine the particle-size distribution function at the stage of Ostwald ripening. This modified distribution function essentially differs from the universal Lifshitz-Slyozov (LS) solutions for several mass transfer mechanisms. Namely, its maximum lies below and is shifted to the left in comparison with the LS asymptotic distribution function. In addition, the right branch of the particle-size distribution lies above and is shifted to the right of the LS blocking point. It is shown that the initial 'tail' of the particle-size distribution function completely determines its behaviour at the concluding stage of Ostwald ripening. The present theory agrees well with experimental data. This article is part of the theme issue 'Patterns in soft and biological matters'.

Altering the X-ray Scattering Contrast of Triton X-100 Micelles and Its Trapping in a Supercooled Solvent.

The structure of core-shell micelles formed by nonionic surfactant Triton X-100 (TX-100) in a supercooled glucose-urea melt is investigated by contrast variation small-angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and HR-TEM. Cooling a molten mixture of glucose-urea (weight ratio of 3:2) to room temperature yields a supercooled solvent without crystallization that can be used for trapping micelles of TX-100. By use of a combination of water and glucose-urea mixture at different proportions as solvent for micellization, the scattering length density (SLD) of the solvent can be tuned to match the shell contrast of the micelles. A systematic analysis of SAXS and SANS data with different SLD of solvent permits a quantitative evaluation of electron density profile of micelles in different matrices. The core of TX-100 micelles shows significant swelling in glucose-urea melt, as compared to that in water. The dimension and morphology of micelles were evaluated by scattering techniques and HR-TEM. Dynamic light scattering (DLS) studies suggest that, unlike micelles in water, the diffusion of micelles in supercooled glucose-urea melt decreased by several orders of magnitude.

Viscous properties of nickel-containing binary metal melts

The paper presents the results of molecular dynamics study of the viscosity of nickel-containing binary metal melts for a wide range of temperatures, including the region of the equilibrium liquid phase and supercooled melt. It is shown that the temperature dependencies of the viscosity of binary metal melts are described by the Kelton’s quasi-universal model. Based on the analysis of the viscosity coefficient of the binary melt composition within the framework of the Rosenfeld’s scale transformations, it has been established that to correctly describe the viscosity of binary/multicomponent metal melts within the framework of entropy models, it is necessary to use a more complex representation of the excess entropy S ex than in the approximation of pair correlation entropy S 2.

Monodisperse Polymer Melts Crystallize via Structurally Polydisperse Nanoscale Clusters: Insights from Polyethylene.

This study demonstrates that monodisperse entangled polymer melts crystallize via the formation of nanoscale nascent polymer crystals (i.e., nuclei) that exhibit substantial variability in terms of their constituent crystalline polymer chain segments (stems). More specifically, large-scale coarse-grain molecular simulations are used to quantify the evolution of stem length distributions and their properties during the formation of polymer nuclei in supercooled prototypical polyethylene melts. Stems can adopt a range of lengths within an individual nucleus (e.g., ∼1–10 nm) while two nuclei of comparable size can have markedly different stem distributions. As such, the attainment of chemically monodisperse polymer specimens is not sufficient to achieve physical uniformity and consistency. Furthermore, stem length distributions and their evolution indicate that polymer crystal nucleation (i.e., the initial emergence of a nascent crystal) is phenomenologically distinct from crystal growth. These results highlight that the tailoring of polymeric materials requires strategies for controlling polymer crystal nucleation and growth at the nanoscale.

Mathematical modeling of the supercooled steel ingot formation. Problem statement

The article considers hydrodynamic and heat exchange processes taking place in a supercooled melt, which is located in a stationary chill mold. We share the developed physical and mathematical model of a supercooled ingot solidification on the basis of the solidification process theory. A method for numerical realization of the obtained model is described. The article assesses the effect of circulating flows caused by forced convection on the solidification of a melt which is in a fixed chill. Studies show that only at the initial stage of solidification, certain mixing effect can be observed. The further influence of mixing practically does not affect the temperature distribution in the ingot body.

On the theory of phase transformation process in a binary supercooled melt

The processes of bulk crystallization of a supercooled binary melt where the nucleation and growth of solid particles occur are considered with allowance for different mechanisms of nucleation kinetics. A nonlinear set of integro-differential equations is analytically solved by means of the saddle-point technique. The crystal-radius distribution function, temperature, solute concentration, and supercooling of a binary liquid are found taking into account the nonlinear growth rates of spherical crystals. The supercooling represents a decreasing function of time while the distribution function increases up to a maximum size of crystals in a metastable melt.

Collective Motion of Atoms in a Superheated Crystal and a Supercooled Melt of a Simple Metal

Stable and metastable states of a crystal and a melt have been studied near the equilibrium crystal–melt phase transition point using a four-point correlation coefficient previously proposed to reveal collective motions of atoms in a liquid. The embedded copper atom model has been considered as an example with the molecular dynamics method. Strong hysteresis of the correlation coefficient values is detected along the paths of isochoric superheating of the crystal until its melting near the spinodal and the inverse supercooling of the melt to its crystallization. The capabilities of this coefficient for a quantitative estimate of the character of the near-range order, the degree of correlation of the motion of neighboring atoms, and the radius of action of dynamic collective effects in condensed matter are revealed.