Numerical studies of the effect of the temperature drop in the crucible - melt - cooled disk system on the shapes of crystallization fronts

The crystallization process on a cooled disk located on the free surface of a water layer is studied numerically. The influence of thermal gravitational-capillary and mixed convection on the shape of the crystallization front is investigated. In mixed convection modes, the speed of uniform rotation of the disk is set. The calculations were carried out in an axisymmetric formulation of the problem by the finite element method using an adaptive triangular grid and taking into account the latent heat of crystallization and the inverse dependence of density on temperature.

Molecular dynamics study of the influence of supercooling temperature and orientation of the crystallization front on its velocity in silver

The molecular dynamics method was used to study the influence of the supercooling temperature and the orientation of the crystallization front relative to the growing crystal on the front velocity in silver. According to the data obtained, the crystallization velocity with an increase in the supercooling temperature does not increase monotonically, but has a maximum at about 0.7-Tm (Tm is melting temperature), after which it gradually decreases, which is explained by a decrease in the diffusion mobility of atoms in the amorphous phase. Crystallization proceeds faster with the orientation of the front plane (100), slower - with the (110) and (111) orientations.

Application of the liquid bridges theory to find the melt menisci shape when growing cylindrical crystals

A vertical liquid bridge of small volume between a conical shaper and a convex crystallization front was investigated. Two variants of the front crystallization shape selection are considered: a conical and a spherical fronts. A variational statement of the original problem is given. As the boundary conditions we used the condition of engagement at the edge of the shaper and a given growth angle at the crystallization front. The Bond number was assumed to be small, and to find a solution of the problem the asymptotic approach was applied. The calculations are carried out for small diameter cylindrical sapphire crystals, grown from the melt by the Stepanov method. The results of the menisci shapes calculations are presented. The comparison of the results of calculations for conical and spherical crystallization fronts is carried out.

Simulation of heat transfer processes during the growth of crystals of the NiFeGaCo alloy

Long crystals of NiFeGaCo alloy with shape memory effect, including magnetically controlled ones, were obtained by the methods of Czochralski and Stepanov. A strong influence on the properties of crystals of dendritic formations, especially noticeable in the initial part of the crystal, has been revealed. In order to optimize the growth experiments, the heat transfer process in the thermal growth zone was simulated. It is shown that the formation of dendrites is due to a change in heat transfer during growth, which leads to an increase in the axial temperature gradient near the crystallization front as the crystal grows. This fits into the framework of the classical concepts of the transition from dendritic growth to normal growth.

THE USE OF EXTERNAL PHYSICAL INFLUENCE TO REGULATE THE FORMATION OF THE STRUCTURE OF STEEL INGOTS

В настоящей работе исследовано влияние температурных и силовых параметров внешнего воздействия на кристаллизацию и структурообразование слитков из модельных и реальных сплавов. Показано, что скоростью зарождения и роста кристаллов, а также размерами структурных зон в слитках можно управлять, изменяя температурный градиент, возникающий в жидкой фазе перед границей затвердевания. Управлять градиентом температур по сечению затвердевающего расплава можно, регулируя интенсивность теплоотвода, а также дифференцируя теплоотвод по периметру формирующегося слитка. Установлено, что от соотношения градиента температуры к скорости кристаллизации - G/R зависит формирование той или иной структурной зоны слитков. Наглядно доказана эффективность влияния вибрации на формирование структуры исследуемых слитков. Определен механизм воздействия вибрации на процессы кристаллизации и формирования структур слитков, который заключается в проявлении следующих эффектов: в разрушении и дроблении дендритов, растущих на фронте кристаллизации, под действием знакопеременных изгибающих давлений упругой волны; в зарождении кристаллов в объеме кристаллизующегося расплава вследствие кавитации; в разрушении дендритов в расплаве и на фронте кристаллизации под действием давлений, возникающих в результате схлопывания кавитационных каверн. In this paper, the influence of temperature and force parameters of external influence on the crystallization and structure formation of ingots from model and real alloys is investigated. It is shown that the rate of crystal nucleation and growth, as well as the size of the structural zones in the ingots, can be controlled by changing the temperature gradient that occurs in the liquid phase before the solidification boundary. You can control the temperature gradient along the cross-section of the solidifying melt by adjusting the intensity of the heat sink, as well as differentiating the heat sink along the perimeter of the forming ingot. It is established that the ratio of the temperature gradient to the crystallization rate - G/R depends on the formation of a particular structural zone of ingots. The effectiveness of the influence of vibration on the formation of the structure of the ingots under study is clearly proved. The mechanism of vibration influence on the processes of crystallization and formation of ingot structures is determined, which consists in the manifestation of the following effects: in the destruction and crushing of dendrites growing at the crystallization front under the action of alternating bending pressures of an elastic wave; in the nucleation of crystals in the volume of the crystallizing melt due to cavitation; in the destruction of dendrites in the melt and at the crystallization front under the action of pressures resulting from the collapse of cavitation cavities.

Spontaneous reheating of crystallizing lava

We show that recalescence, or spontaneous reheating of a cooling material due to rapid release of latent heat, can occur during disequilibrium crystallization of depolymerized Mg-rich melts. This can only happen at fast cooling rates, where the melt becomes undercooled by tens to hundreds of degrees before crystallization begins. Using a forward-looking infrared (FLIR) camera, we documented recalescence in pyroxene (Fe, Mg)SiO3 and komatiite lavas that initially cooled at 25–50 °C s–1. Local heating at the crystallization front exceeds 150 °C for the pyroxene and 10 °C for komatiite and lasts for several seconds as the crystallization front migrates through. We determined the latent heat release by differential scanning calorimetry to be 440 J g–1 for pyroxene and 275 J g–1 for komatiite with a brief power output of ~100 W g–1 or ~300 MW m–3. Recalescence may be a widespread process in the solar system, particularly in lava fountains, and cooling histories of mafic pyroclasts should not be assumed a priori to be monotonic.

Analysis of hydrogen behavior by crystallization of secondary aluminum alloy

Methodology. There was conducted the process modeling of hydrogen by crystallization of secondary aluminum alloy. Findings. There was conducted an analysis of hydrogen behavior by indurating secondary aluminum alloy castings. There were obtained expressions for calculation of the current value in the process of secondary aluminum alloy crystallization of the hydrogen density in the boundary diffused layer, on the crystallization front, in the volume of the remaining melt, and the effective hydrogen distribution coefficient. Originality. For the first time, there was obtained an arithmetic model for the forecasting of hydrogen behavior by indurating secondary aluminum alloy castings. Practical value. The results of the study can be implemented with the purpose of the flow optimization process and the more efficient use of expensive equipment for getting high-quality secondary aluminum alloy castings. Keywords: aluminum, secondary aluminum alloys, indurating, hydrogen, crystallization front, distribution coefficient, boundary diffusion layer.

Расчет динамики границы аморфная фаза-кристалл при твердофазной взрывной кристаллизации

The nonlinear differential equation described a dynamics of solid-phase explosive crystallization front in a much larger parameters domain in comparison with the theoretical results available in literature was obtained. The features of the self-oscillating mode transition of the front motion to the mode of its self-propagation with a constant velocity was numerically studied in detail.

Modeling of electrodynamic processes in self-organization of fractal-convective cloud structures

В работе рассмотрена модель электроо-динамического процесса возникновения больших потенциалов на фронте кристаллизации при самоорганизации конвективных облачных структур, в котором учитывается фрактальность облачной среды. В аналитическом виде найдено решение уравнения модели. С помощью численных расчетов показано зависимость разности потенциала от фрактальности среды и зависимость уровня фрактальной зоны от радиуса частицы. The paper considers a model of fractal-dynamic process of occurrence of large potentials on the crystallization front in the self-organization of convective cloud structures, which takes into account the fractality of the cloud environment. The solution of the model equation is found in the analytical form. With the help of numerical calculations demonstrate the dependence of the potential difference from the fractality of environment and the dependence of the level of the fractal of the radius of the particle.