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. [email protected]

Dynamic Crystallization of a Haplogranitic Melt: Application to Pegmatites

Both 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.

Особенности микрокристаллизации 50 % двухкомпонентных металлических расплавов в модели переходной двухфазной зоны в диффузионно-релаксационном режиме

В диффузионно-релаксационном режиме кристаллизации 50% двухкомпонентных металлических расплавов в модели переходной двухфазной зоны оценены термодинамические параметры, при которых возможно образование полностью разупорядоченной двухкомпонентной кристаллической фазы с простой кубической элементарной решеткой стехиометрического состава. Исследована область вблизи точки разупорядочения кристаллической фазы и установлен закон стремления параметра дальнего порядка к нулю при достижении критической температуры (переохлаждения) системы двухкомпонентный расплав-кристалл. Установлены кинетические особенности роста кристаллической фазы в точке разупорядочения. Установлен закон восстановления упорядоченной двухкомпонентной кристаллической фазы во времени. Оценены возможные значения времен релаксации при переходе из разупорядоченной двухкомпонентной кристаллической фазы с простой кубической элементарной ячейкой к полностью упорядоченной. Установлены кинетические особенности образования полностью упорядоченного двухкомпонентного кристалла. REFERENCES Sarkisov P. D., Baikov Yu. A., Meshalkin V. P. Matematicheskoe modelirovanie kristallizatsii odno- i dvukhkomponentnykh metallicheskikh rasplavov [The one- and binary metallic melts mathematical mode ling crystallization]. Moscow, Physmatlit Publ., 2003. 378 p. (in Russ.) Baikov Y. A., Petrov N. I. Structure of the Transitive Two-Phase Zone in Crystallization of Two-Component Metal Melts. Russian Physics Journal, 2014, v. 57(4), pp. 459–468. https://doi.org/10.1007/s11182-014-0262-2 Baikov Yu. A., Petrov N. I. Special Features of disordering in Crystallization of Two-Component Metal Melts in the Model of Two-Phase Transitive Zone. Russian Physics Journal, 2014, v. 57(5), pp. 598–614. https://doi.org/10.1007/s11182-014-0282-y Petrov N. I. The Crystal Disordering Study When Growing From the Binary Metallic Melts. National University of Science and Technology «MISiS» Dis. Cand. Phys.-Mat. Sci. Moscow, 2017, 180 p. URL: http:// misis.ru/fi les/6902/Petrov_AR.pdf (in Russ.) Sarkissov P. D., Baikov Yu. A., Meshalkin V. P. Order-disorder processes in crystals when crystallizing binary metallic melts. Doklady Physics, 2003, v. 48(6), pp. 290–295. https://doi.org/10.1134/1.1591316 Chistyakov Yu. D., Baikov Yu. A., Schneider H. G., Ruth V. The order-disorder transformation at supercooled melt/crystal transition region of binary melts (I) the master equation. Crystal Research and Technology, 1985, v. 20(8), pp. 1007–1014. https://doi.org/10.1002/crat.2170200802 Chistyakov Yu. D., Baikov Yu. A., Schneider H. G., Ruth V. The order-disorder transformation at supercooled melt/crystal transition region of binary melts (II) the steady state solution // Crystal Research and Technology, 1985, v. 20(9), pp. 1149–1156. https://doi.org/10.1002/crat.2170200903 Guinier A. J., Griffoul R. Compte Rendu, 1945, v. 221, pp. 121. Guinier A. J. Imperfections of crystal lattices as investigated by the study of X-ray diffuse scattering // Proceedings of the Physical Society, 1945, v. 57(4), pp. 310–324. https://doi.org/10.1088/0959-5309/57/4/306 Schneider H. G. Collection: Advances in Epitaxy and Endotaxy. Akademiai Kiado, Budapest, 1976, p. 23. Chistyakov Yu. D., Baikov Yu. A. Collection: Advances in Epitaxy and Endotaxy. Akademiai Kiado, Budapest, 1976, p. 159. Chistyakov Yu. D., Baikov Yu. A. Collection: Advances in Epitaxy and Endotaxy. Akademiai Kiado, Budapest, 1976, p. 257.

Growing of Thick Single-Crystalline La-Substituted Yttrium-Iron Garnet Films with Reproducible Parameters

Basic principles of growing the thick (35–60 мm) single-crystalline La-substituted yttrium-iron garnet (Y3−xLaxFe5O12, La :YIG) films with reproducible parameters have been formulated. La :YIG films are grown on gallium-gadolinium garnet (Gd3Ga5O12) substrates from a supercooled melt-solution (MS) consisting of Y2O3, La2O3, and Fe2O3 oxides and the PbO–B2O3 solvent. In order to minimize the implantation of Pb2+ ions into the films, which degrades the film quality, the epitaxy has to be performed at high temperatures and a low MS supercooling. It is found that, in order to maintain a constant growth rate of La :YIG films with reproducible parameters, a large MS mass (10–16 kg) has to be used, and the MS temperature has to be permanently lowered at a rate of 0.042 K/min.