Specifics of crystallization of highly refractory alloys based on zirconium, yttrium, and erbium oxides

Abstract

In production of melted ingots of highly refractory alloys in systems with zirconium, yttrium, and erbium oxides we have identified the phenomenon of an increasing specific volume in the course of primary crystallization under unilateral center-symmetric heating. The positive values of the volume crystallization effect are presumably due to the emergence of point defect complexes in the course of formation of a hexagonal structure in solutions of nonisovalent metals (zirconium and rare-earth elements) oxides.It is known that high-temperature ceramics such as zirconium and rare-earth element (REE) oxides and their alloys have a number of unique physicochemical properties, such as high melting temperature, thermodynamic stability, strength and crack resistance and can be used in developing special-purpose refractories, fuel elements, etc. Consequently, these materials are of special interest for research and practical application. However, due to their high refractoriness, the production and studies of these compounds involves certain difficulties. One of the methods for studying these materials is synthesis of alloys by melting in the focal zone of an optical furnace [1].It has been established in the production of alloy ingots in systems with zirconium, yttrium, and erbium oxides that some compositions of the considered systems under crystallization in the focal zone of an optical furnace form a bulging cone on the ingot surface. At the same time, the majority of alloys of the considered systems, including those whose compositions are close to the "anomalous" ones, under solidification and crystallization form a classical shrinkage cone on the ingot surface. Therefore, this phenomenon of the formation of a bulging cone could only be explained by assuming that the volume of the "anomalous" compositions grows in crystallization [2].It is known that solidification of materials involves the transition from a random motion of atoms (molecules) in a liquid to an oscillatory motion localized near the crystalline lattice nodes; in this case the mean distance between the atoms (molecules) of the material, as a rule, decreases, which produces a corresponding decrease in the volume of the material. Therefore, crystallization of a melted material in a crucible involves shrinkage of the ingot volume directed from the crucible walls toward the central zone, which is well known to metallurgists. However, several natural materials are known, whose volume grows in crystallization. These chemical compounds include H 2 O, AlSb, Mg 2 Sn; CH 3 COOH (acetic acid); elementary materials Sb, Bi, Ga, Si, Ge, Pu, and some alloys of these elements [3,4]. The directed crystallization of Ge from the melted zone perimeter to its center under cooling leads to the formation of a protruding conical surface [5]. A similar phenomenon is observed in crystallization of Bi in a cylindrical crucible.The anomalous behavior of water under crystallization is related to the formation of oriented hydrogen bonds between the ice molecules, which determines a ...

Peculiarities of crystallization of alumina, yttria and rare-earth element oxides of yttria subgroup in an optical furnace

“…32,33 These results contradict those obtained by the levitation method. 10,15 The method for the estimation of volume change upon crystallization under the conditions of a center-symmetrical temperature field in the focal zone of an optical furnace has been proposed.…”

“…10,15 The method for the estimation of volume change upon crystallization under the conditions of a center-symmetrical temperature field in the focal zone of an optical furnace has been proposed. [32][33][34] It has significant limitations and cannot be used for substances with high melt viscosity, disposed to the formation of internal hollow spaces, to which Al 2 O 3 is related. Nevertheless, the second task was to thoroughly research the solidification process of alumina in the focal zone of an optical furnace, and comparing the peculiarities of the alumina crystallization with the crystallization processes in REE oxides under the same conditions, and to estimate the influence of their peculiarities on the reliability of the reported results.…”

Method for evaluating the change in specific volume of refratory substances during crystallization

“…The suitability of the method developed was checked in silicon specimens ((V L /V so ) < 1). In order to create a flat radially-symmetrical bath of molten silicon (T m = 1414°C) and lithium niobate (T m = 1253°C), an optical furnace described in [2] was used. Crystallization of colored water was carried out in a chamber with an atmosphere of liquid nitrogen vapor at about -30°C, and here the centrally-symmetrical heating was provided by means of a radiation heater from a projection lamp with a power of 100 W placed at the focus of an ellipsoidal reflector.…”

“…Results of evaluating the change in specific volume of Er 2 O 3 -ZrO 2 alloys during crystallization are provided below: In the system Er 2 O 3 -ZrO 2 alloy with a content of 50% ZrO 2 gives shrinkage during crystallization, and for alloys with a zirconium dioxide content of 10 and 15% there is a typical increase in specific volume during crystallization [2]. There are no published data for high-temperature x-ray structural analysis of Er 2 O 3 -ZrO 2 alloys and therefore it is impossible to estimate the density of the liquid phase for these alloys.…”

“…a solid form of an ingot surface crystallized under conditions of centrally-symmetrical heating. At the basis of the method is contactless heating by a concentrated light flux in the focal zone of an optical furnace [2]. In carrying out crystallization of a flat bath of melt a shrinkage cone forms (in rare cases it is projecting) in the direction of the boundary of the molten bath towards the center at the surface of an ingot.…”