Neutron-diffraction study of the cubic-tetragonal phase structural transition in the single crystals of the solid solutions of zirconium and yttrium oxides

Objectives. The determination of the phase relations, crystallographic characteristics, microstructure features, and atomic crystal structure of zirconium oxide crystals that are partially and completely stabilized by yttrium oxide additives, and the identification of the crystallographic and crystal-chemical correlations with the physicochemical properties of single crystals.Methods. The neutron structure of the crystals was studied using the neutron time-of-flight and constant wavelength methods using a high-resolution Fourier diffractometer on the IBR-2 pulsed fast reactor and a four-circle neutron diffractometer “Syntex.” Single crystals were grown by directed crystallization from the melts of mixtures (1 − х)ZrO2 ∙хY2O3 , х = 0.03 and х = 0.12 with different growth rates (10 and 40 mm/h).Results. It was observed that when growing single crystals with x = 0.03–0.05, the crystal was stratified into cubic and tetragonal phases, and the ratio between the phases depended on the growth rate. At a growth rate of 40 mm/h, the content of the cubic phase was insignificant. In the crystals of partially stabilized zirconium dioxide (ZrO2) with the additions of 3 mol % Y2O3, the coherent coexistence of cubic and tetragonal phases was established, and the twin law for a tetragonal component (rotation of unit cell axis by 90° around the a (b) axis) that was observed during the phase transition from high-temperature cubic phase to tetragonal phase was determined. For the fully stabilized zirconium oxide of the cubic symmetry (with 12 mol % Y2O3), the 0.3 Å displacements of oxygen atoms from their partial structural positions in the directions [100] and [111] were determined. These displacements correlated with the directions of the ion transport.Conclusions. Previous studies have shown that the ratio between the cubic and tetragonal phases of the single crystals of the ZrO2 –Y2O3 system depends on the growth rate of the single crystals. The content of Y2O3 in the cubic and tetragonal phases of a single crystal was determined using the non-destructive neutronography method on the same volume sample of a solid solution of this system. Moreover, the displacements of oxygen atoms from the main position of the crystal were determined.

Microstructural Changes in Ni-Al-Cr-Based Heat-Resistant Alloy with Re Addition

This paper presents scanning and transmission electron microscope investigations of the structure, phase composition, and morphology of a heat-resistant alloy modified by thermal treatment and additionally alloyed by rhenium. The rhenium alloy was obtained by using the directional crystallization technique. The structural investigations were carried out for two states of the alloy, i.e., (1) original (after the directional crystallization); (2) after the directional crystallization with 1150 °C annealing for 1 h and 1100 °C annealing for 480 h. It is shown that fcc-based γ- and γ′-phases are primary in all states of the alloy. The γ′-phase has an L12 structure, while γ-phase is a disordered phase. It was found that after directed crystallization, the volume fraction of the γ′ phase is ~85%, the fraction of the γ-phase is less than 10%. Annealing leads to an increase in the γ′- phase up to 90%, the proportion of the γ-phase practically does not change. Rhenium is a phase-formation element. The investigations show that high-temperature annealing modifies the structural and phase conditions of the heat-resistant alloy.

Solvent-Dependent Thermoelectric Performance of PC-=SUB=-70-=/SUB=-BM

Based on the electrophysical and structural data, a crystallophysical model of the ionic transport in superionic conductors Ba1-xLaxF2+x and Ca1-xYxF2+x (space group Fm¯3m), in which the charge carriers are mobile interstitial ions F− resulting from heterovalent substitutions of fluorite fragments of [M14F64] (M = Ca, Ba) by structural clusters of [M8R6F69] (R = La, Y). Single crystals of Ca1-xYxF2+x (0.02<x<0.16) and Ba1-xLaxF2+x (x = 0.31) solid solutions were obtained by directed crystallization of the melt. The mobility of ionic carriers in isostructural superionics Ba0.69La0.31F2.31, Ca0.84Y0.16F2.16, Pb0.67Cd0.33F2, and Pb0.9Sc0.1F2.1 are compared. Crystals of Ba1-xLaxF2+x and Ca1−xYxF2+x with improved conductometric and mechanical characteristics are promising active to replace the traditional CaF2 electrolyte in galvanic cells for thermodynamic research of chemicals.

Polymer-directed Crystallization of HMX to Construct Nano-/Microstructured Aggregates with Tunable Polymorph and Microstructure

The nano-/microstructured energetic materials (EMs) often display a unique combination of low-sensitivity and high-stability, but their structural control across multiple-length scales has proven difficult. Here, a polyvinyl pyrrolidone (PVP) polymer-directed...

Creation of high-temperature heat-resistant alloys based on refractory matrices and natural composites

The scientific, technical and technological aspects in the field of creating new high-temperature materials for the hot tract parts of gas turbine engines (GTE) with operating temperatures exceeding those existing in the GTE are considered. Investigated more refractory metal materials to create new high-temperature alloys used in the manufacture of working and nozzle blades and other parts of promising gas turbine engines based on Co – Cr, Pt – Al, Nb – Si, Mo – Si – B systems. In Co – Cr alloys, heat resistance is mainly ensured by hardening the Co matrix, including dispersed precipitates of the carbide phase (TaC) and the boride phase Cr2B. In alloys of the Pt – Al system, due to the doping of Cr, Al, Ti, Re ... and precipitates of the coherently embedded Pt3Al phase. In eutectic alloys of the Nb-Si system, this is due to complex hardening of the Nb solid solution and Nb5Si3 silicide, as well as the natural compositional structure. In Mo – Si – B alloys, high strength is achieved by doping a-Mo solid solution and the formation of intermetallic phases Mo3Si, Mo5SiB2, carbides Mo2C, TiC. Compositions were selected, analysis of their smelting methods was carried out, including directed crystallization, which provides a natural compositional structure, mechanical properties at room and high temperatures, oxidation resistance were evaluated, structural features were investigated, information was provided on technological equipment and the possibility of obtaining parts in various ways. It is shown that, depending on the composition of the selected matrix, the working temperature of heat-resistant alloys can increase to 1300 – 1500 °C, which significantly exceeds the existing nickel heat-resistant alloys. It is concluded that the materials under study are promising for use in aircraft engine building and the aerospace industry.

Unification of instrumental units in the position of mechanical processing of alloys with directional crystallization of eutecnic structures on multi"stream automatic lines

Optimization of machining of parts made of alloys with directed crystallization of eutectic structures by instrumental units on a multi-stream automatic line consisting of one-way automatic machines is proposed. Keywords composite material, eutectic structure, automatic line, tool block, machining. [email protected]

Analysis of heat transfer processes for sapphire growth by horizontal directed crystallization method

This research summarizes the analytical and experimental results of heat-transfer processes influence on defects formation during sapphire crystal growth by horizontal directed crystallization method (HDC). The shape of solid-melt interface significantly influences the process of sapphire crystals growth by this method. We receive the Stefan problem solution for sapphire crystals growth. It allows investigating the crystal growth process and the related factors (thermal stresses on different stages of growth process), their influence on defects formation. We investigate the main reasons for the formation of defective structures of the solid phase of sapphire crystals and the influence of thermal unit construction, the crystal geometry on the quality of the resulting sapphire crystal. We study the structure formation process, impurity distribution, and the nature of the defects in the crystal during it growth.

PRODUCTION OF HYIGH-PURITY ZINC SINGLE CRYSTALS BY VERTICAL DIRECTED CRYSTALLIZATION METHOD

A seedless process has been developed to produce high-purity Zn single crystals by the method of vertical di-rected crystallization from a melt. The output of a single crystal structure is from 60 to 80%. Crystals with different growth directions were obtained: [1015] and [0002]. The deviation angles of the growth direction plane relative to the normal to the axis of the sample are 0.5...6°. Microhardness, crystalline perfection of single crystals and micro-structure have been determined. The impurity composition of the start and end parts of single crystals produced from initial grades of zinc of various purities was studied. The developed process can be used to grow single crystals low-melting metals, such as Cd, Pb, Te, In, Bi, Sn, etc.