Corundum Grains Research Articles

Influence of stabilized zirconium dioxide and high hydrostatic pressure on the kinetics of sintering nanopowders of metastable aluminum oxide

The paper presents an analysis of the effect of processing powders with high hydrostatic pressure (HHP) (300, 500, 700 MPa) and stabilized zirconium dioxide (ZrO2 + 3 mol.% Y2O3, (YSZ)) doping on the compaction of metastable nanopowders mixture with γ+θ-Al2O3 + n% YSZ (n = 0, 1, 5, 10, 15 wt%) composition during sintering. The behavior of nanopowders at the initial stage of sintering was studied by dilatometry at a constant heating rate of 5 °C/min, in temperature range from 20 °C to 1500 °C. It has been determined that sintering of compacts occurs in two stages: compaction of γ + θ-Al2O3 (Ⅰ-st stage metastable phases), compaction of the stable α-Al2O3 (ⅠⅠ-nd stage), accompanied by inhibition between them. An “effect of mutual protection against crystallization” of powder mixtures was determined using the X-ray diffraction method. This is characteristic of systems obtained by co-precipitation. A connection between the above mentioned inhibition and an increase in the YSZ additive was found. That, also, correlates to the effect of “mutual protection against crystallization” of Al2O3 – YSZ powder mixtures. A study of the mechanisms and activation energies of γ+θ-Al2O3 + n% YSZ systems’ (n = 0, 1, 5, 10, 15 wt%) sintering depending on the YSZ additive amount and the HHP value showed that a dopant and pressure increase leads to activation energy decrease of the initial sintering stage, and the prevailing sintering mechanism is volumetric diffusion in corundum grains. Also, the work shows that for γ+θ-Al2O3 + ≥5 wt% YSZ systems, the optimal compaction pressure is HHP ≥500 MPa, which corresponds to the transition of the sintering mechanism from grain-boundary diffusion to volumetric diffusion.

Corundum ± magnesium‐deltalumite ± hibonite‐bearing objects in the CH chondrite Sayh al Uhaymir 290

AbstractIsolated corundum grains and corundum ± Mg‐deltalumite [(Al,Mg)(Al,◻)2O4] ± hibonite assemblages were investigated in the CH3.0 metal‐rich carbonaceous chondrite Sayh al Uhaymir (SaU) 290. Although very refractory inclusions containing abundant Zr‐ and Sc‐rich oxides and silicates, hibonite, grossite, or perovskite have been previously described in CH chondrites, this is the first discovery of corundum and Mg‐deltalumite in CHs and the first discovery of Mg‐deltalumite in nature. Magnesium‐deltalumite can be indexed by the Fd3m spinel‐type structure and gives a perfect fit to the synthetic Al‐rich spinel cells. Corundum‐Mg‐deltalumite grains, 5–20 μm in size, are occasionally rimmed by a thin layer of hibonite replacing corundum. Some corundum grains contain tiny inclusions of ultrarefractory Zr,Sc‐rich minerals and platinum‐group element (PGE) nuggets. All corundum, hibonite, and Mg‐deltalumite grains studied have 16O‐rich compositions (average Δ17O ± 2SD = −22 ± 3‰). Two corundum grains show evidence for significant mass‐dependent fractionation of oxygen isotopes: Δ18O ~ +34‰ and ~ +19‰. We suggest that the SaU 290 corundum‐rich objects were formed by evaporation and/or condensation in a hot nebular region close to the proto‐sun where the ambient temperature was close to the condensation temperature of corundum. A corundum grain with tiny inclusions of Zr‐ and Sc‐rich phases and PGE metal nuggets recorded formation temperatures higher than the condensation temperature of corundum. Two corundum‐rich objects with highly fractionated oxygen isotopes must have crystallized from a melt that experienced evaporation. Corundum grains corroded by hibonite recorded gas–solid interaction in this region during its cooling. The Mg‐deltalumite ± corundum ± hibonite objects were formed by rapid crystallization of high‐temperature (>2000°C) refractory melts. The lack of minerals with condensation temperatures below those of corundum and hibonite in the SaU 290 corundum‐rich objects suggests that after formation, these objects were rapidly removed from the hot nebular region by disk wind and/or by turbulent diffusion and disk spreading.

Toledoite, TiFeSi, a New Mineral from Inclusions in Corundum Xenocrysts from Mount Carmel, Israel

During our nanomineralogical investigation of melt inclusions in corundum xenocrysts from the Mount Carmel area, Israel, seven new oxide and alloy minerals have been discovered since 2021. Herein, we report toledoite (TiFeSi; IMA 2022-036), a new alloy mineral. Toledoite occurs as irregular crystals 2–16 μm in size, with gupeiite (Fe3Si), jingsuiite (TiB2), ziroite (ZrO2), osbornite (TiN), xifengite (Fe5Si3), and naquite (FeSi) in corundum Grain WG1124E-1. Toledoite has an empirical formula (Ti0.83Cr0.07Mn0.06V0.02)(Fe0.96Mn0.04)(Si0.99P0.04) and an orthorhombic Ima2 TiFeSi-type structure with the following cell parameters: a = 7.00(1) Å, b = 10.83(1) Å, c = 6.29(1) Å, V = 477(1) Å3, Z = 12. Toledoite is a high-temperature alloy phase, formed under extremely reduced conditions in melt pockets in corundum xenocrysts derived from the upper mantle beneath Mount Carmel in Israel. The name was given in honor of Vered Toledo, of Shefa Gems Ltd. for her support and for providing corundum xenocrysts from the Mount Carmel region for this investigation of new minerals.

New Minerals from Inclusions in Corundum Xenocrysts from Mt. Carmel, Israel: Magnéliite, Ziroite, Sassite, Mizraite-(Ce) and Yeite.

Our nanomineralogical investigation of melt inclusions in corundum xenocrysts from the Mt. Carmel area, Israel has revealed seven IMA-approved new minerals since 2021. We report here four new oxide minerals and one new alloy mineral. Magnéliite (Ti3+2Ti4+2O7; IMA 2021-111) occurs as subhedral crystals, ~4 μm in size, with alabandite, zirconolite, Ti,Al,Zr-oxide, and hibonite in corundum Grain 767-1. Magnéliite has an empirical formula (Ti3+1.66Al0.13Ti4+0.15Mg0.10Ca0.01Sc0.01)Σ2.06 (Ti4+1.93Zr0.08)Σ2.01O7 and the triclinic P1¯ Ti4O7-type structure with the cell parameters: a = 5.60(1) Å, b = 7.13(1) Å, c = 12.47(1) Å, α = 95.1(1)°, β = 95.2(1)°, γ = 108.7(1)°, V = 466(2) Å3, Z = 4. Ziroite (ZrO2; IMA 2022-013) occurs as irregular crystals, ~1-4 μm in size, with baddeleyite, hibonite, and Ti,Al,Zr-oxide in corundum Grain 479-1a. Ziroite has an empirical formula (Zr0.72Ti4+0.26Mg0.02Al0.02Hf0.01)Σ1.03O2 and the tetragonal P42/nmc zirconia(HT)-type structure with the cell parameters: a = 3.60(1) Å, c = 5.18(1) Å, V = 67.1(3) Å3, Z = 2. Sassite (Ti3+2Ti4+O5; IMA 2022-014) occurs as subhedral-euhedral crystals, ~4-16 μm in size, with Ti,Al,Zr-oxide, mullite, osbornite, baddeleyite, alabandite, and glass in corundum Grain 1125C1. Sassite has an empirical formula (Ti3+1.35Al0.49Ti4+0.08Mg0.07)Σ1.99(Ti4+0.93Zr0.06Si0.01)Σ1.00O5 and the orthorhombic Cmcm pseudobrookite-type structure with the cell parameters: a = 3.80(1) Å, b = 9.85(1) Å, c = 9.99(1) Å, V = 374(1) Å3, Z = 4. Mizraite-(Ce) (Ce(Al11Mg)O19; IMA 2022-027) occurs as euhedral crystals, <1-14 μm in size, with Ce-silicate, Ti-sulfide, Ti,Al,Zr-oxide, ziroite, and thorianite in corundum Grain 198-8. Mizraite-(Ce) has an empirical formula (Ce0.76Ca0.10La0.07Nd0.01)Σ0.94(Al10.43Mg0.84Ti3+0.60Si0.09Zr0.04)Σ12.00O19 and the hexagonal P63/mmc magnetoplumbite-type structure with the cell parameters: a = 5.61(1) Å, c = 22.29(1) Å, V = 608(2) Å3, Z = 2. Yeite (TiSi; IMA 2022-079) occurs as irregular-subhedral crystals, 1.2-3.5 μm in size, along with wenjiite (Ti5Si3) and zhiqinite (TiSi2) in Ti-Si alloy inclusions in corundum Grain 198c. Yeite has an empirical formula (Ti0.995Mn0.003V0.001Cr0.001)(Si0.996P0.004) and the orthorhombic Pnma FeB-type structure with the cell parameters: a = 6.55(1) Å, b = 3.64(1) Å, c = 4.99(1) Å, V = 119.0(4) Å3, Z = 4. The five minerals are high-temperature oxide or alloy phases, formed in melt pockets in corundum xenocrysts derived from the upper mantle beneath Mt. Carmel.

Alumina precipitate along dislocations in rutile single crystal of diffusion couple during cooling from 1200° to 1400°C

Diffusion couples of rutile and corundum single crystals were prepared and examined for the solid solution and precipitation of corundum in rutile during cooling. Alumina dissolves in rutile according to the following equation, Al2O3→2AlTi′+3OO+VO∙∙. In the sample air-quenched from 1200 °C, the supersaturated oxide ion vacancies, VO⋅⋅, reacted with the edge component of screw dislocation parallel to c axis to form helical dislocations, along which corundum was precipitated. When air-quenched from 1300 °C and 1400 °C, corundum was precipitated in the form of stacked rectangular loops. Corundum grains with eight lobes extending in the < 101 > directions were precipitated in the samples cooled at 10 °C/min from 1300 °C and 1400 °C. The solid solution reaction increased the volume of rutile single crystals and caused voids outside the reaction region at the interface. Both the corundum and rutile components of the voids also contributed to the solid solution reaction.

Griffinite, Al2TiO5: A New Oxide Mineral from Inclusions in Corundum Xenocrysts from the Mount Carmel Area, Israel

Griffinite (IMA 2021-110), ideally Al2TiO5, is a new mineral from inclusions in corundum xenocrysts from the Mount Carmel area, Israel. It occurs as subhedral crystals, ~1–4 μm in size, together with Zr-rich rutile within a corundum grain. In this study, a mean of eight electron probe microanalyses gave TiO2 44.41 (24), Al2O3 55.13 (18), FeO 0.47 (5), and MgO 0.37 (2), totaling 100.38 wt%, which corresponded, on the basis of a total of five oxygen atoms, to (Al1.97Mg0.02Fe0.01)Ti1.01O5. Electron back-scatter diffraction studies revealed that griffinite is orthorhombic and in the space group Cmcm, with a = 3.58 (2) Å, b = 9.44 (1) Å, c = 9.65 (1) Å, and V = 326 (2) Å3 with Z = 4. The six strongest calculated powder diffraction lines [d in Å (I/I0) (hkl)] are 3.347 (100) (110); 2.658 (90) (023); 4.720 (77) (020); 1.903 (57) (043); 1.790 (55) (200); and 1.688 (44) (134). In the crystal structure, Al3+ and Ti4+ are disordered into two distinct distorted octahedra, which form edge-sharing double chains. Griffinite is a high-temperature oxide mineral, formed in melt pockets in corundum-aggregate xenoliths derived from the upper mantle beneath Mount Carmel, Israel. The new mineral is named after William L. Griffin, a geologist at Macquarie University, Australia.

First Terrestrial Occurrence of Kaitianite (Ti3+2Ti4+O5) from the Upper Mantle beneath Mount Carmel, Israel

Our nanomineralogical investigation of melt inclusions in corundum xenoliths from the Mount Carmel area, Israel, has revealed seven IMA-approved new minerals since 2021. We report here the first terrestrial occurrence of kaitianite (Ti3+2Ti4+O5). Kaitianite occurs as exsolution lamellae in tistarite (Ti2O3), in a melt inclusions together with a Ti,Al,Zr-oxide, a MgTi3+2Al4SiO12 phase, spinel, sapphirine, Ti-sulfide, alabandite, and Si-rich glass in a corundum grain (Grain 1125C2). The chemical composition of kaitianite using electron probe microanalysis is (wt%) Ti2O3 58.04, TiO2 37.82, Al2O3 2.87, MgO 0.85, ZrO2 0.10, CaO 0.02, SiO2 0.02, sum 99.73, yielding an empirical formula of (Ti3+1.78Al0.12Ti4+0.05Mg0.05)(Ti4+1.00)O5, with the Ti3+ and Ti4+ partitioned, assuming a stoichiometry of three cations and five oxygen anions pfu. Electron back-scatter diffraction reveals that kaitianite has the monoclinic C2/c γ-Ti3O5-type structure with cell parameters: a = 10.12 Å, b = 5.07 Å, c = 7.18 Å, β = 112°, V = 342 Å3, and Z = 4. Kaitianite is a high-temperature oxide phase, formed in melt pockets under reduced conditions in corundum-aggregate xenoliths derived from the upper mantle beneath Mount Carmel, Israel.

Abrasive Wear Resistance of Ultrafine Ausferritic Ductile Iron Intended for the Manufacture of Gears for Mining Machinery.

The purpose of this study was to experimentally determine the abrasion wear properties of ausferritic ductile iron austempered at 250 °C in order to obtain cast iron of class EN-GJS-1400-1. It has been found that such a cast iron grade makes it possible to create structures for material conveyors used for short-distance transport purposes, required to perform in terms of abrasion resistance under extreme conditions. The wear tests addressed in the paper were conducted at a ring-on-ring type of test rig. The test samples were examined under the conditions of slide mating, where the main destructive process was surface microcutting via loose corundum grains. The mass loss of the examined samples was measured as a parameter characteristic of the wear. The volume loss values thus obtained were plotted as a function of initial hardness. Based on these results, it has been found that prolonged heat treatment (of more than 6 h) causes only an insignificant increase in the resistance to abrasive wear.

Силовой анализ процесса резания единичным зерном сферокорунда при шлифовании древесины

The productivity of the wood grinding process can be increased by using abrasive wheels with spherical corundum grains. They also expand the area of application of rigid abrasive instruments by reducing soiling and eliminating burns on the treated surface. The studies revealed several advantages of the abrasive wheels over sanding paper: very precise processing, achievement of the desired grade of the treated surface, high durability of the instrument, and low cost. A force analysis of a grinding surface with micro-cuttings made of spherical corundum grains was performed depending on the degree of its wear. The results revealed that chip removal by the spherical corundum grains was possible only under certain conditions when the cutting edges of the grains were exposed. Besides, the theoretical dependencies of forces in micro-cutting were obtained from every principal condition for the process of grinding wood and wooden materials: properties of the spheres, treatment modes, and material-related factors. The special properties were analyzed by the chip formation process and self-sharpening conditions of the spherical corundum wheels. The grain size had the greatest influence on the thickness of chips which were cut by one grain. The grain content and bonding, like other properties of the internal volumetric structure, had a less substantial impact. The abrasive tool characteristics must complement the processed material, such as grain size, hardness, and structure. On the other hand, spherical corundum has optimal physical and mechanical properties for grinding, particularly the thickness of the grain’s edges, which primarily determine the material’s tensile strength. The grinding depth and feed rate mostly define the parameters of the grinding performance of wood and wood-based materials. An increase in those leads to a rise in cutting forces, roughness of the ground surface, a reduction in the grinding length over the period of the wheel’s durability, and the grinding coefficient. If the grinding depth and feed rate are constant, the performance can be improved by increasing the cutting rate. For citation: Sergeevichev A.V., Sokolova V.A., Mikhailova A.E., Ovcharova E.O., Voinash S.A. Force Analysis of a Single Grain Cutting Process in Wood Grinding Using Spherical Corundum. Lesnoy Zhurnal = Russian Forestry Journal, 2023, no. 3, pp. 140–154. (In Russ.). https://doi.org/10.37482/0536-1036-2023-3-140-154

A Method for Detrital Corundum Characterization in Sediments: Case Study of the Gem Mountain Mine Placer Sapphire Deposit (Rock Creek, Montana, USA)

AbstractA new, comprehensive approach to characterizing detrital corundum in sediments and placer deposits is outlined and applied to a sample of poorly sorted, mud-supported colluvium rich in rhyolite tuff clasts from the sapphire-producing layer at Gem Mountain Mine, Rock Creek, Montana. High angularity and the predominance of inherited surface features indicate that corundum was not abraded during transport, consistent with deposition by mass wasting. Corundum grain abundances per 10 kg of sediment (rounded to nearest whole number) are as follows: 4–8 mm (1 grain), 2–4 mm (12 grains), 1.0–2.0 mm (10 grains), 0.5–1.0 mm (43 grains), 0.25–0.50 mm (90 grains), 0.125–0.250 mm (480 grains), and &amp;lt;0.125 mm (55 grains; underrepresentation due to poor recovery). The ∼ 50× abundance of corundum in &amp;lt;2 mm size fractions relative to the &amp;gt;2 mm size fraction of potential economic interest suggests that fine-grained corundum may have potential as a pathfinder for gem corundum deposits in heavy mineral surveys. Corundum grains are transparent and divided into four types by surface texture and extent of crystal face development: (1) rounded with frosted surface, anhedral/subhedral; (2) angular with hillocky/irregular surface, anhedral/subhedral; (3) angular subhedral grains with glassy curved faces; and (4) angular, idiomorphic blue grains (all &amp;lt;1 mm). Surface texture Types 1–3 occur in pastel-colored corundum (green, yellowish-green, yellow, colorless) and are the result of dissolution subsequent to euhedral crystal growth. Mineral inclusions in corundum include: pastel corundum – biotite (variable Fe, Mg, Ti, F contents), plagioclase feldspar (andesine-labradorite), rutile, hercynite; blue corundum – alkali feldspar, rutile, zircon, ilmenite, apatite, spinel. Corundum uncommonly occurs in composite grains with biotite, plagioclase, and K-feldspar. Rhyolite occurs as a partial encrustation on one corundum grain. Rock Creek corundum may have originated from partially melted metapelite in a metamorphic basement, and incorporated and transported to the surface by rhyolitic magmas. Due to scant geologic context and an absence of radiometric age data, the proposed genetic model remains highly speculative.

Geopolymer-based grinding stones utilizable in metal machining

This article presents partial results of a project focused on the Research and development of new grinding tools fixed with hybrid binder based on inorganic polymers. The clay-slag geopolymer with corundum abrasive grains was first time used to prepare novel abrasive tools for metal grinding. The effect of brown and white corundum grains of different grain sizes and content has been investigated. The experiments performed on a rusted steel tube on a lathe along with subsequent scanning electron analysis (SEM) have confirmed the grinding properties of geopolymer grinding stones. The grinding traces detected by SEM analysis correspond to the incorporated amount of the abrasive and the size of the grain used. The results have proven that, in case of metal grinding, a geopolymer matrix can substitute for ceramic binder in grinding tools. The geopolymer grinding stones are an opportunity to sustainable development of a new generation of abrasive materials which utilize waste materials, save primary raw material resources and energy consumption and has a minimum impact on the environment.

The microstructures and properties of Fe2O3 and TiO2 co-doped corundum ceramics for solar thermal absorbing materials

Solar thermal absorbing materials are the key components of concentrating solar power. In this study, Fe2O3 and TiO2 co-doped corundum ceramics were prepared by pressureless sintering. The effects of different Fe2O3/TiO2 ratios on the phase composition, microstructure, thermal shock resistance and solar absorptance were investigated via XRD and EPMA testing. The results showed that, with the decrease of Fe2O3/TiO2 ratio, the appropriate amount of FeAlTiO5 would decompose into ferrite particles, which played a bridging role between the corundum grains making the samples have excellent thermal shock resistance. A6 (90% bauxite, 9.5% Fe2O3 and 0.5% TiO2) sintered at 1460 °C had the optimum comprehensive properties, with a bending strength of 154.80 MPa and an absorptance of 89.20% in the spectral range from 0.3 to 2.5 μm. After 30 thermal shock cycles (1000 °C–25 °C, air-cooled), the bending strength of A6 was 222.05 MPa, and the absorptance was 90.40%, which were 43.44% and 1.35% higher than those before thermal shock, respectively. Therefore, it was suitable as an excellent solar thermal absorbing materials.

Chemical and Mechanical Properties of Metakaolin-Based Geopolymers with Waste Corundum Powder Resulting from Erosion Testing.

Alkali activated binders, based on an aluminosilicate powder that is activated by an alkaline solution, have been proven to encapsulate a wide number of different wastes, both in the form of liquids and solids. In this study, we investigated the effect that the addition of a spent abrasive powder, mainly composed of corundum grains (RC), had on the mechanical, physical, and chemical properties of metakaolin-based geopolymers. The waste was introduced into the geopolymer matrix as a substitute for metakaolin, or added as a filler to the geopolymeric paste. The 3D cross-linking of the geopolymer structure, with and without the presence of the corundum, was investigated via Fourier transform infrared spectroscopy, X-ray diffraction, and ionic conductivity measurements of the eluate that was produced after 24 h of immersion of the sample in water. The RC powder did not significantly modify the matrix reticulation but increased densification, as observed with scanning electron microscopy, and there was increased resistance to compression by 10 wt% addition of RC, and also when added to the paste as a filler at 20 wt%.

FERROHÖGBOMITE-2S2N IN THE DIATEXITES OF WESTERN SANGILEN, SOUTH-EASTERN TUVA, RUSSIA

In the Western Sangilen (Tuva–Mongolia microcontinent, Central Asian Orogenic Belt, Russia) rare mineral ferrohögbomite-2S2N, for the first time, was found in a deep-seated contact aureole of the early Paleozoic Bayan-Kol gabbro-monzodiorite intrusion. The rocks are diatexites of roof pendant, where metamorphics contact gabbronorite. Diatexites formed as a result of strong melting and desilication of quartz-bearing kyanite-staurolite schists (M1) during progressive thermal metamorphism (M2) near the Bayan-Kol intrusion with peak at 950 °C, 7–8 kbars. Ferrohögbomite-2S2N grains up to 50 μm in size found in ilmenite-corundum-magnetite-hercynite pseudomorphs. The later formed during progressive stage of thermal metamorphism M2 after regional staurolite M1 and are in cordierite-gedrite-plagioclase restite blocks of diatexites. Ferrohögbomite also appeared as thin (1–5 μm) rims around individual grains of hercynite, corundum and magnetite in a rock matrix. According to microtextural features ferrohögbomite grains formed during post-peak metamorphic stage after minerals of pseudomorphs. A univariant mineral reaction of ferrohögbomite-2S2N formation calculated quantitatively in Fe-Al-Ti-Zn-H2O system with program Mathematic 9.0 using electron microprobe chemical composition of minerals of pseudomorphs. The reaction is 0.96Spl+0.194Ilm+0.116Mag+0.036Crn+0.163H2O=0.163Hgb. A local equilibrium value during formation of ferrohögbomite was probably correlated with pseudomorphs size and thus do not transcend 1 mm3. Thermobarometry yields metamorphic temperature and pressure of ferrohögbomite formation as T=665 °C, P=5.8 kbar, which correspond to late retrograde stage of thermal metamorphism of diatexites. To present day in Russia, högbomite group mineral was known only on South Urals and Aldan Shield in Eastern Siberia.

Corrosion Resistance of L120G13 Steel Castings Zone-Reinforced with Al2O3.

The aim of the study was to determine the effect of zone reinforcement of cast steel L120G13 with Al2O3 macro-particles on the corrosion resistance of the composite obtained in this way. The obtained results allow us to conclude that strengthening of cast steel with corundum, the aim of which was to significantly increase the abrasive wear resistance, did not significantly deteriorate corrosion resistance. SEM tests show that a permanent diffusion layer interface is formed at the boundary between cast steel and corundum. In this area, simple manganese segregation and reverse iron and chromium segregation were found. These elements pass from the liquid alloy to the surface layer of the corundum particles, causing the aluminium and oxygen to be pushed deep into the corundum grains. Corrosion tests indicate comparable corrosion resistance of cast steel L120G13 and the composite L120G13 + Al2O3. Moreover, no intergranular corrosion was found in the matrix of the composite and no signs of pitting corrosion were found in the areas of the interface between the phases of the composite. This information is extremely important from the point of view of the material’s service life. Observations of breakthrough of both materials obtained during fracture after potentiodynamic corrosion tests, immediately after freezing in liquid nitrogen, indicate cracking with plastic features and increased resistance to dynamic forces of cast steel L120G13 and the composite L120G13 + Al2O3.

Production of corundum armored ceramics

The purpose of this study was to develop the compositions and technology of corundum armored ceramics with high ballistic characteristics based on aluminum oxide with an α-Al2O3 content of more than 99 wt.%. The results of the study of the physicochemical processes of obtaining corundum ceramics modified with complex additives consisting of magnesium - aluminosilicate eutectic mixture and oxides of magnesium, yttrium, titanium, zirconium are presented. The introduction of complex additives into the composition of corundum ceramics provides a significant reduction (100-150°C) of the sintering temperature of the product. In this case, additives of yttrium and zirconium oxides contribute to the formation of a uniform-grained microstructure of ceramics, and in the case of adding titanium oxide, a collective recrystallization of corundum grains is noted. The use of small additives (0.3-0.5 wt.%) of magnesium and yttrium oxides together with a eutectic mixture in the compositions of corundum ceramics based on high-quality alumina contributes to the formation of a uniform-grained, dense structure of the material and giving it a high level of physical and mechanical properties as a result of directional the action of each component of the additive on the physicochemical processes of phase formation of the crystalline matrix. A model idea of the mechanism of action of additives on the formation of the microstructure of ceramics during stage-by-stage heating from 1350 to 1650°C in the sintering mode of products is presented. The developed compositions and technologies of corundum armored ceramics have a production focus and implementation in practice.

Microstructure and mechanical properties of melt-grown alumina-mullite/glass composites fabricated by directed laser deposition

Melt-grown alumina-based composites are receiving increasing attention due to their potential for aerospace applications; however, the rapid preparation of high-performance components remains a challenge. Herein, a novel route for 3D printing dense (< 99.4%) high-performance melt-grown alumina-mullite/glass composites using directed laser deposition (DLD) is proposed. Key issues on the composites, including phase composition, microstructure formation/evolution, densification, and mechanical properties, are systematically investigated. The toughening and strengthening mechanisms are analyzed using classical fracture mechanics, Griffith strength theory, and solid/glass interface infiltration theory. It is demonstrated that the composites are composed of corundum, mullite, and glass, or corundum and glass. With the increase of alumina content in the initial powder, corundum grains gradually evolve from near-equiaxed dendrite to columnar dendrite and cellular structures due to the weakening of constitutional undercooling and small nucleation undercooling. The microhardness and fracture toughness are the highest at 92.5 mol% alumina, with 18.39±0.38 GPa and 3.07±0.13 MPa·m1/2, respectively. The maximum strength is 310.1±36.5 MPa at 95 mol% alumina. Strength enhancement is attributed to the improved densification due to the trace silica doping and the relief of residual stresses. The method unravels the potential of preparing dense high-performance melt-grown alumina-based composites by the DLD technology.

Effects of flake-shape and content of nano-mullite on mechanical properties and fracture process of corundum composite ceramics

ABSTRACT Coal gangue is used to prepare flake nano-mullite with a molten salt method. In order to increase the utilization value, corundum composite ceramics were prepared from the nanomullite and alumina. The effect of particle size of the raw material size and the content of mullite on mechanical properties of the corundum composite ceramics was studied. It is found that the mullite with appropriate sizes can be filled in between the corundum grains, thus increasing the density of the samples. The mechanical properties were enhanced due to the variation in the crack propagation mode. The optimized sample exhibited flexural strength and fracture toughness of 263.7 MPa and 5.0 MPa·m1/2, respectively. Griffith fracture theory analysis results indicated that the critical crack size reached 173.9 μm and the fracture energy was 127.6 J/m2. Coal gangue has realized deep resource utilization, and the mechanical properties of composite ceramics are higher than alumina ceramics prepared under the same conditions. Flake nanomullite contributes to mechanical properties.

Jingsuiite, TiB2, a new mineral from the Cr-11 podiform chromitite orebody, Luobusa ophiolite, Tibet, China: Implications for recycling of boron

Abstract The new mineral jingsuiite (TiB2, IMA-2018-117b), together with osbornite-khamrabaevite solid solution (TiN-TiC), deltalumite, and a potential new mineral, hexagonal Ti10(Si,P,☐)7, constitute four inclusions up to 50 μm across in corundum recovered from the Cr-11 podiform chromitite orebody near Kangjinla, Luobusa ophiolite, Tibet, China. EELS, EDS, and 3D electron diffraction were applied to study the phases. In one inclusion, jingsuiite forms a rounded grain 40 μm across. Associated osbornite-khamrabaevite solid solution forms an irregular mass up to 10 μm across having the composition Ti(N0.5C0.5) and the Ti10(Si,P,☐)7 phase forms an incomplete overgrowth up to 20 μm thick around the grain of jingsuiite. In a second inclusion, jingsuiite, osbornite-khamrabaevite solid solution, Ti10(Si,P,☐)7 and deltalumite form a lamellar intergrowth 100 μm long composed of tablets of the four phases up to 50 μm long × 4 μm in thickness. Jingsuiite has a primitive hexagonal cell with a = 3.04(6), b = 3.04(6), c = 3.22(6) Å, α = 90°, β = 90°, γ = 120°, V = 25.8 (9) Å3, space group P6/mmm, Z = 1. Its structure was determined ab initio and dynamically refined on the basis of three-dimensional electron diffraction data; it is equivalent to that of synthetic TiB2. Results of EELS analyses of jingsuiite in foil no. 5357 (N = 20) gave B 61.87(1.22), C 1.53 (1.26), Ti 36.62 (1.45) at% from which an empirical formula of Ti1.10(B1.86C0.05)Σ1.91 was calculated on the basis of 3 atoms. The ideal formula is TiB2. Our preferred scenario is that corundum with entrapped Ti-Si-P-Fe intermetallic melts was precipitated from basaltic magmas during exhumation following deep subduction. Enrichment of B in the melt pockets is attributed to the highly reducing conditions that led to the segregation of siderophile elements into intermetallic melts and to the siderophile behavior of B, thereby concentrating it in the intermetallic melts in preference to silicate melt. Experimental work on the Ti-Fe-Si system indicates that minerals enclosed in corundum grains such as Ti, FeTiSi2, and TiSi2 could have crystallized from alloy melts at the lowest T accessible on the liquidus, i.e., &amp;lt;1300 °C. The presence of TiB2 in four inclusions in the Cr-11 orebody suggests incorporation of crustal sediments in the ophiolite followed by deep subduction to the Transition Zone where qingsongite (cubic BN) is inferred to have crystallized and subsequently exhumed to shallower levels where hexagonal BN and jingsuiite presumably crystallized.

Influence research of temperature heat treatment on the microstructure of alumina ceramics

The JSC "URIR named after A. S. Berezhnoy" developed and introduced the technology of highly refractory alumina extra-dense ceramics. It is known that, the structure and properties of alumina ceramics in many cases depend on the temperature of its heat treatment, the holding time at the final temperature, the thermal unit, and a number of other factors. In order to study the processes of structure formation during of heat treatment the microstructure of alumina ceramic samples of two compositions (composition 1 with a moisture content of 30 %, as well as with dispersing and hardening additives, and composition 2 with a moisture content of 20 % only with dispersing additive) which were made by slip casting aqueous alumina slips based on reactive alumina α–from with Al2O3 content over 99 %, specific surface ~ 9 m2/g by electron microscopic analysis was studied. It was established that, after heat treatment at 1000 °C the beginning of sintering and recrystallization of corundum grains are observed, after heat treatment at 1200 °C take place packing of structure and recrystallization of α–Al2O3. After firing at 1400 °C the microstructure of samples composition 1 is represented to a large extent by crystals of tabular corundum with a particle size ~ 0.5—5.5 μm, predominant — ~ 2.5—3.0 μm. After firing at 1400 °C the microstructure of samples composition 2 is represented — by individual particles of a rounded and oval form with a size of ~ 0.5—3.5 μm, predominant — ~ 1.5—2.0 μm which are interconnected by intercrystalline layers. The microstructure of alumina ceramic samples of both compositions after firing at 1500 °C is similar and is composed of recrystallized corundum grains with a size of ~ 0.5—5.5 μm, predominant — ~ 1.5—2.0 μm, among which there are also larger α–Al2O3 grains, with a size of ~ 2—8 μm. After firing at 1580 °C the microstructure of samples is represented by a dense fine-grained structure of well-formed practically defect-free corundum grains, with a size of ~ 0.5—3.0 μm, among which grains of ~ 5—10 μm.