N. A. Alad’ev

Titanium-to-rutile oxidation kinetics in the direct-oxidation fabrication of thin-wall ceramics

The oxidation kinetics of titanium preforms for the direct-oxidation fabrication of thin-wall ceramics have been studied in the temperature range 750–1100°C using titanium plates up to 5 mm in thickness. Empirical relations for the kinetics of these processes have been obtained. The preform weight has been shown for the first time to influence the oxidation rate of titanium and the amount of forming rutile, which is inconsistent with the known patterns of metal oxide formation. One possible mechanism of rutile formation during the direct-oxidation fabrication of thin-wall ceramics is considered.

Microstructure and phase composition of ZrO2-CeO2-Al2O3 materials modified with MgO and Y2O3

This paper presents our findings on phase formation processes during heat treatment of sol-gel synthesis products with the composition (mol %) 65(88ZrO2 + 12CeO2) + 35Al2O3 modified with 1 mol % MgO or Y2O3. The composites modified with 1 mol % MgO have been shown to differ significantly in phase composition from the parent nanopowders. Sintering is accompanied by partial decomposition of the tetragonal zirconia (T-ZrO2) based solid solution and the formation of a monoclinic zirconia (M-ZrO2) based solid solution and two aluminum-containing phases: corundum and the mixed oxide MgAl11CeO19. The addition of 1 mol % Y2O3 leads to successive formation of T-ZrO2 and corundum and improvement of their structural perfection. The observed differences in phase formation during heat treatment result in different grain size compositions in the microstructure of the composites.

Colloidal and transparent opal-matrix nanocomposites filled with europium-doped silica sols

Three-dimensional ordered opal-matrix composites filled with europium-doped silica sols have been produced using methods of colloid chemistry. According to elemental analysis data, the Eu concentration in the nanocomposites was ∼30 ppm. A uniform europium distribution over the tetrahedral and octahedral pores of the 3D opal matrix was ensured by repeatedly filling the opal pores with silica sols doped with europium salts or europium oxide. Varying high-temperature annealing conditions, one can control the microstructure of 3D ordered nanocomposites, producing opaline and transparent photonic crystals. The microstructure of opal photonic crystals has the form of an ordered fcc lattice of amorphous silica spheres, whose tetrahedral and octahedral pores are filled with mesoporous europium-doped glass. Partial sintering of the silica spheres and mesoporous glass in transparent photonic crystals results in a periodic arrangement of quantum dots enriched in Eu-doped silica.

α-Al2O3 powders from amorphous alumina gel

We have studied the properties of α-Al2O3 powders prepared from amorphous alumina gel. The powders have a small particle size owing to the low synthesis temperature. The sintering temperature of compacts produced by pressing the powders is lower than that in the case of powders prepared from hydroxide precursors. The sintered material has a homogeneous, fine-grained microstructure, which ensures excellent mechanical properties.

Decomposition of the γ solid solution in a nickel-based cast alloy at a high uniform pressure

The formation of γ′ particles in an as-cast nickel alloy at barothermal treatment temperatures of 1235, 1260, and 1320°C and a pressure of 175 MPa for an action time of 2 h (isobar-isothermal holding (IIH)) is studied by quantitative metallography using optical and electron microscopy and image processing computer programs. An analysis of images of the dendritic structure of the alloy, which is formed by the morphology and sizes of γ′ particles, demonstrates an increase in the degree of homogeneity of the alloy with the IIH temperature. The concentration of γ′ particles is determined in the initial alloy and after barothermal treatment (BTT) at temperatures of 1235, 1260, and 1320°C, and the volume fraction of the γ′ phase is found to significantly decrease as the IIH temperature increases. An activation mechanism is suggested for the formation of the nucleation centers of γ′ particles in the γ solid solution, and the activation energy of the dissolution/coalescence of γ′ precipitates is determined. The fraction of nonequilibrium γ′ particles is determined in the material in the initial state and after BTT at three IIH temperatures. The precipitation of γ′ particles is characterized by a bimodal character with the formation of nanoparticles in the initial material and upon BTT at temperatures of 1235 and 1260°C.

Effect of the Thermal Component of Barothermal Treatment on the Carbide Phase Component of a Cast Nickel Alloy

Electron-probe microanalysis, scanning electron microscopy, and optical quantitative metallography are used to study the behavior of the carbides in a corrosion-resistant nickel superalloy during a barothermal action at a fixed pressure and exposure time (190 MPa and 210 min, respectively) and a sequential increase in the barothermal treatment (BTT) temperature from 1235 to 1320°C. The BTT temperature is found to significantly affect the chemical composition of the carbides, and this effect manifests itself in an increase in the titanium, molybdenum, or tungsten content and a decreases in the nickel or chromium content. The granulomteric composition of the carbide skeleton in the alloy is studied: it is found to depend substantially on the temperature of barothermal action. A model is proposed to describe the change in the chemical composition of the carbide structural constituent of a cast nickel alloy at a high pressure and temperature.

Preparation of silicon carbide whiskers from silicon nitride

Silicon carbide whiskers have been prepared by sintering silicon nitride powder in a graphite reactor at 1800°C under a nitrogen atmosphere. The whiskers differ in morphology: tubular needles, hollow faceted fibers with a square cross section, and solid fibers with a triangular cross section. The average diameter of the needles is 0.5−5 μm, and that of the faceted fibers is up to 20 μm. The fibers range in length up to several millimeters. Such silicon carbide whiskers can be used as reinforcing agents for structural ceramics based on nonoxide materials.

Dendrite segregation in Ni3Al-based intermetallic single crystals alloyed with Cr, Mo, W, Ti, Co, and Re

The character of dendrite segregation in Ni3Al-based intermetallic VKNA-type alloy single crystals with a dendritic–cellular structure is studied. Distribution coefficient kd of an alloying element (AE) in the alloy during solidification kd = cd.a.I/c0 (c0 is the AE content in the alloy (liquid phase composition), cd.a.I is the AE content in primary dendrite arms of the alloy (in the solid phase)) and segregation coefficient ks = cd.a.I/ci.d (ci.d is the AE content in the interdendritic space) have been found. A comparative study of the dendrite segregation parameters in VKNA-nype Ni3Al-based intermetallic alloys and the well-known ZhS36-type nickel superalloy shows that the intermetallic alloys satisfy to the rule deduced for two- and three-component nickel-based superalloys: if an introduced AE increases the melting temperature of the basic metal, we have kd > 1 (Co, W, Re); if it decreases the melting temperature, we have kd < 1 (Al, Ti, Cr, Mo). Dendrite segregation coefficients ks are dependent on the proportion of the AE contents in the alloys. In nickel superalloys, the dendrite segregation of aluminum, tungsten, and rhenium is higher than that in the intermetallic alloys. The dendrite segregation coefficients of tungsten and rhenium is higher by a factor of 1.5–2 than that in the VKNA-type intermetallic alloys with a low content of refractory metals. This can be due to the retardation of diffusion of refractory metals in the solid phase of a nickel superalloy highly alloyed with these elements.

Low-cycle fatigue of an Ni 3 Al-Based VKNA-25 alloy at room temperature

The life of 〈001〉, 〈011〉, and 〈111〉 single crystals made of a heterophase (γ’ + γ) structural cast γ’-Ni3Al-based superalloy having an ordered crystal structure and optimally alloyed with refractory metals is studied under low-cycle fatigue testing conditions at axial pulsed stresses on smooth specimens at room temperature. A correlation is revealed between the type of fracture surface and the structure of the heterophase alloy. The alloy behaves like a composite material in which only ductile structural constituent γ, which represents precipitates of an fcc disordered nickel-based solid solution, has a sufficient ductility margin. Real plastic deformation during tests at 20°C develops only in this structural constituent, which is indicated by slightly elongated Γ layer regions tensioned during fracture and the propagation of a main crack and secondary cracks deep into a specimen.

Nanofilaments of Si3N4

Silicon nitride nanofilaments were synthesized at 1600°C in nitrogen on the surface of a ceramic substrate made of amorphous silicon oxynitride obtained by hexamethyl disilasane pyrolysis. The diameter of filaments represented by different morphological types (filaments, ribbons, needles) was 100–500 nm, and the length was several millimeters.