N. P. Brodnikovskii

Synthesis and properties of ceramics in the SiC — B4C — MeB2 system

The effect of impurities and additives of titanium and zirconium borides on the structure and mechanical properties of SiC — B4C ceramics over a broad temperature range has been investigated. The ceramics was fabricated by hot pressing without a protective medium. Introduction of borides is accompanied by improvement in all the studied mechanical properties at room temperature, and the nature of hardening of the ceramics is practically independent of the type of SiC powders used. At high temperatures, the mechanical behavior of the ceramics is determined by the impurity composition: the ceramics obtained using abrasive powders loses strength beginning at 600°C, while using powders with decreased impurity content makes it possible to preserve the strength of the material up to a temperature of 1400°C.

Features of Brittle Material Powder Compaction During Pressing

The compaction of (i) titanium, vanadium, and molybdenum carbide powders, (ii) mixtures of these powders with the addition of nickel, chromium, and niobium carbide powders, and (iii) coarse titanium and tungsten carbides as well as diamond powders clad with cobalt under compressive loading with a constant rate on the powder in a die at room temperature is studied. Based on the variation of the relative density of the compacts with current pressure, the variation of stresses in the matrix (forming a porous body) with its strain is determined. This enables to reveal the features of compaction, strain hardening, and fracture of brittle powder particles during pressing. It is established that for the molybdenum carbide powder, consisting of powder particles with pronounced irregular shape, the initial elastic deformation of the matrix abruptly transits into the stage of plastic deformation with almost linear strain hardening. At the final stage of the powder compaction, the particles are fractured. At the initial stage of pressing, an increase in the packing density of the powder particles and in their strain hardening occurs for titanium and vanadium carbide powders, whose particle shape is close to the rounded. With the increase in the density of the porous body, it is observed an almost linear strain hardening of the matrix, which changes into decay with decreasing shear stress, when the powder body approaches its non-porous state. As a result of pressing, the size of the coherent X-ray scattering areas decreased to 62 nm and the dislocation density in the titanium carbide particles grew to 8.3 · 1010 cm–2. With an increasing content of plastic metallic particles in the matrix with the particles of brittle materials, it is the metallic particles that predominantly undergo the plastic strain with strain hardening. This causes a sharp increase in the total strain hardening, reducing the density of the porous body during pressing. It is established an effective compacting of coarse titanium and tungsten carbide powders with cobalt-clad particles 200–600 μm in size. In this case the variation of the stress in the matrix with the strain of the matrix has a sharp yield point associated with high elastic limit, significantly exceeding the yield stress, and the time delay in the transition from a purely elastic deformation of the body to its plastic flow.

The structurization and properties of Fe–Cr–C alloys with a liquid phase vanishing during sintering

The possibility of increasing the wear resistance and bending strength of Fe–Cr–C alloys with a high chromium carbide content produced by powder metallurgy methods is studied. Regularities of the structurization and mechanical properties of the alloys are determined in relation to their composition and production conditions. The conditions of sintering and the composition of the alloys with a high chromium carbide content ensuring high abrasive wear resistance, which is 1.5 times greater than that of VK8 alloy, are found. The structure of the alloy represents a fine-grained chromium carbide composite formed at a certain amount of a liquid phase vanishing during sintering.

Effect of aluminum, chromium, and iron doping on the heat resistance of zirconium

The influence of aluminum, chromium, and iron doping on the heat resistance of zirconium is studied. It is shown that a dense oxide film starts forming in the oxidation of the alloy containing 8 wt.% aluminum at 700°C regardless of its phase composition and ensures high heat resistance. At lower temperatures, to achieve the heat resistance comparable with that of the É110 commercial alloy, the content of the Zr–Al solid solution based on α-Zr, which should not contain more than 4 wt.% Al, is to be minimum. Additions of 1% Cr and 1% Fe can be used to strengthen the Zr–8Al alloy without a significant decrease in its heat resistance.

High fracture toughness of powder chromium sintered in magnesium vapor

The fracture toughness of powder chromium sintered in magnesium vapor is higher by a factor of 53 than that of powder chromium sintered in hydrogen and by a factor of 5 than that of deformed low-alloy chromium VKh2K castings. This high fracture toughness is due to the skeleton formed of plastic interlayers of high-purity chromium. Chromium becomes highly ductile after fine purification in Cr–MgO alloys to remove interstitial impurities. The interlayers form on the surface of chromium powder particles under the refining action of magnesium vapor. Auger electron microscopy and data on fracture, chemical composition, and etching resistance lead to the conclusion that there are interlayers made of pure chromium. The high fracture toughness remains after annealing for 1 h at 1500°C.

Fractographic features of failure in polycrystalline chromium on uniaxial stretching

Conclusions1.The main fractographic feature in the brittle failure of chromium is cleavage crack branching. The cracks deviate from the initial direction by ±35°.2.The crack length at branching ranges from 60 to 80 μm. The crack lengths at branching can differ by a factor of four even in a single specimen if several cracks are generated.3.In completely brittle failure, cracks branch in cast chromium with grain size 400 μm. Branching is not observed in recrystallized chromium with grain sizes of 36 and 14 μm.4.Deformation either preliminary or directly preceding failure favors branching, e.g., when a specimen is loaded in the brittle-plastic transition region.5.A cleavage crack passes through the porous part of a specimen produced by plastic strain and causes pore coalescence, i.e., the porosity causes a change in the failure mechanism.6.The cracking resistance has been estimated from the branching as follows: in coarsegrained cast chromium 2 J/m2 (−50°C), recrystallized 19.5 J/m2 (150°C), and deformed 39.5 J/m2 (200°C), 36 J/m2 (300°C), and 58 J/m2 (400°C).