Synthesis of Sialon from Kaolin and Its Phase Formation

The main phase transformations during synthesis of β‐sialon from kaolin of the Prosyanovskii deposit (Ukraine) under test-industrial production conditions have been studied. Sialon was prepared by carbothermal reduction in graphite-tube furnaces in nitrogen. It is shown that the process is characterized by the simultaneous occurrence of several chemical reactions with formation of intermediate phases (Si₂ON₂, X₁‐phase), silicon carbide, and (apart from β‐sialon) sialons based on AlN polytypes. The phase composition of the final product is determined not only by the charge composition but also by the set of production parameters. Results of the work may provide a basis for commercial preparation of sialon powder.     

Compaction, structure formation, and properties of materials based on boron carbide

The effect of adding aluminum and titanium on the compactibility kinetics, structure, and properties of materials in the systems B₄C-Al and B₄C-(Ti-Al) is studied. The micromechanical properties (microhardness, microbrittleness, microstrength, and crack resistance factor (K_1c) of composite materials are determined. The influence of high pressure on the compactibility, structure, and properties of B₄C-based materials with additions of titanium and aluminum is investigated.     

Effects of Nitrogen Deficiency on the Kinetics and Mechanism of Electrolytic ZrN x Oxidation

Use has been made of potentiodynamic polarization curves, XRD, and scanning electron microscopy (SEM) in the electrolytic oxidation in 3% NaCl solution for specimens of nitrogen-deficient zirconium nitride (ZrN_0.67, ZrN_0.77, ZrN_0.87, and ZrN_0.97), as well as pure zirconium. In all cases, the anodic polarization curves have several stages which characterize during oxidation both active dissolution of ZrN _x and Zr in the electrolyte as well as the formation of surface layers of ZrOCl₂, ZrN _x O _y , and α‐ZrO₂ of monoclinic form. The corrosion resistance of single-phase ZrN _x specimens in 3% NaCl solution decreases in the sequence ZrN_0.97 → ZrN_0.87 → ZrN_0.77, and the initial stages of interaction between the specimen surface and the electrolyte largely determine the subsequent behavior of specimens. It is found that ZrN _x containing a large number of nitrogen atom vacancies, in particular ZrN_0.77, is closer in corrosion behavior to metallic zirconium than it is to stoichiometric ZrN (the reduction in the corrosion resistance is undoubtedly due to the reduction in the ionic-covalent components of the bonds in ZrN _x ).     

Contact interaction of double titanium and chromium diboride with Fe-Cr alloys

The kinetics of wetting double titanium and chromium diboride by iron-based alloys with different chromium content is studied. It is shown that contact angles are formed in such systems over the range 0–10°. The energy parameters of wetting are calculated. It is established that chromium as a surface-active element promotes Fe-Cr alloy spreading on the diboride surface. The microstructure of the interaction area is studied. It is shown that terminal solid solutions and eutectics are formed in the TiCrB₂-(Fe-Cr) system. The optimum compositions of the metallic binder are determined for TiCrB₂-based composite materials. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 109–115, 2007     

Mechanical and Corrosion Properties of ZrB2–SiC Composite Ceramics with Oxide Additions

Powder Metallurgy and Metal Ceramics - Hot pressing was used to produce compact ceramic samples with the following composition (wt.%): 60 ZrB2 + 20 SiC + 20 (Al2O3 + 32 t-ZrO2). The tetragonal...     

Deformation approach to the evaluation of the period of initiation and growth of fatigue macrocracks

We performed experimental investigation of the opening displacements of the contours of stress concentrators (notches and cracks) for various amplitudes of cyclic loading. On the basis of experimental results, we propose a new deformation parameter _t which is a function of the notch (crack) tip opening displacement , namely, _t /(+d*), where is the radius of the tip of the notch andd* is the characteristic size of the prefracture zone. It is shown that this parameter uniquely determines the number of cyclesN _l to the initiation of a fatigue macrocrack independently of the geometry of the specimens and stress concentrators in elastic and elastoplastic materials, i.e., the dependence of _t onN ₁ is a characteristic of the material. It is experimentally demonstrated that this dependence enables one to quantitatively describe the process of fatigue fracture both in the stage of initiation of macrocracks and their propagation.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 5, pp. 7–21, September – October, 1995.     

Fracture mechanics and strength of materials: advances and prospects

We analyze the principal stages of the development of fracture mechanics and strength of materials in the second half of the last century. Our attention is mainly focused on the analysis of the computational models of limiting equilibrium of deformable solid bodies with sharp stress concentrators (cracks), on the development of the methods aimed at evaluation of the stress intensity factors, on the methods and means of experimental determination of the characteristics of crack resistance of the material, on the concepts of initiation and propagation of fatigue cracks, and on the processes of cracking of materials in the zone of cyclic contact of two bodies. New approaches to the evaluation of the period of initiation of fatigue macrocracks near stress concentrators based on the use of the conventional v-K diagrams for a given material are proposed, the influence of working media on the corrosion crack-growth resistance of structural materials is analyzed, and the basic (computational) fatigue crack growth curves for the evaluation of the strength characteristics of high-pressure vessels are constructed. Some promising problems in this field of science are formulated.Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 3, pp. 5–18, May–June, 2004.     

Effect of current pulse frequency on the structure and properties of electric-spark alloyed coatings of the Ti - Al - N system

The mass transfer kinetics, composition, structure, and properties of coatings, formed by high-frequency electric-spark alloying of 45 and 40X steels with the Ti - Al (3:1, 1:1, 1:3) intermetallics and TiN - AIN (1:1) nitride were studied as a function of the current pulse frequency(ν = 1200 and 1600 Hz). A decrease in ν was found to lead to a higher mass transfer coefficient and higher microhardness and Young’s modulus of the coating as well. This is due to the decrease in the Ti/Al ratio on the surface. The phase composition of intermetallic and nitride coatings differed only insignificantly and did not depend on the value of ν. The main phases of the coatings were Fe - Ti - O and Al - Ti - O solid solutions. The nitride and intermetallic coatings had similar friction coefficients f and wear rates I (f = 0.26 and 0.28, I = 5.4 and 5.9 µm/km), despite the difference in their structure. We can assume that the coating phase composition which defines the composition of the secondary structure under dry friction, is the main factor controlling tribological behavior.