Structure and some properties of niobium carbide single crystals

Conclusions High-purity niobium carbide single crystals of different compositions within the range of homogeneity of the carbide were grown by crystallization from a melt. X-ray diffraction studies and metallographic examinations revealed that the single crystals obtained contained both perfect regions and regions distorted by thermal stresses and composition heterogeneities. As a result of x-ray diffraction studies, the presence of substructures of the first and second orders in the single crystals was established. Maximum disorientation was found to attain 0.2–0.5° for blocks of the first order and to be about 0.05° for blocks of the second order. The density of dislocations in the niobium single carbides was on the order of 10⁵ cm^–2. Within the limits of niobium carbide compositions investigated no differences in mechanical properties (microhardness and resistance to cracking) were detected. It was established that in single crystals of compositions NbC_0.78-NbC_0.82 room-temperature deformation took place over the {111} 110 system. The room-temperature fracture of the single crystals was mainly brittle in character.Translated from Poroshkovaya Metallurgiya, No. 12(228), pp. 81–86, December, 1981.     

Effect of niobium carbide on the properties of tungsten-free hard alloys

Conclusions A study was made of the effects of alloying with niobium carbide of the carbide phase of a TiC-Ni-Mo hard alloy (in amounts equal to 2,5,10, and 25 mole% relative to the TiC content) upon the physicomechanical properties of the alloy. It is shown that raising the amount of NbC in the alloy increases its transverse rupture strength (to 132 kg/mm²) and impact strength, leaving its hardness virtually unchanged (91 HRA); the improvement in properties is attributable to a slight increase in the ductility of the grains of the hard phase — a complex TiC-NbC carbide.Translated from Poroshkovaya Metallurgiya, No. 9 (129), pp. 83–86, September, 1973.     

Character of plastic deformation in niobium carbide powders subjected to vibratory milling

Conclusions A study of the fine-structure parameters of niobium carbide powders has shown that nonstoichiometric carbides are more brittle than stoichiometric niobium carbide. This is due, firstly, to the formation of vacancy complexes in carbon-deficient sublattices and to the generation of Frank dislocations, which markedly hinder the motion of existing and nucleation of new dislocations, and, secondly, to the increase in Peierls-Nabarro forces brought about by a fall in the carbon content of niobium carbide.Translated from Poroshkovaya Metallurgiya, No. 3(171), pp. 7–11, March, 1977.     

Electrophysical properties of niobium carbide single crystals

Conclusions An investigation was carried out into the temperature dependence of the electrical resistivity, thermal conductivity, and coefficient of thermo-emf of single-crystal niobium carbide. It is shown that the electrical resistivity of single-crystal niobium carbide, a material of high purity and perfect structure, is higher than that of polycrystalline materials.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 59–62, October, 1982.     

Sintering kinetics of niobium carbide powders of nonstoichiometric composition

Conclusions The sintering densification kinetics of NbC_x powders depends to a large extent on their degree of dispersion and nonstoichiometry; a characteristic feature of the densification kinetics is that the main contribution to densification, which grows further with increasing degree of dispersion and defectiveness (activity) of NbC_x phases, is made by the initial stage of sintering, apparently involving threshold mechanisms of mass transport, which are activated by oxycarbide decomposition processes in the temperature range 1600–2420°K. Because of this, in the sintering of NbC_x and ZrC_x phases the principal mechanisms of mass transport change in different temperature ranges.Translated from Poroshkovaya Metallurgiya, No. 5 (185), pp. 47–50, May, 1978.     

Effect of charge composition on the structure and electrophysical properties of self-bonded silicon carbide

Computer treatment of experimental data using the method of Kriging permitted an analysis of the structure, phase composition, and electrophysical properties of silicon carbide materials produced by liquid-phase reactive sintering over a broad range of initial charge compositions. The optimal values of the electrophysical properties were determined, and the possibility of predicting the charge composition for the production of material with given electrophysical properties demonstrated.Institute of Materials Science Problems, Ukrainian Academy of Science, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4, pp. 57–62, April–May, 1994.     

Composition dependence of the abrasive properties of fused niobium carbide powder

Conclusions A study was made of the abrasive characteristics of fused niobium carbide phases. The effects of carbon content variation upon the properties of the carbide phases were determined. Of the materials investigated, the carbide phase NbC_0.82 shows greatest promise for the manufacture of abrasive tools.Translated from Poroshkovaya Metallurgiya, No. 1 (145), pp. 90–93, January, 1975.     

Effect of niobium,molybdenum, and tungsten on some properties of iron-boride materials

Conclusions A study was made of some key physicomechanical properties of iron-boride materials alloyed with Nb, Mo, and W. The greatest strengthening effect (_t = 62–75 kgf/mm² at 30–33 HRC) is achieved with iron-tungsten matrices infiltrated, using alloyed powders produced by coreduction from oxides, with eutectic Fe-Mo-B or Fe-Nb-Mo-B alloys. The presence of Fe(Nb, Mo)_xB_y type borides in the eutectic alloys substantially increases the heat resistance of the infiltrated materials.Translated from Poroshkovaya Metallurgiya, No. 2(206), pp. 79–82, February, 1980.     

Solubility of niobium carbide and niobium carbonitride in alloyed austenite and ferrite

The available data on the solubility of niobium carbide and niobium carbonitride in plain carbon and alloyed austenite has been analyzedvia dilute solution thermodynamics with a view to establishing a consistent set of interaction parameters for predicting austenite + niobium carbonitride equilibria. The computation algorithm includes the prediction of phase mass fractions as a function of alloy composition and temperature between 900° and 1300 °C (tie lines). Analogous ferrite equilibria are included.     

Influence of milling media on the structure and agglomeration behaviour of some hardmetal powder

This paper presents the influence of milling media on the structure and morphology of WC–Co powders for sintered tools. WC–Co powders have been milled in two different organic media, acetone and isohexane. The characterisation of the as-milled samples has been done by X-ray diffraction, scanning electron microscopy (SEM) and differential scanning calorimetry. The mean crystallite size decreases with milling time for the samples processed in acetone, while in the case of isohexane, the crystallite size stays at a relatively constant value when increasing the milling duration. The calculated mean crystallite size is below 50?nm for any milling duration, with larger values for acetone (16–46 nm) as compared to isohexane samples (10–12 nm). The SEM revealed that the dried powder is composed of agglomerated submicronic particles, and at high magnification, the presence of nanoparticles is noticed. The density of the aggregates was linked to the polarity of the solvents, which influences the sedimentation kinetics.     

Effect of composition and amount of nickel-molybdenum binder on the properties of a titanium carbide hard metal

Conclusions In titanium carbide hard metals of high metallic binder content, during hot pressing in graphite die sets part of the binder is forced out of their surface layers into gaps in the sets. Under these conditions, the molybdenum content of the alloys in the early stages of hot pressing appreciably falls. The optimum amount of molybdenum in the Ni-Mo binder of hot-pressed titanium carbide hard metals is 30 vol. %. In a large-sized part strength properties vary over its cross section: Maximum strength is exhibited by the central portion, and with increasing distance from it _tr decreases.Translated from Poroshkovaya Metallurgiya, No. 4(268), pp. 18–21, April, 1985.     

Structure and some properties of sintered tantalum carbide

Conclusions A study was made of the microstructure, transverse rupture strength, and electrical resistivity of tantalum carbide specimens produced by sintering at various temperatures. It is shown that, to obtain sintered tantalum carbide of fine-grained structure, low porosity, and adequate strength, it is necessary to employ very fine starting powders. Raising the sintering temperature and increasing the duration of isothermal holding lower the porosity and electrical resistivity of sintered tantalum carbide, but at the same time induce substantial grain growth.Translated from Poroshkovaya Metallurgiya, No. 11(239), pp. 76–79, November, 1982.