Examination of the physical properties of ZrC-W carbide-metallic alloys

Conclusions The heat conductivity of the two-phase alloys of ZrC-W system increases with an increase of the tungsten content and with increase in temperature. This is caused by the electron contribution to the heat conductivity of the crbide phase.The electrical resistance of these alloys decreases with increase in tungsten content and increases with increase in temperature. At a mass constant of ZrC > 25% the temperature dependence of the electrical resistance of the alloys is nonlinear as a result of slight overlapping of the valency band by the conduction band in the carbide phase.The mean coefficient of thermal expansion ZrC-75% (wt.) W alloy increases with increase in temperature from 5.5·10^–6 in the range 300–600 to 7.05·10^–6 K^–1 in the range 300–2300°K.The spectral emission factor =0.65 mm of the ZrC-W alloys increases with an increase of the zirconium carbide content. With increase in temperature decreases for tungsten, zirconium alloy, and alloys with a mass content of W < 40%. For the alloys with a tungsten content of 45–75% depends only slightly on temperature. This can be explained by the presence of tungsten carbides in the subsurface layer. The critical wavelength of these carbides (_X=500–600 nm) is close to the wavelength in pyrometric measurements.The fracture tensile stress of the specimens of the alloys with a mass constant of tungsten of 75% increases with increase in temperature as a result of utilization of a certain ductility margin of the brittle material.Translated from Poroshkovaya Metallurgiya, No. 6(330), pp. 93–100, June, 1990.     

Structural investigations of detonation-deposited tungsten carbide-cobalt coatings

Conclusions The phase composition of detonation-deposited VK type coatings differs from that of the starting mixtures. A deposited layer contains about 40–50% of the tungsten carbide WC present in the starting mixture, metallic tungsten and cobalt, the intermetallic compounds Co₇W₆ and Co₃W, traces of W₂C, and a small amount of complex carbides. The amount and particle size of the tungsten carbide in a coating are determined by the particle size distribution of the powder being deposited and detonation process parameters. Our investigations have shown that the optimum powder particle size range for the process of detonation deposition of hard-metal mixtures may be taken to be 5–40 m. During detonation deposition under the conditions investigated the usual hard-metal structure and composition are not obtained. Most of the starting tungsten carbide decomposes with the formation of tungsten and cobalt intermetallic compounds. Attempts should now be made to improve deposition conditions so as to increase the phase and structural homogeneity of layers being deposited. In coatings deposited by an optimum method the ductile Co binder will be present in an amount close to that in a sintered VK type hard metal. The fracture of coatings and basis metal on the antivibration shelves of compressor blades has a fatigue character. Crack initiation takes place in the surface and inner layers of detonation-deposited coatings as a result of their considerable brittleness, which is due to the presence of metastable intermetallic phases as well as of defects in the form of pores and blowholes.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 24–29, October, 1982.     

Sintering of alloys of zirconium diboride with molybdenum disilicide

Summary The sintering of zirconium diboride with molybdenum disilicide is accompanied by the formation of a solid solution based on zirconium diboride, formation of a liquid phase at temperatures above 1800°C, and partial vaporization of silicon in the ZrB₂+15% MoSi₂ alloy. At temperatures up to 1800°C, solidphase sintering takes place; at low temperatures, this is accompanied by specimen growth due to heterodiffusion processes resulting from the difference in the partial diffusion coefficients of the components and to the vaporization of excess silicon in the case of the ZrB₂+15% MoSi₂ alloy.At temperatures above 1800°C, shrinkage is caused by the formation of a liquid phase, which disappears during sintering. Under these conditions, grain recrystallization and growth in the solid solution of Mo and Si in zirconium diboride in the case of 15% MoSi₂ alloys are not completed even after 4-h holding at temperatures of 1800, 1900, and 2000°C.Translated from Poroshkovaya Metallurgiya, No. 9(45), pp. 11–16, September, 1966.     

Use of x-ray photoelectron spectroscopy for the investigation of the surfaces of hard-alloy parts machined with polycrystalline superhard materials

Conclusions The thin (<600 Å) surface layers forming on VK20 and VK20K hard alloys during machining with polycrystalline superhard tool materials and diamond grinding under recommended conditions consist chiefly of tungsten and cobalt oxides, cobalt containing dissolved tungsten and carbon, and pure tungsten (or the compound W₂C). The formation of such layers must be regarded as a beneficial phenomenon, because the presence on the surface of a tool material of a thin layer of oxides of the elements of which the material is composed helps reduce tool wear.Translated from Poroshkovaya Metallurgiya, No. 11(191), pp. 73–79, November, 1978.     

Particle growth during the reduction of tungsten and molybdenum and activation of the sintering of these metals

Conclusions Particle growth during the reduction of tungsten and molybdenum powders and the shrinkage kinetics of these powders depend on two processes taking place simultaneously, namely, partial melting and vaporization of surface layers of the dioxides of these metals. During high-temperature reduction (> 1100°C), fine particles of tungsten and molybdenum dioxides are vaporized and redeposited on large particles; on the surface of the particles, a liquid phase may form, resulting in the generation of capillary forces which create a bond between separate particles. Melting of the oxides covering the surface of metallic tungsten and molybdenum particles promotes shrinkage during sintering, while conversely vaporization of the oxides inhibits the shrinkage of these metals.Translated from Poroshkovaya Metallurgiya, No. 9(69), pp. 34–38, September, 1968.     

Reactions at phase boundaries in pseudoalloys based on tungsten (molybdenum) and copper

Conclusions The reaction between tungsten and copper in the presence of nickel under conditions close to those of pseudoalloy formation is characterized by the formation of a two-layer zone in the copper. Next to the tungsten there is a layer of a phase containing W, Ni, and Cu. The reaction between molybdenum and copper under the same conditions is not accompanied by the formation of a new phase. It is possible to activate the reactions at the phase boundaries of such a pseudoalloy without increasing its electrical resistivity by decreasing the amount of nickel added to it and ensuring that the addition is evenly distributed over the surface of the particle of the pseudoalloy's refractory component. The addition of cobalt to a molybdenum-copper pseudoalloy leads to the appearance of layers of a Mo₆Co₇-base phase at its phase boundaries.Translated from Poroshkovaya Metallurgiya, No. 12(216), pp. 39–44, December, 1980.     

Production of a complex zirconium-tungsten carbide on a pilot-plant scale

Conclusions A study was made of the conditions of preparation of a complex zirconium-tungsten carbide on a pilot-plant scale. It was established that the most effective method of producing Zr_0.7W_0.3C is to sinter a charge consisting of mixed zirconium carbide, tungsten, and carbon black powders for 2 h at a temperature of 2300°C in an electric resistance furnace with a graphite heating element, using a hydrogen atmosphere.Translated from Poroshkovaya Metallurgiya, No. 7 (139), pp. 80–83, July, 1974.     

Stability of refractory compounds in fused basalt

Summary A study was made of the reaction of titanium, zirconium, niobium, molybdenum, and tungsten carbides, aluminum, zirconium, and silicon nitrides, titanium, zirconium, chromium, and molybdenum borides, molybdenum disilicide, chromic oxide, and refractories based on silicon nitride and carbide with molten basalt at a temperature of 1400°C. It has been established that molybdenum disilicide exhibits the highest stability in molten basalt.Translated from Poroshkovaya Metallurgiya, No. 2 (50), pp. 47–49, February, 1967.     

Regularities of sintering of zirconium diboride-molybenum alloys

Summary In the process of sintering of mixtures of zirconium diboride with 5, 10, and 15% Mo, specimen growth resulting from heterodiffusion is observed at the instant of formation of solid solution of Mo in ZrB₂ during slow heating to high temperatures or during the initial period of isothermal holding in the case of very rapid heating. At temperatures of up to 1700–1750°C, growth predominates over shrinkage, and specimen dimensions increase with increasing holding time; at temperatures above 1800°C, positive shrinkage takes place, but is very slight in the case of rapid heating to the isothermal holding temperature.During isothermal holding in the temperature range 1800–2200°C, very intensive shrinkage is observed during the initial period (20–30 min). Subsequently, this shrinkage slows down, and may be described as viscous flow caused by diffusional processes. The energy of activation of the densification process, calculated from the shear viscosity values obtained, was found to be 367±48, 352±28, and 379±46 kJ/mole for alloys of ZrB₂ with 5, 10, and 15% Mo, respectively, i.e., less than the energy of activation of densification of zirconium diboride (678±55 kJ/mole).Thus, the presence of molybdenum activates diffusion processes during sintering.     

Diffusion Interaction in Fiber-Reinforced Composites of the Chromium Alloy – Silicon Carbide System

Diffusion interaction in the Cr – ZrC and Cr – HfC systems has been studied in the temperature range 1200-1400°C. The impurities contained in chromium or carbides were found to affect the interaction in the systems. Almost no interaction was observed when the systems were held at temperatures up to 1300°C for 100 h. At higher temperatures the carbides were reduced to ZrO₂, HfO₂, and Cr₂₃C₆ because of the presence of a slight amount of oxygen. Thermodynamic calculations indicated no interaction in these systems at temperatures up to 1600°C. A study of the interaction in the systems SiC – ZrC and SiC – HfC showed that a transition zone formed already in the stage of sample preparation by diffusion welding in vacuum (1300°C, vacuum of 10^_3 Pa, with a load applied for 20 min). During annealing (1300°C, 50 h) the transition zone stratified, forming a solid solution of silicon in Zr(Hf)C and SiC inclusions in the SiC – Zr(Hf)C solid solution. A transition zone formed on the zirconium carbide side when SiC interacted with Zr(Hf)C. The interaction in SiC – Zr(Hf)C casts doubt on the use of them as a barrier without antidiffusion layers.     

Compressive creep of alloys of the ZrC-ZrB2 and TiC-TiB2 systems

Conclusions A study was made of the compressive creep of two-phase alloys of the ZrC-ZrB₂ and TiC-TiB₂ systems at temperatures of 1700–2420°C and stresses of 5–30 MPa. In the ZrC-ZrB₂ system two-phase alloys in a wide range of carbide phase concentrations — from 20 to 70 mole% are characterized by a creep rate exceeding by one to two orders the creep rates of their individual components, while in the TiC-TiB₂ system the highest creep rate is exhibited by alloys with carbide contents of 30–50 mole %. The effect of contamination with tungsten carbide on the creep of alloys of the systems investigated was determined. In TiC-TiB₂ alloys it is possible for coherent phase boundaries to form and for superplastic creep phenomena to manifest themselves, involving also the operation of threshold mechanisms of plastic deformation.Translated from Poroshkovaya Metallurgiya, No. 12(228), pp. 70–75, December, 1981.     

High temperature vacuum sintering of plasmochemical powders based on ZrO2

The phase composition and mechanical properties of ZrO₂-based ceramics prepared by sintering plasmochemical powders of complex morphology in air and in vacuum were compared. Sintering at a high temperatures in vacuum produced material exhibiting high density and good mechanical properties, in which the zirconium dioxide was entirely in the tetragonal form.Institute of the Physics of Strength and Materials Science, Siberian Branch, Russian Academy of Sciences. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 26–30, January–February, 1994.     

Oxidation processes for alloys in the Ni - Zr system. III. oxidation of NiZr

We used the continuous weighing method to study the oxidation kinetics in air for the alloy NiZr at 500–1000°C. We used x-ray diffraction and metallography for layer-by-layer phase analysis of the scale. We have established that the oxidation kinetics is described by a parabolic equation q² = K_p (where q is the mass gain per unit area of the sample, K_p is the rate constant, is the time). The value of K_p periodically decreased on the kinetic isotherms. In the scale, the phase components are distributed over the layers as follows: top layer, cubic and monoclinic ZrO₂, NiO; inner layer, monoclinic ZrO₂, Ni, and (or) Ni₅Zr. At the boundary with the scale, the alloy layer (underscale) is depleted in zirconium. We have established that oxidation of NiZr is accomplished by predominant diffusion of oxygen through the oxygen vacancies in the lattice of monoclinic ZrO₂. The decrease in q and K_p as the temperature rises from 600°C to 850°C is explained by a reduced concentration of these vacancies and (or) slowdown of their mobility. For T850°C, the oxidation mechanism changes: counterdiffusion of Zr⁴⁺ also occurs through interstices in the lattice of monoclinic ZrO₂ . The outer layer (NiO), saturated by zirconium dioxide, loses any protective properties and diffusion of oxygen is facilitated. For this reason, both q and K_p increase as the temperature rises to 1000°C.     

High-temperature mobility of atoms of the components in the system molybdenum-tungsten

Summary The diffusion of both components in the system molybdenum-tungsten was investigated on alloys containing 0.1, 15, 20, 25, 35, 50, 65, 70, 80, 85, and 99.9 at.% tungsten. Experiments were carried out with the aid of Mo⁹⁹ and W¹⁸⁵ radioactive tracers in the temperature range 600–1000°C. The values of D₀ and E were calculated for all the alloys investigated.Translated from Poroshkovaya Metallurgiya, No. 11 (47), pp. 57–61, November, 1966.     

Reaction of Tungsten with Melts in the Cu - Co System

We have studied the solubility at 1200°C of tungsten in Cu - Co melts and the growth kinetics of a W₆Co₇ layer at the tungsten — melt interface. We have established the composition of the melt in the three-phase equilibrium tungsten — W₆Co₇ — melt: 0.0195 Co, 4.8 · 10⁻⁵ W, the rest is Cu (in atomic fractions). In the studied composition range for the melt, the solubility of tungsten is described well by the expression: lgX_W = −7.117 + 25.7 · X_Co ^1/2 − 41.06 · X_Co, where X_W and X_Co are the atomic fractions of the corresponding elements in the melt. We have determined the correlation between the growth rate for a layer of tungsten-containing phase at the tungsten — melt interface and the thermodynamic characteristics of the melt. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(443), pp. 81–86, May–June, 2005.     

Preparation of technical zirconium diboride by the carbothermic reduction of mixtures of zirconium and boron oxides

Conclusions A study was made of the preparation of technical zirconium diboride by the reduction of mixtures of zirconium and boron oxides with carbon under industrial conditions. It is shown that the optimum conditions for the preparation of zirconium diboride by the carbothermic method are established when a charge having the composition ZrO₂+1.2 B₂O₃+5 C (i.e., with a 20% excess of boric anhydride over the stoichiometric composition) is reduced at 2000°C in a hydrogen or converted gas atmosphere. The resultant ZrB₂ contains 18–19% B (compared with the theoretical boron content of 19.25%) and not more than 0.8% C.Translated from Poroshkovaya Metallurgiya, No. 11 (131), pp. 80–84, November, 1973.     

Dry friction and wear of alloys of diboride-nitride systems at normal temperatures

Conclusions A study was made of the room-temperature dry friction and wear of alloys of the systems ZrB₂-ZrN and HfB₂- HfN in air. It was found that all the materials investigated possess high wear resistance. The lowest values of coefficient of friction (0.3–0.4) and increased wear resistance at a sliding speed of 1 m/sec are exhibited by composites based on hafnium diboride and those containing 40–70 mole % zirconium nitride.Translated from Poroshkovaya Metallurgiya, No. 7 (103), pp. 63–67, July, 1971.     

Reaction of Tungsten with Liquid Co ― Sn Alloys

The solubility of tungsten in Co Sn melts and the growth kinetics of a W₆Co₇ phase layer at the tungsten melt interface were studied at 1200°C. The liquid alloys composition in the three-phase equilibrium W W₆Co₇ melt was established as (at. fraction) 0.51 Co, 0.49 Sn, 2.3·10^–3 W. The solubility of tungsten in the investigated range of melt compositions is well represented by the equation lgx _W = –0.964-3.420x _Sn, where x _W and x _Sn are atomic fractions of the elements in the melt. The calculated thermodynamic properties can be used for the analysis of other systems which include cobalt and tungsten.     

Electrochemical synthesis of tungsten and molybdenum borides in a dispersed condition

Electrolytic deposition of tungsten and molybdenum boride particles from ionic melts is studied. Conditions are found for preparing different boride phases. If the anode material is graphite and the voltage in the bath does not exceed 2.5 V the cathode deposit consists mainly of tungsten and molybdenum metals. A mixture of phases (M, M₂B, MB, MB₂, M₂B₅) is produced on the cathode with U=2.5–3.5 V, while with U=2.3–3.5 V, while with U=3.5–4.5 V the deposit consists of the higher boride MB₄. On the whole the process of electrochemical synthesis for molybdenum and tungsten borides is governed by the following interconnected parameters: electrolytic bath composition, voltage in the bath, temperature, and duration of electrolysis.Institute of General and Inorganic Chemistry, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, No. 1 (361), pp. 8–11, January, 1993.     

Structural and phase transformations with friction in a bronze-tungsten disulfide composite

The nature of secondary structures which form protective layers on the friction track of a (bronze - tungsten disulfide composite) — steel pair under vacuum friction conditions using a screen cooled with liquid nitrogen and the action of an additional current is studied. It is established that in all of the cases in question there is no reciprocal mass transfer of the materials in contact. A homogeneous protective layer covering the working friction surface under the action of an additional current is the interaction product of dispersed initial components of the bronze - tungsten disulfide composite and a layer of new chemical compounds of the Me₂C type with similar parameters to a high-temperature modification of the compound Cu₂S. It is assumed that formation of this compound is the result of intense pulse action of microvolumes of the material at points of contact of the body with the counterbody.Institute of Metal Physics, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, No. 4(364), pp. 30–36, April, 1993.