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.     

High-temperature x-ray equipment for the study of powder and dense objects

Conclusions The existing experimental models of both Debye cameras and camera attachments for x-ray diffraction apparatus enable the atomic and crystal structure of metals and alloys to be studied on heating to very high temperatures (occasionally of the order of 3000°C) in a vacuum of 10^–6 torr. The camera attachments currently produced by Soviet industry are suitable for operation at temperatures up to1500°C in a vacuum of about 10^–4 torr.It is now necessary to organize production in the Soviet Union of attachments with a temperature limit of 2000°C in a vacuum of 10^–6 to 10^–7 torr and high-temperature vacuum Debye cameras designed for temperatures up to 1700°C and a vacuum of 10^–5 to 10^–5 torr.Translated from Poroshkovaya Metallurgiya, No. 5(89), pp. 88–102, May, 1970.     

Properties and structure of niobium carbide heating elements

Conclusions In operation at 1900°C in a carbon-free atmosphere at a residual pressure of 10^–2-2· 10^–3 Pa a heating element of composition NbC + Nb₂C + Nb experiences a steady loss of carbon at a rate of 0.5–1 m/h from its surface, with the formation of a metallic phase. Heating elements of this composition may be recommended for service in vacuum resistance furnaces at temperatures of up to 1900°C, where they can be expected to have a useful life of the order of 1000 h. Niobium monocarbide heating elements operating at 2100–2500°C in a carbon-containing atmosphere under a pressure of 10^–2-2·10^–3 Pa experience no change in phase composition or shape and only minor changes in structure, and possess good thermal fatigue resistance. They can thus be employed in vacuum electric furnaces under these conditions. Long experience (more than two years) with the operation of a laboratory vacuum electric furnace with a niobium carbide heating element in the temperature range 1000–1500°C has demonstrated that the use of niobium carbide heating elements widens scope for new electro-thermal equipment.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 92–97, March, 1982.     

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.     

Magnetic powder lubricants and their use for increasing the effective life of dry gear transmissions

Conclusions The most effective method of prolonging the useful life of dry gear transmissions operating in a vacuum, carbon dioxide, or air is to use powdered lubricants and supply them by magnetic means to the rubbing zone. Use of magnetic powder lubricants enables toothed reduction gears to operate satisfactorily under Hertzian contact loads of up to 1100 MPa (11,000 kgf/cm²) at sliding speeds of up to 1.4 m/sec in the temperature range from –110 to +250°C.Translated from Poroshkovaya Metallurgiya, No. 10(214), pp. 61–65, October, 1980.     

Diffusion welding of heaters made of molybdenum disilicide

Conclusions It has been established that with diffusion welding in a vacuum and under optimal conditions, a physical interface is completely absent between the welded materials, i.e., the structure of the welding zone does not differ from the structure of the starting molybdenum disilicide. The porosity is 2.1%. The size of the pores varies from 2 to 70, and the zone of the welded bond of molybdenum disilicide is equal in strength to the base material.Tests under industrial conditions showed that the molybdenum disilicide heater, welded by diffusion welding in a vacuum, worked on a par with those imported, and lasted 3000 h at a temperature of 1650–1700°C.As a result of developing and introducing the new technology of producing molybdenum disilicide heating rods, the Moscow Hard Alloys Combine has obtained a great economic saving, of the order of 350,000 rubles in 1961–1962.     

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.     

Rheological and thermal activation analyses of the sintering kinetics of tungsten powders

Conclusions At any given sintering temperature the relative rate of densificaron of compacts of porosities 29–44% from reduced and atomized tungsten powders of particle sizes 1.5–5m is determined by their capillary pressure, the character of this dependence being close to linear. Densification during sintering is a threshold process, i.e., porous skeletons behave during sintering like Bingham solids. For each sintering temperature there exists a critical pore size above which the rate of densification sharply changes, approaching zero. Assessments of coefficients of Bingham viscosity have shown that at 1600 and 1900°C they are linear functions of sintering time. The energy of activation for the densification of the powders investigated during sintering in the range 1600–2000°C is equal to the energy of activation for grain-boundary self-diffusion in tungsten and amounts to 90 ± 5 kcal/mole.Translated from Poroshkovaya Metallurgiya, No. 10(262), pp. 27–32, October, 1984.     

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.     

Investigation into the sintering of metals

Conclusions The authors studied the dependence of the strength of compounds obtained by sintering samples of technically pure iron on the duration, temperature and pressure.Sintering was carried out in a vacuum 1.33–0.13 KN/m² over a temperature range of 525–1060°C, a pressure range of 1.47–607.6 MN/m² and a duration of 3 min to 10 h.It is shown that a rise in pressure, temperature and duration of sintering raises the strength of the compound, which depends on the actual area of contact and the nondiffusive process, leading to the formation of bonds between the metal surfaces.To attain durable bonds it is necessary to have not only a close contact between the surface layers but also a nondiffusive local rearrangement of the crystal lattice on the contact surface. The activation energy of the displacement of atoms during rearrangement comprises 35.1 kJ/mole which is considerably less than the activation energy of the diffusion of iron.     

Some mechanical properties of silicon carbide fibers

Conclusions Electroerosion of silicon carbide fibers in contaminated mercury electrical contacts decreases their strength by between one-half and two-thirds. During prolonged holding under load at room temperature the creep of silicon carbide fibers is close to zero, while at 1173°K l/l 10^–7 l/day. X-ray structural analysis of silicon carbide fibers annealed for (1.8–10.8)·10³ sec in a hydrogen atmosphere at 1273–1773°K and for (3.6–18)·10³ sec in a nitrogen atmosphere at 1773–1973°K revealed no recrystallization in (4–5)·10^–7-m-thick layers of polycrystalline SiC deposited from a gaseous phase. A correlation was found between the strength of silicon carbide fibers and the number of flashes forming during their fracture.Translated from Poroghkovaya Metallurgiya, No. 1(253), pp. 55–59, January, 1984.     

Reaction between carbon fibers and nickel through a zirconium nitride or titanium carbide diffusion barrier

Conclusions Carbon fibers in contact with nickel suffer a pronounced loss of strength at temperatures above 600°C as a result of surface attack by the nickel (at temperatures of 800–900°C and treatment times of 10–20 h) or recrystallization (during heating for more than l h at 1000°C). There is no unique correlation between fiber structure and strength. A zirconium nitride coating fails to prevent carbon fibers from reacting with nickel. A titanium carbide coating reduces, even at high temperatures, the strength loss experienced by carbon fibers by decreasing the rate of dissolution of the material of the fibers in nickel and by slowing down the fiber recrystallization process.Translated from Poroshkovaya Metallurgiya, No. 7(175), pp. 32–35, July, 1977.     

Hot pressing and vacuum sintering of fine titanium nitride powder

Conclusions Increasing the specific surface of titanium nitride from 18 to 90 m²/g lowers the initial recrystallization temperature of loosely poured powder from 1300 to 600°K. The temperature at which blanks attain practically 100% density in the hot pressing of finely divided titanium nitride (a starting powder particle size of 0.05–0.07 m) is 1600°K, which is 500–700°K below the temperature level of full sintering of relatively coarsegrained powders (a particle size of about 0.5 m). At hot-pressing temperatures above 1800°K a fall in the density of sintered compacts is observed, which is apparently attributable to the beginning of nitrogen evolution from the nitride and also to the formation of microcracks. In vacuum sintering without a plasticizer, crack formation lowering the density of specimens by 3–4% is characteristic of the whole sintering temperature range. The grain size in hot pressing and vacuum sintering is practically the same, being determined chiefly by the sintering temperature and time. At the maximum specimen density the maximum grain size is 20 m.Translated from Poroshkovaya Metallurgiya, No. 12(204), pp. 27–32, December, 1979.The authors wish to thank V. I. Berestenko, T. N. Miller, and D. I. Medvedev for the provision of titanium nitride specimens.     

Investigation on pumping oxygen characteristics of （Bi2O3）0.73（Y2O3）0.27 solid electrolyte

（Bi2O3）0.73（Y2O3）0.27 fine powders prepared by wet chemical precipitation method were cold isostatically pressed to form solid electrolyte tubes, and sintered at 900 ℃ for 10 h in the air. Their pumping oxygen characteristics in non-dehydrated Ar gas were investigated, where a ZrO2 （Y2O3 stabilized） oxygen sensor was used to measure the oxygen partial pressure Po2. The results showed that the Po2 value reached magnitudes of 1×10^-2-1×10^-10 Pa at the applied pumping oxygen voltage of 0.5 V, 1×10^-37-1×10^-27 Pa at 1.0 V and 1×10^-53-1×10^47 Pa at 2.0 V within the temperature range from 550 to 650 ℃. Moreover, no cracks were found in the tested solid electrolyte tubes. Thus, the Bi2O3-Y2O3 system might be used in solid electrolyte oxygen pump for purifying gases.     

Solubility of molybdenum in copper-nickel melts

Conclusions Determinations were made of the solubility of molybdenum in copper-nickel melts at temperatues of 1200 and 1300°C. The solubility is described approximately by the expressions log N_MO = –5.858 + 7.613 N _Ni ^' (at 1200°C and 0.09Ni ^' <0.21) and log N_Mo = –5.536 + 7.242 N _Ni ^' (at 1300°C and 0.09Ni ^' <0.32). In a copper-nickel molybdenum melt with a copper-tonickel ratio smaller than (or equal to) 0.47 molybdenum is in equilibrium with the melt at temperatures from 1200 to 1300°C.Translated from Poroshkovaya Metallurgiya, No. 4(208), pp. 95–98, April, 1980.     

Chromium ― Copper System: Adhesion Characteristics,Doping, the Structure of Phase Boundary and Composites

The adhesive characteristics and the formation of phase boundaries in the Cr Cu system in a vacuum of (2-4)·10^–3 Pa in the temperature range 1100-1300°C have been studied. The microstructure of composites (50:50 by mass) made by copper impregnation of porous samples of Cr electrolytic powders and Cr obtained by calcium hydride reduction. The effect of Cr, Fe, Co, Ni, Si additions on above characteristics has been investigated.     

The effect of helium on the stress-rupture behavior of type 316 stainless steel

Flat tensile samples of type 316 stainless steels with helium contents of 1.5 × 10⁻⁶ and 4.0 × 10⁻⁵ atom fraction, were stress-rupture tested in vacuum at 700°C. The presence of helium caused large reductions in rupture life and in elongation at failure. The amount of strain produced within the grains of helium-containing samples was a small fraction of the measured total elongation. The difference in these values is accounted for by extensive intergranular cracking.     

Theory and technology of sintering processes,thermal and thermochemical treatment reaction kinetics for interaction of tungsten with copper-zirconium melts

We investigated the growth kinetics of layers of W₂Zr phase on tungsten interacting with copper-silicon melts at 1150°C. We determined the limiting value of the zirconium content in the copper melt (40.1 at. %). Below this value, we do not observe the indicated layer. We discuss the mechanism of reactive diffusion in the system and the effect of zirconium additives on the liquid phase sintering of the copper-tungsten materials. We have discovered well-defined differences between the growth rates of the layers on polycrystalline and single-crystal specimens of tungsten.Institute of Problems in Materials Science, National Academy of Sciences of Ukraine. Kiev. Translated from Poroshkovaya Metallurgiya. Nos. 3–4(384). pp. 25–29. March–April 1996. Original article submitted October 25, 1994.     

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.