Sintering kinetics of tantalum carbide

Conclusions Tantalum carbide sinters at a temperature above 2500C. Decreasing the powder particle size activates the sintering process, but even with a powder of 0.17-m particle size specimens sintered at 2700C have a porosity of 11%. Coarse powders (> 7–8 m) sinter, without densification, at 2000–2200C by a surface self-diffusion mechanism. Fine powders (<7–8 m) undergo densification already at temperatures above 1400C by a diffusion-viscous flow and a volume self-diffusion mechanism during long holding periods and also probably by an activated grain-boundary sliding mechanism in the initial stage of sintering after rapid heating.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 16–19, October, 1982.     

Physicochemical processes occurring in nitride and oxide-nitride composites with phosphate binders during heating

Conclusions The hardening of compositions based on aluminum and silicon nitrides is a result of the chemical reaction of their phosphate binder with the nitrides, while additions of, e.g.,-corundum act as inert fillers up to a temperature of 570K. The reaction products are amorphous acid mono- and disubstituted aluminum and silicon phosphates, which on heating at first condense, forming poly- and metaphosphates, and then, at temperatures above 870K, decompose, forming (depending on their composition) aluminum orthophosphate or silicon pyrophosphate.Aluminum nitride initially, after being combined with the phosphate binder, vigorously reacts with it, which may lead to clotting or rapid swelling of the composition. However, gradual mixing under conditions of cooling (or without it) enables masses of any consistency to be obtained.In the mechanism of hardening of the compositions investigated a dominant role is played by the hydrogen bond. With rise in temperature the OH groups evaporate and the role of the hydrogen bonds diminishes, but a new, more powerful mechanism becomes operative, linked with the polymerization of the phosphates. Above 970K the polymer structure, which attains its maximum development at 870–970K, gradually disintegrates, but at the same time a new process-sintering-commences. Accordingly, as has been demonstrated by our investigation into the temperature dependence of the strength of the compositions, all the materials investigated pass through stages of hardening (470K), strengthening (470–970K), partial strength loss (by 30–40% in the range 970–1270K), and high-temperature sintering.The end products of the thermal decomposition of the polyphosphates up to 1520K are (depending on the nature of the nitride filler) aluminum orthophosphate or silicon pyrophosphate, present in amounts of 5–10%.At 1270–1370K a liquid phase (aluminum metaphosphate or silicon polyphosphate) forms in the compositions and then gradually disappears as a result of its further reaction with the fillers. Under these conditions an additional amount of AlPO₄ or SiP₂O₇ is formed.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 50–54, May, 1982.     

Reaction between zirconium carbide and silicon powders

Conclusions A study was made of the reaction of zirconium carbide and silicon powders over the temperature range 800–1700C.Translated from Poroshkovaya Metallurgiya, No. 11(83), pp. 61–65, November, 1969.     

Influence of grain size and porosity on the high-temperature friction of titanium carbide

Conclusions It was established that in friction of a TiC-TiC pair in vacuum the coefficient of friction at 250C and the wear rate at 1250C are practically independent of grain size. At higher temperatures these characteristics have an inverse relationship to grain size.It was shown that with an increase in porosity both the wear rate and the coefficient of friction increase. With an increase in temperature the influence of porosity on the wear rate decreases.With variations in porosity in the 1–10% range, in grain size in the 1–50 m range, and in temperature in the 20–1500C range the wear rate changes within limits of 10–45% and the coefficient of friction within limits of 3–35%.Translated from Poroshkovaya Metallurgiya, No. 9(297), pp. 56–61, September, 1987.     

Effect of sintering conditions on the structure and mechanical properties of P/M aluminum base alloys

Conclusions A magnesium addition promotes densification of Al-Cu alloy specimens sintered for short periods of time in the range 595–635C. Longer sintering at 615 and 635C results in higher specimen porosity. At a temperature above 595C Al-Cu alloys experience severe coarsening. Alloying with magnesium does not significantly affect the structure of the alloys. A magnesium addition improves the mechanical properties of an Al-Cu alloy. The extent to which magnesium alters the mechanical characteristics of heat-treated Al-Cu alloys depends on sintering conditions. The highest strength — 340 MPa and =6% — is exhibited by specimens sintered for 45 min at 595C.Deceased.Translated from Poroshkovaya Metallurgiya, No. 9(297), pp. 29–34, September, 1987.     

Effect of charge fractional composition on high-speed steel powder sintering

The fractional composition of gas-atomized high-speed steel R6M5K5 is studied for its effect on the main features of activated sintering of compacts. A mixture consisting of nickel powder and 30% (wt.) boron is used as an activating addition. It is established that a reduction in average particle size increases shrinkage after sintering, but the dependence of compact shrinkage on fractional composition is different for monodispersed and polydispersed compositions. The maximum size of powder particles for polydispersed compositions has little effect on shrinkage, whereas in the case of monodispersed mixtures there is a marked increase in compact density after sintering with a reduction in particle size. There is a marked increase in density providing closed porosity (90–93%) with a sintering temperature of 1200C, whereas in order to achieve a similar density in the mixture without activator the sintering temperature should be at least 1230C.Translated from Poroshkovaya Metallurgiya, No. 10, pp. 15–20, October, 1992.     

On the strength of cermet alloys

Summary The results of mechanical tests obtained on alloys of TiC-Co, TiC-Ni, TiC-Mo, close by their nature to the alloy WC-Co at temperatures of 20 and 1200C, confirm G. S. Kreimer's view that refractory compoundsmetal alloys are fragile within wide ranges of change in bond and temperature as opposed to V. A. Ivensen's assertion (Powder Metallurgy No. 5, 1961).It is shown that skeletal structure of material of the SiC-C type secures greater strength than the structure of the scattered type.     

Diffusion processes in the sintering of nickel alloy powders with spherical particles under pressure

Conclusions Hot gasostatic pressing and subsequent annealing at 1200C result in the formation of 160–200-m diffusion zones between particles, which extend over practically the whole particle widths. During sintering under conditions of hot hydrostatic pressing, which is characterized by a shorter period of annealing at any given temperature and pressure, the widths of the diffusion zones after sintering and annealing attain 50–65m. The optimum diffusion zone width in the sintering of particles making good contact with one another (pure and even surfaces) is about 30–40m. However, the presence of oxides, carbide phases, and more complex compounds on the surfaces of particles substantially increases the optimum width of this zone. In such a case the optimum width is determined by the need for diffusion to cross interparticle contacts with diffusion-inhibiting barriers. In view of this, for particles of the nickel alloys investigated the optimum diffusion zone width may be taken to be 100m. At this width there are no areas in which the diffusion zone width is less than 40m, i.e., the minimum thickness at which strong bonds can be expected to form between particles.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 38–41, May, 1982.     

Kinetics of hydrochlorination of oxides during the production of alloyed powders by diffusional impregnation from point sources

Conclusions The optimum hydrochlorination temperature of chromium and manganese oxides in mixtures designed to yield powders containing up to 12% chromium and up to 6% manganese is 1150C. Diffusional impregnation with the use of oxides is not recommended as a method of producing powders of higher alloying element content because the degree of hydrochlorination of the oxides in these circumstances is not high.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 1–5, October, 1982.     

Method and apparatus for the rapid determination of the thermal diffusivity of refractory materials over a wide temperature range

Summary Using the proposed method, it is possible to determine the coefficient of thermal diffusivity and conductivity over a wide temperature range.In estimating the errors, it is necessary take into account the fact that owing to differences in structure and to inconstancy of composition, it is practically impossible to prepare two absolutely identical specimens. In a large number of experiments, however, it is always possible to obtain a sufficiently reliable result.In the furnace of the construction we have proposed, specimens may be heated to a temperature of the order of 1600C. If the experimental conditions require heating the specimen to a temperature not exceeding 1000C, an ordinary laboratory tubular electric furnace may be used.     

Vacuum-dynamic disintegration of melts. III. Experimental examination of the operation of a vacuum-dynamic atomization unit

Conclusions A vacuum-dynamic unit operates in a stable manner in the atomization of metals with melting points ranging from 230 to 1200C. The key control parameters of the device are the blast pressure and the gap between the nozzles. To obtain finer powders, the former should be increased and the latter decreased.Translated from Poroshkovaya Metallurgiya, No. 10(274), pp. 63–65, October, 1985.     

Surface reinforcement of iron-base materials

Conclusions A study was made of the application of a wear-resistant TiC-Kh12M steel layer to type 45 steel by methods consisting in simultaneous pressing and sintering and in the sinterbonding of a pressed or sintered layer to a sintered base. It has been established that the alloying of the basis material with 1% of Cu substantially increases its density and adhesion to the coating. It is shown that a layer can be applied to a sintered base with or without a substrate. It has been found that the highest bond strength, (10–14.3)·10⁷ Pa, is attained when a layer and a base containing 1% of Cu are simultaneously pressed under a pressure of 3.10⁸ Pa and then simultaneously sintered at 1673K.Translated from Poroshkovaya Metallurgiya, No. 10(238), pp. 33–38, October, 1982.     

Effects of silicon powder structure of nitriding

A study has been made on how the structural state of the silicon affects the nitriding at 1200–1400C. As the initial powder becomes more defective, the nitriding accelerates, and there is an increase in the importance of reactions in the gas phase, with an increase in the proportion of the a modification of silicon nitride, and a tendency for elongated crystals to form.Translated from Poroshkovaya Metallurgiya, No. 10, pp. 1–7, October, 1992.     

Phase diagram of the system chromium-germanium

Conclusions Using the methods of metallographic, x-ray diffraction, and differential thermal analyses, the authors investigated chromium-germanium alloys in the whole range of concentration. The existence of five intermetallic phases -Cr₁₁Ge₁₉, CrGe, Cr₁₁Ge₈, Cr₅Ge₃, and Cr₃Ge- is demonstrated; the first four of these compounds are formed by peritectic reactions at temperatures of 955, 1025, 1160, and 1250C, respectively. The lowest germanide, Cr₃Ge, melts congruently, the highest melting temperature being shown by the alloy with 22 at%Ge.Translated from Poroshkovaya Metallurgiya, No. 8(44), pp. 55–60, August, 1966.     

Addition of vanadium to silicide master alloys for plasma-deposited coatings

Conclusions An investigation was carried out into the effect of vanadium on the structure and phase composition of some alloys of the Si-Ti-Cr system. The microstructure of resultant alloys is the same as that of similar vanadium-free materials. Vanadium is concentrated in the refractory phase, entering into the composition of a complex (Cr, Ti, V)Si₂ disilicide. The remaining structural components are, as in the vanadium-free ternary alloys, free silicon and a eutectic. A study was made also of the effect of vanadium on the oxidation kinetics of alloys of the Si-Ti-Cr system exposed for 100 h to air at 1300C. Alloying with vanadium substantially slows down the rate of oxidation of these alloys and changes the law of oxidation from parabolic, characterizing the ternary alloys, to logarithmic for the vanadium-containing alloys. The 100-h rate of oxidation of an alloy of optimum composition at 1300C is 0.04 mg/cm² · h. The optimum particle size of Si-Ti-Cr-V alloy powders intended for the application of coatings by gasothermic spray-deposition has been established.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 46–50, May, 1982.     

Development of tungsten-carbide-base corrosion-resistant composite materials with a binder of PKh8N15 stainless steel

Conclusions The bend strength of VNS20 and VNS32 composites is determined by the quantity of ₁phase and depends little upon the content of binder (PKhl8N15 steel). The hardness, on the other hand, depends substantially upon both the ₁-phase content and upon the quantity of binder.The VNS20 and VNS32 composites sintered at the temperatures of the start of formation of the liquid phase in the tungsten carbide—PKhl8N15 steel system, when compaction is completed and the rate of formation of ₁-phase is still insignificant, have the optimum properties.With a satisfactory level of physicomechanical properties, the VNS20 and VNS32 composites show high corrosion resistance in a medium of nitrine with additions of nitric acid.An increase in the strength of VNS type composites may be obtained by reducing the diffusion reaction of tungsten carbide with stainless steel.Translated from Poroshkovaya Metallurgiya, No. 5(281), pp. 69–75, May, 1986.     

Structuring of Multiphase Compacts in the SiC′ ― Carbon System. Part 2. Structuring during Sintering

We have studied the effect of particle sizes in the starting powders in the silicon carbide carbon system, and also effects on the structure and phase composition of self-bonded silicon carbide (SBSC) that are associated with structural interactions between the starting components.     

Effect of milling in various media on the composition and some properties of niobium carbide

Conclusions Comminution of carbides in various media which do not prevent the well-developed surfaces of their particles from coming into contact with water and oxygen leads to surface oxidation of these particles. This is reflected in the chemical composition of carbides, which almost continually changes during sintering in the usual temperature range, right up to 2200C, and in the densification kinetics and properties of carbide-base materials. Because of the instability of oxycarbides of the Group V metals in a vacuum at these temperatures, with the carbides Me^vC (NbC and TaC) the surface oxidation experienced by their particles during milling in certain dispersing media may have an activating effect on the sintering process and shift the composition of the carbides toward the lower limits of their homogeneity ranges.Translated from Poroshkovaya Metallurgiya, No. 2(170), pp. 23–28, February, 1977.     

Phase formation processes and densification kinetics of the β-Si3N4 based sialon charges during sintering

Conclusions We studied the densification and phase formation processes during reactive sintering of the -sialon charges based on the -modification of silicon nitride with an insignificant weight content of aluminum. Densification of the charge occurs according to the regularities of the liquid phase sintering process during which the liquid phase disappears. The kinetics of densification and phase formation and the degree of completion of these processes depend on the composition of the original charge and the degree of dispersion of the powders of the additives. Under the present sintering conditions, the process of -sialon formation from -Si₃N₄ occurs according to the mechanism of dissolution and redeposition through a liquid phase with a significant contribution from the processes of mass transport through a gaseous phase.Translated from Poroshkovaya Metallurgiya, No. 8(296), pp. 56–62, August, 1987.     

Electric Explosion Comminution of Silicon Carbide in an Elastic Shell

Comminution of silicon carbide powder in an elastic shell by electric explosion is less efficient than powder comminution in an elastic shell by mechanical impact because of its greater energy consumption. The level of silicon carbide powder comminution is the same by both methods and it is 40 m with an original size of _300+160 m. Comparative analysis showed that for more efficient powder comminution in elastic shells by pulsed methods it is desirable to match the duration of applied loads with frequency characteristics of the shell.