“Activity” of powders in the sintering of single-component compacts

Summary A study has been made of the shrinkage characteristics of compacts obtained by pressing: 1) active powders of electrolytic origin, 2) powders thoroughly deactivated by high-temperature annealing, and 3) a mixture of equal amounts of the above two powders. It is shown that the observed shrinkage characteristics can be qualitatively explained by assuming that, in addition to the usual mechanism of deactivation operating during annealing, there is also a mechanism of activity loss resulting from defect migration from active to deactivated particles.Translated from Poroshkovaya Metallurgiya, No. 4 (64), pp. 18–21, April, 1968.     

The phase equilibria in W-Fe-Co-Ni system alloys. II. Isotherms of the solidus and the phase composition of alloys containing 10 and 20% (Fe + Co + Ni) at temperatures above 1200°C

Conclusions The isostructural intermediate -phases Fe₇W₅ and Co₇W₆ in the W—Fe—Co system form a continuous series of -solid solutions. In the 1640–1630°C range the L + (Fe₇W₆) peritectic equilibrium in this system changes to a similar L + (Co₇W₆) equilibrium, where is the tungsten-base boundary solution.In the W-Fe-Co-Ni polythermal tetrahedron in the 1470–1460°C range conversion of the L +(Fe₇W₆)+, peritectic equilibrium into the similar L + (Co₇W₆) + , where is the nickel-, -ironcobalt-base boundary solution, occurs.Upon completion of crystallization at 1400°C, the W-Fe-Co system alloys with 10–20% (Fe + Co) have a + phase composition, while the W-Fe-Co-Ni system alloys with 10–20% (Fe + Co + Ni) accordingly have + , + + or + . At temperatures below 1215°C in alloys rich in iron, FeW may be formed instead of -phase and therefore the alloys may have an + FeW, + + FeW, + + + FeW and + + FeW phase composition.Translated from Poroshkovaya Metallurgiya, No. 5(281), pp. 86–89, May, 1986.     

Electric-spark alloying of steels with WC-Co hard metals

Conclusions A study was made of the electroerosion and cathode weight increase accompanying the ESA of steels with WC-Co hard metals and of some properties of resultant alloyed layers. In ESA the best results are obtained with VK20 alloy under soft conditions and with VK15 alloy under hard condition. The optimum alloying time is 5–10 min/cm² in process No. 2 and 3–5 min/cm² in process No. 5. Under these conditions ESA by the soft process (with VK20 alloy) ensures the formation of a hard (2200 kgf/mm²) white layer of the same thickness (40 m).Translated from Poroshkovaya Metallurgiya, No. 8(212), pp. 67–71, August, 1980.     

Polythermal Ti-∑MoNi (9∶1) section of the ternary system Ti-Mo-Ni

Conclusions As a result of an investigation of the titanium corner of the system Ti-Mo-Ni, a partial phase diagram of the Ti + (0–55%) MoNi (91) section was constructed. The solubility of Mo and Ni in-Ti is 0.65% MoNi. Thea + phase field is bounded by MoNi contents of 0.65 and 17% at 600°C, 0.5 and 14% at 700°C, and 0.4 and 6% at 800°C. At MoNi contents of 55% and higher, there is a + Ti₂Ni field, the existence of which was confirmed by x-ray structural studies.Translated from Poroshkovaya Metallurgiya, No. 2 (110), pp. 33–37, February, 1972.     

Comparison of diffusion calculations for two models of powder mixtures

Summary The advantages and drawbacks are discussed of two models of powder mixtures: The model based on single-particle approximation and the concentric sphere model. Calculated values obtained by using both models are compared with experimental data.Translated from Poroshkovaya Metallurgiya, No. 1, pp. 23–24, January, 1968.     

Phase relations in the Ti-TiNi-NbNi-Nb region of the ternary system Ti-Nb-Ni

We have used microstructural, differential thermal, x-ray phase, and electron probe analysis to study alloys in the Ti-TiNi-NbNi-Nb region of the ternary Ti-Nb-Ni system, both as-cast and annealed at 900°C. We have located a pseudobinary TiNi- section, where is a Nb-rich (Nb, Ti)-based solidsolution. The pseudobinary eutectic point parameters are 1170°C and 38 Ni 26 Nb (at. %). We have found two invariant peritectic four-phase equilibria for crystallization of the alloys in the region of interest: L + TiNi + Ti₂Ni (950 °C) and L + TiNi + NbNi (1140°C). We have not confirmed the existence of a ternary compound Ti₃(Ni,Nb)₂.Institute of Materials Science Problems, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3/4, pp. 48–54, March–April, 1995.     

Buckingham-potential parameters and relations between them for solids

Parameters m and n in the Buckingham potential in the form =[U₀mn/(m – n)]{(1/m)Y^–m – (1/n)exp[n(1–Y)]} have been calculated for 79 elements and 35 compounds, which have been compared with data on =c_v/(U₀) and =KV₀/c_v, in which U₀ is the cohesion energy, Y=(V/V₀)_1/3, V a volume with equilibrium value V₀, ... c_v the specific heat, the thermal-expansion coefficient, and K is the bulk elastic modulus. It is found that m is dependent essentially on and n on . These relationships have been approximated, and conclusions are drawn from them about the most reliable input data, particularly K because of the large spread in published values. A study is made on the relation of m and n to the positions of the elements in the periodic system and the electron structures.Institute of Problems of Materials Sciences, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 5–6, pp. 118–126, May–June, 1994.     

Stability of liquid metallic interlayers during the sintering of heterophase composites. I. Two-phase systems

Conclusions For composite materials of the first type (_ss > 2_s), there exists a critical liquid interlayer thickness above which they do not form or are unstable. At a channel thickness smaller than the critical, liquid interlayers at first form spontaneously by migration of molten metal from within the part into the channel, and subsequently they become filled by growing refractory particles. In composite materials of the second type (_ss 2_s), liquid metallic interlayers of all sizes are stable.Translated from Poroshkovaya Metallurgiya, No. 1(337), pp. 11–17, January, 1991.     

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.     

Densification of metal powders during heating under high pressures of up to 80 kbar

Conclusions An experimental study was made of the densification of metal powders during heating under pressures of up to 80 kbar. The character and kinetics of densification of powders during heating were found to differ markedly depending on whether the pressure applied to them was higher or lower than the critical pressure. It is demonstrated that the minimum necessary condition for attaining the density of the nonporous metal in the densification of a powder under the critical and higher pressures is the development of thermally activated dislocation motion processes in the range (0.3–0.4)T_m. The densification of powders at pressures below the critical is controlled by particle flow processes, its kinetics being strongly dependent on the applied pressure and temperature.Translated from Poroshkovaya Metallurgiya, No. 11(191), pp. 28–33, November, 1978.     

Formation of detonation coatings applied on an iron base

Conclusions The results of x-ray diffraction analysis indicate a complex multiphase composition of the areas adjoining the zone of contact of the coating with the base related to occurrence of the and -structural transformations. Mutual mixing of the materials of the coating and base in the liquid phase leads to alloying and stabilization of the high temperature modification of iron (-Fe) and cobalt (-Co).The structural transformation ( -Fe) has a significant influence on the processes of formation of detonation coatings of powders of pure metals (Co-Ni) applied on iron and leads to a reduction in the level of mechanical properties of the joint.In the coating itself significant mixing of the sprayed metals occurs including different mechanisms of mass transfer such as mass transfer in the liquid and solid phases [1] with a depth of penetration of more than 10 m.On the boundary of the joint of the coating with the iron base relatively weak diffusion interaction of the contacting metals is observed. The width of this zone (5–10 m) is significantly less than that in spraying of a Co-Ni coating on an aluminum base [1].Translated from Poroshkovaya Metallurgiya, No. 10(298), pp. 60–65, October, 1987.     

Automatic airtight sealing of a product in a container during hydrostatic compaction

A hydrostat based on the dry bag principle is proposed for hydrostatic compaction of powder metallurgy products. The hydrostat design is a combination of a direct action device with the press mold mounted in the container channel, where a bare briquette is loaded into the mold. The device is made airtight automatically as soon as the face end of the moving plunger touches the mold flanging. The hydrostat can eliminate manual labor and improve efficiency.Physicotechnical Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 7–10, May–June, 1996. Original article submitted June 27, 1994.     

Creep of highly-porous iron compacts under conditions of cyclic α ↔ γ-transformations

Features of the creep of highly-porous iron compacts with cyclic -transformation are studied. Possible reasons are explored for acceleration of deformation during -transformation and sintering of compacts with -transformation.Kharkov University. Translated from Poroshkovaya Metallurgiya, Nos. 3–4, pp. 19–23, March–April, 1994.     

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.     

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.     

Metal pressure in cold rolling in the elastic aftereffect zone of porous strip

Conclusions The metal pressure at the end of the forward slip zone and the beginning of the elastic aftereffect zone of a porous strip being rolled is given by the expression p=_t. This pressure differs from that normally employed in theoretical calculations by the coefficient , which is greater than unity and grows appreciably with increasing strip density and Poisson's ratio of the corresponding nonporous material.Translated from Poroshkovaya Metallurgiya, No. 3(195), pp. 7–10, March, 1979.     

Development of a mathematical model of centrifugal dispersion of a melt

A model is developed for a new process of centrifugal melt atomization from a consumable ingot surface under the action of a condensed heat source.AO Fiko, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5/6, pp. 16–21, May–June, 1995.     

Special features of the ductile-brittle transition in iron-based powder materials. II. Physicomechanical model of the ductile-brittle transition in cracking resistance tests

Conclusions The presence of pores in the material leads to the redistribution of stresses in the material and to localization of strains in small volumes between the pores. The susceptibility to ductile failure increases with increasing porosity.At –196°C, the failure mechanism of the porous materials based on iron changes from ductile to brittle (cleavage) with decreasing porosity. The dependence of cracking resistance on the porosity of these materials is nonmonotonic and the maximum cracking resistance is recorded at porosity values at which the failure mechanism changes. The nonmonotonic nature of cracking resistance reflects the high sensitivity of this characteristic to the change of the stress state in the material. The change of the stress state in the ductile-brittle transition temperature range can be described by the condition _f = _T. In the porous materials examined this condition is valid because of the specific form of the _f- and _T- dependences. The model of the ductile-brittle transition proposed in this work can be used to determine the analogy between the cold brittleness temperature T_br and the critical value of porosity _c corresponding to the point of intersection of the _f- and _T- curves for the porous materials.Translated from Poroshkovaya Metallurgiya, No. 3(303), pp. 39–42, March, 1988.     

Kinetics and mechanism of oxidation of β-sialons

Conclusions We examined the behavior of two materials during high-temperature oxidation: the singlephase -sialon and the material based on -sialon containing up to 8% residual -Si₃N₄ and the glass phase produced by reaction sintering of the charge based on -modification of silicon nitride. The results show that the maximum oxidation resistance in the temperature range 800–1300°C is exhibited by the single-phase -sialon Si_5.5Al_0.5O_0.5N_7.5. The oxidation resistance decreases in the presence of the residual -Si₃N₄ and the glass phase in the material. The oxidation kinetics are also determined by the structure and composition of the oxide film. The kinetic curves are governed by a parabolic law which indicates that oxidation of the sialons is limited by the diffusion processes. No marked changes were detected in the lattice spacing of the sialon after oxidation.Translated from Poroshkovaya Metallurgiya, No. 6(306), pp. 69–73, June, 1988.     

Phase equilibria in W-Fe-Co-Ni system alloys. I. Alloys containing 10% (Fe + Co + Ni) at 1400–1200°C

Conclusions The isostructural intermediate -phases of Fe₇W₆ and Co₇W₆ in the W-Fe-Co system form a continuous series of solid solutions and transformation of the L + peritectic equilibrium into, the similar L + (Co₇W₆), occurring in a narrow temperature range (1640–1630°C) is observed. In the W-Fe-Co-Ni system in the 1470–1460°C range transition of the L + (Fe₇W₆)+ peritectic equilibrium into the similar. L + (Co₇W₆+ is also observed.Upon completion of crystallization and at temperatures of 1400–1200°C alloys of the primary section with 10% (Fe + Co + Ni) have a two-( + or +) or three-phase (+ +) structure. In alloys rich in iron at temperatures below 1215°C FeW may form instead of (Fe₇W₆) phase.Translated from Poroshkovaya Metallurgiya, No. 4(280), pp. 60–64, April, 1986.