Effect of liquid phase on W-Ni-Sn and W-Co-Sn pseudoalloys in liquid-phase sintering

The effect of the liquid phase of different compositions on the compaction of W-Ni-Sn and W-Co-Sn pseudoalloys in liquid-phase sintering is studied. It is established that the compaction of W-Ni-Sn pseudoalloys enhances with increasing liquid-to-solid phase volume ratio. Changes in volume of W-Co-Sn pseudoalloys with a varying amount of the liquid phase in sintering depend on the melt composition: the compaction increases for 20 wt.% Co and 80 wt.% Sn and the samples grow regardless of the liquid phase amount for 50 wt.% Co and 50 wt.% Sn. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 3–4 (454), pp. 3–10, 2007.     

Diffusion-based model for isothermal solidification kinetics during transient liquid-phase sintering

Results from a diffusion-based model for transient liquid-phase sintering (TLPS) were used to predict the influence that the solute diffusivity (D), base-metal particle size (a), base-metal grain size (d), alloy partition coefficient (k), and the extent of solute saturation (X _max/X _o) had on the rate of isothermal solidification (where both X _max/X _o and k determine the initial liquid weight fraction in the system W _{A o}). The solidification rate increases with an increase in D and a decrease in a, d, X _o, and W _{A o}, but decreases with an increase in k. Model predictions are close to, but slightly underestimate, results for the solidification rate measured in a Pb-Sn TLPS system.     

Effect of phase and structural transformations occurring during liquid-phase sintering on the magnetic properties of SmCo5 magnets

Conclusions A study of phase and structural transformations occurring in magnets during liquid-phase sintering has shown that the following factors have a beneficial effect on magnetic properties: single-phase structure after sintering, consisting of the compound SmCo₅ alone; presence of surface segregation zones in the grains of magnets; presence of epitaxial regions preventing magnets from growing and their texture perfection from diminishing; isolation of pores and a high relative density of magnets, which protect the latter's grains against oxidation.Translated from Poroshkovaya Metallurgiya, No. 5(185), pp. 81–87, May, 1978.     

Densification kinetics of Mo-Ni-Sn composites in liquid-phase sintering

The paper examines the densification kinetics in liquid-phase sintering of Mo-Ni-Sn samples containing 90 wt.% of molybdenum and 10 wt.% of liquid phase in which the content of nickel varies from 5 to 67.5 wt.%. The samples continuously densify with increasing nickel content. It is examined how the amount of the 28 wt.% Ni-72 wt.% Sn liquid phase influences the densification kinetics of samples with varying content of molybdenum. The densification parameter increases with the amount of the liquid phase.     

Consolidation processes in the sintering of fine-grained iron-copper pseudo-alloys

The properties of fine-grained iron-copper pseudo-alloys (ICPA) and consolidation processes that occur during their sintering are studied. The grains are no larger than 0.5 μm. It was established that the specimens undergo shrinkage, not growth, when fine iron-copper mixtures are sintered within the range 600–1130 °C. This occurs as a result of active consolidation of the dispersed powder mixture, shortening of the diffusion paths, and the active formation of solid solutions based on iron and copper. Sintered ICPAs have a stable fine-grained microstructure with a maximum grain size of 0.5 μm only when they are sintered and treated in the solid phase. A coarse-grained structure is formed when ICPAs are heated to temperatures at which a liquid phase appears. Fine-grained ICPAs are also characterized by high hardness (up to 240–260 HB). Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 22–29, January–February, 2000.     

Effect of porosity on the volume changes experienced by Al-Cu compacts during liquid-phase sintering

Conclusions In the Al-Cu system a linear relationship between the end and starting porosities exists only at porosities exceeding 20%. At a smaller pore content the linearity is disturbed by the presence of an irreducible oxide phase on the aluminum particles. Decreasing the starting compact porosity results in greater growth in the first stage of sintering and smaller shrinkage in the second. The extent of the compact growth preceding the shrinkage may markedly exceed that due to copper and aluminum atom diffusion under conditions of uniform reaction of the liquid phase on all the surfaces of the particles. The anomalously large compact growth in the first stage of sintering is due to a negative regrouping of particles resulting from an uneven Kirkendall flow of material inside the particles, bringing about a change in their shape. The extent of shrinkage is not apparently linked with the structure forming during sintering, but depends on the starting porosity. Pores in a compact affect grain growth during sintering by inhibiting it.Translated from Poroshkovaya Metallurgiya, No. 7(295), pp. 22–26, July, 1987.     

Gravitational effects on grain coarsening during liquid-phase sintering

Grain growth in liquid-phase sintering (LPS) is often treated as an extension of Ostwald ripening. However, the inherent grain contact at high solid contents leads to coalescence and other conflicts with most coarsening theories. Consequently, grain growth rates during LPS significantly deviate from the predictions of most coarsening theories. This study compares grain sizes of tungsten heavy alloys sintered on Earth and under microgravity conditions. The microgravity samples consistently produced a smaller grain size. A case is made for a gravity contribution to coarsening during LPS. Such a concept provides a rationalization for previously reported differences in sintered grain size with location in compacts sintered on Earth.     

Thermal analysis of isothermal solidification kinetics duirng transient liquid-phase sintering

A technique for experimentally measuring the kinetics of isothermal solidification during transient liquid-phase sintering (TLPS) has been successfully developed using differential scanning calorimetry (DSC). Comparison of these data with a model based on diffusion of Sn solute into an array of spherical Pb particles reveals very good agreement between predicted and measured solidification rates. The DSC technique and the solid-state diffusion model represent valuable tools in investigating the parameters that control TLPS, such as powder size, solder composition, process temperature, and heating rate.     

Effect of the growth memory and shrinkage during liquid-phase sintering of Al-Cu powdered bodies

Bulk variations of powdered bodies Al-6 wt % Cu during liquid-phase sintering are investigated by a dilatometry method. It is shown that with the help of rapid cooling, it is possible for an indefinitely prolonged time to stop the occurring variations that accompany heating. Manifestation of the “memory” effect of growth and shrinkage in a two-stage liquid-phase sintering of the powdered Al-Cu bodies is conditioned by the diffusion nature of the interaction of two metals, one of which is in the liquid state.     

Deformation of aluminum-copper powder solids during liquid-phase sintering

Conclusions During sintering with the participation of a liquid phase Al-Cu powder solids experience growth deformation in the first stage and shrinkage in the second. When the copper concentration in the mixture does not exceed the limit of solid-phase solubility at the sintering temperature, powder solids experience only growth; when the liquid phase is preserved during the whole period of sintering, growth is replaced by shrinkage. The growth of such powder solids during liquid-phase sintering is due to diffusion of copper atoms from the melt into aluminum particles, with the formation of solid solutions in these particles and of a liquid phase at grain boundaries. The deformation of powder solids in the shrinkage stage is brought about by dissolution of particles of the solid phase in the liquid and the particle regrouping process. The sign and magnitude of the deformation of powder solids during liquid-phase sintering are determined by Eq. (1).Translated from Poroshkovaya Metallurgiya, No. 8 (272), pp. 39–43, August, 1985.     

Modeling of distortion after densification during liquid-phase sintering

Liquid-phase sintering (LPS) is a technique widely used to sinter hard and heavy metals such as tungsten carbide and tungsten heavy alloys. LPS involves formation of a liquid phase during sintering that promotes fast densification. However, the ratio of liquid to solid, microstructure and external forces (gravity, component/substrate friction) act to promote distortion as a function of sintering time and temperature. To understand and control distortion during LPS, a numerical model is being developed to solve continuity and momentum equations using a finite-element technique. In this article, transient distortion under gravity is calculated as a function of surface tension, density, and viscosity of the material. The effect of the friction force due to the component support during isothermal sintering is also evaluated and compared with experimental data acquired by in-situ recording of distortion during sintering.     

Effect of powder particle size on the growth of titanium compacts during liquid-phase sintering with aluminum

Conclusions In the liquid-phase sintering of a Ti+30 at. % Al powder mixture compact growth increases with increasing mean particle size. The volume of a compact from such a mixture exhibits an anomalous increase (surpassing its growth due to Kirkendall flow during unipolar diffusion of aluminum atoms from the molten phase to the titanium). The increase is attributable to growth of layers of an intermetallic compound, which sets up a stress in the particles disturbing the continuity of their material. A fall in the density of particles linked with the disturbance of the continuity of their material is characteristic of particles exceeding in size a certain critical value — in the case under consideration, about 45m. Smaller particles grow only as a result of Kirkendall flow, whose extent is determined by the concentration of aluminum in the mixture. The reason why no disturbance in the continuity of material occurs in fine particles is apparently that the intermetallic compound layers forming on them are very thin, so that the stress set up in the particles does not reach a level sufficient for the initiation and growth of discontinuities.Translated from Poroshkovaya Metallurgiya, No. 9 (225), pp. 33–37, September, 1981.     

Effect of structural vacancies in interstitial phases on their sintering shrinkage

Conclusions The effect of departures from stoichiometric composition on shrinkage in the sintering of zirconium carbide is different in different temperature ranges. This is attributable to the manifestation of collective recrystallization, which is particularly pronounced in strongly nonstoichiometric carbides. Differences are observed in the variation of sintering shrinkage, the sinter bonding of spherical models, and creep with composition in the homogeneity ranges of interstitial phases.Translated from Poroshkovaya Metallurgiya, No. 6(174), pp. 22–26, June, 1977.     

Nature of the growth of Al-Cu powder compacts during liquid-phase sintering

Conclusions Arresting the process of liquid-phase sintering of aluminum-copper system powder compacts by rapid cooling does not affect the character of the volume changes experienced by them during subsequent sintering under the same temperature conditions. In the growth stage a decrease in the crystal lattice parameter of aluminum and an appreciable broadening of an x-ray line have been observed, caused by the formation of aluminum base solid solutions. These findings bear out the hypothesis that the growth of aluminum-copper powder compacts above their eutectic melting point is mainly due to diffusion of copper from the liquid phase into aluminum particles.Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 16–19, May, 1988.