Properties of porous tungsten-copper and molybdenum-copper pseudoalloys

Conclusions We obtained a complex of properties (hydraulic resistance, thermal conductivity, heat transfer coefficient) of the pseudoalloys W-Cu and Mo-Cu in the range of porosity 0.49–0.84 for different ratios of the high melting metal and copper. It was experimentally confirmed for the first time that the dependence of the heat transfer coefficient on porosity is extremal. It was shown that Koh's dependence for the calculation of the thermal conductivity of porous powdered metals can also be used for pseudoalloys with the same empirical coeifficient.Translated from Poroshkovaya Metallurgiya, No. 2(290), pp. 47–50, February, 1987.     

Effect of nickel additions on the sintering and properties of biporous molybdenum-copper pseudoalloys

The addition of small amounts (1%) of nickel were sufficient to activate liquid-phase sintering and improve phase interface adhesion and the mechanical properties of molybdenum-copper pseudoalloys with a complex bimodal pore structure. The pseudoalloys retained high porosity (up to 75%) after sintering. This was achieved by lowering the microporosity in the biporous materials, thus decreasing the total amount of shrinkage which occurred during the sintering process.     

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.     

Densification kinetics of W-Fe-Sn pseudoalloys in liquid phase sintering

The densification kinetics in liquid phase sintering of W-Fe-Sn powder composites containing 90 wt.% of a refractory component and 10 wt.% of a low-melting component is strongly dependent on the iron content in the melt. As the iron content in composites increases, the concentration dependence of their densification shows a maximum. Samples containing more than 1.5 wt.% Fe grow intensively because of the formation of W₆Fe₇ intermetallide whose decomposition temperature is higher than that of liquid phase sintering. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 22–29, 2007.     

Sintering behavior of biporous molybdenum-copper pseudoalloys

Conclusions Solid- and liquid-phase sintering experiments on a biporous molybdenum-copper material have been found to be accompanied by an increase in pore size at the macroporosity level, which is a manifestation of the law of zonal separation. It is shown that in the sintering of a biporous material processes taking place at the microporosity level determine the structural characteristics of the material at the macroporosity level. Some characteristics of the solid- and liquid-phase sintering behavior of the material investigated have been established.Translated from Poroshkovaya Metallurgiya, No. 9(273), pp. 11–14, September, 1985.     

Grain growth and densification during liquid phase sintering of W-Ni

Mixtures of single crystalline spherical W particles of 200–250 μm dia., fine W powder of 10 μm size and fine Ni powder were sintered at 167°C. Depending on the amount of liquid phase present complete densification is obtained after different sintering times and this is always accompanied by pronounced growth of the large W spheres at the expense of the smaller grains. With limited amounts of liquid phase present, under the influence of the densification force arising from the pores, grain growth occurs preferentially away from the areas where the grains are in close contact. Thus grain shape accommodation occurs during growth and this enables complete densification. In order to achieve rapid densification during liquid phase sintering, conditions of rapid grain growth are therefore favourable.     

Kinetics of densification and growth of refractory phase grains in the liquid-phase sintering of very finely divided tungsten-copper materials

Conclusions The kinetics of tungsten grain growth during the LPS of a very finely divided W-20% Cu mixture and of densification of the composite in the middle stage of LPS is determined by a process of transport of the refractory component through the liquid phase. Under conditions of full separation of the refractory grains by the second phase the character of the variation of the viscosity and rate of local nonuniform deformation of the two-phase material indicates that the densification of such a composite is due to a superplasticity mechanism. An expression has been derived, taking into account the kinetics of refractory phase grain growth, relating the porosity ofa very finely divided composite to time of LPS under conditions of superplastic flow.Translated from Poroshkovaya Metallurgiya, No. 9(285), pp. 30–37, September, 1986.The authors wish to thank V. V. Skorokhod and V. V. Panichkina for helpful discussion.     

Structural inhomogeneity and localization of densification in the liquid-phase sintering of tungsten-copper powder mixtures

Conclusions The size of the relative phase interface of a finely divided tungsten-copper mixture provides a measure of the uniformity of distribution of the phases in the mixture. A mixture with a large phase interface is not prone to localization of densification during LPS. In the particle size range of the refractory phase ensuring the necessary level of capillary forces, a high starting degree of homogeneity of a mixture with a small concentration of the low-melting-point component is an essential condition of its effective densification during LPS.Translated from Poroshkovaya Metallurgiya, No. 8(284), pp. 14–19, August, 1986.     

A theoretical analysis of solution-precipitation controlled densification during liquid phase sintering

Models for solution-precipitation controlled, intermediate stage liquid phase sintering (LPS) have been derived by assuming a tetrakaidecahedron grain geometry with cylindrical pore channels along the grain edges and incorporating the liquid content and dihedral angle by adapting Wray's two phase grain model. The stress distribution of a thin liquid film between the grains is modelled as a hydrostatic squeeze film. The model predicts that a thin viscous liquid film is stable and suppors the normal stress arising from the Laplacian force at the liquid-vapor interface. The derived equations for both diffusion and interface reaction controlled liquid phase sintering show significant differences with respects to Kingery's LPS models, but are in good agreement with those based on liquid phase controlled creep models. Comparative analyses of the derived models indicate that the rate controlling mechanism during solution-precipitation shifts from diffusion to interface reaction control or vice versa as a function of particle size and/ or grain growth.