Effect of interaction between the structural components of W-Co-Sn pseudo-alloys on the kinetics of their compaction during liquid-phase sintering

X-ray analysis (in Cu and Mo radiation) is used to examine the phase composition of pseudoalloys for the first time. The pseudoalloys are formed in the liquid-phase sintering of W, Co, and Sn powders in hydrogen at 1200°C. After cooling, all pseudoalloys contain tungsten (intensive reflections) and W 6 Co 7 (weak reflections) phases, γ′-phases of Co 3 Sn 2 , and CoSn 2 .Studying the liquid-phase sintering of refractory metal powders is of great importance. Moreover, the production of dense compacts by liquid-phase sintering at relatively low temperatures is very promising. The kinetics and mechanism of liquid-phase sintering of tungsten were studied in [1][2][3][4][5][6].This paper deals with x-ray diffraction analysis of composites compacted and sintered from W, Co, and Sn powders for 1 h at 1200°C in hydrogen followed by cooling in a furnace. The composites were as follows, wt.%: 90 W-7 Co-3 Sn, 90 W-3 Co-7 Sn, and 90 W-2 Co-8 Sn.Diffraction patterns of the sintered samples were taken in molybdenum radiation using a DRON-3 diffractometer. Simultaneously, x-ray patterns of the starting powders and those produced from the composites were taken in copper radiation. The x-ray patterns were taken in a Debye camera of 150-mm diameter. Tables 1-3 summarize the results from analysis of x-ray patterns and some diffraction patterns and present sin 2 ϑ (ϑ is the Bragg angle) and their relative intensities visually evaluated on a ten-point scale for each reflection.To identify x-ray and diffraction patterns, we used data on the crystal lattices of potential (sought) phases [7-10]. Based on these data, typical sin 2 ϑ values were calculated for each phase. The phase analysis was carried out by comparing experimental and calculated sin 2 ϑ values. Tables 1-3 give reflection indices (hkl) of the phases thus found.Using the relevant quadratic formula [8], we calculated the lattice parameters for all hkl typical of this phase. The calculated values of each parameter matched, so it was confirmed that the analysis was correct.According to [9], tin does not interact with tungsten. Therefore, composites formed in the sintering of W, Co, and Sn powders may contain compounds resulting from the interaction of tungsten with cobalt or cobalt with tin.The W-Co phase diagram [9, 10] is evidence of two compounds: Co 3 W (25 at.% W) and W 6 Co 7 (46 at.% W). Since the amount of tungsten is maximum in the composites (90%), the presence of W 6 Co 7 would be naturally

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confidence: 98%

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confidence: 99%

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X-ray analysis of the phase composition of liquid-phase interaction products of W, Co, and Sn powders

Chuprina

Shalya

2008

“…The W-Fe-Sn composites analyzed in this paper differ from the W-Ni-Sn and W-Co-Sn pseudoalloys that we studied earlier [12][13][14] in how the liquid-phase components interact with each other. In the Co-Sn system, which constitutes the liquid phase, both cobalt and nickel in Ni-Sn melts negatively deviate from the ideal behavior practically over the entire region of compositions.…”

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confidence: 72%

“…Such composites include the W-Ni-Sn and W-Co-Sn systems [12][13][14] that we have studied. The shrinkage of such composites in liquid phase sintering is dependent on the thermodynamic activity of the activator in the carrier metal: the higher the thermodynamic activity, the denser the samples.…”

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confidence: 99%

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

Nizhenko

Petrishchev

Skorokhod

2007

Self Cite

V. I. Nizhenko, V. Ya. Petrishchev, and V. V. Skorokhod UDC 621.762 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 W 6 Fe 7 intermetallide whose decomposition temperature is higher than that of liquid phase sintering.Great attention has been recently focused on the effect of interactions in complex composites on the kinetics of their densification in liquid phase sintering. The interacting systems are those whose components react with each other to form a liquid (solid) solution or new compounds. An important task of materials science is to identify basic patterns in forming powder metallurgy materials of interacting mixtures of fine powders.Experimental data on sintering of interacting disperse systems even now permit identifying a number of factors that may underlie the formation of materials with a certain porous structure and needed properties. An important factor is the liquid phase. It serves as a kind of activator in a disperse system of interacting metals since, along with bulk diffusion, the intermetallide formed in the system enables rapid diffusion transport of components through the melt, in which the diffusion coefficient is four to five orders of magnitude higher than in the solid phase [1].Of special interest are the papers [2-8] presenting a systematic experimental study of reactive liquid phase sintering accompanied by the synthesis of TiFe and TiNi intermetallides. The former intermetallide is capable of reversely sorbing hydrogen (hydrogen accumulator) and holds much promise for hydrogen energy [9] and the latter is a shape memory material.When the liquid phase forms, the intermetallide originates from contact melting at the interface of dissimilar particles; a substantial amount of heat is released in the process. The rate of nonisothermal heating of a billet is a factor that permits maintaining the balance between heat release and dissipation in reactive sintering. A heating rate of no greater than 15 deg/min should be kept in synthesizing titanium nickelide in controlled reactive sintering [2]. However, higher heating temperatures are permitted in synthesizing equiatomic TiFe intermetallide, whose heat of formation is almost half that of TiNi [3].Moreover, reactive sintering may be controlled by activating and deactivating additions. For example, 1 to 3 at.% Fe added to the Ti-Ni system decreases the formation rate of titanium nickelide and acts as an inhibitor (retarder) of reactive diffusion [4]. If nickel is introduced into the Ti-Fe system, the intermetallide is formed at a greater rate primarily owing to a decrease in the eutectic temperature from 1085 to 1040°C [3].

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

Nizhenko

Petrishchev

Skorokhod

2008

Self Cite

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.Liquid-phase sintering of multicomponent disperse systems whose components interact is a process that has been examined inadequately. The papers [1-5] analyze the densification mechanism in sintering of ternary pseudoalloys in which tungsten is a refractory component. A number of features in liquid-phase sintering resulting from the interaction between the refractory compounds and liquid phase and the interaction between the liquidphase components have been found in the W-Ni-Sn, W-Co-Sn, and W-Fe-Sn systems. Of interest is to examine these features when tungsten is replaced by molybdenum.This paper examines the densification kinetics of Mo-Ni-Sn pseudoalloys in liquid-phase sintering.We have found no data on the ternary Mo-Ni-Sn phase diagram. The Mo-Ni system has three incongruently melting intermediate phases, a wide region of solubility of molybdenum in solid nickel, and a very narrow region of nickel solid solutions in molybdenum [6]. In contrast to W-Ni, none of the Mo-Ni intermediate phases (namely, MoNi) decomposes at 1362°C, which raises the temperature of isothermal holding in sintering.The Mo-Sn phase diagram has not been finally established. Intermetallic MoSn 2 that decomposes at 800 ± 100°C and intermetallic Mo 3 Sn that decomposes at 300 ± 100°C are surely known to exist. There is one intermediate phase between these intermetallics which decomposes at 1200 ± 100°C but its composition has not yet been established (either Mo 2 Sn 3 or Mo 3 Sn 2 ) [6].The densification kinetics was examined with dilatometry. The displacement of the connecting rod resulting from shrinkage of the sample is converted by an inductive pickup into an electric signal proportional to the displacement [7]. The error of measurement is 2%.