Stage Of Liquid Phase Sintering Research Articles

Development of High Strength Mo<sub>2</sub>NiB<sub>2</sub> Ternary Boride Base Cermets Produced by Reaction Boronizing Sintering

The effects of V substitution for Cr to the sintering behavior of Cr containing Mo2NiB2 ternary boride base cermets and Mn addition to the mechanical properties and microstructure of the Cr and V containing cermets were investigated by using Ni-5.0B-51.0Mo-(17.5-x)Cr-xV (mass%) and Ni-5.0B-51.0Mo-12.5Cr-5.0V-(0-1.5)Mn (mass%) model cermets. 10mass%V substitution for Cr in the Cr containing cermets markedly improved transverse rupture strength and hardness from 2.27 to 2.94GPa and from 85.3 to 87.2RA respectively and refined the microstructure by retarding the progress of sintering especially at liquid phase sintering stage. Small amount of Mn addition to the Cr and V containing Mo2NiB2 base cermets significantly improved the sinterability and increased the mechanical properties of the cermets.

Computer simulation of solution and growth processes during the initial stage of liquid phase sintering of tungsten heavy metal

In the initial stage of liquid phase sintering, particle dissolution and growth processes occur. The melting matrix penetrates the solid particle agglomerates. A fraction of the particles dissolves in the liquid matrix phase and the original powder size will initially be reduced. At the same time the agglomerates of particles are effectively separated. In a second stage larger particles grow in equilibrium with the matrix, whilst smaller, pure particles dissolve into the matrix and the mean particle size increases. When an equilibrium is reached, the solid particles start to grow in the liquid matrix phase in accordance with the ripening process. The initial stages of liquid phase sintering have been investigated by short time sintering under microgravity and are presented in a series of micrographs. Mathematical models for the 3 stages have been developed.Computer simulations of these different stages in the solution and growth processes have been used to visualise the change in particle size distribution.

Liquid Phase Sintering of Alumina, I. Microstructure Evolution and Densification

The microstructure evolution and densification of alumina containing 10 vol% calcium aluminosilicate glass and 0.5 wt% magnesium oxide sintered at 1600°C were quantified by measuring the evolution of pore‐size distribution, the redistribution of liquid phase, and the fraction of closed and open pores. The densification stopped at a limiting relative density during the final stage of sintering, and the small and large pores were filled simultaneously by glass during sintering. In addition, the results indicate that the pressure build‐up of the trapped gases in pores causes a significantly negative contribution to the driving force, and consequently the observed reduction in densification during the final stage of liquid phase sintering.

Homogenization modeling of capillary forces in selective laser sintering

We study the initial stage of liquid phase sintering when the particles rearrangement due to densification forces is over. The heterogeneous medium is a mixture of powders, connected by liquid necks. It gives a fluid-structure problem built on a periodic domain, with three phases: an isotropic elastic solid, a newtonian, incompressible, weakly viscous liquid and a barotropic viscous gas. We study this equilibrium under the action of capillary forces on solid–liquid boundary. We penalize the divergence free condition in the liquid and by the energy method, we obtain the homogenized problem satisfied by the macroscopic displacement field in the homogenized medium. The result gets close to a Stokes problem where the capillary forces act as a density of body forces and the viscosity effects vanish.

Three-Dimensional Modeling of the Grain Growth by Coalescence in the Initial Stage of Liquid Phase Sintering

A Monte Carlo method was developed to simulate the three-dimensional grain growth by coalescence in the initial stage of liquid phase sintering. The simulated grain, including a cluster of bonded particles, was treated in an appropriate three-dimensional multi-particle arrangement of the powder compact, and each cluster was assumed to be coalesced to reduce the system energy. The probability model also incorporated the energy-misorientation relationship assigned to randomly generated neighboring particles. Simulation results indicate that the size distribution of agglomerated particles are broadened by either an increase in the standard deviation of particle size distributions or a decrease in the volume fraction of the liquid, mainly due to the increasing of the probability of particle contacts. The findings of the simulation are favorably compared with past experimental observations on W-Ni-Fe alloys.

Three-Dimensional Modeling of Coordination Number and Contiguity in the Incipient Stage of Liquid Phase Sintering

Three-Dimensional Modeling of Coordination Number and Contiguity in the Incipient Stage of Liquid Phase Sintering

Structure and properties of extruded aluminium alloy-aluminium nitride composite

Understanding the microstructure of W–Cu nanocomposite powder is essential for elucidating its sintering mechanism. In this study, the effect of milling time on the structural characteristics and densification behavior of W-Cu composite powders synthesized from WO3-CuO powder mixtures was investigated. The mixture of WO3 and CuO powders was ball-milled in a bead mill for 1 h and 10 h followed by reduction by heat-treating the mixture at 800 °C in H2 atmosphere with a heating rate of 2 °C/min to produce W-Cu composite powder. The microstructure analysis of the reduced powder obtained by milling for 1 h revealed the formation of W–Cu powder consisting of W nanoparticle-attached Cu microparticles. However, Cu-coated W nanocomposite powder consisting of W nanoparticles coated with a Cu layer was formed when the mixture was milled for 10 h. Cu-coated W nanopowder exhibited an excellent sinterability not only in the solid-phase sintering stage (SPS) but also in the liquid-phase sintering stage (LPS). A high relative sintered density of 96.0% was obtained at 1050 °C with a full densification occurring on sintering the sample at 1100 °C. The 1 h-milled W-Cu powder exhibited a high sinterability only in the LPS stage to achieve a nearly full densification at 1200 °C.

The K Value Distribution of Liquid Phase Sintered Microstructures

A complex geometry of a powder compact in the three-dimensional multiple particle arrangement was generated by a Monte Carlo method, and the K value (a constant links the two-dimensional connectivity to the three-dimensional coordination number) distributions of liquid phase sintered systems, parameters linked the two-dimensional connectivity to the three-dimensional coordination number, were solved by the numerical computation. With this probability model, irregular packing of particles in three-dimensional space could be formulated with the variations of the particle size distribution. Unlike previous analytical models in which the grain boundary energy was set to a constant value, a relationship between the grain boundary energy and the misorientation angle between neighboring grains could be incorporated into this model in a probability manner. Simulation results indicate that the mean K value in the initial stage of liquid phase sintering increases with an increasing volume fraction of liquid phase, an increasing ratio of the mean base particle size to the mean additive particle size, and a decreasing standard deviation of the particle size distribution. The findings of the simulation are favorably compared with previous experimental observations on W-Ni-Fe alloys.

Initial stage of liquid phase sintering: liquid reaction and particle growth in W–Ni–Fe–(Co)

The initial stage of liquid phase sintering, involving liquid reaction and particle growth, has been investigated under microgravity in experiments using tungsten heavy alloys for short periods of time (8–14 seconds). The influence of different factors, such as alloy composition, plastic deformation, and non-equilibrium conditions, have been evaluated. During the liquid phase sintering of tungsten heavy alloys at about 1470°C, the liquid matrix penetrates the tungsten particle agglomerates. A fraction of the tungsten particles goes into solution in the liquid phase and the original tungsten powder size will initially be reduced. At the same time, the agglomerates of tungsten particles are effectively separated. In a second stage, larger particles grow in equilibrium with the matrix while pure tungsten particles are dissolved into the matrix. When equilibrium is reached, the tungsten particles start to grow in the liquid Ni–Fe–W matrix phase in accordance with the Ostwald ripening process. A theoretical treatment of the particle solution and growth during these stages is proposed.The addition of iron and cobalt to the W–Ni system reduces the rate of penetration and growth. Non-equilibrium conditions during the formation of a liquid phase have a marked effect on the tungsten particle separation. Milling of the tungsten powder increases the initial growth of tungsten particles.

Initial stage of liquid phase sintering: liquid reaction and particle growth in W–Ni–Fe–(Co)

The initial stage of liquid phase sintering, involving liquid reaction and particle growth, has been investigated under microgravity in experiments using tungsten heavy alloys for short periods of time (8–14 seconds). The influence of different factors, such as alloy composition, plastic deformation, and non-equilibrium conditions, have been evaluated. During the liquid phase sintering of tungsten heavy alloys at about 1470°C, the liquid matrix penetrates the tungsten particle agglomerates. A fraction of the tungsten particles goes into solution in the liquid phase and the original tungsten powder size will initially be reduced. At the same time, the agglomerates of tungsten particles are effectively separated. In a second stage, larger particles grow in equilibrium with the matrix while pure tungsten particles are dissolved into the matrix. When equilibrium is reached, the tungsten particles start to grow in the liquid Ni–Fe–W matrix phase in accordance with the Ostwald ripening process. A theoretical treatment of the particle solution and growth during these stages is proposed.The addition of iron and cobalt to the W–Ni system reduces the rate of penetration and growth. Non-equilibrium conditions during the formation of a liquid phase have a marked effect on the tungsten particle separation. Milling of the tungsten powder increases the initial growth of tungsten particles.

A model of liquid phase sintering by the homogenization

We study the first stage of liquid phase sintering, when the particles rearrangement due to capillary forces is over. We give the boundary value problem satisfied by the displacement field of points of the medium in the phase of elastic compression of solid particles, for given capillary forces acting as a density of external forces, by using the homogenization method and we characterize the mechanical behavior of this constrained medium from the material properties of each elementary components.

A micromechanical model for the initial rearrangement stage of liquid phase sintering

A constitutive model for powders in the initial rearrangement stage of liquid phase sintering (LPS), where capillary force is the driving force for densification, is developed in this paper. The formulation is based on certain theories in micromechanics of granular materials (see Mehrabadi et al., 1992a; Mehrabadi et al., 1992b; Nemat-Nasser and Mehrabadi, 1983; Nemat-Nasser and Mehrabadi, 1984; Christoffersen et al., 1981). The anisotropic mechanical behavior of powder system is analyzed by considering the behavior of interparticle force and fabric which is represented by the distribution of contact normals. The proposed model is used to determine the dependence of the overall volume change on such factors as local liquid volume, uniformity of liquid distribution, contact angle, initial particle distance, initial confining pressure, particle size and viscosity. The numerical and theoretical analyses in this paper indicate that small particle size, small contact angle, low viscosity, even distribution of liquid phase and loose green powder mass enhance the amount of volume change in the initial stage of LPS. It is also found that a higher confining pressure increases the densification rate but it lowers the amount of net volume shrinkage at the end of the initial stage. The anisotropic behavior of the sample is analyzed by studying the interparticle force change and fabric change during the sintering.

The effect of Mo addition on the liquid-phase sintering of W heavy alloy

The morphological and compositional changes of grains have been investigated in the initial stage of liquid-phase sintering of W-Mo-Ni-Fe powder compacts. Both large (5.4-μm) and small (1.3-μm) W powders have been used to vary their time of dissolution in the liquid matrix. When 8OW-10M0-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-μm) Mo, Ni, and Fe and coarse (5.4-μm) W powders are liquid-phase sintered at 1500 °C, the Mo powder and a fraction of the W powder rapidly dissolve in the Ni-Fe liquid matrix. The W-Mo grains (containing small amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the liquid-phase sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the liquid matrix during the sintering, the W content in the precipitated solid phase also increases. The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusion at the surface. During sintering, the W particles continuously dissolve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-μm) W powder, the W grains dissolve more rapidly, in about 1 hour, and only W-Mo grains remain. These observations show that the morphological evolution of grains during liquid-phase sintering can be strongly influenced by the chemical equilibrium process.

Certain regularities of phase formation and structure evolution during hot-pressing of ultrafine powders

We studied the processes of structure evolution and phase formation in the material during hot-pressing of ultrafine (highly dispersed) composite powders of the [Si3N4-10% Y2O3] system. Densification of the powder occurs under the conditions facilitating chemical interaction in the complex heterogeneous system. The sintering mechanism of UFCP consists of a stage of self-activated rapid grain growth in the solid phase mainly in the 1120–1150°C range and a stage of liquid phase sintering in the 1150–1650°C range under the conditions of varying composition, viscosity, and content of the liquid phase. It was established that densification is initiated by a low-melting yttrium silicate type liquid phase that disappears after participating in the process leading to the formation of a high-melting liquid phase having a composition close to that of yttrium-silicon oxynitrides. The revealed regularities in the behavior of UFCP during the process of hot-pressing offer the possibility of controlling the structure evolution in them purposefully in order to obtain the required set of properties. The results of our studies are being used for obtaining engineering components.

鉄ほう化物系硬質合金の焼結機構

The ternary Fe-6%B-x%Mo alloys have a composite microstructure consisting mainly of a complex boride Mo2FeB2 as a hard phase and a ferritic binder. The mechanism of liquid phase sintering of this alloy was investigated by means of dilatometry, differential thermal analysis, X-ray diffraction, and scanning electron microscopy. A fine composite microstructure of Mo2FeB2 and ferrite was produced from powders of Fe, Mo and FeB by a reaction sintering process involving two liquid phases. The hard phase Mo2FeB2 is produced in the compact prior to liquid formation. Above 1, 365K, considerable densification results from the initial stage rearrangement of the solid phases (austenite and Mo2FeB2) coexisting with the first formed liquid phase (L1). Another liquid (L2) which forms above 1, 415K, where Mo2FeB2 is the only coexisting solid phase, is required for complete densification. L2 having a high solubility for Mo2FeB2, provides for solution/reprecipitation process which characterize the intermediate stage of liquid phase sintering. The effective separation of the initial and intermediate stages by L1 and L2 is considered essential for the control of the sintered microstrucure of the ternary alloy.

Densification kinetics of the initial stage of liquid-phase sintering: glass-cordierite system

Densification kinetics of the initial stage of liquid-phase sintering: glass-cordierite system

Reaction sintering of an Fe-6 Wt Pct B-48 Wt Pct Mo alloy in the presence of liquid phases

The mechanism of liquid phase sintering of an Fe-6 wt pct B-48 wt pct Mo alloy was investigated by means of thermal dilatometry, differential thermal analysis (DTA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). A fine composite microstructure of Mo2FeB2 and ferrite was produced from powders of Fe, Mo, and FeB by a reaction sintering process involving two liquid phases. The hard phase Mo2FeB2 is produced in the compact prior to liquid formation initially by the reaction 2Mo+2FeB=Mo2FeB2+Fe and later by the reaction 2Mo+2Fe2B=Mo2FeB2+ 3Fe. Above 1365K, considerable densification results from the initial stage rearrangement of the solid phases (austenite and Mo2FeB2) coexisting with the first formed liquid phase (L1). Another liquid (L2) which forms above 1415K, where Mo2FeB2 is the only coexisting solid phase, is required for complete densification. L2, having a high solubility for Mo2FeB2, provides for solution/reprecipitation processes which characterize the intermediate stage of liquid phase sintering. The effective separation of the initial and intermediate stages by L1 and L2 is considered essential for the control of the sintered microstructure of the ternary alloy.

Capillary forces between spheres during agglomeration and liquid phase sintering

The capillary force due to a wetting liquid between solid particles is responsible for agglomeration and the rearrangement stage of liquid phase sintering. The capillary force has been calculated for several situations using a numerical technique. Included in the calculations are variations in particle size, contact angle, liquid volume, and particle separation. The capillary force obtained from these calculations is more accurate than prior estimates using a circular profile for the interparticle liquid bridge. A large attractive force exists between particles with small contact angles, particle sizes, and liquid volumes. Rupture of the liquid bridge is predicted using an energy analysis. At large contact angles, a zero force condition exists at an intermediate particle separation.

Sintered properties of WCrNi heavy alloys

Heavy alloys with high hardness values are essential for certain applications. To meet this requirement CrNi binder has been selected for the liquid phase sintering of 90W10(CrNi) heavy alloys in which the binder compositions are varied. Sintering has been carried out at 1500 °C in hydrogen (dew point −35 °C) for various periods. Densification, microhardness, ultimate tensile strength and percentage elongation have been measured with respect to the binder composition. In addition, spheroidization kinetics of the tungsten spheroids have also been studied. The results are explained on the basis of different stages of liquid phase sintering.

Liquid-phase sintering of SmCo5

The initial sintering kinetics of stoichiometric SmCo5 powder containing a samarium-rich sintering addition (60 wt% Sm plus 40 wt% Co) have been investigated as a function of amount of liquid phase, time, temperature, and particle size. The shrinkage as a function of time exhibits the classical three stages of liquid-phase sintering, i.e., rearrangement, solution precipitation, and solid phase. The rate-controlling step during the solution-precipitation stage corresponds to a phase-boundary (solid/liquid) reaction leading to dissolution. Evidence for this conclusion is partially based on the logarithm-shrinkage—logarithm-time slopes being equal to 1/2 instead of 1/3. A 1/3 slope is predicted by the liquid-diffusion-controlled sintering model while the phase-boundary sintering model predicts a slope of 1/2. An activation energy of 52.8 kcal/mole was obtained for the temperature dependence, and this also suggests a phase-boundary reaction rather than rate control by diffusion in the liquid phase. The rate of sintering follows an r−1 particle-size dependence instead of an r−4/3 dependence, again suggesting that the solution-precipitation stage of sintering of SmCo5 is controlled by a phase-boundary reaction leading to dissolution.