The critical grain size for liquid flow into pores during liquid phase sintering

A model of grain-liquid mixture containing a spherical pore is analyzed to describe the pore filling during liquid phase sintering. Since the radius of liquid menisci around a pore increases linearly with the grain size, the menisci form a spherical bubble at a critical grain size and the pore is rapidly filled with liquid flowing from numerous menisci at the specimen surface. The values of the critical grain size for pore filling are calculated for various dihedral angles, wetting angles, and liquid contents. Although the predicted critical grain size is subject to errors because of assumptions in the models, the values agree in an order of magnitude with some experimental results. The effect of entrapped gas on pore filling and the expected behavior of irregular pores are also briefly discussed.     

Effect of entrapped inert gas on pore filling during liquid phase sintering

The effect of an inert gas entrapped in isolated pores on liquid flow into them during liquid phase sintering has been studied. An analysis of the balance between the capillary pressure of the liquid menisci and the gas pressure shows that the entrapped gas delays the pore filling and produces bubbles. If the gas pressure exceeds a critical level, the pores remain intact and the critical point for their filling will never be reached. These predictions are confirmed by experimental observations on large spherical pores produced artificially in an Fe-Cu alloy. Argon gas is trapped in the pore by first sintering in Ar-H₂ mixture gas and then in H₂ after the isolated pores are formed. The entrapped inert gas of even low pressure is thus shown to cause a substantial porosity in liquid phase sintered specimens. Formerly a Doctoral Student at the Korea Advanced Institute of Science and Technology.     

Carbide composition change during liquid phase sintering of a wear resistant alloy

Constitutive liquid phase sintering is used to obtain fully dense parts of powdered STELLITE Alloy No. 6 PM (Co-29Cr-4.5W-l.2C- < 1B) with excellent wear resistance at elevated temperature. This alloy is characterized by a cobalt-rich fcc solid solution and interdendritic carbide phases in the as-atomized state. Compositional changes in the carbides prior to, and during, the liquid phase sintering were investigatedvia X-ray diffraction, optical microscopy, and Auger electron spectroscopy. The rejection of boron and cobalt by an M₂₃C₆-type carbide was identified as leading to the local formation of the liquid phase. A mechanism for the interactive role of the carbide composition change and the constitutive liquid phase sintering is proposed. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME.     

Retarded grain boundary mobility in activated sintered molybdenum

Rapid grain growth accompanies the enhanced sintering of molybdenum treated with nickel additions. Grain growth is detrimental to sintering kinetics and mechanical properties. A sintering model is developed to illustrate that reducing grain boundary mobility is a means to increase the densification rate. A fine silica dispersion is added to molybdenum powder which is activated by the addition of nickel. This powder exhibits a long term sintering benefit due to retarded grain growth which is attributed to dispersoid drag effects on grain boundaries. These experimental powders are further analyzed through precision dilatometry, showing a characteristic shift in shrinkage rate during constant heating rate experiments. The shrinkage rate of molybdenum is increased by a factor of 10 at 1000°C when activated with 0.37 pct Ni. The shrinkage rate of nickel activated molybdenum is further increased by 67 pct with 1400 ppm silica dispersed at the interparticle grain boundaries. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME. Formerly Research Assistant in the Rensselaer Polytechnic Institute, Materials Engineering Department     

Microstructural change during liquid phase sintering of W−Ni−Fe alloy

The changes of bulk density and microstructures during heating and liquid phase sintering of 98W-1Ni-1Fe compacts prepared from 1 and 5 μm W powders have been observed in order to characterize the densification behavior. The compact prepared from a fine (1 μm) W powder begins to densify rapidly at about 1200°C in the solid state during heating, attaining about 95 pct density upon reaching the liquid phase sintering temperature of 1460°C. The compact prepared from a coarse (5 μm) W powder begins to densify rapidly at about 1400°C in the solid state, attaining about 87 pct density upon reaching the liquid phase sintering temperature. Thus, the skeleton of grains is already formed prior to liquid formation. During the isothermal liquid phase sintering, substantial grain growth occurs, and the liquid flows into both open and closed pores, filling them sequentially from the regions with small cross-sections. The grains subsequently grow, into, the liquid pockets which have been formed at the pore sites. The sequential pore filling by first liquid thus is shown to be the dominant densification process during the liquid phase sintering of this alloy, as has been demonstrated earlier with spherical model pores and as predicted theoretically.     

An analysis of the surface menisci in a mixture of liquid and deformable grains

The capillary force due to liquid menisci at the surface of a mixture of deformable grains and a limited amount of liquid (exemplified by liquid phase sintered alloys) is analyzed. Geometrical models for the grains and the menisci at the specimen surface are described. The menisci curvature required to keep the grains in the anhedral (contact flattened) shape with limited liquid content is calculated from the condition that the capillary force is counterbalanced by the sphering force of the grains. The radius of the menisci at equilibrium increases with liquid content. Its dependence on the dihedral angle, on the wetting angle, and on the ratio of the interfacial energies between the liquid-vapor and solid-liquid phases is also described. The grain-meniscus system maintains a shape geometrically similar with respect to change of grain size; hence, the meniscus radius increases in proportion to the grain radius. It is proposed that the difference between the capillary force and the sphering force is the meaningful driving force for grain shape accommodation during liquid phase sintering. Finally, some experimental evidence supporting the results of these analyses is discussed.     

The influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size <63 μm) in the temperature range from 1000°C to 1600°C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atrnospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (10^-3 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (0T > 3000 min^-1) to temperatures above the melting point of Nb₂O₅ (Tm = 1495 °C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME     

The instability of solid-liquid interface in MoNi alloy induced by diffusional coherency strain

In an alloy of 85Mo-15Ni by weight prepared by liquid phase sintering for a long time, the relatively large spherical MoNi grains are in chemical equilibrium with the surrounding liquid matrix and are separated from each other by bulk liquid. When the chemical equilibrium between the grain and the liquid is broken either by heat-treating at a temperature different from that of the sintering or by adding Fe to the liquid, some grain surface zones become convex by reprecipitation of a new equilibrium solid, while others become concave by dissolution of the initial solid solution. An instability of the grain-liquid interface in the form of an undulating structure thus develops. It is demonstrated that the driving force for this solution-reprecipitation process arises from the coherency strain energy in the solute diffusion zone at the dissolving grain surface.     

Ostwald ripening during liquid phase sintering—Effect of volume fraction on coarsening kinetics

Phase coarsening, also termed Ostwald ripening, is generally thought to be a slow, diffusion-controlled process which occurs subsequent to phase separation under extremely small under- or over-saturation levels. The theory due to Lifshitz, Slyozov, and Wagner (LSW), which predicts the coarsening kinetics and the particle distribution function, are applicable todilute systems only, in which particle-particle interactions are unimportant. Most liquid phase sintered systems, however, have large enough volume fractions of the dispersed phase to violate the essential assumptions of LSW theory. Recent progress will be described on simulating Ostwald ripening in randomly dispersed, high volume fraction systems. A fast algorithm for solving the multiparticle diffusion problem (MDP) will be described, permitting simulation of coarsening dynamics by cyclic time-stepping and updating the diffusion solution for large random particle arrays. The rate constants, controlling the growth of the average particle, and the particle distribution functions were obtained by numerical simulations up to a volume fraction of 0.55. A new statistical mean field theory has now been developed which reproduces the MDP simulation data accurately, and finally makes clear how the linear mean-field approximations employed by LSW theory must be modified to describe real systems. The predictions of the mean field are found to compare favorably with experimental measurements made over a wide range of volume fraction solid of the kinetics of Ostwald ripening in liquid phase sintered Fe-Cu alloys. The new theory provides a comprehensive approach to understanding microstructural coarsening in liquid phase sintered systems. This paper is based on a presentation delivered at the symposium “Activated and Liquid Phase Sintering of Refractory Metals and Their Compounds” held at the annual meeting of the AIME in Atlanta, Georgia on March 9, 1983, under the sponsorship of the TMS Refractory Metals Committee of AIME.     

On the removal of pores from castings by sintering

The causes, sizes, and distribution of porosity in castings have been reviewed and quantitatively evaluated for several important modes of alloy solidification. In general, gas exsolution is found to be the most probable cause of porosity in castings which solidify in either a cellular or dendritic fashion. On the other hand, solidification alone may cause porosity creation if the interdendritic liquid metal cannot feed the solidification shrinkage. This effect may be enhanced by gas exsolution. Removal of porosity by “sintering” after solidification requires that the grain size be of the order of, or smaller than, the pore spacing, and that the pores be small (>1 μ) for removal within reasonable times (tens of hours). When gas exsolution is the cause of pore creation, the gas must be diffused out of the sample to permit pore shrinkage. Small ingot sizes (>10 cm) and rapidly diffusible gases (H₂) are required for pore elimination within reasonable times (tens of hours). The application of low pressure (>20 atm) during sintering increases the rate, or the size (to >10 mμ) of the pores which can be eliminated within >20 hr. This paper is based on an invited talk presented at a symposium on Homogenization of Alloys, sponsored by the IMD Heat Treatment Committee, and held on May 11, 1970, at the spring meeting of The Metallurgical Society of AIME, in Las Vegas, Nev.     

Kinetics of grain coarsening during sintering of Co-Cu and Fe-Cu Alloys with low liquid contents

The coarsening of grains immersed in varying amount of liquid matrix is investigated in Fe-Cu and Co-Cu alloys at the liquid phase sintering temperatures. Specimens containing 20, 30, and 50 wt pct Cu have been prepared by compacting and sintering mixtures of fine powders. With 50 wt pct of Cu, spherical grains are dispersed in the liquid matrix. With 20 wt pct of Cu, anhedral grains are in contact with the neighbors across grain boundaries or thin liquid films, and the liquid matrix forms continuous prisms along the three grain contacts. The form of the rate law for grain coarsening at all compositions agrees with predictions of the diffusion controlled Ostwald ripening theories of Lifshitz, Slyozov, Wagner, and others. The coarsening rate also increases with decreasing matrix content. The activation energy for grain coarsening does not vary with specimen composition. Therefore, the rate controlling mechanism for coarsening of the anhedral grains in contact with each other appears to be the solution and reprecipitation of solute atoms by diffusion through the liquid matrix. SU SOK KANG, formerly a student in the Department of Materials Science at the Korea Advanced Institute of Science and Technology     

Fe-Mn-Si系烧结钢显微组织的观察与分析

观察了含硅锰母合金烧结钢的组织变化和合金元素锰、硅的分布情况。发现:添加硅锰母合金后,在1100℃以上为液相烧结,但只有小尺寸母合金颗粒熔化后留下的孔洞趋于球形,较大尺寸母合金颗粒则在其周围留下环形孔洞。那些较大的原硅锰母合金颗粒的组织随温度的升高而变化。合金元素锰硅向基体内扩散的过程会由于液相的出现而大大地加快。     

A theoretical study of grain growth in porous solids during sintering

The processes of grain boundary migration, pore drag and pore/boundary separation are described on the basis of the phenomenological equations for boundary migration and surface diffusion. Cylindrical pores on triple grain junctions are assumed to represent the open porosity during intermediate-stage sintering. It is found that cylindrical pores can hardly detach from migrating boundaries. Three-dimensional closed pores, however, which predominate during final stage sintering, can separate from migrating grain junctions. The separation process is modelled numerically and the conditions for separation are formulated. Analytical approximations for the pore mobility are shown to describe the numerical results well. They serve to establish effective mobilities of grain boundaries bearing pores in various configurations. Classical theories of grain coarsening are modified by using these effective mobilities. Mechanical constitutive models of sintering contain the grain size as an internal variable. The present analysis leads to an evolution equation for the average grain size, which depends on the volume fraction of the pores and on their configuration.     

Contact formation during the liquid-phase sintering of Fe-Cu and Cu-P alloy powders

Conclusions During the liquid-phase sintering of Fe-Cu and Cu-P alloy powders a dominant role in the formation of particle contact structure is played by capillary forces, which ensure migration of the liquid phase along the grain boundaries of the solid phase. In the course of prior heat treatment of these alloy powders emergence of the liquid phase to their surfaces under the influence of excess intergranular pressure is favored energetically.Translated from Poroshkovaya Metallurgiya, No. 12(276), pp. 33–37, December, 1985.     

The discontinuous precipitation of a liquid phase in MoNi induced by diffusional coherency strain

When a liquid phase sintered MoNi alloy is heat-treated at a temperature lower than that used for sintering where the solid and liquid phases coexist, the liquid films and grain boundaries between the grains migrate, leaving behind a new solid solution somewhat depleted of Ni. Since some liquid is produced during this migration, it resembles a discontinuous precipitation of the liquid phase. It is demonstrated experimentally that the driving force for this discontinuous precipitation arises from the coherency strain produced by Ni atom diffusion out of the grains. When two solute atom species simultaneously diffuse into or out of crystals in a ternary system, the resulting coherency strain depends on the size and concentration of the diffusing atoms and can be varied independently of the free energy of mixing. In particular, the coherency strain can be reduced to zero when the strain effects of the two atomic species exactly cancel each other. In this study, series of 90Mo10Ni alloy (by wt%) have been prepared by liquid phase sintering at temperatures between 1400 and 1520°C in order to produce solid MoNi grains of varying Ni concentration that are in equilibrium with the surrounding liquid matrix. The migration of liquid films and grain boundaries in the sintered specimens is induced by heat-treating them at 1400°C after adding various amounts of Fe to the liquid matrix. The migration does not occur when the estimated coherency strain is close to 0, although the free energy of mixing is finite. This result is a definitive demonstration that the driving force for the steady state migration is the coherency strain energy. The evidence that the grain boundaries exist in the sintered specimens and remain as grain boundaries during the migration is discussed.     

The role of pore evolution during supersolidus liquid phase sintering of prealloyed brass powder

ABSTRACT

The aim of this research is to study the pore structure as well as to assess the liquid phase sintering behaviour of Cu-28Zn powder specimens at different green density levels and temperatures. For this purpose, samples were compacted to obtain six different green densities and then sintered at 870°C, 890°C and in part at 930°C for 30?min. The results revealed that the spherical pores which are formed inside the grains can be swept by grain boundaries due to grain growth and join to primary pores so that secondary intragranular pores are eliminated and intergranular pores enlarged at higher temperatures. Also, the pores move upwards to the top of sample due to buoyancy forces. The role of pore structure in distortion is more tangible at higher temperatures (930°C) so that O-shape and X-shape distortions were observed at high and low green density samples, respectively.     

Microstructural coarsening during supersolidus liquid phase sintering of alpha brass

Coarsening of the grains and pores during sintering has a pronouncedly negative effect on the densification of prealloyed brass powder compacts. This investigation examines the role of sintering variables in realising the complicated effects on densification and microstructure. Experiments were designed to model and evaluate the effect of sintering parameters such as temperature, time and furnace atmosphere on densification, grain and pore intercept as well as pore number. The study of microstructures suggests that there is a good correlation between grain and pore intercepts. It is concluded that pore coarsening is a result of supersolidus liquid phase sintering of Cu28Zn powder, and it can retard densification, which is in acceptable agreement with the experimental data.     

细粒级铁矿粉对烧结液相和矿结构的影响

利用基础烧结设备检测了细粒级铁矿粉同化速度、流动能力,并通过微型烧结杯模拟料层下部单元点烧结过程的方法来研究配加15%细粒级矿粉的烧结矿结构变化,有效分析了3种细粒级矿粉在烧结时的液相行为及对烧结矿结构和性能的影响。通过比较生产用混匀矿与配加质量分数为15%的A、B、C粉的烧结矿结构表明:A粉有利于减少烧结矿内部孔洞的尺寸,减少核颗粒和液相间较大孔洞的数量,并能促进针铁矿发展;B粉会增加烧结矿内部大孔洞,增加柱状或片状铁酸钙的生成;C粉同化速度慢,液相流动能力差,粘结效果差,会使液相与核颗粒间孔洞尺寸和数量增加。烧结杯试验结果表明:在生产用混匀矿中使用质量分数为15%的A粉,烧结矿的转鼓指数提高2.94%,低温还原粉化指数(RDI)降低3.37%。     

Formation of Cu pockets in Fe grains during the sintering of Fe-Cu alloys

The mechanisms for forming liquid pockets in grains during the liquid phase sintering of Fe-Cu alloys are examined. The number of liquid pockets is observed to increase with a slower rate of heating to the sintering temperature and with a higher compacting pressure. These results are consistent with two processes which occur in the solid state: sweeping of Fe grain boundaries across solid Cu particles and formation of C-shaped Fe grains. The liquid pockets are therefore formed when an extensive grain growth occurs in the solid state during heating.     

Collapse of interconnected open

The closing behavior of interconnected open pores has been investigated in W-Ni systems where the dominant sintering mechanism is grain boundary diffusion. The samples were prepared by conventional powder metallurgy technique with large W particles containing Ni contents of 0.03 to 0.1 wt pct. The microstructure analysis indicated that pore collapse predominantly occurs at the three grain junction midpoints and produces closed pores at the four grain corners. The observed critical porosity, below which all the pores exist in isolated form, was found to be about twice as great as the critical porosity predicted by morphological perturbation theory. From these results, it is concluded that the grain boundary diffusion flux across the pore surface, as well as the capillary force, plays an important role in the closing behavior of interconnected open pores.