Microstructural evolution during liquid phase sintering: Part I. Development of microstructure

During liquid phase sintering, numerous solid-solid particle contacts can be generated by particle motion within the fluid. It is shown that, somewhat surprisingly, Brownian motion can produce such contacts. If such contacts are accompanied by particle adherence, the particles can then subsequently fuse into one (i.e., coalesce) by the liquid state analog of the evaporation-condensation mechanism of sintering. An isolated microstructure will develop if the time for particle coalescence is much less than the time between contacts. A highly skeletal arrangement of particles will form under the converse condition. Using these principles, a “microstructure map” is calculated in which the expected morphology of microstructure (i.e., skeletal or isolated) is related to the solid particle volume fraction, the kinetic and thermodynamic parameters affecting particle coalescence, and the frequency of particle contacts by Brownian motion. Some discussion of the thermodynamic and morphological factors affecting the probability of particle adherence after contact is presented.     

Structurization in the sintering of heterophase materials TiN-Ni

Conclusions As a result of the investigation of the structurization in the quasibinary system TiN-Ni after high-temperature liquid-phase and low-temperature solid-phase sintering it was established that structurization in short-term liquid-phase sintering is accompanied by the denitration of TiN, decrease of its microhardness, change of the compositon of its metal bond and of its amount in consequence of diffusion of Ti from TiN (formation of the eutectic Ti(Ni) + Ni₃Ti).The considerable increase of microhardness of grains of TiN and the linear increase of its lattice period after lengthy isothermal sintering are due to the dissolution of nickel in TiN. In the metal bond there appear excess crystals of Ni₃Ti and of the eutectic Ni₃Ti + TiNi.The growth of TiN grains in the heterophase material TiN-Ni proceeds by the mechanism of coalescence and dissolution-segregation. After solid-phase sintering the phase composition of Ni and TiN did not change, the grain size of TiN remained stable.Translated from Poroshkovaya Metallurgiya, No. 4(328), pp. 25–30, April, 1990.     

Structuring on sintering in the presence of liquid phase in Cr-Cu-iron group metal systems. I. Cr-Cu system

Structuring has been examined for Cr-Cu composites under conditions of impregnation and subsequent liquid-phase sintering at 1200°C in a vacuum of (2–4) · 10⁻³ Pa with reduced and electrolytic chromium powders. The size distribution for the particles of the refractory component in the microstructure containing the reduced chromium on liquid-phase sintering for 60 min corresponds to a logarithmic normal distribution; the distribution parameters are sensitive to the volume fraction of refractory particles. The calculated values for the dihedral angle are close to one of the modes of the distribution for the dihedral angles in the microstructure for specimens made of electrolytic chromium (115°). At 1200°C, the equilibrium Cr_s-Cu_l system obeys the condition . This indicates the probability of formation or preservation of framework structure elements during the liquid-phase sintering, which are observed by experiment in specimens containing reduced chromium. __________ Translated from Poroshkovaya Metallurgiya, Nos. 5–6(449), pp. 3–9, May–June, 2006.     

Compaction and structure formation during sintering of powder heterophase materials of the system titanium nitride-chromium-nickel alloy

Institute of Materials Science Problems, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, No. 6(366), pp. 32–38, June, 1993.     

Modeling of supersolidus liquid phase sintering: I. Capillary force

Prealloyed powders subjected to supersolidus liquid phase sintering (SLPS) exhibit partial melting. A liquid phase forms along the grain boundaries, at the interparticle neck region, or on the particle surface. Densification involves an increase in the liquid volume fraction along with simultaneous coalescence of the particles. A two-particle model is developed that takes into account an increasing liquid content with coalescence, as well as a shape change of the merging particles (initially spher-ical). The variation in capillary force with processing parameters such as temperature and liquid volume fraction give improved predictions over prior models. The model predicts that the capillary force is affected strongly by the amount of liquid phase, wetting angle, and the shape of the coa-lescing particles.     

Fiber fragmentation during processing of metallic matrix composites

Fiber fragmentation is a problem frequently encountered during the processing of metallic matrix composites. In this study, we examine the fragmentation of continuous fibers during a common composite consolidation process based on hot pressing in an open-ended channel die with fibers aligned parallel to the die walls. During the latter stages of consolidation, flow of the matrix along the die cavity may occur such that the resulting load transfer to the fibers can cause their fracture even in the absence of bending. This study analyzes the combination of conditions necessary for both matrix flow along the die cavity and the shear-lag loading of the fibers to a level that causes fragmentation. In order to validate the analysis, we model the fragmentation of fibers during elevated temperature hot pressing of Ni-base composites by the room-temperature consolidation of degraded sapphire fibers in a tin matrix. The observed fiber fragmentation behavior is in good agreement with theoretical predictions. The analysis also indicates that this mode of fiber fragmentation is confined either to low volume-fraction fiber composites or to the ends of panels of high volume-fraction fiber composites.     

Role of solid-state skeletal sintering during processing of Mo-Cu composites

Mo-Cu composites with Mo contents up to 85 wt pct can be processed by either infiltration of a presintered Mo skeleton with liquid Cu or by liquid-phase sintering of mixed Mo and Cu powders. For both cases, the effects of particle size, sintering temperature, and sintering time on densification and microstructural evolution are compared. The effects of transition metal additions on the densification of Mo-Cu are also investigated. The liquid-phase sintering densification rate of Mo-Cu is much slower than in traditional liquid-phase sintering and is similar to the solid-state densification rate of elemental Mo. Furthermore, the poor densification behavior and absence of slumping for compositions up to 50 vol pct Cu indicate that the high dihedral angle of the Mo-Cu system stabilizes the formation of a rigid Mo skeleton during liquid-phase sintering. Results from a computer simulation that takes into account mass transport via both solid-state and liquid-phase mechanisms show that the solubility of Mo in Cu is sufficient for rapid densification, but confirm that the sintering behavior of Mo-20 vol pct Cu is best described by solid-state skeletal sintering. In this case, the liquid phase promotes microstructural coarsening by solution reprecipitation but contributes little to densification because of the rigid Mo skeleton.     

Microstructural evolution during liquid phase sintering: Part II. Microstructural coarsening

During liquid phase sintering, microstructural coarsening takes place. One mechanism by which this occurs is Ostwald ripening. Alternatively, particle coalescence also leads to a concomitant reduction in the solid particle surface area per unit volume. In isolated structures in which particle-particle contacts are made, the rate of coarsening by coalescence is limited by the time between particle contacts, for this is long compared to the time to fuse two particles together. In skeletal structures the “coalescence time” limits coarsening by coalescence since this is long in comparison to the time between contacts. Expressions for the rate of particle coarsening are developed for the different mechanisms and different particle morphologies. The results of these calculations are combined with the microstructure maps developed in Part I of this paper to refine these maps so that they predict both the morphology developed and the dominant mechanism of coarsening in liquid phase sintered systems.     

铁矿粉烧结液相流动特性

在铁矿粉烧结过程中,铁矿粉与CaO反应生成的液相的流动特性,是衡量烧结混合料能否有效粘结的重要指标之一.采用微型烧结法和基于流动面积的粘度测量法,以10种常用进口铁矿粉为对象,研究其液相流动特性及其影响因素.分析了铁矿粉的液相流动特性对实际烧结过程的影响.结果表明:不同种类的铁矿粉由于其自身特性的不同,所生成的液相的流动能力及其随温度、碱度的变化规律有很大差异.     

Deformation behavior of metallic composites with brittle fibers during bending

Conclusions The aluminum-boron system was chosen for studying the mechanism and conditions of deformation of metallic composite materials reinforced with brittle fibers. It was established that, unlike that of an orthodox metallic material, the bending of a composite is accompanied by displacement of a neutral layer in the direction of compression. The causes and character of the displacement of this layer in the course of deformation were determined. The role of processing pads in bending was examined. A study was made of the fracture of fiber-reinforced composites on the attainment of critical values of strain.Translated from Poroshkovaya Metallurgiya, No. 11(239), pp. 80–85, November, 1982.     

Finite element modeling of distortion during liquid phase sintering

Liquid phase sintering (LPS) is a common technique to consolidate materials that are difficult to process by fusion techniques, such as tungsten heavy alloys. One of the major processing difficulties associated with liquid phase sintered alloys is component distortion and loss of component shape. In LPS, this distortion is the result of viscous flow driven by curvature effects and gravity. A finite element model is developed for viscous flow of the semisolid sintering structure using Stokes equations. This model considers solid volume fraction and effective viscosity of the solid-liquid mixture. The simulation predictions are compared to distortion results for microgravity and ground-based sintering experiments, and they show good agreement. The model results indicate that the effective semisolid viscosity is significantly greater than the liquid metal viscosity. Hence, future work needs to quantitatively examine the factors controlling viscosity and the benefits from such high viscosities in liquid phase sintered systems.     

A reanalysis of the kinetics of neck growth during liquid phase sintering

During liquid phase sintering, solid particles make contact and can subsequently coalesce into one particle. This coalescence phenomena can affect the type of microstructure formed and its subsequent coarsening behavior during liquid phase sintering. The mechanism of particle coalescence is assumed to be the liquid state analog of the evaporation-condensation mechanism of sintering. In this work, a detailed study of the geometry appropriate for analysis of the coalescence phenomena during liquid phase sintering is made. It is found that in the early stages of particle coalescence, the neck between the particles acts as a geometrical barrier to diffusion and the neck between the particles grows approximately ast ^1/5,i.e., the same kinetics appropriate for solid state sintering are obtained. At longer times, the neck area no longer restricts diffusive flow and at ^1/6 dependency of neck growth is obtained. The use of numerical techniques also allows the analysis to be carried out with fewer geometrical restrictions than in the original analysis of the evaporation-condensation mechanism.     

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.     

Particle growth by coalescence during liquid phase sintering of Fe-Cu

Particle growth during liquid phase sintering of Fe-30 wt% Cu and Fe-60 wt% Cu was measured and compared with a recently developed coalescence theory. Measured normalized particle size distribution, time-dependence of average particle size and dependence of growth rate on solid phase volume fraction agree well with calculations based on the assumption of a diffusion-controlled coalescence process where all component particles are assumed to have equal coalescence frequency. Slight discrepancies can be explained as the results of nonspherical particle shape and of favored coalescence opportunity for larger particles.     

Particle sedimentation during processing of liquid metal-matrix composites

A novel electrical resistance probe technique to measure thein situ volume fraction of ceramic particles in molten metals was applied to the measurement of sedimentation rates of 90-μm-diameter silicon carbide particles in molten aluminum. The results indicate that the rate strongly depends on volume fraction; the time to clarify a 0.15-m depth increased from approximately 60 to 500 seconds as the particle volume fraction increased from 0.05 to 0.30. Maps showing the changes in volume fraction throughout the melt were generated. A multiphase hydrodynamic model was developed to describe the sedimentation. Using volume fraction-dependent drag coefficients from work in aqueous systems, the model was able to simulate the experimental results remarkably well. The experimental and modeling results indicate that there was little agglomeration or network formation during sedimentation. The implications of the results for solidification and particle pushing are discussed. Formerly Graduate Student, McMaster University     

The effect of gravity on solution-reprecipitation during liquid phase sintering

Solution-reprecipitation during liquid phase sintering can lead to gravity-aligned gradients in the amount of refractory phase as a result of the interaction between gravitational forces and capillary forces. We provide an anlaysis of this mechanism for gradient formation and show that for most important engineering materials, solution-reprecipitation does not cause substantial gravity-induced gradients. This conclusion is in agreement with published data for tungsten heavy alloy materials containing volume fractions of solids greater than about 0.7 at the sintering temperature. Macrostructural gradients in liquid phase sintered materials have been reported in the literature; however, these materials contain sufficient liquid at the sintering temperature that solid grains settle within the liquid, perhaps contributing to the observed gradients.