Influence of gravity level and interfacial energies on dispersion-forming tendencies in hypermonotectic Cu-Pb-Al alloys

Nondirectional solidification experiments involving several hypermonotectic Cu-Pb-Al alloys were carried out aboard NASA's KC-135 zero-g aircraft in order to determine the influence of interfacial energies and gravity levels on dispersion-forming tendencies. For Cu-Pb-Al alloys, changes in Al content are thought to result in variations in the interfacial energy between the two liquid phases. It has been postulated that the interfacial energy between the two liquid phases may have a strong influence on the ability to form well-dispersed structures in these systems. In order to study the influence of interfacial energies, the Al content was systematically varied in the alloys. To eliminate gravity driven sedimentation of the more dense immiscible liquid phase during solidification, experimentation was carried out aboard NASA's KC-135 zero-g aircraft. The resulting structures have been analyzed and the dispersion-forming ability related to the gravity level during solidification, the interfacial energy between the immiscible phases, and the tendency for the minority immiscible phase to wet the walls of the crucible. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

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.     

Modeling of distortion after densification during liquid-phase sintering

Liquid-phase sintering (LPS) is a technique widely used to sinter hard and heavy metals such as tungsten carbide and tungsten heavy alloys. LPS involves formation of a liquid phase during sintering that promotes fast densification. However, the ratio of liquid to solid, microstructure and external forces (gravity, component/substrate friction) act to promote distortion as a function of sintering time and temperature. To understand and control distortion during LPS, a numerical model is being developed to solve continuity and momentum equations using a finite-element technique. In this article, transient distortion under gravity is calculated as a function of surface tension, density, and viscosity of the material. The effect of the friction force due to the component support during isothermal sintering is also evaluated and compared with experimental data acquired by in-situ recording of distortion during sintering.     

Directional solidification of lead-copper immiscible alloys in a cyclic gravity environment

Hypermonotectic copper-lead alloys were directionally solidified at unit gravity on earth and also in the cyclic gravitational environment attainable during flight of NASA's KC-135 aircraft. In both cases macrosegregation developed that consisted of an initial lead-rich phase above which an aligned composite structure of apparent monotectic composition grew. Differences within these regions are examined, and the suitability of the KC-135 environment for directional solidification of monotectic alloys is discussed. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–229, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Directional solidification of Al- Ni/SiC composites during parabolic trajectories

Aluminum-6.1 wt pct nickel-silicon carbide composites containing varying volume fractions and particle sizes of SiC were directionally solidified at different translation rates and temperature gradi-ents, under variable gravity levels. The gravity level was changed by solidifying the composites in a Bridgman type directional solidification furnace aboard NASA KC-135 aircraft, flying on parabolic trajectories. It was observed that high gravity, high volume fractions of the particles or high effec-tive viscosity of the liquid favors the engulfment of particles by the melt-interface. Solidification in low gravity seems to deflocculate the SiC particle agglomerates while opposite results are obtained when solidifying under high gravity. Intercellular spacings are found to be higher in low gravity so-lidification as compared to high gravity solidification. These results are discussed in terms of the influence of gravity on various physical phenomena involved in the solidification process of the above composite. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermo-dynamic Data Committee and the Material Processing Committee.     

A mathematical model for gravity-induced distortion during liquid-phase sintering

Liquid-phase-sintered materials consist of interconnected crystalline grains in a homogeneous matrix phase that forms a liquid during sintering. These composites exhibit viscous flow during sintering that allows densification. Gravitational forces give rise to compact distortion when there is a large amount of liquid at a high temperature. This article treats kinetic aspects of distortion during sintering of tungsten heavy alloys (W-Ni-Fe). The mathematical model predicts distortion and highlights the important variables influencing this phenomenon. The results provide guidelines for minimizing distortion due to gravity. Experiments conducted at several different sintering times show reasonably good agreement with theoretical predictions using the liquid-phase viscosity as a single adjustable parameter. Theoretical predictions of the model are crucial to designing microgravity experiments aimed at understanding dimensional stability.     

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.     

Pore filling process in liquid phase sintering

Models for liquid flow into isolated pores during liquid phase sintering are described qualitatively. The grains are assumed to maintain an equilibrium shape determined by a balance between their tendency to become spherical and a negative capillary pressure in the liquid due to menisci at the specimen surface and the pore. With an increase of grain size, the grain sphering force decreases while the radius of liquid menisci increases to maintain the force equilibrium. When grain growth reaches a critical point, the liquid menisci around a pore become spherical and the driving force for filling the pore rapidly increases as liquid flows into it. The critical grain size required for filling a pore increases linearly with pore size. Experimentally, filling of isolated pores has been investigated in Fe-Cu powder mixture after liquid phase sintering treatment and after dipping into a molten matrix alloy. The observed pore filling behaviors agree with the qualitative predictions based on the models. In Fe-Cu alloy, pore filling is terminated by gas bubbles formed in liquid pockets. 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.     

Mixing fuel particles for space combustion research using acoustics

Part of the microgravity science to be conducted aboard the Shuttle (STS) involves combustion using solids, particles, and liquid droplets. The central experimental facts needed for characterization of premixed quiescent particle cloud flames cannot be adequately established by normal gravity studies alone. This paper describes the experimental results to date of acoustically mixing a prototypical particulate, lycopodium, in a 5 cm diameter by 75 cm long flame tube aboard a Learjet aircraft flying a 20-sec low-gravity trajectory. Photographic and light detector instrumentation combine to measure and characterize particle cloud uniformity. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermo-dynamic Data Committee and the Material Processing Committee.     

Fluid Oscillation in the drop tower

An interfluid meniscus oscillates within a cylindrical container when suddenly released from earth's gravity and taken into a microgravity environment. Oscillations damp out from energy dissipative mechanisms such as viscosity and interfacial friction. Damping out of the oscillations by the latter mechanism is affected by the nature of the interfacial junction between the fluid-fluid interface and the container wall. Perfluoromethylcyclohexane and isopropanol in glass were the materials used for the experiment. The wetting condition of the fluids against the wall changes at the critical wetting transition temperature. This change in wetting causes a change in the damping characteristics. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

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.     

Volume changes exhibited by Cu-Al compacts during liquid-phase sintering

Conclusions The volume changes occurring during the liquid-phase sintering of Cu-Al compacts obey Eq. (1), which reflects the fact that diffusion from the liquid phase into the solid precedes and then accompanies the migration of the solid phase into the melt. In dilatometric investigations into sintering in the presence of a liquid phase, compacts were found to grow in volume. Such growth is due to diffusion of atoms into the solid phase from the melt, and precedes shrinkage linked with dissolution of particles in the liquid phase. When the solid phase constitutes about one-third of the whole volume of a compact, the latter's shrinkage during the formation of the liquid phase takes place without prior growth. This shows that in principle it is possible for the rigid skeleton to be destroyed and for shrinkage to occur under these conditions as a result of a regrouping of particles without a substantial change in compact shape.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 31–37, May, 1982.     

Gravity induced solid grain packing during liquid phase sintering

The effects of gravity on the solid grain packing in liquid phase sintering have been investigated by both theoretical and experimental analyses. This treatment relies on quantitative microstructural measurements to determine the solid volume fraction variation along the direction of gravity. The model assumes that the grain packing coordination is proportional to the gravitational pressure and density difference between solid and liquid phases. It is confirmed by sintering experiments on W-Ni, and is consistent with a computer simulation model previously reported by German.     

Radiation as a tool in understanding phase transformations

Radiation may affect the course of a phase transformation in many ways, ranging from such a simple effect as enhanced diffusion or solute segregation to more complex effects such as dissolving thermally stable precipitates, inducing compositional instabilities, or altering phase boundaries. In the case of Invar-type Fe-35Ni-XCr alloys, irradiation-enhanced diffusion of all elements and segregation of nickel at sinks accelerate and make observable a spinodal reaction otherwise observable only after extended thermal annealing. Moreover, irradiation apparently also expands the boundaries of the miscibility gap. Irradiation may also be used to obtain structures unattainable thermally, even through such techniques as rapid quenching from the vapor or liquid. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Radiation as a tool in understanding phase transformations

Radiation may affect the course of a phase transformation in many ways, ranging from such a simple effect as enhanced diffusion or solute segregation to more complex effects such as dissolving thermally stable precipitates, inducing compositional instabilities, or altering phase boundaries. In the case of Invar-type Fe-35Ni-XCr alloys, irradiation-enhanced diffusion of all elements and segregation of nickel at sinks accelerate and make observable a spinodal reaction otherwise observable only after extended thermal annealing. Moreover, irradiation apparently also expands the boundaries of the miscibility gap. Irradiation may also be used to obtain structures unattainable thermally, even through such techniques as rapid quenching from the vapor or liquid. This paper is based on a presentation made in the symposium “Irradiation-Enhanced Materials Science and Engineering” presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25–29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD.     

Nature of the growth of Al-Cu powder compacts during liquid-phase sintering

Conclusions Arresting the process of liquid-phase sintering of aluminum-copper system powder compacts by rapid cooling does not affect the character of the volume changes experienced by them during subsequent sintering under the same temperature conditions. In the growth stage a decrease in the crystal lattice parameter of aluminum and an appreciable broadening of an x-ray line have been observed, caused by the formation of aluminum base solid solutions. These findings bear out the hypothesis that the growth of aluminum-copper powder compacts above their eutectic melting point is mainly due to diffusion of copper from the liquid phase into aluminum particles.Translated from Poroshkovaya Metallurgiya, No. 5(305), pp. 16–19, May, 1988.     

Influence of microgravity on the morphology of the directionally solidified front in an alSi alloy

One aim of the experiments carried out in the GFQ during the German Spacelab Mission D1 was to study the influence of convection on the coarsening of secondary dendrite arm spacing in an AlSi 7.0 alloy during normal crystallization at constant velocitiesv _SF and the temperature gradientG _SF with quenching of the residual melt. When, under μg and 1 g conditions, the same temperature gradientG _SF ≈ 16 K/mm and two different velocities (5 and 8 mm/min) were used, dendrite arm coarsening was shown to be lower than in the 1 g reference experiments atv _SF ≈ 5 mm/min and nearly identical with the reference results atv _SF ≈ 8 mm/min. A separation of the different kinds of convection, gravity-driven convection and convection driven by the volume jump, was tried using the coarsening factorM. The influence of gravity convection on the dendrite spacings seems to be high, if the velocity of crystallization is low. This influence fades away, if the velocity is high(e.g., >8 mm/min). This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

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.     

Dendritic solidification of alloys in low gravity

Gravity-driven convective flow influences dendrite morphology, interdendritic fluid flow, dendrite interface morphology, casting macrosegregation, formation of channel type casting defects, and casting grain structure. Dendritic solidification experiments during multiple parabolic aircraft maneuvers for iron-carbon type alloys and superalloys show increased dendritic spacing in low-gravity periods. Larger dendrite spacings for low-gravity solidification have also been reported for sounding rocket and space laboratory experiments for metal-model and binary alloys. Convection decreases local solidification time and increases the rate of interdendritic solute removal. The elimination of convection in low gravity is thus expected to increase dendritic spacing. Convection's effect on dendritic arm coarsening is expected to be dependent on the coarsening mechanism. Decreased coarsening in low gravity found for Al-Cu is indicative of coarsening predominately by arm coalescence. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

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.