J.B. Andrews

Analysis of monotectic growth: infinite diffusion in the L2 phase

The Jackson-Hunt model of eutectic solidification is applied to monotectic solidification in which a liquid (L1) transforms into rods of a different liquid (L2) in a solid matrix. Limiting cases of no diffusion and infinite diffusion (complete mixing) in the L2 phase are considered. An adaptive refinement and multigrid algorithm (MGGHAT) is used to obtain numerical solutions for the concentration field in the L1 phase; this allows consideration of a general phase diagram. Density differences between the three phases, which cause fluid flow, are treated approximately. Specific calculations are carried out for aluminum-indium alloys. Infinite diffusion in the L2 phase has only a small effect on the relationship between interface undercooling and rod spacing.

Effect of flow due to density change on eutectic growth

Abstract The Jackson–Hunt model of eutectic growth is extended to allow for different densities of the phases. The density differences give rise to fluid flow which is calculated from a series solution of the fluid flow equations in the Stokes flow approximation. The solute diffusion equation with flow terms is then solved numerically using an adaptive refinement and multigrid algorithm. The interface undercoolings and volume fractions are calculated as a function of spacing for tin–lead and iron–carbon eutectic alloys and for an aluminum–indium monotectic alloy. The numerical results are compared with various approximations based on the Jackson–Hunt analysis.

The influence of gravity level during directional solidification of immiscible alloys

Abstract During directional solidification of immiscible (hypermonotectic) alloys itis theoretically possible to establish a stable macroscopically-planar solidification front, and thus avoid sedimentation. Unfortunately, convective instabilities often occur which interfere with the directional solidification process. In this paper, stability conditions are discussed and results presented from directional solidification studies carried out aboard NASAs KC-135 zero-g aircraft. Samples were directionally solidified while the effective gravity level was varied from approximately 0.01g for 25 s to 1.8g for 45 s. Dramatic variations in microstructure were observed with gravity level during solidification.

Directional solidification in immiscible systems: The influence of gravity

Abstract To take maximum advantage of immiscible alloys for proposed applications, it is desirable to produce aligned fibrous composite microstructures which contain a high volume fraction of the immiscible phase, L 2 /1–4/. Production of these microstructures should be possible through the directional solidification of hypermonotectic alloys. Analysis indicates that convection which may result from density differences in the melt will make cooperative growth of the fibrous structure in a hypermonotectic alloy very difficult under one-g conditions. Results from one-g experimentation support this conclusion /1,4–6/. However, results from directional solidification experiments aboard NASAs KC-135 zero-g aircraft and from transparent model systems indicate fibrous composite formation may indeed be possible in hypermonotectic alloys. /7/. Long duration low-g experimentation appears to be warranted in this area.

Microgravity Solidification of Al-In Alloys

Several aluminum-indium alloy samples were directionally solidified under microgravity conditions during the Life and Microgravity Spacelab Mission in 1996. These samples were processed as part of the Coupled Growth in Hypermonotectics flight experiment which is scheduled for completion aboard the International Space Station. The overall objective of this flight experiment is to obtain a fundamental understanding of the physics controlling solidification processes in immiscible alloy systems. Convective instability is anticipated in the Al-In alloys processed due to the low density solute depleted boundary layer that forms in advance of the solidification front. The three samples were processed using the Advanced Gradient Heating Facility and consisted of a 17.3 wt%In monotectic composition sample and two hypermonotectic composition samples, one containing 18.5 wt%In and the other 19.7 wt%In. Post flight analysis revealed the presence of small voids in two of the flight samples which may have had an impact on the ability to maintain steady state growth conditions. Precision density measurements revealed compositional variations along the length of ground processed samples which were representative of results anticipated due to convective mixing in the melt. Flight samples showed an initial compositional variation indicative of minimal mixing in the melt. Interface stability was obtained in one of the hypermonotectic flight samples over a region of the sample. Microstructural comparisons between flight samples indicate the increased volume fraction of the aligned rod-like phase in hypermonotectic samples is accommodated by an increased number of rods in the structure but a minimal change in rod diameter.

Solidification in immiscible alloys

Abstract Low-gravity processing of immiscible alloys is usually associated with an attempt to prevent sedimentation of the higher density liquid phase in order to form a dispersed microstructure. However, there are many factors in addition to gravity level which can influence the structures produced. These factors include crucible wetting by one of the immiscible phases during low-gravity processing, which often leads to complete separation of the immiscible phases. Attempts at containerless, low-gravity processing indicate that alloy wetting behavior may have a dramatic influence on the ability to form a dispersed microstructure. For directional solidification, alloy composition, thermal gradient, solidification rate, miscibility gap height, and convective stability all have a significant influence on the final structures obtained. This paper discusses the conditions necessary for the production of desired microstructures in each of these processing techniques.

The Effect of Processing Conditions on Solidified Structures in Immiscible Systems

The incentive for low-gravity processing of immiscible alloys is usually tied to an attempt to eliminate sedimentation of the more dense immiscible liquid phase during processing. However, many other factors can influence the solidified structure of these alloys. For example, crucible wetting by one of the immiscible liquid phases during normal (non-directional) low-gravity solidification in a container has been shown to often lead to a completely separated structure. For containerless, low-gravity processing, alloy composition has a tremendous affect on the structure formed. During directional solidification, alloy composition, thermal gradient, solidification rate, convective instability and the miscibility gap height of the system all have a major effect on the structures obtained. This paper discusses the conditions necessary for the production of desired microstructures in each of these processing techniques. Examples are shown of samples processed under both desirable and undesirable conditions.

Monotectic Growth: Unanswered Questions

Early solidification experiments in immiscible alloy systems almost immediately led to conflicting findings between investigators. Investigations revealed that several factors usually considered unimportant, especially the interfacial energy relationships between phases, could have a dramatic influence on the types of microstructures produced in immiscible alloy systems. During the 1980s, work concentrated on the influence interfacial energy on microstructure. However, some findings raised new questions. In the mid 1990s and continuing through today, most efforts have focused on modeling the monotectic growth process and on obtaining steady state coupled growth conditions in hypermonotectic alloys. This paper focuses on some of the advances that have been made to date in understanding solidification in immiscible alloy systems and some of the questions that remain to be answered.

Effect of Convective Instability in Directionally Solidified Hypermonotectic Al-In Alloys

Aligned composite growth of In-rich fibers in an Al-rich matrix can be achieved in hypermonotectic Al-In alloys through directional solidification under interfacially stable conditions, i.e. a sufficiently high thermal gradient to growth rate ratio. During vertically upwards directional solidification, however, a solute depleted boundary layer is expected to develop at the solidification front. In the Al-In system and most other immiscible alloy systems, this solute depleted boundary layer results in an unfavorable density gradient with a more dense liquid above a less dense liquid. This convectively unstable situation is expected to lead to flow iii the liquid in advance of the solidification front. The effect of this flow on the morphology and compositional uniformity along the length of hypermonotectic Al-In samples will be presented. In addition, this study will investigate the feasibility of directionally solidifying vertically downwards to eliminate convection caused by the destabilizing solutal field.

Microgravity processing of immiscible aluminum-indium alloys

Abstract This paper addresses the Coupled Growth in Hypermonotectics experiment carried out aboard NASAs Life and Microgravity Spacelab Mission which took place from June 10 to July 7, 1996. The experiment involved the directional solidification of hypermonotectic (immiscible) alloys in an attempt to obtain steady-state, coupled growth conditions by processing under microgravity conditions. Processing was carried out utilizing the Advanced Gradient Heating Facility directional solidification furnace in order to obtain the high thermal gradients and controlled growth rates required for stability. Solidification under the proper growth conditions was expected to lead to interface stability and the production of a desired aligned fibrous microstructure even for hypermonotectic alloys. The paper covers the objectives of the experiment and a discussion of the anticipated results. While microstructural information is not yet available at the time of writing, results will be presented on the thermal gradients and growth rates obtained and how these compared with predictions.