Microgravity Solidification of Al-In Alloys

Abstract

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.