Microgravity solidification of immiscible alloys

This paper covers findings obtained from the microgravity directional solidification of immiscible aluminum-indium alloys during the Life and Microgravity Spacelab Mission in 1996. Three alloys, one of monotectic composition and two alloys containing an excess of indium above the monotectic (i.e., hypermonotectic compositions) were solidified using the Advanced Gradient Heating Facility (AGHF). Samples were processed in specialized ampoule assemblies containing pistons and a high temperature spring in a partially successful attempt to prevent void formation due to thermal contraction of the melt and solidification shrinkage. A comparison of compositional variations between microgravity processed and ground processed samples 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. However, a discontinuity in the microstructure was observed which coincided with the presence of a void in the flight sample.