Microstructure and Macrosegregation Study of Directionally Solidified Al-7Si Samples Processed Terrestrially and Aboard the International Space Station

This talk reports research that has been carried out under the aegis of NASA as part of a collaboration between ESA and NASA for solidification experiments on the International Space Station (ISS). The focus has been on the effect of convection on the microstructural evolution and macrosegregation in hypoeutectic AlSi alloys during directional solidification (DS). The DSexperiments have been carried out under 1g at Cleveland State University (CSU) and under lowg on the International Space Station (ISS). The thermal processinghistory of the experiments is well defined for both the terrestriallyprocessed samples and the ISSprocessed samples. We have observed that the primary dendrite arm spacings of two samples grown in the lowg environment of the ISS show good agreement with a dendritegrowth model based on diffusion controlled growth. The gravitydriven convection (i.e., thermosolutal convection) in terrestrially grown samples has the effect of decreasing the primary dendrite arm spacings and causes macrosgregation. In order to process DSsamples aboard the ISS, dendriticseed crystals have to partially remelted in a stationary thermal gradient before the DS is carried out. Microstructural changes and macrosegregation effects during this period are described

Evaluation of the MICAST #2-12 AI-7wt%Si Sample Directionally Solidified Aboard the International Space Station

The US team of the European led "MIcrostructure Formation in CASTing of Technical Alloys under Diffusive and Magnetically Controlled Convective Conditions" (MICAST) program recently received a third Aluminum - 7wt% silicon alloy that was processed in the microgravity environment aboard the International Space Station. The sample, designated MICAST#2-12, was directionally solidified in the Solidification with Quench Furnace (SQF) at a constant rate of 40micometers/s through an imposed temperature gradient of 31K/cm. Procedures taken to evaluate the state of the sample prior to sectioning for metallographic analysis are reviewed and rational for measuring the microstructural constituents, in particular the primary dendrite arm spacing (Lambda (sub1)), is given. The data are presented, put in context with the earlier samples, and evaluated in view of a relevant theoretical model