Primary dendrite array morphology in Al-7 wt% Si alloy samples directionally solidified aboard the International Space Station

Abstract

Under a NASA (National Aeronautics and Space Agency)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions), three Al-7 wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified aboard the International Space Station (ISS) to determine the effect of mitigating convection on the primary dendrite array morphology. MICAST-6 was processed with a step-increase in the growth speed from 5 to 50 µm s−1, MICAST-7 with a step-decrease from 20 to 10 µm s−1, and MICAST2-12 with constant speed of 40 µm s−1. Nearest-neighbor primary dendrite arm spacings and primary dendrite trunk diameters were measured on transverse sections taken at approximately 5 mm intervals along the length of the MICAST samples. In MICAST samples, especially in the steady-state growth regime, the observed primary dendrite nearest-neighbor spacings show a good agreement with predictions from the Hunt-Lu numerical model. The maximum to minimum (highest 10% to lowest 10%) nearest-neighbor spacing ratio in MICAST samples is 2.10 ± 0.27 in agreement with Hunt-Lu model, which assumes pure diffusive transport. Trunk diameters in MICAST samples also agree well with predictions from a coarsening based model. The terrestrial-grown equivalent samples, on the other hand, display smaller nearest-neighbor spacing, and larger primary dendrite trunk diameters than those predicted from the models. Thermosolutal convection appears to increase the dendrite tip radius, causing the trunk diameter to increase, while simultaneously reducing the primary dendrite spacing. This requires verification-experiments involving directional solidification followed by rapid quenching of the liquid-solid interface in metallic alloys in a low gravity environment. The International Space Station currently does not have on-board hardware to conduct such experiments.