Structures in directionally solidified Al-7 wt.% Si alloys: Benchmark experiments under microgravity

Abstract : Microgravity offers a unique opportunity to achieve solidification in the limit of diffusive transport. Directional solidification experiments in Al-7 wt.% Si alloys were carried out under such conditions on board the International Space Station in orbit around the Earth. Microstructural characterizations include the elongation factor and equivalent diameter of the dendritic grains, together with the dendrite arm spacing and the percentage of eutectic. The experimental investigations reveal that coarse randomly oriented dendritic grains promote non-uniform distribution of eutectic and enhance intergranular segregation. The columnar-to-equiaxed transition (CET) observed in the dendritic grain structure of the refined alloys, sharp or progressive, is defined and characterized based on the profile of the averaged elongation factor. Progressive CET is revealed by an intermediate zone where elongated and equiaxed dendritic grains coexist, sandwiched between the columnar and the equiaxed zones. Fragmentation is also observed in non-refined alloy experiments by electron backscattered diffraction analyses. Capillary-driven detachment during coarsening is suggested to explain this finding, while dendrite fragments cannot cause CET under microgravity because of the absence of convection. This unique set of well-characterized experiments serve as benchmark data for direct numerical simulation of structures and segregations.