Defect And Disorder In Dendritic Arrays Solidified on Earth And on the Space Station

Under a NASA (National Aeronautics and Space Administration)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in the casting of technical alloys under diffusive and magnetically controlled convection conditions), three Al-7wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified at growth speeds varying from 10 to 50 pm s-1 aboard the International Space Station to determine the effect ofmitigating convection on the primary dendrite array. The purpose of this research is to examine the ordering in the pattern formation during dendritic array growth of binary metallic alloys and explore if natural convection affects the extent of the disorder. Contrary to the expectations the MICAST samples also show some defects, such as misoriented primary dendrites or macrosegregation usually attributed to natural convection. It is observed that all of the primary dendrites on a cross-section do not have identical shape and morphology. Natural convection during terrestrial growth introduces more scatter in their morphology and distribution. Fast Fourier Transform analysis of the transverse images should be investigated as another tool to quantitatively determine the extent of disorder in the mushysone introduced by natural convection