Microstructure Evolution in Immiscible AlSiBi Alloys Under Reduced Gravity Conditions

摘要

Strip cast Al-Si-Bi alloys were directionally melted and solidified in the Isother-mal Heating Facility (IHF) of the Werkstofflabor on board the Spacelab mission D2. The main objective of the experiment was to investigate the microstructure evolution during melting and solidification in alloys exhibiting a miscibility gap in the liquid state. The microstructure rearranges from an evenly distributed dis-persion of soild Bismuth particles in a solid AISi matrix due to thermocapillary motion of Bi-rich droplets during melting and solidification in microgravity. The strip casting process provides a material with a well-defined particle dispersion. Two experiment runs were performed successfully. Samples with a Bi-content of 7 wt.% and two different melting rates were melted and solidified. Their as-solidified microstructures clearly show that the Bi droplets moved by ther-mocapillary motion as anticipated. The mean size of the droplets increased by a factor of two and the droplet size distribution exhibits a maximum with very big droplets, revealing that the coalescence of drops contributed to the coarsening of the dispersion. The results are analysed with a numerical modell for such alloys: the Discrete Multi-Particle Approach (DMPA) is presented in some detail and applied to the results.