Nucleation and Growth Dynamics of Equiaxed Dendrites in Thin Metallic Al-Cu and Al-Ge Samples in Microgravity and on Earth

In this study, the nucleation and growth characteristics of equiaxed dendrites in near-isothermal solidification experiments, performed during two sounding rocket missions, MAPHEUS-6 and 7, are investigated. Two samples of the composition Al-15 wt pct Cu and one sample of the composition Al-46 wt pct Ge were processed in microgravity and several samples on ground. In situ X-radiography was performed to observe the nucleation dynamics and microstructure evolution during solidification of the 200 mu m thin, disc-shaped samples. The measured dendritic growth rates and observed concentration distributions in the liquid indicate no difference between microgravity and on-ground horizontal experiments, in line with previous observations. On the contrary, a difference in the nucleation behavior was found. In all microgravity experiments bursts of nucleation were observed, which is different from comparable on-ground solidification experiments, where the nucleation occurs continuously. The combined analyses suggest that small variations of the concentration fields that probably originate from convective flow and buoyancy of the grains in the on-ground experiments, lead to a spatially variable nucleation undercooling distribution and hence to continuous nucleation. In contrast, the absence of convective flow in microgravity results in a more uniform nucleation undercooling distribution. The grains therefore nucleate in bursts at the surface walls, when the necessary nucleation undercooling is reached.