5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Experiments on the International Space Station (ISS) lead to the idea of a special growth mechanism in Al-rich Al-Ni melts. Previous analyses of dendrite growth velocity as a function of undercooling in these alloys showed anomalous behaviour, particularly a decreasing velocity for increasing undercoolings [1]. The experiments on the ISS reproduced the results obtained on earth but with an unprecedented clarity. During rapid solidification, the visible growth front does not consist of dendrite tips that grow along the surface of the sample, but is formed by a sequence of nucleation events propagating perpendicularly to the actual dendrite growth direction. The front progression is thus a superposition of growth of existing surface nuclei and new nucleation events in the vicinity of the existing nuclei. Microstructure analysis strongly supports the theory of this growth mechanism. It shows dendrites growing towards the sample centre and provides clear evidence of nucleation at the sample surface. However, the question concerning the anomalous behaviour (slower apparent growth with higher undercoolings) is not yet fully answered by the nucleation mechanism. For this, a time resolved analysis of nucleation and surface growth is presented to investigate the relation between the nucleation rate, front propagation and undercooling. Acknowledgements ---------------- This work was financially supported by the DFG project HE 1601/26-3 and ESA-MAP project AO-2009-0829. The authors thank the team from the Microgravity User Support Center at DLR, Cologne, and ESA for support with the experiments in the ISS-EML. Reference --------- [1] R. Lengsdorf, D. Holland-Moritz, D. M. Herlach, Scripta Materialia 62 (2010) 365-367