MEASUREMENT OF DENDRITE GROWTH ON Al-Ni ALLOYS IN REDUCED GRAVITY

Kurzfassung

It is well known that the growth kinetics in metallic melts controls microstructure evolution. If the melt is cooled below its equilibrium melting temperature prior to solidification, the state of a metastable undercooled melt is created. An undercooled melt possesses an enhanced free energy that enables the liquid to choose solidification pathways into various metastable solids of properties being different to their stable counterparts. A very efficient method to undercool a metallic liquid is the application of containerless processing a liquid drop such that heterogeneous crystal nucleation on container walls is completely avoided. Electro-magnetic levitation is a power-full technique to produce a freely suspended drop without any contact to a solid or liquid medium with the extra benefit that it is accessible for direct observation of solidification far from equilib-rium by proper diagnostic means. Under terrestrial conditions, strong electromagnetic fields are needed to compensate the gravitational force. That, in turn, causes forced convection inside the liquid drop and influences mass and heat transport, and consequently, crystal growth in undercooled melts. If the reduced gravity environment is utilized the forces to compensate residual accelerations are several orders of magnitude smaller than the levitation force on Earth. In the present paper we report on results obtained in the Earth laboratory, during parabolic flight missions and during TEXUS 44 flight in 2008 using the TEMPUS facility for containerless processing of metals in space. The results are discussed within dendrite growth theory and give evidence for strong effects of gravitational driven effects in the solidification dynamics.