Growth of dendrites into undercooled melts under microgravitational conditions

Abstract

The application of cntainerless processing by electromagnetic levitation gives access to liquid drops of metals and alloys to be undercooled over a large temperature range below their melting points. In such a way a metastable liquid is created, which possesses an enhanced free enthalpy. This can be used by the system to choose between different solidification pathways in various metastable solid states which may differ in their physical properties from the stable counterpart. In the present, the velocity of crystal growth in undercooled melts is investigated. Crystal growth in undercooled melts takes place by rapid propagation of dendrites into the liquid. Dendrite growth is controlled by heat-and in case of alloys by mass-redistribution in front of the solid / liquid interface. Since heat-and mass transport are influenced by convection it is essential to determine the influence of convection on dendrite growth dynamics. Comparative experiments on measurements of dendrite growth velocity as a function of undercooling under terrestrial conditions and in reduced gravity are presented. Mesoscopic models of dendrite growth are extended in order to describe growth dynamics without and with convection. These models are applied to analyze the experimental results. Finally, an outlook is given for future experiments to be performed on board the International Space Station, which are currently in preparation.