Undercooling and solidification of liquid metals under different conditions of convection

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Undercooling of liquid alloys leads to non-equilibrium solidification with the formation of metastable materials of properties being different to those of their stable counterparts. Solidification of undercooled melts is controlled by heat and mass transport in front of the solid-liquid interface. In the case of undercooled melts any planar interface is unstable and one has to deal with dendritic morphology of the solidification front. Both size and morphology of dendrites are essentially influenced by fluid flow. Hence, fluid flow motion controls dendrite growth dynamics. The changes of dendrite growth dynamics are observed by measurements of the dendrite growth velocities as a function of undercooling under different conditions of convection. Since dendrite growth dynamics governs microstructure evolution, effects of convection are expected on the morphological instability and the fragmentation of growing dendrites. The break up of dendrites leads to the formation of grain refined and equiaxed microstructures in contrast to coarse-grained dendritic microstructure. In the present project it is aimed to test various parameters to control the interdendritic fluid flow motion during solidification of undercooled melts. It is envisaged to measure dendrite growth velocity as a function of undercooling and to study the evolution of microstructures as a function of undercooling under the conditions of strongly forced convection in alternating electromagnetic fields by electromagnetic levitation, natural convection by processing the melt in a melt fluxing medium and with highly reduced convection by performing undercooling and solidification experiments in reduced gravity by using an electromagnetic positioning facility (TEMPUS) during parabolic flight campaigns offered by the German Aerospace Center (DLR) and the European Space Agency (ESA). In addition to that our Chinese project partner, Professor Jianrong Gao from Key Laboratory of Electromagnetic Processing of Materials, North-Eastern University, Shenyang will perform comparative experiments, however, utilizing strong electromagnetic fields to test their influence of damping of fluid flow motion for control of dendrite growth velocities and microstructure evolution of undercooled melts. The experimental results will be analysed within current theories of dendrite growth and introducing fluid flow induced by convection as an additional parameter in the description of growth dynamics and microstructure evolution.