Dendrite growth in undercooled melts of Al–Ni alloys solidified on Earth and under reduced gravity

Fluid flow decisively influences the heat and mass transport during solidification of melts and consequently the properties of the as-solidified materials. In this work we present investigations on the influence of convection on the non-equilibrium solidification of different undercooled Al-Ni alloy melts. The dendrite growth velocities in Al-Ni melts containerlessly processed by electromagnetic levitation have been measured on Earth and under the conditions of reduced gravity. While under terrestrial conditions strong electromagnetic fields are required for levitation against gravity, the fields necessary to compensate disturbing accelerations under reduced gravity conditions are smaller by orders of magnitude. Consequently, convective fluid flow induced by electromagnetic stirring effects is significantly decreased in experiments performed under reduced gravity. Comparative experiments on congruently melting Al50Ni50 alloys revealed that the dendrite growth velocities measured at small undercoolings in the electromagnetic positioning facility TEMPUS during parabolic flight campaigns are significantly lower as compared to terrestrial results [1]. At elevated undercoolings, when the growth velocity is exceeding the fluid flow velocity, the influence of convection becomes negligible. Finally, at very high undercoolings and growth velocities a transition from growth of the ordered superlattice structure of the intermetallic AlNi phase to solidification of a disordered superlattice structure due to the nonequilibrium effect of disorder trapping is observed for Al50Ni50 [2]. This transition is associated with a strong rise of the growth velocity. The growth velocity - undercooling relation measured on Earth and during parabolic flight, respectively, is well described by modelling of the dedrite growth using a sharp interface approach with and without considering convective fluid flow. While on the Ni-rich side of the Al-Ni phase diagram and around the equiatomic composition the growth velocity during non-equilibrium solidification is increasing with increasing undercooling, Al-rich Al-Ni alloys show an unusual decrease of the growth velocity with increasing undercooling in terrestrial investigations. The comparison with complementary microgravity experiments performed during the sounding rocket mission TEXUS 44 suggests that the anomalous growth behavior of Al-rich Al–Ni alloys may be caused by fluid-flow related processes [3]. Support by ESA within contract numbers 15236/02/NL/SH (NEQUISOL), by DLR Space Agency under contract number 50 WM 036, by Deutsche Forschungsgemeinschaft (DFG) under contract number HE1601/18 and by the European Commission EC under contract FP6-500635-2 (IMPRESS) is gratefully acknowledged. References: [1] S. Reutzel, H. Hartmann, P.K. Galenko, S. Schneider, and D.M. Herlach, Appl. Phys. Lett. 91, 041913 (2007). [2] H. Hartmann, D. Holland-Moritz, P.K. Galenko, and D.M. Herlach, Europhys. Lett. 87, 40007 (2009). [3] R. Lengsdorf, D. Holland-Moritz, and D.M. Herlach, Scripta Materialia 62, 365 (2010)