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The purpose of the present work was to measure the velocity of dendrite growth in undercooled Ni–Al alloy melts as a function of undercooling. The experiments were performed both by containerless electromagnetic levitation on Earth and under reduced gravity conditions during parabolic flight campaigns. While under terrestrial conditions, strong magnetic fields are required to compensate the gravitational force, the forces to compensate disturbing accelerations are decreased by orders of magnitude in reduced gravity. In turn, the alternating electromagnetic fields induce convection, which is strong under terrestrial conditions while much weaker in reduced gravity. The heat and mass transport in front of the solid-liquid interface during solidification controls the dynamics of dendrite growth. By comparing results obtained on Earth and in reduced gravity, it was demonstrated that the change of transport conditions by convection significantly alters the kinetics of solidification and the evolution of grain refined microstructures at undercoolings less than 100K.

Abstract. We report on investigations on the atomic dynamics in melts of different binary ZrNi alloys and of the ternary glass-forming Zr 60 Ni 25 Al 15 alloy. The liquids are containerlessly processed in an electromagnetic levitator that is combined with quasielastic neutron scattering at the time of flight spectrometer TOFTOF of the FRM II. Ni self-diffusion coefficients are determined that exhibit an Arrhenius-type temperature dependence with comparatively large activation energies ranging between 0.64 and 0.90 eV. Although glass forming abilities and melting temperatures for these alloys exhibit large differences, the absolute values of the selfdiffusion coefficients are similar at same temperature. IntroductionAmong numerous multi-component alloys forming (bulk) metallic glasses [1], one important class of alloys is based on the binary Zr-Ni system. The solidification behaviour of melts and consequently the glass formation process is strongly affected by the the atomic dynamics in the liquid. While for glassforming Pd-(Ni-Cu-)P melts the viscosity is coupled with the diffusivity by the Stokes-Einstein relation above the critical temperature T c of mode-coupling theory [2], for Zr-Ti-Cu-Ni-Be melts viscosity and diffusivity are decoupled above T c and the liquidus temperature T L [3]. Moreover these liquids exhibit considerably smaller values of the Ni self-diffusivity as Pd-(Ni-Cu-)P alloys at similar temperatures. In this work we present investigations on the Ni self-diffusion in melts of different binary Zr-Ni alloys and of the ternary glass forming alloy Zr 60 Ni 25 Al 15 by quasielastic neutron scattering. In order to avoid reactions of the Zr-based melts with crucible materaials and in order to cover a large temperature range including the metastable regime of undercooled liquids at temperature below the melting temperature, the samples are containerlessly processed by application of the electromagnetic levitation technique.

The dendrite growth velocity during solidification is measured on liquid drops of the intermetallic compound Ni50Al50 undercooled by levitation up to 265 K. A sharp increase of the growth velocity is found at a critical undercooling ∆T * ≈ 250 K. In situ diffraction of synchrotron radiation on levitation-processed samples unambiguously shows a transition from ordered to disordered growth at ∆T * . The sharp interface model is extended to describe the transition from ordered to disordered dendrite growth by taking into account the velocity dependence of the order parameter and the kinetic growth coefficient.