Conventional techniques to measure diffusion coefficients in liquid metals and alloys are hampered by buoyancy-driven convective fluid flow and chemical reactions of the liquids with container material. To overcome these obstacles we combined containerless processing via electromagnetic levitation with quasielastic neutron scattering. This combination allowed us to study the atomic self-motion in liquid nickel within a broad temperature range from 200 K above to more than 200 K below the melting point, in the metastable regime of an undercooled melt. Other than in liquid Sn the temperature dependence of the Ni self-diffusion coefficient is well described with an Arrhenius law.Processes on the atomic scale determine the physical properties of mass transport in liquids, as well as crystal growth and microstructure evolution during solidification of alloys. In this respect the knowledge of diffusion coefficients, including their temperature and concentration dependence, as well as the profound understanding of diffusion mechanisms is a necessary prerequisite for modeling of solidification processes 1-4 and, hence, to enable a successful materials design from the melt.From the experimental side, classical diffusion experiments in liquid metals, e.g., using the long-capillary method, 5 are hampered by buoyancy-driven convective flow and chemical reactions of the melt with the capillary. For these reasons experimental diffusion data in liquid alloys are rare, the more so the larger the processing temperatures involved. Quasielastic neutron scattering ͑QNS͒ probes sample dynamics on atomic time and length scales, and therefore, the resulting data are not altered by convective flow. This enables us to derive self-diffusion coefficients on an absolute scale for liquids containing an incoherently scattering element. [6][7][8][9][10] Although special thin-walled SiC ͑Refs. 11-13͒ and Al 2 O 3 ͑Refs. 14-16͒ sample crucibles were used, QNS experiments have been limited so far to low melting and/or chemically fairly inert metallic systems and to temperatures close above the liquidus temperature. In order to give access to the study of refractory and chemically reactive melts, containerless processing techniques are required. The absence of a sample holder makes it not only possible to extend the accessible temperature range to high temperatures of up to 2300 K, but also into the metastable regime of the undercooled liquid several hundreds of Kelvin below the equilibrium melting point, due to the avoidance of heterogeneous nucleation at crucible walls.To enable these investigations we designed an electromagnetic levitation ͑EML͒ device for the use with QNS. For a first measurement on the new neutron time-of-flight ͑TOFTOF͒ spectrometer at the FRM II, 17,18 we have chosen liquid Ni, which is a strong incoherent scatterer ͑ i = 5.2 barn͒. As there are diffusion data available for pure Ni in a thin-walled sample geometry, 14 we could verify the levitation approach, and proof that the larger thickness of the levitated samples, with...

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This work presents investigations on the Ni self-diffusion coefficient in Al-Ni melts forming the technological relevant intermetallic phases Al 50 Ni 50 and Al 25 Ni 75 by quasielastic neutron scattering. We found that the Ni self-diffusivity at constant temperature is nearly independent of the composition within the composition range between 0 and 50 at. % Al. This may be a result of the small composition dependence of the atomic volume observed in the same range of composition.

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 relation between static structure and dynamics as measured through the diffusion coefficients in viscous multicomponent metallic melts is elucidated by the example of the binary alloy Zr64Ni36, by a combination of neutron-scattering experiments and mode-coupling theory of the glass transition. Comparison with a hard-sphere mixture shows that the relation between the different self diffusion coefficients strongly depends on chemical short-range ordering. For the Zr-Ni example, the theory predicts both diffusivities to be practically identical. The kinetics of concentration fluctuations is dramatically slower than that of self-diffusion, but the overall interdiffusion coefficient is equally large or larger due to a purely thermodynamic prefactor. This result is a general feature for non-demixing dense melts, irrespective of chemical short-range order.