Melt Structure and Atomic Diffusion in Multicomponent Metallic Melts

This chapter presents investigations on the atomic dynamics and structure in melts of Ni, Ni80P20, Pd40Ni40P20, Pd43Ni10Cu27P20, Zr64Ni36, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (V1), and Zr46.8Ti8.2Cu7.5Ni10Be27.5 (V4) by quasi elastic neutron scattering and neutron diffraction. In order to avoid chemical reactions of the melts with crucible materials and to cover a large temperature range including the metastable regime of undercooled melts, some of the experiments have been performed using a newly developed experimental setup, in which the containerless processing technique of electromagnetic levitation was combined with quasi elastic neutron scattering at the time-of-flight spectrometer TOFTOF of the Forschungsneutronenquelle Heinz Maier-Leibnitz, FRM II, in Garching, Germany, and with elastic neutron scattering at the diffractometer D20 of the Institut Laue Langevin (ILL) in Grenoble, France. In the multicomponent liquids investigated here, an increasing number of components leads to a continuous decrease of the liquidus temperature, the critical cooling rate for glass formation, and the atomic mobility at the liquidus temperature as compared to the binary Ni-P and Zr-Ni systems. At a constant temperature, the replacement of more than 85% of the Ni atoms in Ni-P by Pd and Cu has no resolvable effect on the Ni self-diffusion coefficient. In these dense liquids, atomic transport is controlled by the packing fraction, which is very similar in these alloys. Zr-Ti-Cu-Ni-Be melts forming bulk metallic glasses are characterized by considerably smaller values of the mean Ni and Ti diffusivity as of the Ni diffusivity in the Ni-P-based liquids at similar temperatures. Our investigations show that undercooled binary Zr-Ni melts exhibit the same Ni self-diffusivities as Zr-Ti-Cu-Ni-Be melts at the same temperature, suggesting that the binary systems show similar mechanisms of atomic diffusion as the complex multicomponent alloys. In order to investigate the influence of the short-range order of the liquids on the atomic dynamics, the quasi elastic neutron scattering experiments are accompanied by measurements of the static structure factor by elastic neutron scattering. For Zr64Ni36 melts, partial structure factors have been derived by isotopic substitution, indicating a pronounced chemical order and a topological short-range order different from the icosahedral short-range order found in most melts of pure metals and metallic alloys with a small difference of the atomic radii of the components. Using the measured partial structure factors, calculations in the framework of mode coupling theory have been performed. These suggest that the self-diffusion coefficients for Ni and Zr diffusion are similar and that the interdiffusion coefficient is only slightly larger than both self-diffusion coefficients. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.