Short-Range Order and Atomic dynamics in Glass-Forming Alloy Melts

Kurzfassung

Despite considerable research effort, the mechanisms of glass-formation are not well understood on the atomic length and time scale. While the metallic alloys showing the best glass-forming ability are multi-component alloys, the less complex binary glass-forming systems are attractive for studies aiming to find a microscopic understanding of the glass-formation process. In this work results of studies on the short-range structure and on the atomic dynamics in different stable and undercooled binary glass-forming metallic melts are presented. In order to undercool the melts deeply below the melting temperature and to avoid chemical reactions of the melts with crucible materials, the samples are containerlessly processed utilizing the electromagnetic or the electrostatic levitation technique. The short-range structure of the containerlessly processed melts is studied by diffraction of neutron- or synchrotron radiation, and the atomic dynamics are investigated by quasielastic neutron scattering. While for most of the experimental studies on the short-range order of glass-forming alloy melts only total X-ray structure factors of limited significance have been measured, here we report investigations on melts of Zr-Ni, Zr-Cu, Zr-Co, Zr-Pd, Nb-Ni and Hf-Ni, from which partial structure factors are directly inferred from the experiment [1-4]. For liquid Zr64Ni36 [1], Nb40.5Ni59.5 [4] and Hf35Ni65 the full set of partial structure factors was measured by neutron diffraction combined with an isotopic substitution technique using samples prepared with natural Ni, 60Ni and 58Ni. For melts of Zr2Cu, Zr50Cu50, Zr7Cu10 and Zr2Pd the combination of neutron scattering with X-ray diffraction allowed for the determination of good approximations of the two Bhatia-Thornton partial structure factors SNN and SNC [2,3] and for liquid Zr74Co26 of the partial Faber-Ziman structure factors S_ZrZr and S_ZrCo. From the number-number structure factors SNN nearest neighbour coordination number, ZNN, have been determined. Characteristic of the studied glass-forming melts are large values of 13.3 ≤ Z_NN ≤ 14.3. This is in contradiction to speculations on a decisive role of icosahedral SRO for glass-formation, because a dominant icosahedral SRO (as well as a fcc- or hcp type SRO) implies Z_NN = 12. The Ni, Co and Cu self-diffusion coefficients in glass-forming Zr-Ni, Zr-Cu, Zr-Co and Hf-Ni melts of different compositions were measured by quasielastic neutron scattering as a function of the temperature [1]. The results indicate high activation energies for the Ni, Cu or Co self-diffusion and consequently sluggish atomic dynamics, favouring glass-formation. The sluggish atomic mobility may find its structural explanation in the large local density of packing suggested by the high nearest neighbour coordination numbers. 1. D. Holland-Moritz, S. Stüber, H. Hartmann, T. Unruh, T. Hansen, and A. Meyer, Phys. Rev. B 79, 064204 (2009). 2. D. Holland-Moritz, F. Yang, T. Kordel, S. Klein, F. Kargl, J. Gegner, T. Hansen, J. Bednarcik, I. Kaban, O. Shuleshova, N. Mattern, and A. Meyer, EPL 100, 56002 (2012). 3. S. Klein, D. Holland-Moritz, D.M. Herlach, N.A. Mauro, and K.F. Kelton, EPL 102, 36001 (2013). 4. D. Holland-Moritz, F. Yang, J. Gegner, T. Hansen, M.D. Ruiz-Martín, and A. Meyer, J. Appl. Phys. 115, 203509 (2014).