The structure-dynamics relationship in binary glass-forming alloy melts

Abstract

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 order 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 neutron diffraction at the high-flux diffractometer D20 of the ILL, and the atomic dynamics are investigated by quasielastic neutron scattering (QNS) at the time-of-flight spectrometer TOFTOF of the FRM-II. For liquid Zr64Ni36 [1], Zr50Ni50, Zr36Ni64, Nb40.5Ni59.5 [2] and Hf35Ni65 the full sets of partial structure factors were determined by neutron diffraction combined with an isotopic substitution technique using samples prepared with natural Ni, 60Ni and 58Ni. Ni self-diffusion coefficients have been determined by QNS for the Zr-Ni melts as function of the temperature [1,3]. While for these alloys QNS gives only access to the Ni self-diffusion coefficients due to the vanishing incoherent scattering cross section of Zr, for Hf-Ni alloys the non-vanishing incoherent scattering cross section of Hf allows also to study the atomic dynamics of the early transition metal component. By using a Hf35Ni65 sample prepared with 60Ni we have measured the Hf self-diffusion coefficient and using a sample prepared with natural Ni the mean Ni/Hf self-diffusion coefficient has been determined. Similar as recently reported from radio-tracer measurements in Ni-rich Zr36Ni64 alloys [4] a decoupling of the diffusion coefficients of the different alloy components has been observed for Hf35Ni65, showing a faster Ni diffusion as compared with the Hf-diffusion. In order to analyze the structure-dynamics relationship, we have calculated the transport coefficients in the framework of the mode coupling theory (MCT) of the glass transition using the experimentally determined partial structure factors as an input. The MCT calculations are able to well reproduce the activation energies for self-diffusion inferred from QNS data as well as the decoupling of the diffusion coefficients of the different alloy components experimentally observed for Ni-rich Zr-Ni [4] and Hf-Ni alloys. This demonstrates that the space- and time-averaged structure of the melt, as described by partial static structure factors, is sufficient for an understanding of the atomic dynamics in metallic melts. 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, J. Gegner, T. Hansen, M.D. Ruiz-Martín, and A. Meyer, J. Appl. Phys. 115, 203509 (2014). 3. D. Holland-Moritz, S. Stüber, H. Hartmann, T. Unruh, and A. Meyer, J. Phys. Conf. Ser. 144, 012119 (2009). 4. S. W. Basuki, F. Yang, E. Gill, K. Rätzke, A. Meyer, and F. Faupel, Phys. Rev. B 95, 024301 (2017).