Dirk Holland-Moritz

Preceding investigations on the atomic dynamics in melts of Ti75Ni25 revealed faster atomic dynamics as compared to that found in melts of Zr64Ni36 and Ni66.7B33.3, despite the fact that all these melts are characterized by similar packing fractions. In order to find a structural explanation for the different dynamic behavior, we have studied the short-range structure of stable and undercooled Ti75Ni25 melts. The melts were containerlessly processed under high-purity conditions by application of the electrostatic levitation technique. Partial structure factors of the liquid alloys have been determined by a combination of neutron diffraction (with isotopic substitution) and synchrotron X-ray diffraction. The studies reveal that Ti75Ni25 melts are characterized by a chemical short-range order, where heterogeneous Ti-Ni nearest neighbors are preferentially formed. This chemical short-range order, however, is less pronounced than the chemical short-range order reported for melts of Zr64Ni36 and Ni66.7B33.3. The less pronounced chemical short-range order may account for the faster atomic dynamics reported for liquid Ti75Ni25. For a further analysis of the structure-dynamics relationship the experimentally determined partial structure factors were used as an input for calculations in the framework of the mode coupling theory of the glass transition. These reveal that the Ni self-diffusion is only slightly faster than the Ti self-diffusion with a ratio of the self-diffusion coefficients DNi/DTi ≈ 1.2. Moreover, the Onsager coefficients calculated by MCT are in good agreement with those estimated by use of Darken's equation indicating that cross-correlation effects are negligible when describing the interdiffusion in liquid Ti75Ni25.

We investigated the change in the structure and dynamics of a Ni–Nb bulk metallic glass upon sulfur addition on both microscopic and macroscopic scales. With the sulfur concentration of 3 at. %, where the composition Ni58Nb39S3 exhibits the best glass forming ability in the investigated sulfur concentration range, both the equilibrium and undercooled melt dynamics remain almost unchanged. Only in the glassy state does sulfur seem to result in mass transport less decoupled to the viscosity of the undercooled liquid, where the measured Ag tracer diffusion coefficient is slower in the ternary alloy. With the structural disorder introduced by the alloying sulfur, the improved glass forming ability is attributed to geometrical frustration, where crystal nucleation requires a depletion of sulfur and hence long range diffusion, as long as no primary sulfur-containing crystalline phase is involved.

Containerless measurements of the thermophysical properties density, viscosity, and self-diffusion by electromagnetic- (EML) and electrostatic levitation (ESL) are compared. The development history of the two techniques is briefly traced. The levitation principles and the measurement techniques for the properties considered are discussed. In the case of the density, data measured by both techniques are available for a liquid NiTi alloy. The results agree within a systematic error of ± 1 %. The data measured in EML exhibit a significant larger scatter. Viscosity data cannot be measured in ground-based electromagnetic levitation, so the comparison is carried out for a NiB alloy investigated in ESL and a classical viscometer. Good agreement was found as well. No significant difference is observed in self-diffusion data of various systems between different levitation techniques.

Neutron diffraction in combination with isotopic substitution on the zero-scatterer ⁶²Ni₄₃ ⁶³Cu₅₇ shows indications for chemical short-range order in the stable liquid as evidenced by oscillations in the concentration–concentration structure factor S _CC(q). This points towards a non-ideal solution behavior of Ni–Cu contrary to common believe but in agreement with measurements of free enthalpy of mixing. The temperature dependence of S _CC at small momentum transfer provides evidence of critical compositional fluctuations in Ni₄₃Cu₅₇ melts.

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