Short-range order and atomic dynamics of Ti75Ni25 melts

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.