Dynamical and structural properties of undercooled Cu-Ti melts investigated by neutron and x-ray diffraction

We investigate temperature-dependent dynamical and structural properties of Cu-Ti melts within a compositional range of 24--69 at. % Ti. Accurate data of viscosity, density, and structure have been obtained by employing the electrostatic levitation technique, which enables containerlessly processing of the samples. Within the Cu-Ti system, the viscosity features a nonmonotonous trend, with the melt of the highest viscosity located at the compositions with intermediate Ti contents. This compositional trend of the liquid dynamics is not reflected in the average packing fraction derived by the macroscopic density, which is almost independent of the Ti concentration. By studying the melt structure using x-ray and neutron diffraction, particularly the direct access of the concentration-concentration structure factor S₂₂, we show that the slowdown of the melt dynamics upon mixing is rather due to chemical effects associated with the preferred formation of Cu-Ti pairs. This leads to a contraction of the interatomic distances, whereas the almost ideal mixing behavior in the molar volume of the melt is a result of the simultaneous decrease of the average coordination number.