Mass transport and structural relaxation in metallic melts as seen by quasielastic neutron scattering

Kurzfassung

We investigate metallic melts ranging from elementary metals to multicomponent glass forming alloys using quasielastic neutron scattering. The intrinsic resolution in the quasielastic neutron scattering technique on the momentum transfer q allows mass transports and microscopic structural relaxation phenomena being studied simultaneously. Combined with containerless processing techniques like e.g.: electrostatic levitation, this technique gives accesses to precise diffusion coefficient data for metallic melts over wide temperature ranges, where the conventional long-capillary method often facies artifacts due to convection and sample-crucible reactions [1-3]. With the accurate data obtained from quasielastic neutron scattering, we are able to study the relation between different dynamic properties in metallic melt, revealing the underlying microscopic mechanism. It is found that in single component metallic melts the empirical Stokes-Einstein relation can predict the diffusion coefficient from the shear viscosity within an accuracy of about 20%, using the covalent atomic radii and applying the slip boundary conditions [1-2]. In contrast, for glass-forming alloy melts, a violation of the Stokes-Einstein relation is observed over a large temperature range, where the self-diffusion D and the melt viscosity $eta$ exhibit the same temperature dependence, and hence D*eta ~ constant [4-6]. This can be attributed to the high packing fraction of the melt, resulting in a dominant microscopic structural relaxation timescale, evident even at temperatures close to the glass transition. This holds also for bulk metallic forming alloys where a mismatch in the high- and low temperature dependence of the melt kinetics exists [7].