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The study of diffusion processes in melts is vital for an understanding of liquid dynamics, nucleation, vitrification, and crystal growth. Diffusion coefficients are an essential input to the modeling of microstructure evolution and serve as a sound benchmark to molecular dynamics (MD) simulation results. Despite its tremendous importance for the understanding of underlying mechanisms and for the input in modeling and simulation of processes alike, accurate experimental diffusion data in liquid metals and alloys are rare. Conventional techniques exhibit several drawbacks that in most cases prevent an accurate measurement of diffusion coefficients – convective contributions during diffusion annealing are the most prominent ones.

As a consequence a thorough understanding of atomic diffusion and its fundamental relations to other properties is still missing. These include: The relation between self- and interdiffusion (Darken’s relation), the relation between self- and impurity diffusion, the elemental self-diffusion coefficients of an alloy, the dependence of diffusion on temperature, the dependence of diffusion on alloy composition, the relation of diffusion to viscous flow (Stokes-Einstein relation), and the relation of diffusion to crystal growth.

For the last six years we have invented and/or further developed an entire suite of new techniques for the investigation of diffusion in liquid metals and alloys at the DLR Institute of Materials Physics in Space. The use of in-situ techniques plays an essential role for advancements in this field: X-ray and neutron radiography for an in-situ monitoring of interdiffusion processes in long-capillary experiments, and quasielastic neutron scattering for accurate measurements of self-diffusion coefficients. Thereby, the use of radiography techniques allows to circumvent artifacts related to melting and solidification of the sample, and to detect convective flow contributions to mass transport. The use of quasielastic neutron scattering enables the measurement of liquid dynamics on atomic length scales and on a picosecond time scale, which is short enough to be undisturbed by the presence of convective flow.

In this special issue we discuss artifacts and pitfalls of long-capillary interdiffusion experiments that have been uncovered with in-situ monitoring of X-ray radiography and how these insights enabled the design of an improved sample environment. Finally, this led to accurate interdiffusion coefficients, which in turn stimulate the further development of a new high-temperature shear-cell that is currently applied to interdiffusion studies of multicomponent alloys and binary alloys with an insufficient X-ray or neutron radiography contrast, respectively. Prospects and limitations of the use of quasielastic neutron scattering for the measurement of self-diffusion coefficients in one-component liquid metals are outlined with the use of isotopic substitution in liquid germanium, and finally experimental results are presented for self-diffusion of transition metals in liquid silicon alloys.

Andreas Meyer
Director Institute of Materials Physics in Space
German Aerospace Center
Cologne, Germany

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