Shear Viscosity of Liquid metals and alloys: Experiments and Atomic Characterization.

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The main objective of this study is to evaluate, characterize and quantify the flow behavior (shear stress as a function of shear rate) and shear viscosity of liquid metals and alloys that hold both academic and commercial significance. The research involves three interacting themes carried out in tandem: rheological experiments, atomic structure characterization through diffraction experiments and modeling through molecular dynamics and first principle formulations. The successful completion of this initiative and characterization of the flow behavior of liquid metals and alloys will have significant and far reaching impact in several areas of fundamental and applied materials science: casting (direct chill, continuous and near net shaped), joining (welding, soldering and brazing), single crystal growth, zone-refining and metal matrix composite processing to name a few. The design of many advanced manufacturing processes involving liquid metals and alloys requires a comprehensive understanding of transport properties in the liquid phases (such as the shear viscosity and associated relaxation time). These transport properties depend intimately on the microscopic structure and atomic composition of these liquids and our knowledge of this correlation is quite limited at present. While phenomenological models, either entirely empirical, or based on certain approximations of the microscopic processes involved in liquid transport have been used to predict transport phenomena in liquids, the gap between experiments and these models remains quite large. Our recent exploratory experiments and publications demonstrate that contrary to conventional wisdom, a wide variety of liquid metals and alloys exhibit non-Newtonian behavior, which are more pronounced at low shear rate regimes; this behavior is attributed to a transition in the governing mechanism from the resistance of the metallic bonding to shear, to momentum transfer between atom layers with increasing strain rate. At low shear rates the resistance offered to the flow of these liquids from the strong atomic bonds existing in the short range atomic order of the liquid structure dominates while at higher shear rates, the momentum transfer between atomic layers dominates the flow mechanism. This work motivates us to examine the fundamental mechanisms governing flow behavior in liquid metals and alloys through rigorous experiments, and atomistic structure evaluation and simulations, with a view to correlate and quantify the shear viscosity and the impact of liquid atomic structure on the same. Currently, our research group has the only high temperature rheometer in the North America, capable of evaluating rheological properties of liquids with a maximum temperature of about 1873 K in an environment chamber; acquired through our recent partnership with the Switzerland rheology equipment manufacturer, Anton Paar. Pure liquid metals and alloys of commercial interests such as Fe, steel, Si, Al alloys, Mg alloys, Cu alloy and Pb free solder alloys would be used as materials for rheological characterization (shear stress as a function of shear rate) with the high temperature rheometer. Diffraction experiments with both the Neutron and Synchrotron beam sources will evaluate structure information of liquid metals under controlled applied shear. Non-equilibrium molecular dynamics simulations using the popular molecular dynamics software packages, including VASP and LAMMPS will be formulated; attention will be restricted to determining and validating shear viscosity in pure liquid metals and simple binary alloys, and interatomic potentials appropriate for a study of shear viscosity will be employed. Through this study, we will gain useful insights into the physics governing this behavior.