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

本研究的主要目的是评估、表征和量化液态金属和合金的流动行为（剪切应力作为剪切速率的函数）和剪切粘度，这具有学术和商业意义。该研究涉及三个相互作用的主题：流变实验，通过衍射实验表征原子结构，通过分子动力学和第一性原理公式建模。这一倡议的成功完成和液态金属和合金流动行为的表征将对基础和应用材料科学的几个领域产生重大而深远的影响：铸造（直接冷却、连续和近净形）、连接（焊接、焊接和钎焊）、单晶生长、区域细化和金属基复合材料加工等等。许多涉及液态金属和合金的先进制造工艺的设计需要对液相中的输运特性（如剪切粘度和相关的松弛时间）有全面的了解。这些输运性质密切依赖于这些液体的微观结构和原子组成，目前我们对这种相关性的认识相当有限。虽然现象学模型，无论是完全经验的，还是基于涉及液体输运的微观过程的某些近似，已被用于预测液体中的输运现象，但实验与这些模型之间的差距仍然很大。我们最近的探索性实验和出版物表明，与传统观点相反，各种各样的液态金属和合金表现出非牛顿行为，这在低剪切速率下更为明显；这种行为归因于控制机制的转变，从金属键对剪切的阻力，到随着应变速率的增加原子层之间的动量传递。在低剪切速率下，这些液体的流动阻力主要来自于存在于液体结构的短程原子序中的强原子键，而在高剪切速率下，原子层之间的动量传递主导了流动机制。这项工作促使我们通过严格的实验，原子结构评估和模拟来研究控制液态金属和合金流动行为的基本机制，以期关联和量化剪切粘度和液体原子结构对其的影响。目前，我们课题组拥有北美唯一的高温流变仪，能够在环境室中评估最高温度约为1873 K的液体的流变特性；通过我们最近与瑞士流变设备制造商安东帕的合作获得。纯液态金属和具有商业价值的合金，如铁、钢、硅、铝合金、镁合金、铜合金和无铅焊料合金，将被用作高温流变仪流变学表征（剪切应力作为剪切速率的函数）的材料。用中子和同步加速器光束源进行的衍射实验将评估受控外加剪切下液态金属的结构信息。利用VASP和LAMMPS等流行的分子动力学软件包进行非平衡分子动力学模拟；重点将限于确定和验证纯液态金属和简单二元合金的剪切粘度，并将采用适合于研究剪切粘度的原子间电位。通过这项研究，我们将对控制这种行为的物理原理获得有用的见解。

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.