Disruptive Solidification Microstructures via Thermoelectric Control

更轻更坚固的合金的发展改变了现代社会。我们发明了一种新方法，潜在地进一步增强机械性能，减轻重量，提高可回收性，同时减少制造过程中的能源消耗。以前的许多改进都归功于对如何在凝固过程中控制合金组织的理解。然而，只有两种常用的控制方法：冷却速度和成分(包括像细化颗粒这样的东西)。我们提出了一种新的附加工具来控制合金微结构的生长。拥有第三种控制方法可能会对英国的金属工业产生革命性的影响，使所有新的产品/性能都能被开发出来。合金是许多元素的组合，这些元素通常凝固为被称为树枝晶的晶体结构。这些树枝晶的形状和它们生长成相连的颗粒产生的微观结构最终决定了材料的整体性能。因此，控制微结构的技术是至关重要的，通过作为EPSRC资助的博士研究的一部分，提出者进行的计算模拟，我们从理论上证明了一种新的机制，通过该机制，磁场可以用来改变或破坏这种微结构。我们提出的新机制利用了热电，这是一种在凝固过程中相对未被研究的现象。基本原理依赖于这样一个事实，即电流在具有不同塞贝克系数的两种材料的界面上循环，只要该界面上存在温度梯度。这种效应在其他领域也有许多实际应用：微电子中的热电制冷器；利用汽车发动机内的温差产生电能的热电材料。这两个例子都使用半导体材料，因为这些材料通常具有较大的塞贝克系数。令人惊讶的是，这种影响在凝固合金固液前沿的微观尺度上也是显着的。事实上，它是这个系统的固有部分。当树枝晶凝固时，潜热被释放出来，产生温度变化，同时一些元素被更多地分配到液体或固相中。这种成分的变化导致塞贝克系数的不连续，在界面上产生了一个电势，导致了热区和冷区之间的热电电流。当凝固受到外部磁场的作用时，这些电流会与之相互作用，从而产生流体运动。这种现象被第一批观察到的研究人员命名为热电磁流体力学(TEMHD)，它导致枝晶臂之间的微观流动和枝晶周围的循环。这种流动会改变枝晶的形状，导致更多的热电流，以此类推。实验证据表明，外加磁场可以导致显微结构的显著变化，但到目前为止，还没有详细分析这种变化是如何发生的。作者的数值模拟让人们对问题的复杂性有了更深入的了解。为了将这一技术应用于实际铸件，我们进行了系统的实验和数值研究。在金刚石光源下对不同磁场下生长微观组织的实时三维观察将明确地证明我们目前唯一的理论假设。数值模拟对于设计这些实验至关重要，可以优化合金和磁场，最大限度地提高对组织结构和性能的影响。关键参数(塞贝克系数、磁场和热梯度)将通过使用完全耦合的3D数值模型和对一系列合金的实验来检查。

Modern society has been transformed by the development of alloys that are lighter and stronger. We have invented a new method to potentially further enhance mechanical properties, reducing weight and increasing recyclability, whilst reducing the energy consumed during manufacturing. Many prior improvements were due to an understanding of how to control alloy microstructure during solidification. However, there are only two commonly utilised methods for control: cooling rate and composition (including things like grain refinement). We propose a novel additional tool to manipulate alloy microstructures as they grow. To have a third method of control could be transformative to the metals industry in the UK, enabling all new products/properties to be developed.Alloys are a combination of many elements that commonly solidify as crystalline structures known as dendrites. The shape of these dendrites and their growth into linked grains produces the microstructure that ultimately determines overall material performance. Techniques for controlling the microstructure are therefore of paramount importance and via computational simulations performed by the proposers as part of an EPSRC funded PhD study, we have theoretically demonstrated that a new mechanism by which magnetic fields can be used to alter, or disrupt this microstructure is possible. The new mechanism we propose utilises thermoelectricity, a relatively unexplored phenomenon in solidification. The fundamental principle relies on the fact that current is caused to circulate around the interface of two materials with different Seebeck coefficients, provided a thermal gradient exists along that interface. This effect has many practical applications in other fields: thermoelectric coolers in microelectronics; thermoelectric materials used to produce electricity from temperature differences within car engines. Both examples use semiconductor materials as these generally have larger Seebeck coefficients. Surprisingly this effect is also significant on the microscopic scale along the solid-liquid front of a solidifying alloy. It is in fact an inherent part of the system. As a dendrite solidifies, latent heat is released creating temperature variations, and simultaneously some elements are partitioned more into either the liquid or solid phase. This compositional variation causes a discontinuity in the Seebeck coefficient, creating a potential across the interface resulting in thermoelectric currents between hot and cold regions. When solidification is subjected to an external magnetic field, these currents interact with it to create fluid motion. This phenomenon -named by the first researchers to observe it as Thermoelectric Magneto-hydrodynamics (TEMHD) - causes microscopic flow between dendrite arms and circulations around the dendrite. This flow can alter the dendrite shape, leading to further thermoelectric currents, and so on. Experimental evidence shows that external magnetic fields can lead to significant changes in microstructure, but so far a detailed analysis of how this occurs has not been conducted. Numerical simulations by the proposers have given some insight into the complex nature of the problem.To harness this technique in real castings, systematic experimental and numerical studies are proposed. Real-time 3D observation of growing microstructure under various magnetic fields at Diamond light source will unequivocally prove our as yet only theoretical hypothesis. Numerical simulations are essential to design these experiments, optimising alloy and magnetic fields, maximising the impact on microstructure and hence properties. Key parameters (Seebeck coefficient, magnetic field and thermal gradient) will be examined through the use of a fully coupled 3D numerical model and experiments for a range of alloys.