Collaborative Research: Annealing and Deformation of Directionally Solidified Alloys, and the Earth's Core

地球的中心，固体内核显示出一些有趣的特性，特别是地震波的速度和衰减取决于地震波的传播方向，自转轴接近对称轴。此外，有证据表明，这种方向性或各向异性在内核的西半球更强。这些地震推断可以让我们深入了解地核的演化和结构，这是本研究将探索的。这项工作将借鉴材料科学和地球物理学的经验，研究凝固、退火和变形的过程，这可能是了解内核地震性质起源的关键。该项目将让不同的本科生参与到工作的各个方面，使本科生有机会参与到一个机构的研究中来，这个机构正在积极努力改善其对不同兴趣和科学背景的学生的科学教育。该项目还包括一名博士后，他/她将花三分之一的时间在本科院校教学和接受指导，在本科院校积累教学经验和管理教学/科研平衡。大多数对弹性各向异性的解释都依赖于可能构成内核的六边形密排铁晶体的排列。对取向的解释大致可分为两类：凝固变形和变形变形。然而，似乎越来越有可能没有一种解释足以理解复杂的内核结构。因此，本研究的目标之一是了解金属合金在凝固过程中的变形。东西不对称的一种可能是，内核在西部凝固，由于对流向东移动，在西部融化。伴随这一转变的是退火，本研究的第二个相关目标是更好地理解定向凝固合金的退火，例如地球内核中的退火。本研究的第一部分将对定向凝固的hcp富锌锡合金的高温变形进行实验研究。定向凝固的铸件将具有柱状、枝晶结构，这是为内核提出的。然后将铸件的薄片加热到较高的相应温度，在此温度下，一小部分枝晶间锡将熔化。当保持在这个温度时，片将被赋予一个不同的扭转以产生恒定的应变率。将在变形前后检查每个薄片的晶体取向、微观结构（形貌和晶粒尺寸）和化学变化，同时在变形过程中测量扭矩，以建立应力-应变关系，从而确定变形机制。本研究的第一个目标将有助于解释内核的弹性和衰减各向异性，并深入了解内核的晶粒尺寸和粘度，这两者都与变形机制有关。先前的工作表明，铁晶体对流穿过内核时的退火可能是东西不对称的原因。因此，本研究的第二个目标是更好地理解定向凝固合金的退火，其中合金元素在初级相中的溶解度非常低，这已被确定为一个关键的，以前未研究的特征。该研究将使用相场建模来更好地理解这类系统的演变，并检查定向凝固的hcp富镁合金的退火，以证实富锌系统中的观察结果。该研究利用材料科学的方法和经验，揭示了地球科学中一个令人困惑的问题。

The Earth's central, solid inner core exhibits some intriguing properties, in particular, seismic wavespeed and attenuation that depend on the propagation direction of the seismic wave, with the spin axis being close to the axis of symmetry. Moreover, there is evidence that this directionality, or anisotropy, is stronger in the western hemisphere of the inner core. These seismic inferences can give us insight into the evolution and structure of the Earth's core, which this study will explore. The work will draw on experience in materials science and geophysics to study the processes of solidification, annealing, and deformation, which are likely key to understanding the origin of the inner core seismic properties. The project will involve diverse undergraduates in all aspects of the work, allowing undergraduates the opportunity to get involved in research at an institution that is trying actively to improve its science education for students across the spectrum in interest and background in science. It will also involve a post-doc who will spend one-third of his/her time teaching and being mentored at an undergraduate institution, gaining experience teaching and managing the teaching/research balance at an undergraduate institution.Most explanations for the elastic anisotropy rely on an alignment of the hexagonal close-packed (hcp) iron crystals that likely compose the inner core. The explanations for the alignment fall broadly into two classes, solidification texturing and deformation texturing. However, it seems increasingly likely that no one explanation may suffice to understand the complex inner core structure. Hence, one goal of this study is to understand deformation of metallic alloys during solidification. One possibility for the east-west asymmetry is that the inner core is solidifying in the west, translating eastward due to convection, and melting in the west. Accompanying this translation is annealing, and a second, related goal of this study is to better understand the annealing of directionally solidified alloys such as that in the Earth's inner core.The first part of this study will examine experimentally the high temperature deformation of an hcp zinc-rich tin alloy that has been directionally solidified. The directionally solidified castings will have the columnar, dendritic structure that has been proposed for the inner core. Slices of the castings will then be heated to a high homologous temperature, at which the small fraction of interdendritic tin will melt. While held at this temperature, a slice will be given a differential twist to produce a constant strain rate. Each slice will be examined before and after deformation for crystalline orientation, microstructure (morphology and grain size), and chemical variations, while the torque will be measured during the deformation in order to establish the stress-strain relationship and hence the deformation mechanism. The first goal of this study will help to interpret inner core elastic and attenuation anisotropies, and to give insight on the grain size and viscosity of the inner core, both of which relate to the deformation mechanism.Previous work has shown that annealing of iron crystals as they convectively transverse the inner core could be responsible for east-west asymmetry. The second goal of this study is thus to better understand the annealing of directionally solidified alloys where an alloying element has a very low solubility in the primary phase, which has been identified as a key, previously unstudied feature. The study will use phase field modeling to better understand the evolution of such systems, and also examine the annealing of an hcp magnesium-rich alloy that has been directionally solidified to confirm the observations in the zinc-rich system. The study uses the methods and experience of materials science to shed light on a puzzling problem in Earth science.