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

地球中央的固体内核表现出一些有趣的特性，特别是地震波的传播速度和衰减取决于地震波的传播方向，自旋轴接近对称轴。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，这是本研究将要探索的。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，这可能是理解内核地震特性起源的关键。该项目将涉及不同的本科生在工作的各个方面，让本科生有机会参与研究的机构，正在积极努力提高其科学教育的学生在整个频谱的兴趣和科学背景。这也将涉及一个博士后谁将花费他/她的时间在本科院校教学和辅导，获得经验的教学和管理教学/研究平衡在本科院校.大多数解释弹性各向异性依赖于排列的六方密堆积（hcp）铁晶体可能构成内核.对齐的解释大致分为两类，凝固纹理和变形纹理。然而，似乎越来越有可能没有一个解释可能足以理解复杂的内核结构。因此，本研究的一个目标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性是内核在西部凝固，由于对流而向东移动，并在西部融化。伴随着这种翻译是退火，第二，本研究的相关目标是更好地了解退火的定向凝固合金，如在地球的内核。本研究的第一部分将实验研究的高温变形的hcp富锌锡合金已定向凝固。定向凝固铸件将具有已被提议用于内芯的柱状枝晶结构。然后将铸件的切片加热到高的同源温度，在该温度下，小部分枝晶间锡将熔化。当保持在这个温度下时，切片将被赋予不同的扭曲以产生恒定的应变速率。将在变形前后检查每个切片的晶体取向、微观结构（形态和晶粒尺寸）和化学变化，同时在变形期间测量扭矩，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，并提供洞察内核的晶粒尺寸和粘度，这两者都涉及到变形mechanism.Previous工作表明，退火的铁晶体，因为它们对流横向内核可以负责东西不对称。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，其中合金元素在主相中具有非常低的溶解度，这已被确定为一个关键的，以前未研究的功能。这项研究将使用相场模型来更好地理解这些系统的演变，并检查hcp富镁合金的退火，该合金已定向凝固，以确认富锌系统中的观察结果。这项研究利用材料科学的方法和经验来阐明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.