Deformation of Directionally Solidifying Alloys, and the Earth's Inner Core

地球吗?S的中心，固体内核显示出一些有趣的地震特性，特别是弹性和衰减取决于地震波的传播方向。这种方向性，或者说各向异性，大概能让我们深入了解地球的演化？S核心，本研究将对其进行探索。弹性和衰减各向异性几乎与旋转轴对齐，尽管地震学家开始揭示一个更复杂的结构。大多数对弹性各向异性的解释依赖于可能构成内核的六角形紧密排列的铁晶体的排列，因为单个各向异性的晶体排列产生非零的平均各向异性。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。本研究的主要目的是了解金属合金在凝固过程中的变形，目的是了解地球不寻常地震特性的起源。S内核。本研究将对定向凝固的六方密排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。每个切片将在变形前后检查晶体取向，微观结构（形态和晶粒尺寸）和化学变化。主要研究方向凝固合金的高温变形机制、变形的相关长度尺度（晶粒尺寸或枝晶间距）、动态恢复和再结晶的作用，以及由此导致的微观组织和晶格择优取向（取向）的变化。该研究将检查一系列高温、应变率和总应变，同时测量扭矩以推断应力。希望这项研究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。虽然这项研究的主要动机是地球物理，但定向凝固合金的变形对材料科学来说也很有趣，因为与柱状枝晶相关的长度尺度的多样性。这项研究将让不同的本科生参与到工作的各个方面，让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科学教育，面向不同兴趣和科学背景的学生。

The Earth?s central, solid inner core exhibits some intriguing seismic properties, in particular, elasticity and attenuation that depend on the propagation direction of the seismic wave. This directionality, or anisotropy, is presumably giving us insight into the evolution of the Earth?s core, which this study will explore. The elastic and attenuation anisotropies are nearly aligned with the rotation axis, though seismologists are beginning to reveal a more complex structure. Most explanations for the elastic anisotropy rely on an alignment of the hexagonal close-packed iron crystals that likely compose the inner core, because an alignment of crystals that are individually anisotropic yields a non-zero average anisotropy. 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. The primary intent of this study is to understand deformation of metallic alloys during solidification, with a goal of understanding the origin of the unusual seismic properties of the Earth?s inner core. This study will examine experimentally the high temperature deformation of a hexagonal close-packed 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. The Principal Investigator intends to understand the high temperature deformation mechanism of directionally solidifying alloys, the relevant lengthscale for deformation (grain size or dendritic spacing), the role of dynamic recovery and recrystallization, and the resulting changes in microstructure and lattice preferred orientation (alignment). The study will examine a range of high temperatures, strain rates, and total strains, while measuring the torque to infer the stress. The hope is that 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. Although the primary motivation for the study is geophysical, the deformation of directionally solidifying alloys is also interesting for materials science, because of the multiplicity of lengthscales associated with columnar dendritic crystals. The study will involve diverse undergraduates in all aspects of the work, allowing undergraduates the opportunity to get involved in research at an institution that is actively trying to improve its science education for students across the spectrum in interest and background in science.