RUI: A laboratory study of ultrasonic scattering attenuation by possible microstructures in Earth's inner core

地球内核是弹性各向异性的，地震波平行于自转轴传播的速度比平行于赤道平面传播的速度快3%。内核还表现出衰减各向异性，其中较快的波具有较小的振幅。现在有证据表明，这两种各向异性的模式更加复杂，表现出深度依赖性，半球形变化和较小尺度的区域变化。各向同性地震速度也可能表现出区域性变化。这些令人困惑的地震推断是关于固体铁合金内核结构和演化的线索。这笔赠款将使PI能够调查内核衰减及其各向异性的原因，从而有助于更好地了解我们星球上最偏远的部分。特别是，在实验室研究中，PI将使用超声波探测具有各种微结构的金属合金，这些微结构已被建议用于内芯，采用与内芯相关的波长与晶粒和亚晶粒长度尺度的比率。通过与地震数据的比较，这将允许PI量化散射衰减与固有衰减（粘弹性）的相对重要性。作为RUI的资助，该项目将让不同的本科生参与研究的各个方面，为他们提供科学研究的培训和经验。大多数对弹性各向异性的解释都依赖于六方密堆积铁晶体的晶格择优取向（织构），铁晶体是内核条件下最有可能的稳定相。织构化的解释大致分为两类，即由于固化和由于变形。金属合金的定向凝固通常导致由初级组成相的初级和次级枝晶形成的柱状晶粒，次级相沿着晶界和枝晶之间，即，晶内的这种微结构已被提出用于内核，并且已被建议为衰减各向异性的原因，散射离开细长晶体的晶界。然而，凝固微观结构并不稳定，并且退火可以导致二次枝晶粗化，同时保持一次枝晶和柱状晶体，或者可能导致再结晶和多边形晶粒生长。后者通常发生在暴露于高温的变形材料中。 PI将检查地球内核中可能存在的三种微观结构的超声波散射衰减：由初级和次级枝晶组成的定向凝固柱状晶粒;定向凝固然后退火的晶粒;以及由于变形和退火而重结晶的多边形晶粒。他将使用Pb-Sn，因为其简单的共晶相图，易于使用，和相对较小的单晶弹性各向异性;和相对波长/散射体尺寸的微观结构被认为是类似于内核中的那些。PI将使用超声波尾波的形状和衰减来确定品质因数Q，并将超声波波形与地震数据进行比较，以推断内核中的区域微观结构，这将使人们了解内核的演变。

Earth's inner core is elastically anisotropic, with seismic waves propagating parallel to the rotation axis about 3% faster than those parallel to the equatorial plane. The inner core also exhibits an attenuation anisotropy, with the faster waves having smaller amplitudes. There is now evidence that the pattern of both anisotropies is more complex, exhibiting depth dependence, hemispherical variations, and smaller scale regional variations. The isotropic seismic velocity may also exhibit regional variations. These puzzling seismic inferences are clues as to the structure and evolution of the solid iron alloy inner core. This grant will allow the PI to investigate the causes of inner core attenuation and its anisotropy, and thus help to better understand the most remote part of our planet. In particular, in laboratory studies the PI will use ultrasonic waves to probe metallic alloys with a variety of microstructures that have been suggested for the inner core, employing ratios of wavelengths to grain and sub-grain lengthscales that are relevant to the inner core. By comparison with seismic data, this will allow the PI to quantify the relative importance of scattering attenuation versus intrinsic attenuation (viscoelasticity). As an RUI grant, the project will involve diverse undergraduates in all aspects of the research, providing them with training and experience in doing science.Most explanations for the elastic anisotropy rely on a lattice preferred orientation (texturing) of hexagonal close-packed iron crystals, the most likely stable phase of iron under inner core conditions. Explanations for the texturing fall broadly into two classes, that due to solidification and that due to deformation. Directional solidification of metallic alloys typically results in columnar grains formed by primary and secondary dendrites of the primary compositional phase, with the secondary phase along grain boundaries and between dendrites, i.e., intragranular. Such microstructure has been proposed for the inner core, and scattering off grain boundaries of elongated crystals has been suggested as a cause for the attenuation anisotropy. Solidification microstructure is not thermodynamically stable, however, and annealing can result in coarsening of secondary dendrites, while maintaining the primary dendrites and columnar crystals, or possibly, in recrystallization and polygonal grain growth. The latter typically occurs in deformed materials exposed to high temperature. The PI will examine the ultrasonic scattering attenuation of three possible microstructures likely in Earth's inner core: directional solidified columnar grains composed of primary and secondary dendrites; directionally solidified and then annealed grains; and polygonal grains that result from recrystallization due to deformation and annealing. He will use Pb-Sn because of its simple eutectic phase diagram, ease of use, and relatively small single crystal elastic anisotropy; and microstructures with relative wavelength/scatterer dimensions thought to be similar to those in the inner core. The PI will use the shape and decay of the ultrasonic coda to determine the quality factor Q, and will compare the ultrasonic waveforms with seismic data to infer regional microstructure in the inner core, which will give insight into the evolution of the inner core.