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的传播速度比平行于赤道平面的传播速度快约3%。内核也表现出衰减各向异性，较快的波具有较小的幅度。现在有证据表明，这两种各向异性的模式更加复杂，表现出深度相关性、半球变化和较小的尺度区域变化。各向同性地震速度也可能表现出区域性变化。这些令人费解的地震推断为固体铁合金内核的结构和演化提供了线索。这笔赠款将使PI能够调查内核衰减的原因及其各向异性，从而有助于更好地了解地球上最偏远的部分。特别是，在实验室研究中，PI将使用超声波探测具有所建议的内芯的各种微观结构的金属合金，使用与内芯相关的波长与颗粒和亚晶长度的比例。通过与地震数据的比较，这将使PI能够量化散射衰减与固有衰减(粘弹性)的相对重要性。作为瑞安基金的一项资助，该项目将让不同的本科生参与研究的各个方面，为他们提供科学方面的培训和经验。对弹性各向异性的大多数解释依赖于六方密排铁晶体的晶格择优取向(织构)，这是铁在内核条件下最有可能稳定的相。对织构的解释大致可分为两类，一类是由于凝固引起的，另一类是由于变形引起的。金属合金定向凝固的典型结果是由一次成分相的一次枝晶和二次枝晶形成的柱状晶，第二相沿晶界和枝晶间形成，即晶内。这种微结构已经被提出用于内芯，并且细长晶体的晶界散射被认为是衰减各向异性的原因。然而，凝固组织在热力学上是不稳定的，退火会导致二次枝晶的粗化，而保留一次枝晶和柱状晶，或者可能导致再结晶和多边形晶长大。后者通常发生在暴露在高温下的变形材料中。PI将检测地球内核中可能存在的三种微结构的超声波散射衰减：由一次和二次树枝晶组成的定向凝固柱状颗粒；定向凝固然后退火的颗粒；以及由于变形和退火而产生的再结晶的多边形颗粒。他将使用铅-锡，因为其简单的共晶相图，易于使用，以及相对较小的单晶弹性各向异性；以及被认为具有与内核相似的相对波长/散射体尺寸的微观结构。PI将利用超声波尾波的形状和衰变来确定品质因数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结构，这将深入了解内核的演化。

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.