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

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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.