M.I. Bergman, S. Agrawal, and M. Carter
Physics Department, Simon's Rock College
The inner core has been inferred to be elastically anisotropic, with the direction parallel to the rotation axis about 3-4 % fast. More detailed seismic studies have revealed that the elastic anisotropy may be nearly absent from the top of the inner core, that there may be longitudinal variations in the elastic anisotropy, and that there may be an attenuation anisotropy such that the fast direction exhibits smaller amplitudes and more complex waveforms. Most explanations for the elastic anisotropy involve a preferred orientation of the inner core hcp Fe grains, but there is not agreement on the texturing mechanism. Some hypotheses involve deformation, some solidification. However, no single, simple hypothesis is fully capable of explaining the range of seismic inferences. Moreover, there is still uncertainty of the elastic constants of Fe under inner core conditions. In the original solidification texturing hypothesis it was assumed, as had been observed during dendritic growth, that the c-axes of the hcp grains lie randomly in the plane transverse to the growth direction, which was assumed to be cylindrically radial. However, recent observations on hcp sea ice solidifying in the presence of a fluid flow in the melt have shown that a transverse texture can also develop. In particular, if the flow is driven externally, such as by an ocean current beneath a solidifying ice sheet, the c-axes tend to lie parallel to the flow. On the other hand, if the flow is driven convectively, such as results from horizontal solidification from a vertical chill, the c-axes tend to lie perpendicular to the flow. These textures are thought to arise from the interaction of the flow with the solute boundary layer within and adjacent to the dendrites, which take on a platelet form in hcp alloys. In these experiments we investigate whether fluid flow during the solidification of hcp Zn alloys also causes a transverse solidification texture. We solidify the metal horizontally from a vertical chill, so that there is a mean downwards flow adjacent and parallel to the solidification interface (as in an analogous experiment where we solidified salt water), and examine the orientation of the grains using X-ray diffraction. We hope to establish whether transverse texturing due to fluid flow is a general phenomenon during the solidification of hcp alloys, as a result of the platelet nature of hcp dendrites. If so, it may also occur during the solidification of the inner core, due to flow at the base of the outer core. This transverse texturing could contribute to the complexity of the inner core anisotropy, though the effects of post-solidification deformation are not known.