Elastic and attenuation anisotropy in directionally solidified (hcp) zinc,and the seismic anisotropy in the Earth's inner core

The inner core of the Earth has been interpreted as being elastically aniso tropic, with the direction parallel to the rotation axis fast. There are al so observations that north-south rays exhibit greater amplitude reduction a nd more complex waveforms, In previous work, we found that directionally so lidified, polycrystalline, tin (Sn)-rich alloys exhibit a solidification te xturing that is measurable ultrasonically, and suggested that such texturin g might be a cause for the observed seismic elastic anisotropy. Here, we fi nd that zinc (Zn)-rich alloys also exhibit a measurable elastic anisotropy that is due to a preferred growth direction. We confirm the grain orientati on by back-reflection X-ray diffraction. Zn solidifies as a hexagonal close st-packed (hcp) metal, albeit one with a c/a ratio greater than the ideal, unlike hcp iron (Fe), and is 60% elastically anisotropic. We also find that the columnar dendritic grains that grow with a preferred orientation durin g directional solidification result in anisotropic scattering off grain bou ndaries, yielding an anisotropic apparent attenuation. In addition, we pres ent results for directionally solidified 100% Zn, which exhibits qualitativ e differences in both the elastic and attenuation anisotropy from Zn alloys . The laboratory results can be interpreted as a cause for both the seismic elastic and attenuation anisotropy if the cooling near the inner core has more nearly cylindrical than spherical symmetry. (C) 2000 Elsevier Science B.V. All rights reserved.