Strength and anisotropy of textured ε-FeSi at extreme conditions

The cores of terrestrial planets are comprised of Fe-Ni alloys, with around 5-10 weight % of the light elements. They account for the observed core density deficit compared to the Fe-Ni alloy at high-pressure high-temperature conditions. Silicon has long been considered as a major light element of the cores due to its high cosmic abundance and ability to incorporate into the Fe-Ni alloy during core formation. Thus, Fe-Si alloys and compounds are likely to be present in the cores of terrestrial planets such as Mercury.

The seismological observations indicate that compressional waves travel slower through the equatorial plane of the Earth’s core compared to the waves traveling along the polar axis. This anisotropy is likely related to the plastic deformation of the inner core material. Yet, the effect of light elements on the plasticity of iron is poorly known.

Here we investigate lattice strains and textures of ε-FeSi up to 50 GPa at 300 K and 1100 K. We employed the radial x-ray diffraction technique using diamond anvil cells as deformation apparatus. We used our data to determine yield strength of ε-FeSi and to assess the sound-velocity anisotropy of the polycrystalline ε-FeSi as a function of pressure and temperature. Upon compression, FeSi aggregate does not develop strong textures and, as a result, shows weak sound-velocity anisotropy. The strength of the material is nearly temperature-independent. Thus, we infer the constraints on the silicon budget in planetary deepest interior by demonstrating that strong sound-velocity anisotropy is incompatible with the high silicon content.