Strength and anisotropy of hexagonal Fe-Si-C alloy in planetary cores

The observed density of planetary cores is lower than in pure iron-nickel alloy at corresponding conditions. Therefore, the cores of terrestrial planets should be composed of iron-nickel alloyed with some lighter elements. These elements should be abundant in the solar system, siderophile, and compatible with iron at high-pressure high-temperature conditions. Si and C comply with these requirements and could be planetary core constituents. Seismic observations of the Earth's inner core revealed anisotropy of seismic wave propagation. For instance, compressional waves travel 1-3% faster along the polar axis compared to waves travelling in the equatorial plane. One of the hypotheses of the origin of the anisotropy is the plastic deformation and development of textures in inner-core materials under pressure. We employed Fe-Si-C alloy to study its yield strength and anisotropy at high-pressure high-temperature conditions to compare its properties with those observed in the Earth's core. The experiments were conducted by radial X-ray diffraction coupled with resistively heated diamond anvil cells that acted as a deformation apparatus. We performed experiments up to 120 GPa pressure with temperatures exceeding 1100 K. The strength values of Fe-Si-C alloy are higher than the strength of pure Fe and Fe-Si alloys. Our results show lower anisotropy of sound-wave velocities in hexagonal Fe-Si-C alloy compared to the seismic observations. We detected the change in main texture orientation upon compression from [0001] to  in Fe-Si-C alloy. In our presentation, we will discuss the dominant mechanisms of plastic deformation, responsible for these observations, and the overall effects of carbon and silicon on the strength and anisotropy of hexagonal iron alloys in planetary cores.