High‐Pressure Deformation of Iron–Nickel–Silicon Alloys and Implications for Earth’s Inner Core

Abstract

Since the discovery of the inner core (Lehmann, 1936) and its identification as a solid iron alloy (Birch, 1952), seismic studies have revealed it to be complexly structured and seismically anisotropic. On average, seismic waves move through the inner core several percent faster on paths parallel to Earth's axis of rotation compared to perpendicular paths (e.g., Deuss, 2014). This anisotropy is well documented by travel time (e.g., Creager, 1992) and normal mode (e.g., Durek & Romanowicz, 1999) observations, and deviations from this large-scale anisotropy have been used to suggest other inner core structural features (e.g.

Strength and seismic anisotropy of textured FeSi at planetary core conditions

“…Our results demonstrate that temperature does not significantly influence the average lattice strain parameter in ε-FeSi, at least within the experimental precision. Brennan et al (2021) reported a decrease of the 〈Q(hkl)〉 with estimated d〈Q〉 dT = -1.6•10 -6 K −1 in iron with 5 wt% silicon in the 43-46 GPa pressure range. This disagreement with our observations may arise from the difference in plastic behavior of hcp Fe-Si alloy and cubic FeSi, but more likely from differences in the experimental strategy.…”

“…This disagreement with our observations may arise from the difference in plastic behavior of hcp Fe-Si alloy and cubic FeSi, but more likely from differences in the experimental strategy. Brennan et al (2021) measured the sample stress while increasing temperature between 300 and 1800 K at 43-46 GPa but did not further deform their sample at high temperature. Their measurement is hence more related to stress relaxation induced by laser heating rather than an effect of temperature on plastic strength.…”

“…Note, however, that none of the regular slip systems reported for cubic metals or ionic crystals can result in textures with a minimum at [001] and maxima at both [011], [111], and [101]. Also note that, in the body-centered cubic Fe-Si alloys, texture develops along [111] direction in compression (Brennan et al, 2021) this behavior is similar to the dominant texture in the B20 ε-FeSi phase. However, the Fe-Si alloy does not show the secondary maximum along [110] we observe here.…”

“…We do not observe any significant difference of the yield strength based on temperature. Figure 4 shows a comparison of the strength of ε-FeSi, Fe-Si alloys (Brennan et al, 2021) with the compositions Fe-5 wt% Si (Fe-5Si) and Fe-9 wt% Si (Fe-9Si), and pure iron (Gleason and Mao, 2013) at 300 K. The ε-FeSi investigated here displays a yield strength about 3 times higher than hcp-Fe and about 2 times higher than that of Fe-9Si and Fe-5Si alloy at 30 GPa. In other words, our result suggests that ε-FeSi is a harder material to deform compared to pure iron and iron-silicon alloys.…”

“…Pressure evolution of the yield strength of ε-FeSi at 300 K and 1100 K along with data for hcp-Fe-5Si, hcp-Fe-9Si alloy (Brennan et al, 2021), and hcp-Fe (Gleason and Mao, 2013) for comparison.…”