Equations of State and Anisotropy of Fe‐Ni‐Si Alloys

We present powder X‐ray diffraction data on body centered cubic (bcc)‐ and hexagonal close packed (hcp)‐structured Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 at 300 K up to 167 and 175 GPa, respectively. The alloys were loaded with tungsten powder as a pressure calibrant and helium as a pressure transmitting...

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Bibliographic Details
Published in:Journal of geophysical research. Solid earth 2018-06, Vol.123 (6), p.4647-4675
Main Authors: Morrison, Rachel A., Jackson, Jennifer M., Sturhahn, Wolfgang, Zhang, Dongzhou, Greenberg, Eran
Format: Article
Language:English
Subjects:
Adiabatic
Adiabatic flow
Alloy powders
Alloying elements
Alloys
Anharmonicity
Anisotropy
Bulk density
Bulk modulus
Composition
Diamond anvil cells
Diamonds
Diffraction
Earth
Earth core
Earth's core
Elastic anisotropy
equation of state
Equations of state
Fe alloys
Ferrous alloys
Geophysics
Helium
high pressure
Iron
MATERIALS SCIENCE
Mathematical analysis
Mathematical models
Nickel
Oxygen
Powder
Pressure
Rangefinding
Ratios
Seismic activity
Silicon
Sound velocity
Sulfur
Sulphur
Thermodynamic properties
Tungsten
Uncertainty
X-ray diffraction
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Summary:We present powder X‐ray diffraction data on body centered cubic (bcc)‐ and hexagonal close packed (hcp)‐structured Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 at 300 K up to 167 and 175 GPa, respectively. The alloys were loaded with tungsten powder as a pressure calibrant and helium as a pressure transmitting medium into diamond anvil cells, and their equations of state and axial ratios were measured with high statistical quality. These equations of state are combined with thermal parameters from previous reports to improve the extrapolation of the density, adiabatic bulk modulus, and bulk sound speed to the pressures and temperatures of Earth's inner core. We propagate uncertainties and place constraints on the composition of Earth's inner core by combining these results with available data on light‐element alloys of iron and seismic observations. For example, the addition of 4.3 to 5.3 wt% silicon to Fe0.95Ni0.05 alone can explain geophysical observations of the inner core boundary, as can up to 7.5 wt% sulfur with negligible amounts of silicon and oxygen. Our findings favor an inner core with less than ∼2 wt% oxygen and less than 1 wt% carbon, although uncertainties in electronic and anharmonic contributions to the equations of state may shift these values. The compositional space widens toward the center of the Earth, considering inner core seismic gradients. We demonstrate that hcp‐Fe0.91Ni0.09 and hcp‐Fe0.8Ni0.1Si0.1 have measurably greater c/a axial ratios than those of hcp‐Fe over the measured pressure range. We further investigate the relationship between the axial ratios, their pressure derivatives, and elastic anisotropy of hcp‐structured materials. Key Points X‐ray diffraction data and equations of state of bcc‐ and hcp‐ Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 are reported at 300 K up to ∼170 GPa Thermal equations of state are combined with seismic observations and error propagation to place constraints on the inner core composition The hcp axial ratios of Fe0.91Ni0.09 and Fe0.8Ni0.1Si0.1 at 300 K and their relation to P wave anisotropy are investigated
ISSN:2169-9313
2169-9356
DOI:10.1029/2017JB015343