Configurational thermodynamics of Fe–Ni alloys at Earth's core conditions

By means of ab-initio calculations, we perform an analysis of the configurational thermodynamics, effects of disorder, and structural energy differences in Fe–Ni alloys at the pressure and temperature conditions of the Earth's core. We show from ab-initio calculations that the ordering energies of fcc and hcp-structured Fe–Ni solid solutions at these conditions depend sensitively on the alloy configuration, i.e., on the degree of chemical disorder, and are on a scale comparable with the structural energy differences. From configurational thermodynamic simulations we find that a distribution of Fe and Ni atoms in the solutions should be very close to completely disordered at these conditions. Using this model of the Fe–Ni system, we have calculated the fcc–hcp structural free energy difference in a wide pressure–temperature range of 120–360 GPa and 1000–6600 K. Our calculations show that alloying of Fe with Ni below 3000 K favours stabilisation of the fcc phase over the hcp, in agreement with experiments. However, above 3000 K the effect is reversed, and at conditions corresponding to those of the Earth's inner core, Ni acts as an agent to stabilise the hcp phase.

Research Highlights
► We consider fcc- and hcp-based Fe–Ni alloys at Earth's core conditions.
► Configurational energy is found to be on the same scale as structural energy differences.
► Fe–Ni alloys should be modelled as random at these conditions.
► At moderate temperature, Ni stabilises the fcc phase.
► At Earth's core temperature, Ni stabilises the hcp phase.