doi:10.1016/j.pepi.2005.11.002
Published by Elsevier B.V.
Phase relations of Fe–Ni alloys at high pressure and temperature
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Wendy L. Maoa, e, 
, 
, , Andrew J. Campbella, b, 1, Dion L. Heinza, c and Guoyin Shend, 2
aDepartment of the Geophysical Sciences, The University of Chicago, Chicago, IL 60637, USA
bChicago Center for Cosmochemistry, The University of Chicago, Chicago, IL 60637, USA
cJames Franck Institute, The University of Chicago, Chicago, IL 60637, USA
dConsortium for Advanced Radiation Sources, The University of Chicago, Chicago, IL 60637, USA
eGeophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA
Received 25 July 2005;
revised 8 November 2005;
accepted 11 November 2005.
Available online 27 December 2005.
Abstract
Using a diamond anvil cell and double-sided laser-heating coupled with synchrotron X-ray diffraction, we determined phase relations for three compositions of Fe-rich FeNi alloys in situ at high pressure and high temperature. We studied Fe with 5, 15, and 20 wt.% Ni to 55, 62, and 72 GPa, respectively, at temperatures up to 
3000 K. Ni stabilizes the face-centered cubic phase to lower temperatures and higher pressure, and this effect increases with increasing pressure. Extrapolation of our experimental results for Fe with 15 wt.% Ni suggests that the stable phase at inner core conditions is hexagonal close packed, although if the temperature at the inner core boundary is higher than 6400 K, a two phase outer region may also exist. Comparison to previous laser-heated diamond anvil cell studies demonstrates the importance of kinetics even at high temperatures.
Keywords: Iron–nickel alloys; Earth's core; High pressure; Diamond anvil cell; Laser-heating; X-ray diffraction
Fig. 1. Representative XRD spectra for Fe5%Ni at 43 GPa. The sample transforms from an initially hcp phase to a mixture of hcp and fcc phases and finally to nearly entirely fcc phase with increasing temperature. Diffraction peaks from the different phases as well as those from the B2 phase of NaCl are labeled.
Fig. 2. Phase relationships for Fe5%Ni, Fe15%Ni, and Fe20%Ni. The solid lines show approximate phase boundaries for the hcp + fcc two phase coexistence region. Symbols show experimental results where we observed different phases: (*) hcp only; (□) hcp + fcc; (
) fcc only; (×) melt. Lines may differ from thermodynamic equilibrium due to kinetics.
Fig. 3. Schematic phase relationships for Fe-rich Fe–Ni compositions for varying T–x at 30, 40, 50, and 60 GPa; (circles) interpolations of our results for Fe5%Ni, Fe15%Ni, and Fe20%Ni; (*) Fe (Ma et al., 2004); (triangles) Fe10%Ni (Lin et al., 2002); (diamonds) Fe30%Ni (Huang et al., 1992). Closed symbols and dotted lines show hcp → hcp + fcc boundary; open symbols and solid lines show hcp + fcc → fcc boundary.
Fig. 4. Portion of XRD spectra for Fe20%Ni taken at 72 GPa and 1400 K at different times during a 50 min heating cycle. At right, Bruker CCD images show that the Debye rings became much smoother with extended heating, indicating the decreasing grain size of the hcp phase equilibrating at high temperature relative to the grain size in the hcp + fcc assemblage initially quenched from higher temperature. It took approximately 30 min to transform the initial mixed hcp + fcc assemblage into hcp only.
Fig. 5. Conditions for the long heating cycle for Fe20%Ni at 72 GPa corresponding to Fig. 4. Before this cycle, a hcp + fcc two phase mixture was synthesized at 1596 K. Dotted line indicates when power to YLF lasers was increased. Symbols show what phases were present in the in situ XRD patterns taken at different times during heating. We crossed the hcp only → hcp + fcc phase boundary on both decreasing and increasing temperature to constrain this boundary within 100 K. Dashed line indicates approximate transition temperature.
Fig. 6. Extrapolation of our Fe15%Ni and previously reported Fe10%Ni (Lin et al., 2002) results to core conditions. Thick solid line: Lindemann law extrapolation of hcp Fe melting curve (Ma et al., 2004); Dotted lines: hcp → hcp + fcc boundaries; thin, solid lines: hcp + fcc → fcc boundaries Shaded areas indicate two phase hcp + fcc regions. Dashed lines show the core–mantle boundary (CMB) and inner core–outer core boundary (ICB) pressures.
Table 1.
ΔVfcc−hcp for different compositions and P–T conditions
Table 2.
Phase boundary slope for varying Ni compositions
Fe10%Ni data is from Lin et al. (2002); Fe30%Ni data from Huang et al. (1992); Note: there was not enough data to estimate a phase boundary for Fe5%Ni and Fe20%Ni.
Corresponding author at: Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Tel.: +1 202 320 2025; fax: +1 505 665 2676.
1 Present address: Department of Geology, University of Maryland, College Park, MD 20742, USA.
2 Present address: HPCAT, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA.
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