In situ X-ray diffraction measurements of the fcc-hcp phase transition boundary of an Fe-Ni alloy in an internally heated diamond anvil cell

摘要

The phase transition boundary between the face-centered cubic (fcc) structure and hexagonal close-packed (hcp) structure in an Fe-Ni alloy was determined at pressures from 25 to 107 GPa by using an internally resistive-heated diamond anvil cell (DAC), combined with in situ synchrotron X-ray diffraction measurements. The fcc-hcp phase transition boundary in Fe-9.7 wt% Ni is located at slightly lower temperatures than that in pure Fe, confirming the previous understanding that the addition of Ni expands the stability field of the fcc phase. The dP/dT slope of the boundary was determined to be 0.0426 GPa/K, which is slightly larger than that of pure Fe. The pressure interval of the two-phase region is about 6 GPa at a constant temperature, implying that the previous estimates by laser-heated DAC experiments of 10-20 GPa were overestimated. The two-phase region of fcc + hcp would be limited to a pressure of about 120 GPa even in Fe-15 wt%Ni, excluding the possibility of the existence of the fcc phase in the inner core if the simple linear extrapolation of the two-phase region is applied. The pressure and temperature dependences of the c/a axial ratio of the hcp phase in Fe-9.7 wt% Ni are generally consistent with those in pure Fe, suggesting that Ni has minor effects on the c/a ratio.