Atomic structure and melting of Ni and Fe36 Ni up to 400 GPa

Iron, nickel, and their alloys are critically important materials for industrial and technological applications due to their unique magnetic properties, strength, and thermal expansion. In this study, lasers were used to compress and heat Fe36Ni alloy (36 wt% Ni) and pure nickel up to the melting temperature using a combination of shock and ramp compression. The structure was measured using nanosecond in situ x-ray diffraction, and simultaneous velocimetry was used to measure the pressure up to 454 GPa. A mixed face-centered-cubic (fcc) solid-liquid phase in Fe36Ni at 311 GPa provides experimental evidence that, compared with pure iron, the incorporation of nickel expands the stability field of the fcc phase to the melting curve. At lower temperatures, a mixed fcc and hexagonal-close-packed (hcp) phase is observed in ramp-compressed Fe36Ni at 278 GPa. At the higher compressions, a structure inconsistent with fcc, hcp, and body-centered cubic is observed. In the case of pure Ni, the fcc phase is stable under ramp compression up to 402 GPa.