Pressure–Composition Phase Diagram of Fe–Ni Alloy

We investigated the structural phase transitions of Fe–Ni alloys under hydrostatic pressure conditions at 298 K by examining the X-ray diffraction patterns of polycrystalline samples with Ni contents of 5%–31.6%, and pressure–composition phase diagram was established for the pressure range of 0–15 GPa. The diagram comprised three structural phases: bcc, fcc, and hcp, and these all coexisted in 25% Ni at 11 GPa during compression and in 23% Ni at 7 GPa during decompression. During both processes, low- and high-pressure phases coexisted. During the pressure-induced structural transitions, significant hysteresis was also observed. The 27% Ni alloy showed the two-stage transition: bcc–fcc–hcp, and the fcc phase was quenched to ambient pressure. For the 5–23% Ni alloys, a bcc–hcp transition was observed at high pressure. As the Ni content increased, the transition pressure decreased, and the volume reduction of the bcc–hcp transition also decreased due to the increase in the atomic volume of the hcp phase. The transition pressure from fcc to hcp rapidly increased with the Ni content. Compression curves of 29.9% Ni and 31.6% Ni alloys exhibited an anomaly at 2.0 and 2.9 GPa, respectively, and the compression anomaly was attributed to the magnetic transition.