The phase diagram and superabundant vacancy formation in Fe-H alloys revisited

Detailed measurements of the p (H2)-x -T relation in the fcc phase of Fe-H alloys revealed the characteristics of supercritical anomalies, with the critical point located at around a hydrogen pressure of 4-5 GPa and a temperature of 400±100° C. The vacancy-hydrogen (Vac-H) cluster concentration in the fcc phase estimated from the lattice contraction was proportional to the hydrogen concentration independently of temperature and hydrogen pressure. A small lattice contraction due to the superabundant vacancy formation was also observed in the bcc phase, in accordance with previous thermal desorption measurements, and showed a similar dependence on the hydrogen concentration.