Hydrogen-induced superabundant vacancies and diffusion enhancement in some FCC metals

Lattice contractions caused by the formation of extremely high concentrations of vacancies (superabundant vacancies of ∼10 at.%) were observed in the fcc phases of Mn-H, Fe-H, Co-H, Ni-H and Pd-H samples at high temperatures ( ≦900°C ) and high H₂ pressures ( ≦ 5 GPa). Comprehensive measurements in the Pd-H system, analyzed in terms of our theory of vacancy- hydrogen ( Vac-H ) cluster formation, have allowed to determine the formation and migration properties of the Vac-H clusters. From the observed lattice contraction process and concomitant diffusion enhancement, it is concluded that most Vac-H clusters are introduced by diffusion from the surface over a long time but some of them are created instantly at internal sources.