Defect recovery and hydrogen-vacancy interactions in nickel between 20 and 650 K

The defect recovery in Ni and the influence of hydrogen on the recovery have been studied by means of the positron lifetime technique. Icosahedral annealings after irradiation at 20 K show the free migration of interstitials in stage I around 55 K. Free migration of vacancies (state III) is seen above 320 K, where the agglomeration of vacancies is clearly observed. Hydrogen, introduced by implantation at 20 K or electrolytically at 295 K, is found to become trapped at the vacancies. The hydrogen-vacancy pairs dissociate around 305 K, below the vacancy migration temperature. The binding energy of hydrogen to a vacancy is deduced to be 0.57+or-0.04 eV.