Effects of Hydrogen on the Vacancy Formation in Magnesium

Quenching and annealing experiments with electrical resistivity measurements were applied to magnesium to investigate the formation of thermal vacancies. Two specimens made from materials differing in impurity contents were examined. One of the specimens quenched in a methanol bath at -80°C from elevated temperatures ranging from 160°C to 500°C revealed a significant decrease in electrical resistance after annealing for 10 min in the bath. Based on the annealing behaviors at low temperatures (-100°C to -60°C) after the quenching from 200°C, this decrease is thought to be due to the presence of hydrogen in solution. The other specimen, presumably containing smaller amounts of hydrogen, was quenched in iced water from elevated temperatures (200°C-560°C) which yielded results characterized by two thermal activation processes. These processes had the activation energies 54.1 kJ/mol (0.56 eV) and 89.8 kJ/mol (0.93 eV) for the lower and higher quenching temperature ranges, respectively. The former is ascribed to the formation energy of a vacancy interacting with hydrogen and the latter to the intrinsic formation energy of a vacancy. The difference between these energies, 35.7 kJ/mol (0.37 eV), is the binding energy between a vacancy and a hydrogen atom.