Effects of hydrogen on the vacancy formation in magnesium

Quenching and annealing experiments with electric 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 that was quenched into a methanol bath at -80°C from elevated temperatures ranging from 160 to 500°C revealed a significant decrease in electrical resistance during subsequent annealing for ten minutes in the bath. This decrease is attributed to the presence of hydrogen in solution, on the basis of annealing behaviors at low temperatures (-100~-60°C) after the quenching from 200°C. Another specimen, presumably containing smaller amounts of hydrogen, was quenched into iced water from elevated temperatures (200~560°C), which yielded results characterized by two thermal activation processes. These processes have the activation energies, 54.1 kJ/mol (0.56 eV) and 89.8 kJ/mol (0.93 eV) for lower and higher quenching temperature range, respectively. The former is ascribed to the formation energy of a vacancy interacting with hydrogen and the latter the intrinsic formation energy of a vacancy. The difference of these energies, namely 35.7 kJ/mol (0.37 eV), can be identified as the binding energy between a vacancy and a hydrogen atom.