Strain-rate dependence of hydrogen-induced defects in pure α-iron by positron annihilation lifetime spectroscopy

The formation of defects in hydrogen-charged pure α-iron deformed at room temperature at different tensile strain rates was investigated by variable-temperature positron annihilation lifetime spectroscopy to clarify the role of hydrogen-induced defects in driving the hydrogen embrittlement mechanism. A distinct strain-rate dependence of the hydrogen-related defects was observed and a clear difference between the defects formed in hydrogen-embrittled iron and non-embrittled iron was detected. In hydrogen-embrittled slowly strained iron, the low-temperature measurements showed the formation of smaller vacancy clusters than in non-embrittled fast strained iron. With increasing annealing temperature, these defects grew into larger vacancy clusters. These results suggest that, in hydrogen-embrittled iron, vacancy clusters stabilized by hydrogen accumulated locally in high concentrations. This condition is found to be the a sufficient and critical condition to trigger the hydrogen embrittlement. This result represents a crucial step forward towards a comprehensive understanding of the hydrogen embrittlement mechanism in iron.