In-situ characterization of hydrogen-induced defects in palladium by positron annihilation and acoustic emission

Positron annihilation spectroscopy was employed for characterization of hydrogen-induced defects in Pd. Positron annihilation studies were performed in-situ during electrochemical hydrogen charging and were combined with measurement of acoustic emission, which is a non-destructive technique capable of monitoring of collective dislocation motion. It was found that hydrogen loading introduced defects into Pd lattice, namely vacancies and dislocations. At low concentrations (α-phase) hydrogen loading created vacancies associated with hydrogen. Stresses induced by growing α'-phase particles led to plastic deformation and introduced dislocations into the sample. Moreover, additional vacancies were introduced into the sample by crossing dislocations. Vickers hardness testing revealed that hydrogen absorbed in interstitial sites causes solid solution hardening. Further hardening was caused by dislocations when α'-phase particles are formed. Pd sample completely transformed into the α'-phase was subsequently unloaded. Decomposition of α'-phase particles during unloading caused further increase of dislocation density and led to an additional hardening. Loading-unloading of Pd sample with hydrogen continuously generates dislocations and makes the sample harder.