Characterization of defects in titanium created by hydrogen charging

In the present work positron annihilation spectroscopy was employed for investigation of defects created in titanium by hydrogen loading. Pure titanium samples were firstly annealed to remove dislocations introduced by cutting and polishing. Subsequently the samples were loaded with hydrogen up to various hydrogen concentrations. Ti samples with different microstructures were compared: (i) conventional coarse grained sample, (ii) ultra fine grained material with microstructure refined by severe plastic deformation. Hydrogen gas loading of coarse grained and ultra fine grained samples was performed at hydrogen gas pressure of 103 bar and temperature of 150 °C. This resulted in formation of δ-TiHx phase in Ti matrix. The hydrogen content absorbed in the samples was determined by thermogravimetric analysis. The phase composition of hydrogen-loaded samples was characterized by X-ray diffraction. Hydrogen loading introduced vacancies which agglomerated in the sample into small vacancy clusters. In addition to vacancies, dislocations were created by α-Ti → δ-TiHx phase transition. Differential thermal analysis revealed that hydrogen is trapped at several kinds of traps characterized by different binding energies. The release of hydrogen from these traps precedes the decomposition of the δ-TiHx phase.