Effects of stress-relief pre-annealing on deuterium trapping and diffusion in tungsten

Positron annihilation Doppler broadening spectroscopy (PA-DBS) with slow positrons was utilized to characterize the change of vacancy-type defects induced by a stress-relief annealing (1273 K for 1 h in vacuum) as pre-annealing can remove not only residual stresses but also most intrinsic defects of the material. Deuterium behavior after low energy (40 eV/D) and high-flux (1024 D/m2 s) plasma exposure was analyzed by a combination of PA-DBS, elastic recoil detection analysis (ERDA) and thermal desorption spectroscopy (TDS). It is found that the vacancy defects density in tungsten is largely decreased by annealing and some intrinsic vacancies clusters to be multi-vacancies during the annealing process. With deuterium plasma exposure, a general decrease of S parameter found in both non- and pre-annealed tungsten suggests the deuterium occupation of vacancy-type defects. In combination of the S parameter change in the non- and pre-annealed tungsten, it is implied that deuterium trapping ability of vacancy-type defects is constrained due to the residual stresses. This confinement effect concentrates on the top surface and it becomes weaker with increasing depth. A similar deuterium distribution in the near surface of non- and pre-annealed tungsten supports this confinement effect. In addition, comparing the amount of deuterium retained in the near surface and in the bulk suggests that the residual stress facilitate deuterium to diffuse into a larger depth in tungsten.