Multiscale Stress-Diffusion Analysis of Notch-Tip Hydrogen Profiles in Zircaloy-4

Hydrogen pick-up in zirconium alloys can lead to their structural failure, which is an important problem in the nuclear industry. This investigation focuses on modelling the accumulation of hydrogen in the vicinity of loaded V-notches in four-point bend Zircaloy-4 specimens. In order to account for the anisotropic diffusivity of hydrogen in hexagonal close-packed α-zirconium, a multiscale methodology is proposed to compute notch-tip hydrogen profiles. This methodology unifies continuum scale stress analysis, using the finite element approach, and atomistic scale stress analysis, using the elastic dipole tensor of point defects. The steady state notch-tip hydrogen profiles are determined for different notch geometries and crystal orientations. It was found that hydrogen enhancement is greater but more localised for sharper notches with a smaller flank angles, which is the expected effect of stress. It was also found that hydrogen enhancement is greater if the notch opening plane coincides with the prism plane as opposed to the basal plane. This anisotropic effect is a consequence of the trigonal symmetry of the hydrogen interstitialcy.

The tetragonal elastic dipole Cr5+ in SrTiO3

Tetragonal Cr⁵⁺ impurity centre in single crystals of the perovskite SrTiO₃. After careful analyses we came to the conclusion that this centre is an off-centre system and no Jahn-Teller impurity. From the previous stress experiments in EPR we calculated the linear stress coupling tensor as β = 3.56×10^-30 m³, which is in agreement with other stress coupling tensors of off-centre systems in SrTiO₃ as well as BaTiO₃.