Trapping of hydrogen and helium at grain boundaries in nickel: An atomistic study

Abstract

Atomistic computer simulations of the trapping of hydrogen and helium at defect free grain boundaries in nickel are presented. Three symmetrical tilt boundaries that encompass a number of compact polyhedra of atoms are considered as regions of potential trapping sites. By employing the structural unit model, these boundaries are shown to be representative of a wide range of grain boundary structures. A general correspondence of trap locations in regions of expansion for both hydrogen and helium has been found; however, the binding energy for helium trapping is much greater than that for hydrogen. Consequently, clean grain boundaries in nickel appear to be important trapping sites for helium, but not significant sites for hydrogen binding. These results are consistent with experimental autoradiography, thermal desorption, and transmission electron microscopy observations. They imply that grain boundary trapping plays an important role in mechanisms of helium embrittlement, but not in hydrogen embrittlement of nickel.