TRAPPING OF HYDROGEN TO LATTICE-DEFECTS IN NICKEL

This paper addresses the energy associated with the trapping of hydrog en to defects in a nickel lattice. Several dislocations and grain boun daries which occur in nickel are studied. The dislocations include an edge, a screw, and a Lomer dislocation in the locked configuration, i. e. a Lomer-Cottrell lock (LCL). For both the edge and screw dislocatio ns, the maximum trap site energy is approximately 0.1 eV occurring in the region where the lattice is in tension approximately 3-4 angstroms from the dislocation core. For the Lomer-Cottrell lock, the maximum b inding energy is 0.33 eV and is located at the core of the a/6(110) di slocation. Several low-index coincident site lattice grain boundaries are investigated, specifically the Sigma 3(112), Sigma 9(221) and Sigm a 11(113) tilt boundaries. The boundaries all show a maximum binding e nergy of approximately 0.25 eV at the tilt boundary. Relaxation of the boundary structures produces an asymmetric atomic structure for both the Sigma 3 and Sigma 9 boundaries and a symmetric structure for the S igma 11 tilt boundary. The results of this study can be compared to re cent experimental studies showing that the activation energy for hydro gen-initiated failure is approximately 0.3-0.4 eV in the Fe-based supe ralloy IN903. From the results of this comparison it can be concluded that the embrittlement process is likely associated with the trapping of hydrogen to grain boundaries and Lomer-Cottrell locks.