The competition between dislocation emission and cleavage at a crack tip was evaluated in the presence of H. At this level, embrittlement was predicted when the critical stress intensity required for emission rises above that needed for cleavage, eliminating crack tip plasticity and blunting as toughening mechanisms. Continuum predictions for emission and cleavage were made using computed generalized stacking fault energies and surface energies in a model Ni–H system, and embrittlement was predicted at a critical H concentration. An atomistic model was then used to investigate actual crack tip behavior in the presence of controlled arrays of H atoms around the crack tip. The continuum models were accurate at low H concentrations, below the embrittlement point, but at higher H concentrations the models deviate from the atomistic behavior due to alternative dislocation emission modes. Additional H configurations were investigated to understand controlling features of the emission process. In no cases does crack propagation occur in preference to dislocation emission in geometries where emission was possible, indicating that embrittlement could be more complicated than envisioned by the basic brittle–ductile transition.
Testing Continuum Concepts for Hydrogen Embrittlement in Metals using Atomistics. J.Song, M.Soare, W.A.Curtin: Modelling and Simulation in Materials Science and Engineering, 2010, 18, 045003