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Environment-assisted intergranular cracking occurs in many polycrystalline materials as a result segregation of impurities to grain boundaries, precipitation of second phases and solute depletion at grain boundaries, or precipitate free zone (PFZ) formation. The presence of a specific environment and sustained tensile stress can render a material susceptible if the required grain boundary metallurgical conditions exist. Models have been developed to describe conditions for intergranular cracking. Such models are usually based on either dissolution-controlled intergranular cracking or hydrogen embrittlement along grain boundary paths. Currently, these models rely on average descriptions of grain boundary segregation level, solute depletion, and PFZ character. However, solute depletion, segregation, and PFZ character can vary from grain boundary to boundary depending on boundary energy, crystallographic mis-orientation, and other factors. The propagation of an continuous intergranular crack through a component requires a high population of such highly susceptible grains, favorable geometrical grain facet orientation relative to direction of uniaxial tensile stress, or high hydrostatic tensile stress to overcome unfavorable geometric grain facet orientation. Statistical theories of fracture and bond percolation provide methods to quantify whether a critical population of susceptible grains is necessary to provoke extensive intergranular cracking. These theories provide insight into whether or not a well-connected intergranular crack can propagate through a structure. Two examples explore the existence of a bond percolation threshold in two substantially different systems that exhibit intergranular cracking by either anodic dissolution or hydrogen embrittlement.
Bond percolation, crystallographic grain misorientation, geometric-grain misorientation, environment-assisted intergranular cracking, hydrogen trapping, solute segregation, solute depletion
Professor, University of Virginia, Charlottesville, VA