Model for the final stage of creep failure due to high temperature helium embrittlement

High temperature helium embrittlement (HTHE) is characterized by nucleation and growth of helium bubbles on grain boundaries (GBs) and on triple grain Junctions (TGJs). The final stage of material failure in HTHE is due to premature crack nucleation and accelerated crack growth on GBs and TGJs containing helium bubbles. Crack nucleation would eventually take place by the coalescence of diffusionaly growing cavities but this process may be accelerated by stress concentrations at cavity arrays on GBs and particularly on TGJs due to slip and GB-sliding occuring during creep. In the present paper these processes are modeled in terms of interaction of cavity arrays with a pile-up of dislocations. The critical crack size is calculated in dependence upon cavity density, cavity size and gas content in cavities on TGJs. Fracture criteria for a material containing helium is formulated.