MODEL FOR THE FINAL STAGE OF CREEP FAILURE DUE TO HIGH-TEMPERATURE HELIUM EMBRITTLEMENT

High temperature helium embrittlement (HTHE) is characterized by nucle ation and growth of helium bubbles on grain boundaries (GBs) and on tr iple grain Junctions (TGJs). The final stage of material failure in HT HE is due to premature crack nucleation and accelerated crack growth o n GBs and TGJs containing helium bubbles. Crack nucleation would event ually take place by the coalescence of diffusionaly growing cavities b ut this process may be accelerated by stress concentrations at cavity arrays on GBs and particularly on TGJs due to slip and GB-sliding occu ring 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 densi ty, cavity size and gas content in cavities on TGJs. Fracture criteria for a material containing helium is formulated.