On the modeling of the high-temperature embrittlement of metals containing helium

Models for premature intergranular fracture of metals in creep tests during He injection and after room temperature He implantation are critically assessed. The creep rupture lifetime and its dependences upon stress, temperature, He generation rate, He concentration and microstructure of the metal are determined by the intrinsic properties of the lifetime-controlling mechanism as well as by the He flux to and the resulting bubble density on the grain boundaries. The He flux on the other hand is controlled by the He generation rate and the bubble density developing within the grains. On the basis of these considerations the differences in the stress and temperature dependences of the lifetimes observed in creep tests during He implantation and after room temperature He implantation are attributed to differences in the bubble nucleation and growth kinetics of the two cases. For creep tests during He injection the approximate agreement between predicted and observed trends in the time to rupture indicate gas driven stable bubble growth up to the stability limit as the life time controlling mechanism. A set of special creep tests to check this indication is suggested. For creep tests after room temperature implantation the lifetime controlling mechanism is less clear because of secondary nucleation and coarsening processes. It could be bubble nucleation or creep controlled bubble growth on grain boundaries. For both types of creep tests the possibilities and limits of solid precipitates for reducing the He embrittlement is discussed. (orig.)