The interaction of creep damage and fatique damage and their temporal development in the high temperature superalloy 800H

The cavitation damage of internal grain boundaries during creep-fatigue experiments needs a nucleation stage and a growth stage. The latter exists for a slow deformation in tension and a fast one in compression. Additionally, experiments have shown that the compression half cycle is also very important for the nucleation of cavities. The shorter the time in compression, i.e., the higher the compression stress, the more nuclei are formed by interfacial decohesion of grain boundary carbides. In all cases the lifetime is determined by fatigue crack propagation, whose rate is controlled (i) by fast athermal deformation, (ii) by creep deformation in tension and (iii) by the interaction with cavities. These three contributions taken into account, a model calculation is able to predict the cyclic lifetime for quite different cycle types within a factor of two. (orig.)