Nucleation of intergranular cracks during steady-state and cyclic creep is analyzed in detail. The basis of the analysis is that grain boundary sliding produces stress concentrations at inclusions, ledges and triple-line junctions which leads to nucleation of cracks. The analysis is built upon different facets of the mechanical behavior of grain boundaries such as: sliding with elastic accommodation, diffusional and power law creep accommodation, and the contribution of sliding to the strain rate under power law creep conditions. Inclusions play an important role in fracture since the energy of the inclusion-matrix interface is usually high and since sliding produces stress concentration at inclusions. Triple line fracture is shown to be important during transient creep conditions. Crack initiation is calculated as a function of the following engineering parameters: temperature, strain rate, grain size, inclusions, cyclic frequency and hold time. The results are in broad agreement with experimental observations.

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