Study on initiation and growth of microstructurally small cracks in creep-fatigue of oxide-dispersion-strengthened superalloy

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In order to elucidate the creep-fatigue failure mechanism of an oxide-dispersion -strengthened (ODS) superalloy with the coarse elongated grain structure, the behavior of crack initiation and propagation was observed under stress- controlled creep-fatigue conditions at 1173K and 1273K using a round bar specimen with smooth surface and a plate specimen with a center notch. Although intergranular traverse small cracks initiated on the smooth surface from the early stage of life, they were arrested immediately after the initiation by adjacent longitudinal grain boundaries. Transgranular small cracks initiated from the middle stage of life and continued to propagate while longitudinal grain boundaries interfered the growth. The macrocrack was subsequently formed by the propagation and the coalescence of the transgranular small cracks. The macrocrack propagation, which was examined by the notched specimen, was locally influenced by an array of longitudinal grain boundaries. However, the fluctuation due to the elongated coarse grain structure was smoothed out in the whole crack propagation throughout the thickness of specimen which included about 15 grains. The propagation rate was correlated well with a nonlinear fracture mechanics parameter, creep J-integral.