Explication of the degradation mechanism of the strength of heat-resistant alloys under random loading at elevated temperatures and the establishment of the prediction method of their lifetime

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The main objectives of this research are 1) Clarification of the acceleration mechanism of the degradation of materials under creep-fatigue loading at elevated temperatures, and 2) Establishment of the quantitative evaluation method of the damage of the materials under random load conditions. Original small dumbbell-shape specimens were manufactured for both the observation of the degradation process under the distribution of the applied load and the direct observation of the change of the local crystallinity of the alloy in a scanning electron microscope. The degradation process of the crystallinity of the representative heat-resistant alloy, Ni-base superalloy, under creep-fatigue loading at elevated temperatures was monitored continuously by using our original intermittent micro tensile test method with electron microscopy. The degradation process of the crystallinity and strength of the material was continuously observed, and it was explicated by applying the modified Arrhenius equation by considering the effect of mechanical stress on the local decrease in the activation energy of atomic diffusion. it was also found that the accumulation of vacancies and dislocations was accelerated only around grain boundaries and thus, the initiation time of intergranular cracking became shorter when the strain rate during loading and unloading became faster. The main reason for the acceleration was found to be the activation of viscoelasticity of the alloy at the elevated temperature.