Degradation of the Strength of Grain Boundaries in Ni-base Superalloy Alloy617 Under Creep-Fatigue Loading at Elevated Temperatures

<p>Ni-base superalloys such as Alloy 617 has been used for boiler pipes or tubes in the A-USC because of its high strength and oxidation and corrosion resistances at elevated temperatures. It is concerned, however, that thermal stress during the operation increases because this alloy has higher coefficient of thermal expansion compared to conventional ferritic steels. Moreover, frequent output change is required for thermal power plants because of co-existence with renewable energies. Therefore, it is necessary to consider the effect of creep-fatigue damage. Even though the conventional linear damage rule has been used for the evaluation of creep-fatigue damage, it was found that the critical damage to final fracture decreases drastically depending on loading conditions. The main reason of this drastic decrease is attributed to the change of crack path from transgranular to intergranular under creep-fatigue loadings. The main reason for the intergranular crack was considered to be the degradation of the crystallinity and strength around grain boundaries due to creep-fatigue damage. In this study, in order to quantitatively evaluate the change of the effective strength of a grain and a grain boundary, EBSD analysis and a micro tensile test system with FIB equipment were applied to single-crystal specimens and bicrystal specimens cut from initial and damaged materials. The IQ value which indicated the total density of defects such as vacancies, dislocations, and local strain was used for evaluating the crystallinity of these micro specimens. As a result, it was quantitatively confirmed that the strength of a grain decreased with the damage. Moreover, the result of bicrystal specimens suggested that the degradation of the IQ value in the vicinity of a grain boundary under creep-fatigue loading caused intergranular cracking.</p>