CREEP-FATIGUE DAMAGE OF HEAT-RESISTANT ALLOYS CAUSED BY THE LOCAL LATTICE MISMATCH-INDUCED ACCELERATION OF THE GENERATION AND ACCUMULATION OF DISLOCATIONS AND VACANCIES

Degradation mechanism of the strength of a grain boundary in Ni-base superalloy under creep-fatigue loading at elevated temperature was investigated by using the modified Arrhenius equation, which explained the stress-induced acceleration of the local generation and diffusion of dislocations and vacancies. EBSD analysis confirmed that dislocations and vacancies started to generate and accumulate around grain boundaries and the interface between precipitates and matrix in grains. The generation and accumulation were accelerated around the interfaces with large difference in the lattice constant between the nearby crystallographic phases and grains. The activation energies of the diffusion of dislocations and vacancies measured under the harsh condition was much lower than those measured under the thermodynamically stable conditions. It was confirmed that there are two main acceleration mechanisms of the degradation of the crystallinity and strength of grain boundaries under a tensile stress at elevated temperatures: the acceleration of the generation and diffusion of dislocations and the acceleration of accumulation of voids due to the outward diffusion of component atoms from the grain boundaries. These phenomena were explained by the modified Arrhenius equations in which the effective activation energies were changed by the summation of the applied nominal stress and the localized internal stress around various interfaces quantitatively.