MOLECULAR DYNAMICS ANALYSIS OF THE ACCELERATION OF THE DEGRADATION OF THE STRENGTH OF A GRAIN BOUNDARY UNDER CREEP-FATIGUE LOADS

Since thermal power plants will be required to operate at random power output to compensate for the output instability of renewable energies, it is essential to consider the degradation damage of materials under creep-fatigue loading at high temperatures. Under creep-fatigue loading, the effective lifetime of Ni-based superalloys is known to decrease drastically, and the main reason for this was believed to be the change in the crack initiation and propagation path from transgranular to intergranular. Therefore, it is very important to identify the dominant factors that accelerate the degradation of the effective strength of grain boundaries under creep-fatigue loading. In this study, molecular dynamics analysis was applied to the analysis of the degradation process of the crystallinity of grain boundaries. Bicrystal structures with different combinations of crystal orientations were modeled by considering the experimental results of the strength of bicrystal structures cut from polycrystalline materials. A strain-controlled creep-fatigue uniaxial load was applied to this bicrystal structure. Clear stress relaxation was observed even when the magnitude of the applied strain was much smaller than the yield criterion. The main reason for the stress relaxation was not only the local generation of dislocations around the grain boundary in the bicrystal structure, but also the accelerated ejection of atoms around the grain boundary, i.e., the grooving of the grain boundary to relieve the local strain concentration due to the lattice mismatch between nearby grains. This grooving behavior corresponds well to the accumulation of fine voids around grain boundaries under creep loads. These results indicate that under creep-fatigue loading, plural damage mechanisms are activated and they accelerate the damage accumulation and degrade the strength of grain boundaries, and thus, accelerate intergranular cracking.