Molecular Dynamics Analysis on the Degradation Mechanism of the Crystallinity and Strength of Grain Boundaries in Heat-Resistant Alloys Under Creep-Fatigue Loading at Elevated Temperature

Abstract

Abstract In this study, we analyzed the degradation phenomena near grain boundaries that cause brittle intergranular cracking under high-temperature creep loading by molecular dynamics analysis of a bicrystal structure consisting of grains with different orientation. The results show that stress relaxation occurs even under uniaxial stress creep loading with a magnitude of 40% of the yield stress of the bicrystal structure. Furthermore, it was observed that the stress relaxation was accelerated by the combination of grain orientations of the grains constituting the bicrystal structure. This stress relaxation was caused by the activation of atomic diffusion around the grain boundary. In particular, when there is a difference in the elastic moduli of the two crystals forming the bicrystal structure, it was found that diffusion outside the grain boundary is activated. In the crystal with the lower elastic modulus, the diffusion of atoms from the grain boundary to the grain inside was found to occur significantly. Then, it was revealed that the significant decrease in atomic density around the grain boundary reduces the effective strength of the grain boundary and promotes grain boundary cracking due to the decrease in crystallinity around the grain boundary.