Molecular Dynamics Analysis of Strength Degradation of Grain Boundaries with δ-Phase Precipitates in Ni-based Superalloy GH4169 under Creep Loading at Elevated Temperatures

抄録

Nickel-based superalloys used in gas turbine disks are required to operate under frequent and random fluctuations in power output at elevated temperatures. Under such high-temperature creep-fatigue loading conditions, the crystallinity near grain boundaries deteriorates rapidly, and intergranular cracking occurs, significantly reducing material life. It is also known that the accumulation of fine voids at the interface between the precipitates and the matrix phase accelerates damage. In this study, the acceleration mechanism of intergranular cracking observed in GH4169, which is one of the representative nickel-based alloys for jet engines, was investigated using molecular dynamics. At temperatures above 650°C, the γ" (Ni₃Nb) phase of the dispersion strengthened precipitation structure coarsens, and the more stable δ (Ni₃Nb) phase precipitates mainly in needle-like form near the grain boundary. In the experiments, fine voids accumulated near the interface between the δ-phase precipitated near grain boundaries and the matrix phase, and intergranular cracking was accelerated. In MD simulations modeling the δ-phase precipitated near the grain boundary of the nickel matrix, the formation of a local stress concentration field near the interface between the δ-phase precipitates and the nickel matrix under creep loading induced the generation of dislocations between the precipitates and the unstable grain boundary. A large lattice mismatch at the interface between the δ-phase and the nickel matrix causes the formation of this stress concentration field. The stress concentration field at the interface between the δ-phase adjacent to a grain boundary and the matrix phase increased the instability of the grain boundary, accelerating the degradation of crystallinity near the grain boundary and deteriorating the grain boundary strength. It was also found that the rate and magnitude of grain boundary strength degradation is non-uniform, depending on the crystallographic orientation of the grain boundary where the δ-phase precipitates.