Theoretical Calculation and Analysis of the Effect of Oxygen Atom on the Grain Boundary of Superalloy Matrices Ni, Co and NiCr

As advanced aero engines and heavy-duty gas turbines require high service temperatures, how to maintain better performance and damage tolerance of superalloys at high service temperatures has emerged as a critical issue in the application of superalloys. Previous research has shown that temperature rise has no effect on the crack growth rate of alloys placed in a vacuum. However, in air, fatigue crack growth rate is observed to significantly depend on temperature. According to the sample fracture, which is an oxide-covered intergranular fracture, oxygen atoms significantly affect the performance of superalloys. As first-principle calculation method has advanced rapidly in recent years, it can eliminate the effect of irrelevant impurity atoms on the weakening of grain boundaries and establish a pure grain boundary system. Thus, this method is an ideal analysis tool for this research. The ideal tensile test combined with ideal separation work and charge density difference of the grain boundary in nickel-based superalloy under different oxygen concentrations, was performed using the first-principle calculation method. The internal reason for the weakening of the Ni grain boundary due to oxygen is given. A similar comparative analysis of Co and NiCr grain boundaries was also performed. Further, the cause of the oxidation and weakening of the grain boundaries is explained. The results demonstrate that the high electronegativity of O atoms weakens the Ni-Ni metal bond at the grain boundary due to the lack of charge. Further, in the stretching process, when the tensile strain reaches 0.10, the strain of the oxygencontaining Ni grain boundary is entirely provided by the Ni-Ni bond present at this boundary. The presence of oxygen significantly accelerates the fracture failure process of the Ni grain boundary. In addition, Co-based alloy has higher strength after grain boundary oxidation and has better oxidation resistance weakening performance than Ni; however, the strain of fracture is small. Although the NiCr-based grain boundary strength is the weakest, the mechanical properties after oxidation are relatively stable. The reason for the weakening of the Ni grain boundary due to oxidation is attributed to structural distortion caused by oxygen atoms, whereas the weakening of Co- and NiCr-based grain boundary due to oxidation is primarily related to changes in the charge density distribution.