Effect of the precipitation of the η-Ni3Al0.5Nb0.5 phase on the microstructure and mechanical properties of ATI 718Plus

Some research showed that the precipitation of η-Ni3Al0.5Nb0.5 phase at grain boundary can optimize the mechanical properties and reduce the notch sensitivity of the nickel-based superalloy ATI 718Plus. Therefore, finding the optimum concentration and morphology of η-Ni3Al0.5Nb0.5 for different operating conditions is of great importance during the microstructure design process. In this study, the relationship between the mechanical properties as well as notch sensitivity and the η-Ni3Al0.5Nb0.5 content in ATI 718Plus was investigated by subjecting the alloy to different heat treatment processes. The stress rupture life of ATI 718Plus was found to improve significantly by decreasing the η-Ni3Al0.5Nb0.5 content. However, the notch sensitivity drastically increased when the η-Ni3Al0.5Nb0.5 mass percentage dropped below approximately 1.1 wt%. The microstructure of the samples was analyzed by field emission scanning electron microscopy and the phase composition was quantitatively determined by electrolytic phase isolation followed by a micro-chemical and X-ray diffraction analysis. The atomic distribution of elements in the alloy on a nanoscale was determined by 3DAP technique. The phase diagram and time-temperature-transformation curve of the different phase were calculated using the JMatPro6.0 software. Furthermore, the precipitation kinetics of the η-Ni3Al0.5Nb0.5 and γ′ phases and the optimum η-Ni3Al0.5Nb0.5 mass percentage in 718Plus were discussed.