Effects of phosphorus and boron on the γ″ precipitate evolution and creep properties of IN718 superalloy during long-term thermal exposure at 680 ℃

A study was conducted to investigate the effect of the combined addition of P and B on the γ″ precipitate and creep properties of IN718 superalloy under long-term thermal exposure at 680 ℃. Atomic probe tomography (APT) analysis and X-ray diffraction (XRD) results indicated that P could segregate in the γ″ precipitates in a substitutional way and then improve the stability of the γ″ precipitates when subjected to prolonged thermal exposure. No segregation of B was found in the γ″ precipitates. The addition of P and B significantly improves the creep properties of the IN718 superalloy during long-term thermal exposure at 680 ℃. By increasing the exposure time, the creep life of the high P and B alloy at 650 ℃/725 MPa initially increased, peaking at 500 h, and then decreased as the exposure time increased. However, the low P and B alloy shows a consistent decrease in creep life with increasing thermal exposure time. During a relatively short thermal exposure, the microstructural analysis indicated that the substantial enhancement in creep life of the high P and B alloy primarily stems from cross twins and Lomer-Cottrell (LC) locks. As the thermal exposure time increases, the more stable γ″ precipitates in the high P and B alloy, which directly improves the creep properties. The dominant deformation mechanism in the two alloys shifts from predominantly twin to a combination of twin, stacking fault (SF) and antiphase boundary-like (APB-like) shear γ″ precipitate with prolonged thermal exposure.