Stability of the microstructure and elevated-temperature mechanical properties of additively manufactured Inconel 718 superalloy subjected to long-term in-service thermal cycling

The present study investigates the thermal stability of the additively manufactured Inconel 718 (IN718) under a long-term thermal cycling exposure, similar to what is encountered in aircraft turbine engines, up to 3000 h. Before thermal cycling, the IN718 parts were heat-treated using two conditions designated, respectively, 2.5H and 4H. The results show that, after the simulated in-service thermal cycling conditions, the 2.5H and 4H treated samples exhibited initially a slight increase in their tensile (TS) and yield strengths (YS) at 650 °C to reach their maximum values after 500 and 1000 h, respectively, and then gradually decreased with further thermal cycling time to reach their lowest values after 3000 h. The microstructural analysis and the X-ray diffraction (XRD) results revealed that a slight initial strength increase is mainly due to a larger precipitation of γ′′ and inter-granular δ-phase during a relatively short-term thermal cycling exposure, whereas the prolonged thermal cycling time resulted in an undesired phase transformation of the metastable γ′′ to intra-granular δ-phase which adversely affected the mechanical properties. However, comparing the tensile results of both treated conditions revealed that the 4H treatment effectively delayed the deteriorations of the material strength until after 2000h as compared to the 2.5H condition for which the strength deterioration occurred after 1000 h due to a slower phase transformation of γ′′ into δ-phase in the former condition. Considering the thermal stability during in-service exposure, it was found that the 4H treated samples demonstrate a higher strength stability than their 2.5H treated equivalents, since after a thermal cycling exposure for 3000 h, the TS and YS values of the former decreased by only 3.3 and 1.3% of the initial state, respectively, as compared to 5.3 and 3.8%, for the latter.