Aging effects on the microstructure and creep behavior of Inconel 718 superalloy

Standard heat treatment (HT1) for Inconel 718 superalloy is solutionizing at 1095 °C, 1 h/AC, then aging at 955 °C, 1 h/AC + 720 °C, 8 h/FC 57 K/h to 620 °C, 8 h/AC. In order to study the aging effects of the δ phase, two more conditions HT2 (no aging condition 955 °C) and HT3 (955 °C, 3.5 h/AC) were studied in this research. Lever arm creep tests were performed at 650 °C under constant stress 625 MPa. Since HT2 produces no δ phase, the stress rupture life, creep elongation to failure and steady state creep rate of HT2 are largest among these three aging conditions. However, increasing the 955 °C aging time, the stress rupture life, creep elongation and steady state creep rate raise slightly as compared to HT1, because platelet δ phase is more uniformly nucleated and more direction oriented at grain boundaries. Fractographs show ductile fracture patterns mostly and, small portion of inter-granular fracture in the HT2 specimens. Generally only inter-granular fracture is observed in the other two cases of HT1 and HT3. Besides twinning and dislocation mechanisms, grain boundary sliding is also activated, so that creep elongation to failure of HT2 specimens could reach 5.6%, whereas 1% for the other two schemes.