The Effects of Cooling Rate after Solutionizing on the Microstructural ‎Evaluation of Cast IN718 Superalloy during Long-Term Thermal Exposure

In the present study, cast IN718 superalloys solutionized at 1050◦C for 150 min. Afterwards, the samples were quenched with three various cooling rates including 240◦C/s, 5◦C/s and 0.1◦C/s. After solutionizing, the samples were aged at 750◦C for 30, 60, and 90 hrs. Microstructural investigation was carried out by optical microscopy (OM), Field Emission Scanning Electron Microscopy (FESEM), and X-ray diffraction (XRD). The results indicated that as the cooling rate increased, the conditions for complete dissolution were provided and only a small amount of MC carbide (TiC) phase remains in the matrix. By reducing the cooling rate and increasing aging time, due to the increase in volume fraction and size of secondary precipitates, the shear transformation of the γ'' to δ increases, and consequently, the δ precipitates increased. After solutionizing, as the cooling rate increased, the hardness due to the decrease in the amount of cooling precipitates decreased. Besides, as the aging time increases, due to the abnormal growth of precipitates, hardness first increased and then experienced a decrease.