Effect of the Thermomechanical History on the Precipitation Characteristics of Inconel 718 Superalloy

Inconel 718 is one of the most commonly used superalloys for turbine applications. This material exhibits excellent creep properties, being able to work at temperatures as high as 700ºC. The mechanical behavior of this alloy relies on complex precipitation phenomena which are properly controlled through specific aging schedules. Currently, the overall processing of Inconel 718 components requires forming, solution treating and aging steps, and long times are involved. Any modification of the process, aiming at reducing the processing times, would require a deep knowledge of the precipitation species, precipitation kinetics, and their dependence on the microstructure generated during forming. In this work, compression samples were conditioned by applying different degrees of deformation at high temperatures to generate microstructures with different recrystallized and/or recovered fractions. After the conditioning of the microstructure, the samples were directly cooled to the aging temperature (900ºC and 850ºC), held at this temperature for one hour and then quenched. The precipitation characteristics were analyzed by Field Emission Scanning Electron Microscopy after proper sample preparation. The results show that precipitation in terms of nature, size and distribution of the precipitates is very sensitive to the initial microestructure and aging temperature.