MICROSTRUCTURE EVOLUTION DURING THERMAL AGING OF INCONEL 718

The Inconel 718 superalloy was subjected to isothermal and cyclic thermal annealing at 750 degrees C to assess its expected use at higher temperatures. Microstructure evaluation during isothermal annealing and thermal cycling was investigated by different imaging techniques. According to High Resolution Scanning and Transmission Electron Microscopy (HRSEM & HRTEM) results, all the annealed samples showed the coarsening of gamma', gamma '' and delta precipitates, however, the total volume fraction of these precipitates remains the same for all heat treatment conditions. Using backscattered electrons images the size of gamma', gamma '' and delta phases were measured and correlated with annealing time. The most striking result is the impact of thermal cycling which accelerates the growth inside the grains of gamma '' phase and at the vicinity of the grain-boundaries leading to its transformation to delta phase, which induces a fast decrease of the mechanical properties of thermally cycled specimens compared to isothermally aged ones. A close investigation showed that the gamma '' phase grows by ledge mechanism. Moreover, Electron backscattered diffraction analysis (EBSD) revealed no significant grain size change during all annealed times.