Microstructure evolution during thermal aging of inconel 718

The Inconel 718 superalloy was subjected to isothermal and cyclic thermal annealing at 750°C to assess its expected use at higher temperatures. Mcrostructure evaluation during isothermal annealing and thermal cycling was investigated by different imaging techniques. According to High Resolution Scanning and Transmission Electron Mcroscopy (HRSEM & HRTEM) results, all the annealed samples showed the coarsening of γ′, γ″ and δ precipitates, however, the total volume fraction of these precipitates remains the same for all heat treatment conditions. Using backscattered electrons images the size of γ′, γ″ and δ 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 γ″ phase and at the vicinity of the grain-boundaries leading to its transformation to δ 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 γ″ phase grows by ledge mechanism. Moreover, Electron backscattered diffraction analysis (EBSD) revealed no significant grain size change during all annealed times.