Microstructure evolution in Alloy 617 B after a long-term creep and thermal aging at 700 °C

Changes in the microstructure of heat-resistant materials may influence their long-term behaviour. For this reason, materials chosen for high temperature-based applications, e.g. advanced ultra-supercritical (A-USC) steam power plants, must exhibit long-term microstructure stability. Therefore, changes in the microstructure of frequently used materials must be determined and correlated with their creep behaviour to assure a reliable operation of components. In this work, a long-term study investigated the microstructure of a creep rupture specimen made of a nickel-based alloy 617 B. The creep tests were carried out at 700 °C for up to 45,148 h. By using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) the virgin, thermal-loaded and creep states were characterized. The precipitate location, number and size have been determined. The experiment results were compared, discussed and correlated to the creep and failure behaviour. Furthermore, the long-term precipitation kinetics of Alloy 617 B was simulated, which considered its thermal history, using the MatCalc software. The simulated precipitates fraction and size were compared with the experimental data obtained in this study. The comparison between the experimental and simulation results demonstrated comparable gamma prime (γ') phases. The simulation results of the carbides identified at the grain boundaries were satisfactory. However, the size of the intragranular fine carbides M₂₃C₆ was not reproduced correctly.