Microstructural characterization and mechanical properties of IN617 superalloy after long-term operation

The microstructural evolutions and mechanical properties of the IN617 superalloy aged at 750°C for 105000 hours were investigated. Microstructural examinations were carried out by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Microstructural evaluations shows that continuous carbides have been formed in all grain boundaries and large carbides have been formed within the grains and twin boundaries, due to long-term aging at high temperature. The volume fraction of carbides increases from 0.5% in the as-received plate to 6.5% in the aged sample. In addition to the three main compounds of M6C, M23C6 and Ti(C,N), the γ' phase and a small amount of harmful δ phase have been identified in the aged specimen. The stability of mechanical properties at high temperature is attributed to the presence of γ' phase after 105000 hours of aging. Based on γ' particles sizes (4-10 nm) as well as their morphology, this phase is in the dissolution stage after 105000 hours. Furthermore, M6C is in the transformation stage to M23C6 carbide and γ' phase, and a small amount of Ti(C,N) has been transformed to carbide. The presence of M23C6 carbides with dimensions less than 200 nm inside the grains indicates that the nucleation process of carbides continues. More than 90% of the carbides have spherical, quasi-spherical and irregular morphologies, and the rest are plate-like and rod-shaped. The morphology and size of the carbides formed at grain boundaries and intragranular regionsindicate that they are in the process of dissolution and agglomeration. The strength and hardness of the aged alloy is better than the as-received plate; However, due to the formation of large carbides in the grain boundaries, the impact energy of the aged alloy has been reduced by more than 75% as compared to the as-received plate. The results shows that IN617 is a suitable alloy for the long-term services at temperature range of 700-800°C .