Microstructure of Long-Term Aged IN617 Ni-Base Superalloy

The microstructure of the Ni-base superalloy IN617 that had undergone prolonged aging (approximately 65,000 hours) at a series of temperatures from 482 °C to 871 °C has been characterized by microhardness measurements, optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Cr₂₃C₆, Mo-rich eta-M₆C, and Ti(C,N) constitute the major primary coarse precipitates both within the grains and along the grain boundaries. The secondary carbides were mostly fine Cr₂₃C₆, which had a cube-on-cube orientation relationship (OR) with the fcc matrix, and at long times were present in cuboidal and plate-shape forms within the grains and as films along the grain boundaries. Fine, eta-M₆C carbides were also observed at low to intermediate temperatures with an OR given by [011] carbide//[011] matrix, \( \left( {\bar{1}\bar{1}1} \right) \) carbide//\( \left( {\bar{1}\bar{1}1} \right) \) matrix. The coarse eta-M₆C carbides increased in extent at 871 °C, whereas the counterpart fine carbides were absent. The γ′ phase was found to be present at all aging temperatures up to 871 °C, with a volume fraction ranging from very low to approximately 5 pct at 593 °C, where the peak in microhardness occurs. The observations have also suggested that the presence of a very small amount of γ′ at temperatures as high as 871 °C at long times may be associated with a reaction between the fine eta-carbides and the γ matrix. Ultrafine precipitates of the intermetallic phase Ni₂(Cr,Mo) with the Pt₂Mo-type structure was observed in addition to γ′ in samples aged for 28,300 hours at the lowest aging temperature of 482 °C. These precipitates were absent in samples aged at higher temperatures. The various observations made have suggested that the long-term thermal stability of the IN617 alloy is reasonably good over a wide temperature range of 538 °C to 704 °C, whereas at higher temperatures (871 °C), the substantial decrease in the volume fraction of γ′ and coarsening and clustering of the carbides lead to a large drop in the microhardness. A modified time-temperature-transformation (TTT) diagram was constructed based on the results of this study and comparison with previous reports.