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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). Cr 23 C 6 , Mo-rich eta-M 6 C, 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 Cr 23 C 6 , 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-M 6 C carbides were also observed at low to intermediate temperatures with an OR given by [011] carbide//[011] matrix, " 1 " 11 À Á carbide// " 1 " 11 À Á matrix. The coarse eta-M 6 C carbides increased in extent at 871°C, whereas the counterpart fine carbides were absent. The c¢ 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 c¢ at temperatures as high as 871°C at long times may be associated with a reaction between the fine eta-carbides and the c matrix. Ultrafine precipitates of the intermetallic phase Ni 2 (Cr,Mo) with the Pt 2 Mo-type structure was observed in addition to c¢ 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 c¢ 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.

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