Microstructure of long-term aged IN617Ni-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 degrees C to 871 degrees 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, ((1) over bar(1) over bar1) carbide//((1) over bar(1) over bar1) matrix. The coarse eta-M6C carbides increased in extent at 871 degrees C, whereas the counterpart fine carbides were absent. The gamma' phase was found to be present at all aging temperatures up to 871 degrees C, with a volume fraction ranging from very low to approximately 5 pct at 593 degrees C, where the peak in microhardness occurs. The observations have also suggested that the presence of a very small amount of gamma' at temperatures as high as 871 degrees C at long times may be associated with a reaction between the fine eta-carbides and the gamma matrix. Ultrafine precipitates of the intermetallic phase Ni-2(Cr,Mo) with the Pt2Mo-type structure was observed in addition to gamma' in samples aged for 28,300 hours at the lowest aging temperature of 482 degrees 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 degrees C to 704 degrees C, whereas at higher temperatures (871 degrees C), the substantial decrease in the volume fraction of gamma' 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.