Microstructure Evolution of 617B Ni-based Superalloy during Long-Term Aging

The 617B alloy was aged for 3000 h at 650, 700, 750 degrees C. The microstructure evolution was characterized by optical microscope (OM), field emission scanning electron microscope (FESEM) and energy dispersive spectrometer (EDS). The result shows that the gamma' phase and Cr, Mo-rich M23C6 are the major precipitates during aging treatment. The gamma' phase precipitates after aging at 650 degrees C for 600 h and gets coarsening with a prolonged time and an increasing temperature. The coarsening of gamma' phase satisfies the LSW theory during aging below 700 degrees C but when exposed above 750 degrees C, agglomeration becomes the primary coarsening mode. The carbides along the grain boundaries transform gradually into continuous network structure by coarsening from fine serration carbides; and the carbides within grains increase in number during aging. When aged at 750 degrees C, the M23C6 carbides interact with the surrounding matrix and some special evolution characteristic could be observed.