Reverse Coarsening Splitting Behavior of the Secondary γ′ in Superalloy FGH 95 during Long-Term Aging

The size, the distribution and the shape of secondary gamma' in powder metallurgy superalloy FGH95 were researched by field emission scanning electron microscopy, the alloy was long-term aged at 750 degrees C for 5000 h, in order to provide a theoretical basis for its application in newly aero-engines. The results show that with prolonging of the aging time, the morphological character of secondary gamma' is unstable, and its size does not follow the law of LSW theory. During 0 similar to 500 h the splitting of reverse coarsening occurs in the second gamma', resulting in splitting of a single butterfly precipitate into a group of eight cuboids. In 500 similar to 1000 h, the split gamma' coarsens to a certain extent and then start to split again after 1000 h. With prolonging of the aging time, they gradually grow after 2000 h, ellipsoidal secondary gamma' phase become more and more. It is proved that the shape evolution strongly depends on the elastic interaction between gamma' precipitates. Shape changes in order to minimize the sum of interfacial and elastic interaction energies.