Nitride precipitation and formation in IN617 superalloy during creep

Abstract

Precipitation behaviors of four types of nitrides (including π phase (Cr3.25Ni1.75N), AlN, Cr2N and TiN) were investigated in IN617 after creep rupture at 1000 °C/16 MPa. Nitride formation mechanisms were discussed based on their relative stabilities, crystallographic characteristics, and orientation relationships with the matrix. The influences of nitrides on creep rupture behaviors were discussed. The results indicated that the π phase, which was predicted unstable at 1000 °C by thermodynamical calculations, only precipitated at high-energy grain boundaries and phase boundaries. Thermodynamically stable AlN, Cr2N and TiN could be formed at relatively low-energy sites, such as inside the matrix, at dislocations and twining boundaries. Particularly, AlN and Cr2N are polar crystals and have hcp structures, much different from the fcc matrix. They precipitated from the habit planes with low atomic misfits and grew into a needle shape to reduce the strain energy. Fcc TiN was easy to nucleate and had good coherence with the matrix, so it could either precipitate from the regions with high-density dislocations in a needle shape, or directly from the matrix in spherical or dendritic shapes to minimize the interfacial energy and strain energy. Among those nitrides, the π phase and AlN formed coarse intergranular networks and were difficult to deform and release stress, dominantly responsible for the brittle rupture mode.