Crack type degradation behavior of Ni-base superalloy used in advanced gas turbine under creep condition

In order to clarify a crack type degradation mechanism of a directionally solidified Ni-base superalloy CM 247 LC, the effects of environment and microstructural changes on the crack initiation and growth behavior under creep condition have been investigated using an optical and scanning electron microscopes. Metallographic examinations revealed that the surface crack initiation and growth were closely associated with the internal oxidation, nitriding and decomposition of grain boundary carbides. The surface crack initiated at AIN/γ-matrix interface and propagated along a γ' and carbide-free grain boundary. It was also found that the grain boundary M ₂₃C₆ carbide was preferential site for creep cavity initiation, and the transformation of M₆C to M₂₃C ₆ during creep was the critical factor of the internal crack type degradation. Furthermore, the transgranular cracking occurred primarily along the γ-channel, which became lamellar constituent perpendicular to the loading direction because of the raft structure formation.