Influence of coarsened and rafted microstructures on the thermomechanical fatigue of a Ni-base superalloy

The aging of the microstructure of Ni-base superalloys during service is primarily characterized by coarsening and rafting of the γ′ precipitates. The influence of these different aged microstructures on thermomechanical fatigue (TMF) under either continuously cycled (CC) and creep-fatigue (CF) was investigated. Three different aged microstructures, generated through accelerated aging and pre-creep treatments, were studied: stress-free coarsened γ′, rafted with orientation perpendicular to loading direction (N-raft), and rafted with orientation parallel to loading direction (P-raft). Under most conditions, the aged microstructures were less resistant to TMF than the virgin microstructure; however, there were exceptions. Both stress-free coarsened and N-raft microstructures resulted in a reduction in TMF life under both CC and CF conditions in comparison to the virgin material. P-raft microstructure also resulted in reduction in TMF life under CC conditions; however, an increase in life over that of the virgin material was observed under CF conditions. These differences are discussed and hypothesized to be related to the interactions of the dislocations in the γ channels with γ′ precipitates.