Morphological Changes in γ’ Phase by Creep, Aging and Aging After Creep for Polycrystalline Nickel-Based Superalloy

This study investigates the relationship between the amount of creep pre-strain and morphological changes in γ’ phase during thermal aging in conventionally cast nickel-based superalloy GTD-111. In the interrupted creep sample with creep strain of 0.4%, rafting of the γ’ phase was not observed, but it was caused by additional thermal aging. This indicated that the rafting of the γ’ phase during thermal aging was affected by the amount of creep pre-strain. Furthermore, in the samples prepared near an in-service blade surface, rafting of the γ’ phase was not observed, but it was caused by thermal aging. This suggested that a comparatively large amount of creep strain accumulated near the blade surface during service. This result qualitatively corresponded to the creep strain distribution of the blade portion obtained by conjugate heat transfer analysis and finite element analysis. Thus, it clarified that the small amounts of creep strain can be evaluated by aging the samples taken from in-service blades in the gas turbine, which can lead to the creep life assessment.