Contribution of Primary γ′ Precipitates in the Deformation Creep Mechanisms in the Ni-Based Polycrystalline AD730TM Superalloy

TEM characterization of the deformation micromechanisms in the case of AD730TM disk superalloy has been performed in order to identify the relevant parameters controlling its creep behavior at 700 °C under 600 or 850 MPa. The creep behavior has been investigated for different microstructures resulting from different heat treatments: a coarse grain (CG) and a fine grain microstructures (FG). The specific influence of the primary γ′Primary ′ precipitates precipitates, which are only present in the fine grain microstructure, is of main focus. TEM observations indicate that, in the first stage of the creep deformation, primary γ′ precipitatesPrimary ′ precipitates may act as dislocation sources. The stability of this phase was confirmed using samples aged at 850 °C for several hundreds of hours. TEM spectroscopy has been used to characterize the local chemical composition after aging. A clear evolution of these primary γ′ precipitatesPrimary ′ precipitates has been evidenced and a dissolution of the secondary γ′ precipitates during aging. The presence of these primary γ′ precipitatesPrimary ′ precipitates induces a strong localization of the deformation. Its detrimental effect on the creep properties in the case of polycrystalline Ni-based superalloy at high temperature may be concluded.