2016 Volume 65 Issue 10 Pages 706-712
The propagation behavers of naturally initiated small crack in a conventional cast polycrystalline Ni-base superalloy were studied under the thermomechanical fatigue conditions. The small crack propagation tests were carried out under the in-phase (IP) and the out-of-phase (OP) type thermomechanical fatigue (TMF) conditions as well as the isothermal low-cycle fatigue (LCF) condition at the maximum temperature of the TMF conditions. The experimental results revealed that the initiation and propagation morphologies of the naturally initiated small crack were affected by the thermomechanical loading condition. Under the LCF and IP-TMF conditions, the small cracks were initiated and propagated at the grain boundaries perpendicular to the loading axis. On the other hand, under the OP-TMF condition, the small cracks were initiated and propagated with the transgranular modes. When the crack growth rates of naturally initiated small crack were correlated with fatigue J-integral range, which is the continuum mechanics parameters for homogeneous and isotropic materials, the crack growth rates under the TMF conditions were almost similar to those under LCF condition even if the propagation morphologies of small cracks were varied with the thermomechanical fatigue conditions. In addition, it could be considered that the crack growth morphologies, grain structure and grain orientation affect the scatter of the small crack growth rate. The prediction method for TMF and LCF lives was proposed based on the small crack propagation considered with the crack opening-closing behavior. The TMF and LCF lives of Ni-base superalloys could be predicted with higher accuracy by the proposed method.
