The effects of directional coarsening of $\gamma$' precipitates on the creep behavior were investigated in a directionally solidified Ni-base superalloy, CM 247 LC developed from the modification of the chemical composition of preexisting MAR M 247. Special attention was given to the observation of dislocation substructure in creep-ruptured specimens in order to study the dominant deformation mechanism using transmission electron microscopy.
In a creep condition at 982℃ and 196 MPa were examined changes in the morphology of the $\gamma$' precipitates. The dimensions of the $\gamma$ and $\gamma$' phases were related to time and strain in an attempt to trace the changing $\gamma$-$\gamma$' morphology. The results showed that directional coarsening of $\gamma$' began during primary creep, and the achievement of a fully developed lamellar structure did not appear to be directly related to the onset of steady-state creep. It was also found that the raft thickness of $\gamma$' in the lamellar $\gamma$/$\gamma$' structure remained equal to the initial size from the start of the creep test up through the onset of tertiary creep.
The creep-rupture behavior was different in terms of the occurrence position of microcracks in each test condition. Microcracks were formed mostly around carbides at 760℃/765 MPa and 871℃/441 MPa. At a given temperature of 982℃ the position of microcrack formation was altered with stress ; microcracks were formed mainly in the eutectic $\gamma$/$\gamma$' region at 982℃/147 MPa and 982℃/196 MPa but were formed nearly around carbides at 982℃/245 MPa, which was thought to be the effect of the more stress concentration at carbides under the higher stress. At 1050℃ and 123 MPa they were formed almost in the $\gamma$' denuded zones, and these zones were expected to be formed by Nabarro-Herring type mass transport, especially Cr atoms stabilizing the $\gamma$ matrix.
The prerafted structure obtained with directional-coarsening $\gamma$' for 40 hours at 982℃ and 196 MPa was inferior in creep properties to the unrafted structure ( this structure means the structure which was not given prerafting treatment prior to creep test ) at 760℃/765 MPa and 871℃/441 MPa, but the reverse relationship was shown at 1050℃/123 MPa. According to TEM microscopy of creep-ruptured specimens this discrepancy in creep behavior between the prerafted and unrafted structures was expected to be caused by the change in dominant deformation mechanism in each creep test condition. The dominant mechanism at 760℃/765 MPa and 871℃/441 MPa was observed to be $\gamma$' shearing and so the prerafted structure was thought to have poorer creep properties than the unrafted structure due to the reduction in $\gamma$/$\gamma$' interface area. It was also expected that the impedance to dislocation climb at 1050℃/123 MPa provided longer rupture lives for the prerafted structure.
