Negative influence of rafted γ′ phases on 750 °C/750 MPa creep in a Ni-based single crystal superalloy with 4% Re addition

Specimens with two different γ′ phase microstructures, named as-heat treated sample with cuboidal γ′ phases and pre-rafted sample with N-type rafted γ′ phases, were compared to investigate the negative influence of the rafted γ′ phases on the creep property in a Ni-based single crystal superalloy with 4% Re addition at 750 °C and 750 MPa condition. The results indicated that the sample with cuboidal γ′ phases exhibited a lower creep rate and a longer rupture life compared to that with rafted γ′ phases. Transmission electron microscope examinations showed that lots of fine γ′ phases impeded first the dislocation movement in the channel of γ matrix in the initial primary creep stage before the cuboidal γ′ phases were cut by stacking faults in as-heat treated sample. However, in pre-rafted sample, some existed superdislocations located in rafted γ′ phases and the mismatch stress release at γ/rafted γ′ interface resulting from pre-rafted treatment not only decreased the creep resistance of rafted γ′ phases, but also weakened the γ/rafted γ′ interface pinning effect for dislocation movement. Therefore, the rafted γ′ phases were easily cut by superdislocations and stacking faults resulting in a higher creep rate and a shorter creep life.