Lattice strain evolution and load partitioning during creep of a Ni-based superalloy single crystal with rafted γ' microstructure
In-situ neutron diffraction measurements were performed on monocrystalline samples of the Ni-based superalloy CMSX-4 during N-type γ' raft formation under the tensile creep conditions of 1150 °C/100 MPa, and subsequently on a rafted sample under the low temperature/high stress creep conditions of 715 °C/825 MPa. During 1150 °C/100 MPa creep, the γ' volume fraction decreased from ~70% to ~50%, the lattice parameter misfit was partly relieved, and the load was transferred from the creeping γ matrix to the γ' precipitates. On cooling back to room temperature, a fine distribution of γ' precipitates formed in the γ channels, and these precipitates were present in the 715 °C/825 MPa creep regime. Under low temperature/high stress creep, the alloy with rafted γ' microstructure exhibited superior creep strength to the cuboidal γ' microstructure produced following a standard heat-treatment. A lengthy creep incubation period was observed, believed to be associated with {111}$\langle$110$\rangle$ dislocations hindering propagation of {112}$\langle$110$\rangle$ dislocations. Following the creep incubation period, extensive macroscopic creep strain accumulated during primary creep as the γ phase yielded. Lastly, the diffraction data suggest a loss of precipitate/matrix coherency in the (0k0) interfaces as creep strain accumulated.