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 gamma' raft formation under the tensile creep conditions of 1150 degrees C/100 MPa, and subsequently on a rafted sample under the low temperature/high stress creep conditions of 715 degrees C/825 MPa. During 1150 degrees C/100 MPa creep, the gamma' volume fraction decreased from similar to 70% to similar to 50%, the lattice parameter misfit was partly relieved, and the load was transferred from the creeping gamma matrix to the gamma' precipitates. On cooling back to room temperature, a fine distribution of gamma' precipitates formed in the gamma channels, and these precipitates were present in the 715 degrees C/825 MPa creep regime. Under low temperature/high stress creep, the alloy with rafted gamma' microstructure exhibited superior creep strength to the cuboidal gamma' microstructure produced following a standard heat-treatment. A lengthy creep incubation period was observed, believed to be associated with (111)(110) dislocations hindering propagation of {111}(112) dislocations. Following the creep incubation period, extensive macroscopic creep strain accumulated during primary creep as the gamma phase yielded. Finally, the diffraction data suggest a loss of precipitate/matrix coherency in the (0k0) interfaces as creep strain accumulated. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.