Lattice strain evolution and load partitioning during creep of a Ni-based superalloy single crystal with rafted microstructure

In-situ neutron diffraction measurements were performed on monocrystalline samples of the Ni-based superalloy CMSX-4 during N-type g0 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 g0 volume fraction decreased from ~70% to ~50%, the lattice parameter misfit was partly relieved, and the load was transferred from the creeping g matrix to the g0 precipitates. On cooling back to room temperature, a fine distribution of g0 precipitates formed in the g channels, and these precipitates were present in the 715 C/825 MPa creep regime. Under low temperature/high stress creep, the alloy with rafted g0 microstructure exhibited superior creep strength to the cuboidal g0 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 g phase yielded. Finally, the diffraction data suggest a loss of precipitate/matrix coherency in the (0k0) interfaces as creep strain accumulated. © 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.