Lattice parameter misfit evolution during creep of a cobalt-based superalloy single crystal with cuboidal and rafted gamma-prime microstructures

A [h00] oriented Co-based superalloy single crystal was crept under tension at 940°C/100MPa, resulting in a P-type raft morphology with extensive particle coalescence along the [h00] loading direction. However, particle coalescence was also observed in two perpendicular directions on the (h00) plane, normal to the loading axis. Tensile creep experiments were performed with in-situ neutron diffraction at 800°C/500MPa on this initially rafted γ′ microstructure, and for comparison at (i) 900°C/260MPa, and at (ii) 750°C/875MPa, both with initially cuboidal γ′ microstructures. The alloy was shown to exhibit a positive lattice parameter misfit, and during the first hour of creep at 900°C/260MPa, the lattice parameter evolution indicated changes in phase composition associated with γ′ dissolution as the alloy achieved phase equilibrium at 900°C. For all three in-situ creep measurements, there was a significant divergence of the γ′ and γ lattice parameters as creep proceeded. The lattice parameter misfit values between the precipitates and the matrix approached their unconstrained values during creep, and were notably large compared to those of Ni-based superalloys. This is indicative of a loss of coherency at the precipitate/matrix interfaces. Such a loss of coherency at the precipitate/matrix interfaces will likely degrade certain mechanical properties such as fatigue resistance, as has been shown for the Ni-based superalloys.

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