Abstract
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.
Original language | English |
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Pages (from-to) | 118-125 |
Number of pages | 8 |
Journal | Acta Materialia |
Volume | 136 |
DOIs | |
State | Published - Sep 1 2017 |
Funding
The contributions of Stephen DiPietro at Exothermics Inc., New Hampshire; and John Koppes at Alcoa-Howmet Research Center, Michigan are acknowledged for material casting and heat-treatments. This work was performed under awards 70NANB14H012 from U.S. Department of Commerce, National Institute of Standards and Technology as part of the Center for Hierarchical Materials Design (ChiMad), and the European Union Seventh Framework Programme under the Marie Curie grant agreement No. 628643. The neutron scattering study at ORNLs Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. This work made use of the EPIC facility of the NUANCE Center at Northwestern University, supported by SHyNE Resource (NSF NNCI-1542205); the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the Keck Foundation; and the State of Illinois, through the IIN.
Keywords
- Creep
- Directional coarsening
- Misfit
- Neutron diffraction
- Superalloy