Abstract
We characterized the microstructural features and mechanical performance of a model Co-5.6Al-5.8W-6.6Ti-0.12B (at.%) alloy consisting of γ(L12)-precipitates in a γ(f.c.c.)-matrix. Scanning electron microscopy (SEM) was used to follow the isothermal aging of the microstructure at 900 and 1000 °C for 256 h, and 950 °C for 1000 h. The compositions of the γ'(L12)-precipitates and γ(f.c.c.)-matrix were evaluated by atom-probe tomography (APT) for solution-treated and air-cooled conditions, as well as in specimens aged at 950 °C for 16 and 100 h. Boron was shown to partition preferentially to the γ'(L12)-precipitates, and profiles taken across the γ(f.c.c.)-matrix channels in both aged specimens revealed confined segregation of Al at one of the two γ(f.c.c.)/γ′(L12) heterophase interfaces. After aging at 950 °C for 16 h, Co-5.6Al-5.8W-6.6Ti-0.12B (at.%) exhibited anomalous flow-strength behavior in the range 625–900 °C with a peak yield stress of 822 MPa between 800 and 825 °C. Compressive creep tests performed at 850 °C demonstrated a creep strength comparable to archival literature results for Co–9Al–9W-0.12B (at.%), despite a smaller γ′(L12)-volume fraction and lack of strengthening borides along the grain boundaries (GBs). The activation energy for creep in the temperature range 800–900 °C was 606 kJ mol−1. The post-creep microstructure consists of rafted γ′(L12)-precipitates perpendicular to the compression axis, consistent with the positive γ(f.c.c.)/γ'(L12) lattice parameter misfit character of this class of alloys. Creep failure could occur due to GB embrittlement caused by deleterious Ti-rich (L21 or B2) and D019 phases formed at the GBs during creep.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 427-438 |
| Number of pages | 12 |
| Journal | Acta Materialia |
| Volume | 174 |
| DOIs | |
| State | Published - Aug 1 2019 |
Funding
This work was performed with financial assistance from award 70NANB14H012 from the U.S. Department of Commerce, National Institute of Standards and Technology, as part of the Center for Hierarchical Materials Design (CHiMaD) at Northwestern University. Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs. NUCAPT received support from the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the SHyNE Resource (NSF ECCS-1542205), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University. This work made use of the shared facilities at the Materials Research Center of Northwestern University (DMR-1121262). We would like to thank Prof. Noam Eliaz (Tel Aviv University) for discussions leading to the selection of alloy compositions as well as for collaborations during the calibration of the APT run parameters utilized in this article. We also thank Dr. Carelyn Campbell (NIST) for useful discussions leading to this article and Dr. Eric Lass (NIST) for performing the DSC experiment. This work was performed with financial assistance from award 70NANB14H012 from the U.S. Department of Commerce , National Institute of Standards and Technology , as part of the Center for Hierarchical Materials Design (CHiMaD) at Northwestern University . Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI ( DMR-0420532 ) and ONR-DURIP ( N00014-0400798 , N00014-0610539 , N00014-0910781 , N00014-1712870 ) programs. NUCAPT received support from the MRSEC program ( NSF DMR-1720139 ) at the Materials Research Center, the SHyNE Resource ( NSF ECCS-1542205 ), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University. This work made use of the shared facilities at the Materials Research Center of Northwestern University ( DMR-1121262 ). We would like to thank Prof. Noam Eliaz (Tel Aviv University) for discussions leading to the selection of alloy compositions as well as for collaborations during the calibration of the APT run parameters utilized in this article. We also thank Dr. Carelyn Campbell (NIST) for useful discussions leading to this article and Dr. Eric Lass (NIST) for performing the DSC experiment.
Keywords
- Atom probe tomography (APT)
- Cobalt-base superalloys
- Creep
- Mechanical properties
- Microstructure
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys