Abstract
The recently-discovered metastable γ′-Co3(Ta0.76V0.24) phase formed on aging in a Co-6Ta-6V (at.%) ternary alloy is stabilized by partial replacement of Ta and V with Al and Ti. In two alloys with composition Co-6Al-3Ta-3V and Co-5Al-3Ta-3V-1Ti with γ+γ′ microstructure, the γ′-precipitates remain stable for up to 168 h at 850 and 900 °C, with no precipitation of additional phases. Adding Ni and B and doubling the Ti concentration produces a γ/γ′ superalloy, Co-10Ni-5Al-3Ta-3V-2Ti-0.04B (at.%), with γ′ precipitates which are stable up to six weeks of aging at 850 °C, while slowly coarsening and coalescing from cuboidal to elongated shapes. After one day of aging at 850 °C, the γ′ nanoprecipitates have a (Co0.83Ni0.17)3(Ta0.42Al0.23Ti0.19V0.15B0.01) composition, with Al and Ti replacing at the same rate both Ta and V in the original metastable Co3(Ta0.76V0.24) phase. To improve oxidation resistance, 4% Cr is added to the new superalloy, resulting in a somewhat higher volume fraction of finer cuboidal γ′ precipitates after one week of aging at 850 °C, but no other deleterious phases. These W- and Mo-free γ/γ′ superalloys show good creep resistance at 850 °C, on par with two other recent Co-base γ/γ′ superalloys: (i) Co-9W-9Al-8Cr (at.%) which has higher density due to its high W content, and (ii) Co-30Ni-10Al-5Mo-2Nb (at.%) which has lower density (as it is W-free) but contains triple the Ni concentration.
Original language | English (US) |
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Pages (from-to) | 44-54 |
Number of pages | 11 |
Journal | Acta Materialia |
Volume | 172 |
DOIs | |
State | Published - Jun 15 2019 |
Funding
This study was supported by 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 via award 70NANB14H012 . F.R.T acknowledges the support of a NSF Graduate Research Fellowship . S.T. acknowledges the support of the NU-MRSEC Research Experience for Undergraduates ( NSF DMR-1720139 ) and the 3M Corporation . This work made use of the MatCI Facility which receives support from the MRSEC Program ( NSF DMR- 1720139 ) of the Materials Research Center at Northwestern University ; the EPIC facility of Northwestern University’s NU ANCE Center , which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource ( NSF ECCS-1542205 ); the MRSEC program ( NSF DMR-1720139 ) at the Materials Research Center ; the International Institute for Nanotechnology (IIN) ; the Keck Foundation ; the State of Illinois , through the IIN . The authors gratefully acknowledge experimental assistance from Mr. Ding-Wen Chung, Ms. Francesca Long, Mr. Richard Michi and Dr. Amir Fakoosh (NU) and useful discussions with Mr. Ding-Wen Chung and Prof. D.N. Seidman (NU).
Keywords
- Atom probe tomography (APT)
- Cobalt-base superalloys
- Creep
- Microstructure
- Precipitates
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Polymers and Plastics
- Metals and Alloys