Abstract
Polycrystalline Co-10Ni-(9 – x)Al-(9 – x)W-2xTi at% (x = 0, 1, 2, 3, 4) alloys with γ(f.c.c.) plus γ′(L12) microstructures are investigated, where the γ′(L12)-formers Al and W are replaced with Ti. Upon aging, the initially cuboidal γ′(L12)-precipitates grow and develop a rounded morphology. After 256 h of aging at 1000 °C, the precipitates in the 6 and 8 at% Ti alloys coalesce and develop an irregular, elongated morphology. After 1000 h of aging, replacement of W and Al with Ti increases both the mean radius, <R>, and volume fraction, ϕ, of the γ′(L12)-phase from <R> = 463 nm and ϕ = 8% for 2 at% Ti to <R> = 722 nm and ϕ = 52% for 8 at% Ti. Composition measurements of the γ(f.c.c.)-matrix and γ′(L12)-precipitates demonstrate that Ti substitutes for W and Al in the γ′(L12)-precipitates, increases the partitioning of W to γ′(L12), and changes the partitioning behavior of Al from a mild γ′(L12)-former to a mild γ(f.c.c.)-former. The grain boundaries in the aged alloys exhibit W-rich precipitates, most likely μ(Co7W6)-type, which do not destabilize the γ(f.c.c.) plus γ′(L12) microstructure within the grains. Four important benefits accrue from replacing W and Al with Ti: (i) the alloys’ mass density decrease; (ii) the γ′(L12)-solvus temperature increases; (iii) the γ′(L12) volume fraction formed during aging at 1273 K (1000 °C) increases; and (iv) the 0.2% offset flow stress increases.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 122-132 |
| Number of pages | 11 |
| Journal | Materials Science and Engineering: A |
| Volume | 705 |
| DOIs | |
| State | Published - Sep 29 2017 |
Funding
This research was supported by the US Department of Energy, Office of Basic Energy Sciences (Dr. John Vetrano, grant monitor) through grant DE-FG02-98ER45721. P.J.B. received partial support from the NASA Aeronautics Scholarship Program and Fixed Wing project. This work made use of the Materials Characterization and Imaging Facility and the Central Laboratory for Materials Mechanical Properties (CLaMMP), which receives support from the MRSEC Program (DMR-112162) at Northwestern University. This research also made use of the Electron Probe Instrumentation Center (EPIC) facility of the Northwestern University Atomic and Nanoscale Characterization Experimental Center (NUANCE), which receives support from the Materials Research Science and Engineering Center (MRSEC) program (NSF DMR-1121262); the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN. The authors also thank Messrs. Jesse Bierer and Grant Feichter (NASA Glenn Research Center, Cleveland, OH) for their assistance with arc-melting and heat-treating the specimens and Dr. Daniel Sauza for his assistance with sample preparation. This research was supported by the US Department of Energy , Office of Basic Energy Sciences (Dr. John Vetrano, grant monitor) through grant DE-FG02-98ER45721 . P.J.B. received partial support from the NASA Aeronautics Scholarship Program and Fixed Wing project. This work made use of the Materials Characterization and Imaging Facility and the Central Laboratory for Materials Mechanical Properties (CLaMMP), which receives support from the MRSEC Program ( DMR-112162 ) at Northwestern University. This research also made use of the Electron Probe Instrumentation Center (EPIC) facility of the Northwestern University Atomic and Nanoscale Characterization Experimental Center (NUANCE), which receives support from the Materials Research Science and Engineering Center ( MRSEC ) program (NSF DMR-1121262 ); the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN. The authors also thank Messrs. Jesse Bierer and Grant Feichter (NASA Glenn Research Center, Cleveland, OH) for their assistance with arc-melting and heat-treating the specimens and Dr. Daniel Sauza for his assistance with sample preparation.
Keywords
- Cobalt superalloy
- Flow stress
- Gamma prime
- High-temperature stability
- Precipitation strengthening
ASJC Scopus subject areas
- General Materials Science
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering