TY - JOUR
T1 - Microstructural and creep properties of boron- and zirconium-containing cobalt-based superalloys
AU - Bocchini, Peter J.
AU - Sudbrack, Chantal K.
AU - Noebe, Ronald D.
AU - Dunand, David C.
AU - Seidman, David N.
N1 - Funding Information:
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 (Grant No. NNX14AF45H ) and Fixed Wing project. APT measurements were performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The APT system was purchased and upgraded with funding from NSF-MRI ( DMR-0420532 ) and ONR-DURIP ( N00014-0400798 , N00014-0610539 and N00014-0910781 ) grants. NUCAPT is a shared facility of the Materials Research Center of NU, supported by the National Science Foundation MRSEC Program ( DMR-1121262 ). The authors also gratefully acknowledge the Initiative for Sustainability and Energy at Northwestern (ISEN) for grants to upgrade the capabilities of NUCAPT. This work made use of Northwestern's Materials Research Center's core facilities funded by the NSF MRSEC program (DMR-112162). TEM research was performed at the Northwestern University Atomic and Nanoscale Characterization Experimental (NUANCE) Center. The authors would like to thank research associate professor D. Isheim for managing NUCAPT, Dr. S.-I. Baik (Northwestern University) for assistance with transmission electron microscopy, Drs. Dave Ellis and Cheryl Bowman (NASA Glenn Research) for assistance with creep experiments, Mr. Grant Feichter and Jesse Bierer for arc melting and heat treating (NASA Glenn Research), and Dr. Tim Gabb (NASA Glenn Research) for helpful discussions.
PY - 2017/1/13
Y1 - 2017/1/13
N2 - The effects of micro-additions of boron and zirconium on grain-boundary (GB) structure and strength in polycrystalline γ(f.c.c.) plus γ'(L12) strengthened Co-9.5Al-7.5W-X at% alloys (X=0-Ternary, 0.05B, 0.01B, 0.05Zr, and 0.005B-0.05Zr at%) are studied. Creep tests performed at 850 °C demonstrate that GB strength and cohesion limit the creep resistance and ductility of the ternary B- and Zr-free alloy due to intergranular fracture. Alloys with 0.05B and 0.005B-0.05Zr both exhibit improved creep strength due to enhanced GB cohesion, compared to the baseline ternary Co-9.5Al-7.5W alloy, but alloys containing 0.01B or 0.05Zr additions display no benefit. Atom-probe tomography (APT) is utilized to measure GB segregation, where B and Zr are demonstrated to segregate at GBs. A Gibbsian interfacial excess of 5.57±1.04 atoms nm−2 was found for B at a GB in the 0.01B alloy and 2.88±0.81 and 2.40±0.84 atoms nm−2 for B and Zr, respectively, for the 0.005B-0.05Zr alloy. The GBs in the highest B-containing (0.05B) alloy exhibit micrometer-sized boride precipitates with adjacent precipitate denuded-zones (PDZs), whereas secondary precipitation at the GBs is absent in the other four alloys. The 0.05B alloy has the smallest room temperature yield strength, by 6%, which is attributed to the PDZs, but it exhibits the largest increase in creep strength (with an ~2.5 order of magnitude decrease in the minimum strain rate for a given stress at 850 °C) over the baseline Co-9.5Al-7.5W alloy.
AB - The effects of micro-additions of boron and zirconium on grain-boundary (GB) structure and strength in polycrystalline γ(f.c.c.) plus γ'(L12) strengthened Co-9.5Al-7.5W-X at% alloys (X=0-Ternary, 0.05B, 0.01B, 0.05Zr, and 0.005B-0.05Zr at%) are studied. Creep tests performed at 850 °C demonstrate that GB strength and cohesion limit the creep resistance and ductility of the ternary B- and Zr-free alloy due to intergranular fracture. Alloys with 0.05B and 0.005B-0.05Zr both exhibit improved creep strength due to enhanced GB cohesion, compared to the baseline ternary Co-9.5Al-7.5W alloy, but alloys containing 0.01B or 0.05Zr additions display no benefit. Atom-probe tomography (APT) is utilized to measure GB segregation, where B and Zr are demonstrated to segregate at GBs. A Gibbsian interfacial excess of 5.57±1.04 atoms nm−2 was found for B at a GB in the 0.01B alloy and 2.88±0.81 and 2.40±0.84 atoms nm−2 for B and Zr, respectively, for the 0.005B-0.05Zr alloy. The GBs in the highest B-containing (0.05B) alloy exhibit micrometer-sized boride precipitates with adjacent precipitate denuded-zones (PDZs), whereas secondary precipitation at the GBs is absent in the other four alloys. The 0.05B alloy has the smallest room temperature yield strength, by 6%, which is attributed to the PDZs, but it exhibits the largest increase in creep strength (with an ~2.5 order of magnitude decrease in the minimum strain rate for a given stress at 850 °C) over the baseline Co-9.5Al-7.5W alloy.
KW - Atom-probe tomography (APT)
KW - Cobalt-base superalloys
KW - Creep
KW - Gamma prime
KW - Grain boundaries
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U2 - 10.1016/j.msea.2016.10.124
DO - 10.1016/j.msea.2016.10.124
M3 - Article
C2 - 32020989
AN - SCOPUS:84998881916
VL - 682
SP - 260
EP - 269
JO - Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing
JF - Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing
SN - 0921-5093
ER -