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Materials Science & Engineering A 625 (2015) 164–168
A new approach to strengthen grain boundaries for creep
improvement of a Ni-Cr-Co-Mo superalloy at 950 ℃
J.W.Leea , D. J. Kimb , H.U. Honga
a Department of Materials Science and Engineering, Changwon National University, 20 Changwondaehak-ro, Changwon, Gyeongnam 641-773,
Republic of Korea
b Nuclear Materials Research Division, Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Republic
of Korea
Advisor:Tzu-Yao Tai
Advisee:Pei-Yu Zhu
Department of Mechanical Engineering
Southern Taiwan University
Date:2015/12/22
Outline
Experimental
- Solution-treated
Standard specimen
- Proprietary heat treatment
Serrated GBs
- Slow cooling process
The GB began to be wavy
- Tensile strain of 5% at room temperature Serrated GBs
Results and Discussion
Conclusions
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Solution-treated
The investigated material
was aMcommercially
available
• Few Cr-rich
23 C6 secondary carbides
Alloy 617 in a hot-rolled plate form of 19.5 mm thickness.
• least 5% γ' volume fraction with an average
The chemical composition of this plate was Ni-22Cr-12.3Coprecipitate size of around 20-30 nm
9.5Mo-0.39Ti-0.99Al-0.08C (wt%).
The as-received Alloy 617 which was solution-treated
from a manufacturer had a mean grain size of 80-100 μm
with Mo-rich M6 C primary carbide stringers parallel to
rolling direction.
Standard specimen containing straight GBs
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Proprietary heat treatment
• Alloy 617 had serrated GBs decorated with thick planar M23 C6 .
The samples were solution-annealed at 1200 ℃ for 10 min
and slow-cooled to 800 ℃ at a cooling rate of 5 ℃/min,
subsequently aged for 2 h at the same temperature.
Serrated GBs
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Slow cooling process
When a specimen was cooled from 1200 ℃ to 1100 ℃ at
5 ℃/min.
The GB began to be wavy
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Tensile strain of 5% at
room temperature
A cylindrical specimen of 6 mm gauge diameter and 25
mm gauge length was employed for creep test with the
condition of 960 ℃/30 MPa in air.
Cold work treatment was carried out by loading the heattreated specimen to a tensile strain of 5% at room
temperature with a strain rate of 5× 10−4 /s before creep
tests.
Results and Discussion
Alloy 617
950℃/30MPa/Air
Alloy 617
950℃/30MPa/Air
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Showing longitudinal sections near the fractured surfaces
after creep at 950 ℃/30 MPa
Fig a-c corresponds to Pt-passivated area for machining
TEM foil by FIB system.
Standard
Standard
Serrated Serrated
Pre-strained/serrated specimens
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Distribution of dislocations and M23 C6 carbides in the
subgrain structures after creep at 950 ℃ /30 MPa for the
various microstructures
Standard
Pre-strained/serrated specimens
Serrated
Ideal microstructures
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Conclusions
In
summary, the proprietary heat treatment led
successfully to the transition of serrated GBs for the first
time in a solid-solution strengthened Alloy 617.
TEM analysis on the serrated GB without precipitates
indicated that the fundamental driving force for the GB
serration stems from lowering interfacial free energy of
GB per unit area.
The serrated GBs with fine/stable intragranular carbides
ensured 2.8 times better creep resistance due to their
significant contribution to GB strengthening.
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Thanks for your attention!
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