Grain boundary characterization in an X750 alloy Page: 2 of 7
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Grain boundary characterization in an X750 alloy
Kevin Fishers, Sebastien Teysseyre2, Emmanuelle A Marquis
1Department of Materials Science and Engineering, University of Michigan, Ann Arbor MI
2 ATR National Scientific User Facility, Idaho National Laboratory, Idaho Falls, ID
ABSTRACT
Grain boundary chemistry in an X750 Ni alloy was analyzed by atom probe tomography
in an effort to clarify the possible roles of elemental segregation and carbide presence on the
stress corrosion cracking behavior of Ni alloys. Two types of cracks are observed: straight cracks
along twin boundaries and wavy cracks at general boundaries. It was found that carbides (M23C6
and TiC) are present at both twin and general boundaries, with comparable B and P segregation
for all types of grain boundaries. Twin boundaries intercept y' precipitates while the general
boundaries wave around the y' and carbide precipitates. Near a crack tip, oxidation takes place
on the periphery of carbide precipitate.
INTRODUCTION
Alloy 750 is a high-strength, precipitation hardened Ni-Cr-Fe superalloy. It is commonly
used in nuclear reactors either for original components (as springs or guide pins), or as repair
hardware (as core shroud support bracket) when high strength and corrosion resistance are
needed. These components are in contact with environment and can be exposed to radiation at
low fluence. Although limited, such components failed in service due to some localized forms of
degradation (stress corrosion cracking, fatigue fatigue, hydrogen assisted cracking) [1]. Repair
hardware, as tie rods and support bracket also showed evidence of degradation by stress
corrosion cracking. There is limited data about the propagation rate of stress corrosion cracks
and about the influence of radiation [2,3]. There is a need for more data but also for a more
comprehensive understanding on the influence of the microstructure features present in X-750 on
crack propagation by stress corrosion cracking mechanism. Although there is no consensus on
the mechanism of cracking, there are features that are known to, or expected to, influence
cracking rate and its evolution under irradiation.
For some heat treatments a prominent feature of the microstructure is the presence of the
intergranular M23C6 carbide precipitate phase. Intergranular carbides can enhance cracking due
to the sensitization of grain boundaries, as known for stainless steels. For nickel alloys, it was
shown that the intergranular carbide precipitation can be beneficial for crack growth due to its
influence on crack tip stress distribution [4,5]. Intragranular y' phase [Ni3(Al,Ti)] precipitation
will influence the local mechanical behavior of the material, i.e crack tip but it may also impact
the progression of oxidation ahead of the crack. Furthermore, under neutron fluence, y'may
dissolve therefore modifying the initial conditions.
EXPERIMENT
The alloy is a X-750 Ni alloy (heat 2750-5-7656) in the HTH heat treatment. This
material comes from an e core shroud upper support bracket manufactured by Haynes
International. Its nominal composition is given in Table 1. It was solution annealed at 2000F for
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Fisher, Kevin; Teysseyre, Sebastien & Marquis, Emmanuelle. Grain boundary characterization in an X750 alloy, article, November 1, 2012; Idaho Falls, Idaho. (https://digital.library.unt.edu/ark:/67531/metadc838853/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.