Multi Scale Characterization of Stress Corrosion Cracking of Alloy X750
- PDF / 5,096,358 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 83 Downloads / 250 Views
Multi Scale Characterization of Stress Corrosion Cracking of Alloy X750 Kevin Fisher1, Sébastien Teysseyre2, Emmanuelle A Marquis1 1 Department 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 γ’ precipitates while the general boundaries wave around the γ’ 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 γ’ 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, γ’ 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 ma
Data Loading...
