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Initial oxidation behavior of Fe-Cr-Si alloys in 1200 °C steam
Moon, Joonho,Kim, Sungyu,Park, Won Dong,Kim, Tae Yong,McAlpine, Samuel Westcott,Short, Michael P.,Kim, Ji Hyun,Bahn, Chi Bum ELSEVIER 2019 JOURNAL OF NUCLEAR MATERIALS Vol.513 No.-
<P><B>Abstract</B></P> <P>Accident-tolerant fuel (ATF) cladding with high oxidation resistance during severe accidents is of critical importance to light water reactor safety and sustainability. One newly proposed ATF cladding concept, a multi-metallic layered composite (MMLC), hinges upon the oxidation resistance of an outer Fe-Cr-Si layer on top of a Zr-based alloy, separated by barrier layers to avoid Fe-Zr eutectic formation. The initial oxidation resistance of three potential Fe-Cr-Si alloys was evaluated by exposing them to 1200 °C oxidizing steam for up to one hundred seconds, along with a Zr–Nb–Sn alloy as a reference. The oxidation resistance of Fe12Cr2Si and Fe16Cr2Si was poor, exhibiting a porous, incomplete multilayer oxide composed mainly of mixed Fe/Cr/Si spinels. However, Fe20Cr2Si showed excellent oxidation resistance due to a continuous amorphous SiO<SUB>2</SUB> layer formed at the metal–oxide interface, followed by almost fully dense Cr<SUB>2</SUB>O<SUB>3</SUB>. This motivates the consideration of Fe-Cr-Si alloys as an additional ATF design choice, similar to FeCrAl alloys in performance and oxidation resistance mechanism.</P>
다중금속층 복합구조 핵연료 피복관 모관의 열간 등방압 가압법 공정 중 균열 발생원인 분석
바이바오성(Baosheng Bai),문준호(Joonho Moon),Michael P. Short,반치범(Chi Bum Bahn) 대한기계학회 2021 大韓機械學會論文集A Vol.45 No.12
본 연구에서 다중금속층 복합구조 핵연료 피복관 모관 제작을 위해 열간 등방압 가압법을 적용하였다. 이후 모관의 최외곽층을 구성하는 Fe12Cr2Si 합금 용접층 표면에서 균열이 발견되었다. 균열 발생의 원인을 분석하기 위하여 Fe12Cr2Si 합금 용접층에 대한 미세조직 관찰, 균열에 대한 파면 관찰, 인장시험 및 모관 제작 과정을 모사하는 유한요소해석이 수행되었다. 용접층 내 입계에서 Si 농축이나 결함은 발견되지 않았다. 일축 인장시험 결과 연성을 충분히 확보하고 있었으나, 발견된 균열 파면은 연성파괴 특성이 없이 편편한 파면을 보였다. 유한요소해석 결과, 열간 등방압 가압법 공정 중 고온에서 상온으로 냉각하는 과정에서 최외곽층인 Fe12Cr2Si 층에 다축응력이 인가되었다. 따라서 냉각 과정 중 재료층 간 열팽창 계수 차이로 인해 발생한 다축응력이 파괴 변형률을 낮추면서 최외곽층에서 shear fracture를 유발한 것으로 판단된다. Hot isostatic pressing (HIP) was applied to manufacture a multi-metallic layered composite mother tube for accident-tolerant nuclear fuel cladding. After the application, surface cracking was observed on the outermost layer, consisting of Fe12Cr2Si alloy weld. To analyze the root cause, microstructural analysis and uniaxial tension testing with Fe12Cr2Si alloy weld were conducted. Finite element analysis (FEA) was performed to simulate the HIP process, as well as fractography on the fracture surface. No Si segregation was found nor were any defects observed at grain boundaries. Although the sufficient ductility of the Fe12Cr2Si weld was confirmed by the uniaxial tensile testing, flat surfaces without ductile features were revealed by the fractography on the crack. The FEA revealed that a multiaxial stress state was applied on the Fe12Cr2Si layer during cooling. Therefore, it is most likely that the shear fracture on the surface of Fe12Cr2Si layer was caused by the stress triaxiality owing to the difference in thermal expansion coefficients.