Papers by Author: Yong Jun Oh

Abstract: The critical fracture stresses (σ* f(sp)) in various low alloy steels for a reactor pressure vessel(RPV) were evaluated by a small punch test (SP test) and a finite element analysis (FEA) in the cleavage temperature region(-150~-196 °C). The load-displacement curves and distances from the center to the fracture surface (Df) of the FEA results are in good agreement with the experimental results. The maximum principal stresses (SP fracture stresses, σf(SP)) were determined from the FE analysis, when the maximum load was applied to the SP test. The SP critical fracture stress, σ* f(sp) in various reactor pressure vessel (RPV) steels was found to have a linear relationship with the values obtained from the precracked specimens (σ* f(PCVN)). The σ* f(sp) shows a lower value than σ* f(PCVN) because the SP specimen had a lower triaxial stress condition. However, this result indicates that a small punch test could be a useful method to evaluate the cleavage fracture behavior of low alloy steels.

Abstract: The epitaxial growth conditions of CeO2 and Y2O3 single buffer layers on textured Ni tapes were examined using rf magnetron sputtering, and the process conditions for the sequential and mixture buffer layers of these two materials were investigated respectively in order to develop a more simplified buffer architecture. The CeO2 single layer exhibited a well developed (200) epitaxial growth at Ar/10%O2 gas below 450°C, although the epitaxial property was decreased with increasing layer thickness. With regard to the deposition of Y2O3 on Ni, the epitaxial growth was not successful. The epitaxy of Y2O3 on Ni was very sensitive to the O2 gas pressure during sputtering. The repeated sequential architecture of the CeO2 and Y2O3 layers exhibited a good epitaxial property at 400°C/(Ar/10%O2) for the initial CeO2 layer and 700°C/Ar and 700°C/(Ar/10%O2) for the subsequent Y2O3 and CeO2 layers, respectively. The Y-doped CeO2 buffers with (200) epitaxy were successfully obtained by the co-sputtering of Ce and Y metals in a reactive gas condition, and the maximum target Y/Ce ratio for the epitaxy was about 1/10.