발간논문

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Zricaloy-4 specimens were heated in the region of β-phase and then cooled in iced brine, water, oil, air and furnace. From the microstructural observation of the dislocation and twin substructures, the characteristics of martensite, Widmanstatten and massive structure were investigated. At the cooling rate of 2,100℃/s (iced brine quenching), martensite having (1011) twin was formed. And at the cooling rate of 1,200℃/s (water quenching), Widmanstatten structure a having dense dislocations within lath, precipitates at lath boundaries and massive a along prior β grain boundaries were observed. This suggests that both diffusion and diffusionless transformations took place in this case. At a cooling rate lower than 1200℃/s, precipitates appeared along the lath boundaries of Widmanstatten αAs the cooling rate decreased, the crystal structure of the precipitates changed in the following manner : cubic→cubic+hexagonal→hexagonal. From the present study of crystal structures, it is thought that the stable structure of precipitates at room temperature was of HCP ZrCr₂ type. In addition, it was observed that the size of the precipitate and the volume fraction of the precipitate were increased as the cooling rate decreased.

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Effects of ZrO₂ stabilizers such as 3㏖% Y₂O₃, 8㏖% Y₂O₃, and 9㏖% MgO on the joining phenomena of Ni-Cr steel/Al/ZrO₂ joints have been examined by the concept of solid state diffusion bonding. Irrespective of stahilizers used, the Al/ZrO₂ joint whose reaction layer could not be detected showed a good contact while the reaction layer of the Ni-Cr steel/Al was the diffusion layer of Al and Fe. The joint shear strength of this system did not show any distinct differences even though ZrO₂ stabilizers are varying. The maximun shear strength was 27 ㎫ under a pressure of 100 ㎫, temperature of 635℃, and the time of 120min. The residual stress resulting from the thermal expansion mismatch between ZrO₂ and A1 was different depending on the A1 layer depth sites. It was shown that compressive residual stress could be found at the Al/ZrO₂ interface while tensile residual stress at the NI-Cr steel/Al interface.

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