발간논문

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Effect of the internal pressure on the corrosion behavior was investigated for the Zr nuclear fuel cladding tubes which were pressurized up to 2500 psi using Ar gas and corroded in steam of 1500 psi at 400℃. It was found that corrosion resistance of Zr fuel cladding tube was decreased with increasing the internal pressure although corrosion behavior at the early stage was similar irrespective of internal pressure. X-ray diffraction result on the oxide showed that crystal structure was not affected by the applied pressure of 2500 psi. SEM observation on the oxide surface revealed that a number of cracks elongated in axial direction were propagated from surface to interior in the case of 2500 psi. The degradation of corrosion resistance with internal pressure was attributable to the extensive cracking at the oxide surface, which is thought to be originated from the difference of ductility between Zr alloy and oxide when Zr fuel cladding tube was deformed due to the applied internal pressure. However, the increase of corrosion rate with internal pressure was shown to be relatively small. It was therefore suggested that the crack induced at the surface did not propagate to oxide layer in the vicinity of oxide/metal interface which is known to be protective to oxygen diffusion and to control the corrosion behavior.

keyword : Zr alloys, Fuel cladding, Internal pressure, Corrosion, Oxide

Austenite grain size of automotive forging steel was predicted by FEM analysis and dynamic recrystallization model. The hot forging process consisting of rolling, busting, blocking and finishing for lower arm connector was analyzed by commercial code DEFORM-3D. The flow curve and the dynamic recrystallization behavior of 0.43C steel were determined by hot compression test. The process parameters such as temperature, strain and strain rate obtained by the FEM analysis were coupled with the dynamic recrystallization model and they were applied to hot compression test. The variation of the grain size during the entire forging process was calculated by computer programming of the coupled model. The predicted final grain sizes agreed well with the experimentally observed.

keyword : Hot forging, Dynamic recrystallization, Flow stress, Austenite grain size

In order to apply for three-dimensional capacitors of high-density ferroelectric random access memories (FRAMs), the low temperature direct liquid injection metalorganic chemical vapor deposition (DLI-MOCVD) of Pb-based ferroelectric films has been investigated. The combination of Pb(TMHD)₂/Zr(TMHD)(O-i-Pr)₃/Ti(TMHD)₂(Oi- Pr)₂ was selected for metalorganic precursors and n-butylacetate was also selected for solvent. The lowest temperature to grow Pb-based films using those metalorganic precursors was found to be 360℃ at which Ti(TMHD)₂(O-i-Pr)₂ could be sufficiently decomposed. The film grown at 380℃ exhibited the good step coverage characteristic. However, the small variation in the precursor input flow rate ratios led to the large variation in film compositions, which means that the composition control is difficult at the low temperature. The application of pulsed plasma expanded the process window to obtain the stoichiometric films, thereby making the control of film composition somewhat easier.

keyword : Pb(Zr,Ti)O3(PZT), Direct Liquid Injection, DLI, Metalorganic Chemical Vapor Deposition, MOCVD, Ferroelectric Random Access Memory, FRAM, Pulsed Plasma

Fe solubility in AZ91 alloy melts was measured by a new method designed for removal of Al_(8)(Mn,Fe)_(5) inclusions. In the AZ91 alloy melts, Al_(8)(Mn,Fe)_(5) precipitates were formed due to supersaturated Mn and Fe, but all precipitates were not settled down from the melts and hence part of them was included in the samples for measuring Fe solubility. This means that the solubility measured from these samples is higher than the true solubility. In the present study, it was shown that these Al_(8)(Mn,Fe)_(5) inclusions could be separated from the samples by the new method. As a result, the Fe solubility measured by the present method was lower than the previous solubility data. Amount of the unsettled Al_(8)(Mn,Fe)_(5) inclusions was estimated to be 59-148 ppm. It was also observed that Fe contents in Al_(8)(Mn,Fe)_(5) were varied with the Fe and Mn content in AZ91 alloy melts.

keyword : Fe solubility, AZ91 alloy, Magnesium, Precipitate, Inclusion

The nitrogen solubility and nitride capacity of CaO-SiO₂ and CaO-SiO₂-CaF₂ slag systems were measured using gas-liquid equilibration. The nitrogen solubility in CaO-SiO₂ slag system is affected as following factors; (1) the structure and species of polysilicate ions (2) the number of the bonds between nitrogen and polysilicate ions. The detection of Si-N bond in this slag system by XPS analysis proves nitrogen in the slag substitute oxygen ion in polysilicate. With CaF₂ containing slag system nitrogen solubility decreases with increasing CO partial pressure with the liner relationship between nitrogen contents and oxygen partial pressure of -3/4. And the nitrogen solubility first decreases to reach a minimum value and then increases with the increase of CaO contents. This behavior indicates that there are two types of nitrogen dissolution mechanisms, one with an acidic component and the other with a basic component. With low basicity, the addition of CaF₂ is effective on denitogenization, but with high basicity, it`s not.

keyword : Slag, Basicity, Nitrogen solubility