Materials Science Forum Vols. 510-511

Abstract: ZrO2 buffer layer and SrBi2Ta2O9 (SBT) thin films were deposited on the P-type Si(111) substrates by the R.F. magnetron-sputtering method. We studied the effect of the post-annealing of the ZrO2 buffer layer on the MFIS structure. We could conclude that the elements of Zr, Sr, Bi, and Ta etc. were diffused by the post-annealing, and according to the process with and without the post-annealing of the ZrO2 layer, the diffusion amount of Sr, Bi, Ta elements shows slight difference through the glow discharge spectrometer (GDS) analysis. From the results of the XPS analysis, we can confirm that a small amount of SiO2 and metallic Zr exist at the interface, and ZrO2 exists as the amorphous state with the weak binding energy before the post-annealing process. Contrarily, after the post-annealing of the ZrO2/Si structure, SiO2 and metallic Zr are observed at the wide range, but the bonding state of ZrO2 is strengthened.

Abstract: Flow patterns and mixing behaviors in a gas stirred steelmaking ladle with a slag layer were discussed using a water model experiment as well as a numerical simulation. While the water model experiment was performed to investigate the effect of slag on the mixing behavior in ladle, the numerical simulation was carried out to figure out the flow pattern in ladle with a slag layer. Slag viscosity and its thickness in ladle were considered as major variables. It was found that a slag layer made a great change in the flow pattern in ladle, which, in turn, affected on the mixing behavior in ladle. A flow pattern without a slag layer showed that rising bubbles eventually made a recirculation loop at the central area of the ladle and this flow pattern was regarded as a favorable flow pattern for the better mixing behavior. However, a flow pattern with a slag layer showed distorted and localized recirculating loop near side wall below slag layer. This eventually gave a longer mixing time in ladle with a slag layer. Moreover, as the gas flow rate increases, slag existing on top of the ladle was found to be entrained into the melt. Slag viscosity and its thickness were found to be major variables affecting the behavior of slag entrainment. Lower the slag viscosity and thicker the slag layer, much more slag on top of the melt was entrained into the melt.

Abstract: It is generally well known that a steelmaking ladle operation plays an important role in the production of clean steel. A turbulent mixing of melt with Ar gas bubbling from the ladle bottom can homogenize a melt temperature and can control precisely chemical compositions of steel. In order to figure out these phenomena, a quantitative analysis of fluid flow behavior of gas and melt during a ladle operation is required and special concerns should be focused on effects of operating parameters on the perfect mixing time of melt. In this study, as a basic approach, effects of operating parameters such as a melt depth (aspect ratio) and a nozzle type (one-hole or porous plug) on the mixing behavior in ladle operation are investigated. Water model experiments are carried out to simulate these melt behaviors in steelmaking ladle. As a result, it was found that there exist an optimized melt depth and a nozzle type at a given gas flow rate, which affect significantly on the mixing behavior of melt.

Abstract: In the present study, we have attempted to refine a microstructure of conventional AZ31 magnesium alloy using a new combination process including hot extrusion followed by a cold equal channel angular pressing (ECAP). ECAP die was specially designed with an inner die corner angle of 135 degree, the fillet angle of 45 degree and dimensional thermo-coupled elastro-plastic material model in order to understand the change of stress and strain of the deformed material after a cold ECAP. ECAP for the AZ31 alloy, which was extruded in the extrusion ratio 20 to 1 and heat-treated at 623K, was successfully carried out at room temperature. The uniform shear band obtained from experiment was well matched with the zone of effective strain more than 0.533 estimated from calculation. On the basis of the results, it is suggested that the room temperature ECAP makes microstructure to be refined and the basal plane to be rotated slightly from extrusion direction to axis direction. Compressive yield strength of AZ31 alloy can be enhanced up to twice in applying ECAP process. Hall-Petch relations do not fit to the experimental data This can be ascribed to the texture effect. Room temperature ECAP process is very promising in improving mechanical properties of AZ31 alloy in terms of grain refinement and texture control.

Abstract: Non-isothermal behaviors of calcium carbonate using Danyang limestone were investigated. It was attempted to provide non-isothermal data with a precision sufficient for the determination of reliable decomposition behaviors and for the estimation of accurate kinetic parameter. The decomposition temperature of calcium carbonate on the onset, peak and final point were measured. Reaction rate was decreased and maximum reaction temperature was increased with increasing heating rate. Activation energy of Danyang limestone was 45.14㎉/㏖ and 50.80㎉/ ㏖ by Kissinger method and Freeman method, respectively.

Abstract: To determine the causes of the yellowing phenomenon on transparent dielectrics, we investigated a bus electrode and transparent dielectric, varying the amounts of frit content in the Ag paste and experimenting with the thickness of the dielectric. It was found that increasing the frit content in the Ag paste coating caused the transparent dielectric to experience greater discoloration, so that it gradually turned a deep yellow, which suggests that oxygen in the air is a factor causing discoloration of the transparent dielectric during firing.

Abstract: Hot torsion of a C (0.22 wt%)-Mn steel was used to investigate the influence of thermomechanical arameters on the strain induced dynamic transformation (SIDT) of ferrite. The pecimens were strained as a function of strain rate (0.05/sec - 5/sec) and strain (- 5.0) at right bove Ar3 temperature. The critical strain to initiate dynamically transformed ferrite nuclei during deformation increased as increasing the strain rate. On the other hand the completion of SIDT was hifted to larger strain by decreasing strain rate. This is due to the fact that the dynamic ransformation of ferrite was processed in the interior of austenite grain as well as at grain boundary y large stored energy and many nucleation sites for high strain rate. The dynamic transformed micro-structure of ferrite was developed to higher angle and the grain size could be refined to ~3 ㎛ at strain of 3.0 and 5/sec.

Abstract: The low carbon steel of 0.22wt% carbon was tested to estimate the dynamic phase transformation behavior from austenite to ferrite. The samples were deformed at just above Ar3 temperature by hot torsion at condition of strain rate (0.5/sec) and strain (5.0). The flow curve obtained at just above Ar3 significantly differed from others due to dynamic transformation. Based on the analysis of flow stress curve and observation of micro-structure evolution, the initiation and finish points of strain induced dynamic transformation (SIDT) could be determined. An inflection point observed at early deformation range (0.2–0.3) from the work-hardening rate and stress plot meant that new ferrite grains were nucleated in austenite matrix and these nuclei could be also confirmed by optical microscope. Subsequently in strain range of 0.7-1.0, the flow stress had the maximum value and new fine ferrite grains were dynamically generated inside untransformed austenite grains as well as prior austenite grains. The dynamic phase transformation induced by deformation made eventually fine ferrite grains under 3 ㎛ and decreased stress level with a fixed gradient.

Abstract: The relationships between flow stress curve and microstructure evolution in strain induced dynamic phase transformation (SIDT) of low carbon steel (0.22wt.%) were quantitatively investigated. The deformation was carried out at just above Ar3 temperature (710°C) as a function of strain rate (0.01-5/sec). The softening process of SIDT was well agreed with calculated result derived from Avrami’s and constitutive equation at higher strain rate than 0.5/sec. However, the calculated results differed from the experimental curve at strain rate of less than 0.2/sec. This is due to fact that the dynamic transformation from austenite to ferrite can not be completed owing to less stored energy during hot deformation.