Papers by Author: Young Jae Kwon

Abstract: Porous alumina bodies were successfully prepared by spark plasma sintering of alumina powders with different amounts of graphite, and by subsequently burning out the graphite. Highly porous bodies were fabricated by spark plasma sintering at 1000°C for 3 min under a pressure of 30 MPa. The heating rate was 80°C/min, and the pulse pattern (on-off) was 12:2. For example, alumina bodies prepared by the addition of 10 ~ 30 vol% graphite showed high porosity of 50 ~ 57%. Porous alumina bodies prepared by the addition of 10 ~ 30 vol% graphite had a high compressive strength of 200 ± 55 MPa, about 35 times higher than those obtained on samples prepared by pressureless sintering, and about 2.5 times higher than those in samples prepared by hot-pressing. The significant improvement in strength relative to values obtained with conventional sintering was attributed to better sintering resulting from the rapid heating between particles.

Abstract: The nanostructural and optical features of hydrogenated nanocrystalline silicon (nc-Si:H) thin films, which were prepared by plasma enhanced chemical vapor deposition (PECVD), were investigated as a function of deposition conditions. It was found that the crystallite size varied with the relative fraction of Si-H3 bonds in the films, [ ] eger n n n H Si H Si int 3 1 3 / ] [ = = ∑ − − , which was sensitively related with the flow rate of SiH4 reaction gas. The silicon nanocrystallites in the films enlarged from ~2.0 to ~8.0 nm in their size with increasing gas flow rate, while the PL emission energy varied from 2.5 to 1.8 eV; the relative fractions of the Si-H3, Si-H2, and Si-H bonds in the amorphous matrix were also varied sensitively with the SiH4 flow rate. A model for the nanostructure of the nc-Si:H films was suggested to discribe the variations in the size and chemical bonds of the nanocrystallites as well as the amorphous matrix depending on the deposition conditions.

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.

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: 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 high temperature deformation behavior of AISI 430 ferritic stainless steel has been studied over a temperature range of 800 to 1000°C and strain rate of 0.05-5.0/sec. The evolution of flow stress and microstructures showed the characteristics of continuous dynamic recrystallization (CDRX). The flow stress curves gradually decreased with increasing strain over the peak stress until 500% of strain without any steady state shown in typical austenitic stainless steel. Sub-grains of low angle firstly formed along the original high angle grain boundary were propagated into the inside of original grain and transformed to high angle. The CDRX grain sizes of AISI 430 deformed at 1000 °C and 0.5/sec was about 30-35㎛.