Papers by Author: Sun Keun Hwang

Abstract: The recrystallization behavior of cold-rolled, commercial-purity titanium was studied experimentally and with Monte-Carlo (MC) modeling. Utilization of EBSD-OIM as input for MC modeling resulted in realistic predictions of recrystallization kinetics, microstructure and texture, which were in good agreement with experimental results. MC modeling of recrystallization kinetics predicted that the non-uniform stored energy distribution, heterogeneous nucleation of recrystallization and recovery in combination leads to a negative deviation from linear JMAK kinetics. It was found that concurrent recovery that takes place during recrystallization is an important process that controls both the overall recrystallization kinetics and the deviation of linear JMAK kinetics. On the other hand, the non-uniformly distributed stored energy itself has little effect on the negative deviation from JMAK kinetics but intensifies the deviation when heterogeneous nucleation is combined. Modeling results also revealed that heterogeneous nucleation of recrystallized grains and their early impingement in local areas of high deformation are essential for producing a log-normal distribution of grain size and a typical recrystallization texture of rolled titanium.

Abstract: Effect of deformation temperature on subsequent recrystallization characteristics of commercial purity titanium (CP-Ti) was investigated using EBSD and transmission electron microscopy. CP-Ti plates were rolled to at various temperatures between 25°C and 600°C to achieve 60% reduction, and were subsequently annealed at 600°C for various time periods. Increase in rolling temperature resulted in more refined microstructures, which could be attributed to coactivation of secondary slip systems such as basal slip or pyramidal slip in addition to prism slip. During subsequent annealing, a large number of recrystallization nuclei formed in the warm-worked (below 450°C) CP-Ti while nucleation of recrystallization was slower for coldrolled one. This led to the faster recrystallization kinetics and a finer microstructure in asrecrystallized state for warm-worked CP-Ti. Meanwhile, the rate of recrystallization in CP-Ti rolled at 600°C was significantly retarded because a good portion of strain energy stored in deformed matrix was released by dynamic recovery. In the present work, we show that the widely accepted explanation for the influence of the deformation temperature on the recrystallization characteristics, i.e., increase of the deformation temperature retards the recrystallization process, is not always true for the hcp metals since deformation at elevated temperature activates secondary slip systems, providing more sites for nucleation of recrystallization.

Abstract: We present a study of the photoresist (PR) etching and the low-k materials damage using a ferrite-core inductively coupled plasma (ICP) etcher, in order to develop an etching process for the low-k dielectric devices. We reveal that the N2/O2 flow ratio and bias power affected the PR etching rate. By Fourier transform infrared spectroscopy and HF dipping test, we investigated the effect of the gas flow ratio and bias power on the amount of etching damage to the low-k material.

Abstract: To study the effect of grain size on texture and tensile properties of equal channel angular pressed commercially pure Ti, specimens were made to have the grain size ranging from 4 m to 60 m with the severe plastic deformation followed by recrystallization annealing. In this initial condition all the specimens exhibited a strong bimodal split basal texture. During subsequent repressing at 350°C, the texture pattern became randomized through crystal rotations, the phenomenon being more pronounced in coarse-grained specimens. The microstructure of deformed specimens, as examined by electron back scattered diffraction, showed formation of mechanical twins and microstructural inhomogeneity in the coarse-grained specimens. The room temperature tensile properties of the re-pressed specimens showed that the yield strength was remarkably enhanced regardless of the grain size whereas the elongation was reduced as compared to the initial condition, particularly in coarse-grained specimen. It was concluded that microstructural refinement during the severe plastic deformation was the main cause of the improved yield strength while the twinning and microstructural inhomogeneity were responsible for the texture randomization and the impaired ductility.

Abstract: Indium oxide (In2O3) films were successfully grown on LiAlO2 substrates using the triethylindium (TEI) as a precursor in the presence of oxygen in the metalorganic chemical vapor deposition process. We have established the correlation between the substrate temperature and the structural properties. The grain structures were clearly shown on the surface of the films deposited at 350°C. The root mean square (RMS) surface roughness of the In2O3 films increased with increasing the substrate temperature. A photoluminescence measurement at room temperature exhibited a yellow-green emission band centered at 585 nm.

Abstract: This paper reports the fabrication of indium oxide (In2O3) films using a triethylindium and oxygen mixture. The deposition has been carried out on TiAlN substrates (200-350°C). We have established the correlation between the substrate temperature and the structural properties. The films deposited at 300-350°C were polycrystalline, whereas those deposited at 200°C was close to amorphous. XRD analysis and SEM images indicated that the films grown at 350°C had grained structures with the (222) preferred orientation. The room-temperature photoluminescence spectra of the In2O3 films exhibited a visible light emission.

Abstract: Zr702 and commercial purity Ti were grain refined through severe plastic deformation and their microstructure and texture characteristics were studied by optical microscopy, transmission electron microscopy, X-ray diffraction and electron back-scattered diffraction. The equal channel angular pressing was conducted at 350°C using a 90°/20° die. Up to 8 passes of pressing were performed via three different routes, A, C and BC, which resulted in reduction of the grain size down to 0.2-0.5.m. The two materials showed similar evolution characteristics of microstructure and texture. While the specimens pressed via route A showed lamellar grain shapes those via route C or BC exhibited equiaxed grains. A split basal texture was developed in the microstructure obtained by the route A pressing, which was similar to the cold rolling texture, whereas an asymmetric texture was produced in the specimens pressed via route BC. In the case of route C pressing, the texture characteristics were closer to those of the route BC case.

Abstract: Ternary Ti-Zr-N thin films were synthesized by rf-reactive sputtering in Ar–N2 plasma. Effects of the substrate temperature in the sputtering process on the microstructures of Ti-Zr-N thin films were investigated using SEM, TEM, XRD and AES techniques. The hardness of the Ti-Zr-N film increases as the substrate temperature in reactive sputtering increases. The reactive sputtered Ti-Zr-N film is characterized as polycrystalline in nature with two dominant orientations of (111) and (200). A substrate temperature of 300°C is suggested for getting a densely packed film structure with the highest hardness.

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.