발간논문

Home > KJMM 논문 > 발간논문

The mystery of Goss selective abnormal grain growth (AGG) in Fe-3%Si steel has long remained unsolved since its discovery by Goss in 1933. Similarly, the mechanism of secondary recrystallization, which is commonly observed in many metallic systems, has long been a puzzle. Here, attempts to solve this puzzle are reviewed with a focus on solid-state wetting along the triple junction of polycrystalline metals. Especially, the grains with sub-boundaries of very low energy exclusively have a high probability to grow by solid-state wetting and as a result can grow abnormally. The 3-dimensional Monte Carlo (MC) and phase field model (PFM) computer simulations showed that the presence of sub-boundaries induces AGG. The existence of sub-boundaries exclusively in abnormally-growing grains was experimentally confirmed in Fe-3%Si steel and in 5052 Al alloy. The time evolution of abnormally growing grains by ex-situ observation and electron backscattered diffraction (EBSD) analysis of 5052 Al alloy clearly revealed many microstructural evidences for AGG by triple-junction wetting. Misorientation angle measurements by EBSD of the grains belonging to the penetrating morphology showed that the penetrated grain boundaries have high energy and the penetrating grain boundaries tend to have low energy, in agreement with the wetting condition along the triple junction. Parallel three-dimensional MC simulations starting with the experimental misorientationdata, which consider sub-boundaries and precipitates, show many realistic AGG features observed experimentally. (Received July 28, 2013)

keyword : abnormal grain growth, Fe-3%Si steel, 5052 Al alloy, goss grain, secondary recrystallization, solid-state wetting

At the first cycle (n=1), pure MgH2 absorbed hydrogen extremely slowly at 593 K under 12 bar H2, absorbing 0.04 wt% H for 60 min. Activation of the pure MgH2 was completed after five hydriding- dehydriding cycles. At the 6th cycle, the pure MgH2 absorbed 2.41 wt% H for 5 min, 3.00 wt% H for 10 min, and 4.21 wt% H for 60 min, showing that the activated pure MgH2 had a much higher hydriding rate than the activated pure Mg. The pure Mg absorbed 0.10 wt% H for 5 min, 0.38 wt% H for 30 min, and 0.51 wt% H for 60 min at the first cycle. The activated pure Mg, whose activation was also completed after five hydriding- dehydriding cycles, absorbed 1.76 wt% H for 5 min, 2.17 wt% H for 10 min and 3.40 wt% H for 60 min. The XRD pattern of the pure MgH2 after hydriding-dehydriding cycling (n=7) revealed that the sample contained Mg, a small amount of MgO, and a large amount of MgH2, showing that a large fraction of MgH2 remains even after dehydriding in vacuum at 623 K for 2 h. (Received August 27, 2013)

keyword : hydrogen absorbing materials, microstructure, X-ray diffraction, pure magnesium hydride, pure magnesium

A study on grain boundary serration in the Ni-based Alloy 690 was conducted by heat treatments and subsequent microscopic examinations. Carbon had a strong influence on the grain growth behavior of this alloy. The grain boundaries were serrated in a limited temperature range during a controlled cooling. As the average grain size with more carbon content increased and/or the cooling rate decreased, the degree of serration increased with a precipitation of coarse intergranular Cr carbides. Cr carbides in Alloy 690 were identified as Cr23C6, and they grew in a planar shape along the curved grain boundaries. The grain boundaries had a convex shape into the incoherent grain, from which it is believed that the grain boundary shape is closely correlated with the grain boundary migration in this alloy during serration. (Received December 17, 2013)

keyword : alloys, annealing, grain boundary, transmission electron microscopy, serration

To develop an advanced ODS alloy for core structural materials for next-generation nuclear reactor system applications, effects of alloying elements such as Cr, Mo, Al, Zr and Y2O3 on microstructures and tensile properties of ODS ferritic/martensitic alloys were investigated. The ODS alloys were fabricated by mechanical alloying, hot isostatic pressing, and hot rolling processes. The Cr concentration revealed little effect on the tensile strength, but it affected the grain morphology as a result of phase transformation. Mo acted to increase the tensile strength at an elevated temperature. Both grain and oxide particle sizes were significantly coarsened by Al addition. These enlargements led to a decrease in tensile strength but improved the elongation. Minor addition of Zr enhanced the tensile strength, and an increase of Y2O3 addition deteriorated the elongation. These results could be very useful in optimizing the alloying element concentration of ODS alloys with superior out-of-pile mechanical properties. (Received July 18, 2013)

keyword : alloys, mechanical alloying, mechanical properties, tensile test, oxide dispersion strengthened alloy

The effects of nitrogen concentration on the tensile and impact properties of 9Cr-1W steels were evaluated. The 9Cr-1W alloy ingots containing nitrogen concentrations of 110, 280 and 500 ppm were fabricated by a vacuum induction melting process. The alloy ingots, with a 30 kg scale each, were hot-rolled at 1150 ℃ up to 15 mm in thickness, which was followed by normalizing at 1050 ℃ for 1 hour and tempering at 760 ℃ for 1 hour. M23C6, V-rich MX and Ta-rich MC precipitates were observed in all the alloys. We found that the increased nitrogen concentration led to increased V-rich MX carbonitrides. When the nitrogen concentration was increased from 110 ppm to 500 ppm, the tensile strength increased while the impact toughness remained almost constant. We thus concluded that the V-rich MX carbonitrides in 9Cr-1W steels mainly act to improve the tensile strength rather than the impact toughness. (Received October 30, 2013)

keyword : 9Cr-1W steel, V-rich MX, ferritic/martensitic steel, tensile strength, impact toughness

Microhardness, electrical resistivity and microstructures of a mechanically affected zone (MAZ) by cold pressure welding of aluminum rods in terms of joint pressures and the number of strokes were investigated in this study. The MAZ was defined as the area where the materials have been subjected to deformation generating hardening of the joint area. The macroscopic image of the MAZ also reveals that the horizontally aligned lamellar structure of material flowing due to a wire drawing process gradually began to form in a vertical direction while joint pressure was applied. Vickers microhardness tests inside the MAZ showed that higher hardness was obtained in the MAZ, and the highest value was achieved at the center of that area. Similar to the microhardness results, electrical resistivity was also enhanced in the MAZ, compared to the base Al rod. TEM analysis of the MAZ concluded that the extensive elongated subgrains bounded by microbands as expected from a high stacking fault energy metal like aluminum and many dislocations were formed inside the grains as well. Therefore, the behavior of microhardness and electrical resistivity are completely attributed to the change of microstructures. (Received November 12, 2013)

keyword : conductors, welding, microstructure, hardness test, mechanically affected zone

Studies on quantitative forgeability estimation using a deformation processing map was performed to analyze the hot deformation characteristics of SAF2205 duplex stainless steel. To develop the deformation processing map, a dynamic materials model was used on the basis of stress-strain relationships from a Gleeble compression test. As a quantitative forgeability index, the summation of ηi (efficiency of power dissipation for the i-th unit deformation volume) multiplied by the unit deformation volume (Vi) for the whole number of unit deformation volume (n) is suggested. The respective η-values for unit deformation volume can be determined from the deformation processing map for which the temperature and strain rate during the hot forging process can be obtained by tracing the variations of the temperature and strain rate of finite element analysis. To validate this study, finite element analysis at various forging geometries and temperatures was performed. The results show that the proposed quantitative index provide a reliable guide for the design of a hot forging process. (Received November 28, 2013)

keyword : metals, thermomechanical processing, mechanical properties, compression test, deformation processing map

ZnO thin films were deposited on a Si (100) substrate by plasma-assisted molecular beam epitaxy (PA-MBE). Growth of ZnO thin films on a Si substrate is limited due to the large difference between the lattice constants of ZnO and Si. In this research, we studied the effects of the buffer-layer thickness and active-layer growth temperature on the structural and optical properties of ZnO thin films. Scanning electron microscopy, X-ray diffraction, and photoluminescence were carried out to investigate the structural and optical properties of the ZnO thin films. The structural and optical properties of the ZnO thin films were improved when the buffer layers were grown for 20 min and the active layers were grown at 800 ℃.(Received September 9, 2013)

keyword : zinc oxide, molecular beam epitaxy, hydrothermal, x-ray diffraction, photoluminescence

The bomb acid digestion method was used in this study to improve the decomposition efficiency of pure BaTiO3 and Dy-doped BaTiO3. A trace analysis was carried out using inductively coupled plasma atomic emission spectrometry (ICP-AES). The decomposition rate of pure BaTiO3 was obtained at 130 ℃ and above 2 h, and the change of the efficiency according to digestion temperature was identified by the structural stability of BaTiO3 related to a phase transition at around Tc. With pure BaTiO3, optimization of the temperature and conditions increased the analysis efficiency. The bomb acid digestion method was applied to the mixture and compound of Dy-doped BaTiO3 in the same way as with pure BaTiO3. Thedecomposition and recovery rate decreased for the Dy-doped BaTiO3 compound. To clarify this phenomenon, the phase transition of pure and Dy-doped BaTiO3 was characterized through X-ray diffraction (XRD) and differential scanning calorimetry (DSC). It was revealed that the decomposition efficiency was considerably associated with the Curie temperature of Dy-doped BaTiO3. (Received August 27, 2013)

keyword : electronic materials, solutionization, decomposition, X-ray diffraction, bomb acid digestion

This study examined the stability of a Cu/Pt bilayered counter electrode (CE) with the electrolyte and the energy conversion efficiency of dye-sensitized solar cells using a 0.45 cm2 dye-sensitized solar cell (DSSC) device with a glass/FTO/blocking layer/TiO2/N719 (dye)/electrolyte/50 nm-Pt/50 nm-Cu/FTO/glass. For comparison, a 100 nm-thick Pt only CE DSSC was also prepared using the same method. The photovoltaic properties, such as the short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF), energy conversion efficiency (ECE), and impedance, were checked using a solar simulator and potentiostat with time after assembling the DSSC. The microstructure of the Cu/Pt bilayer was examined by optical microscopy after 0~30 minutes and 3 weeks. The ECE of the DSSC using the Pt only CE was 4.60 %, which did not show any time dependence. On the other hand, for the Cu/Pt CE DSSC, the ECEs after 0 minutes, 30 minutes, and 3 weeks were 5.72%, 5.03%, and 1.36%, respectively. Moreover, the interface resistance increased; 6, 7, and 40 Ω at 0 minutes, 30 minutes, and 3 weeks, respectively. The corrosion area of the Cu/Pt CE determined by an optical microscopy after 0 minutes, 30 minutes, and 3 weeks was 0, 23.40, and 51.35%, respectively. These results confirmed that the ECE and catalytic activity of Cu/Pt CE decreased drastically with time. Therefore, a DSSC using a Cu/Pt CE may be superior to the Pt only CE immediately after integrating of the device, but the performance of the former degrades drastically with time. †(Received September 30, 2013)

keyword : solar cells, sputtering, optical properties, solar simulator, bilayer