발간논문

Home > KJMM 논문 > 발간논문

The microstructure and mechanical property degradation of a gas turbine SEV burner made of the nickel-based superalloy Inconel 738LC were investigated with increasing operation times in the range of 24000-65000 equivalent number of operating hours (EOH). The original cuboidal shape of γ? particles with a size of 400-500 nm became spherical with a diameter larger than 1 μm. The γ? coarsening kinetics conformed well to the LSW model suggesting that the variation of r³as a function of exposure time is linear. In the hottest zone of the SEV burner operated for 65000 EOH, a continuous film of carbides along the grain boundaries was observed as a result of the transformation of MC carbide to M_23C_6＋γThis continuous film of M_23C_6 carbides provided a preferential site for intergranular crack initiation and propagation during tensile testing, and reduced accordingly the alloy ductility, whereas it was favorable to tensile strength.

keyword : gas turbine, alloys, degradation, microstructure, intergranular crack

The effects of copper (Cu) addition on the microstructure and mechanical properties of ZK60 alloy were investigated using an optical microscope (OM), a scanning electron microscope (SEM), and electron backscatter diffraction (EBSD) and by performing tensile tests of indirect-extruded ZK60 alloys with 0.5, 1.0, and 1.5 wt% Cu contents. The results revealed that the as-extruded ZK60 alloy had a bimodal grain structure composed of fine recrystallized grains and coarse unrecrystallized grains. The homogeneity of the microstructure was increased by Cu addition due to the promotion of dynamic recrystallization (DRX) by particle stimulated nucleation (PSN) at the Mg-Zn-Cu particles. This enhanced DRX behavior resulted in the reduction of the average grain size and weakening of the basal fiber texture of the as-extruded alloys. The yield and tensile strengths were improved by the Cu addition owing to the decreased grain size and increased number of particles, while the elongation was decreased due to the hard Mg-Zn-Cu particles.

keyword : metals, extrusion, mechanical properties, electron backscattering diffraction, dynamic recrystallization

In this experiment, post-oxidizing treatments were implemented on AISI 316L stainless steel which was plasma nitrocarburized previously at various temperatures. Plasma nitrocarburizing was performed on AISI 316L stainless steel at various temperatures (400℃, 450℃, 500℃) with a gas mixture of O_2 and H_2. After treatment, the behavior of the duplex layer was investigated by optical microscopy, X-ray diffraction, scanning electron microscopy and micro-hardness testing. A Potentiodynamic polarization test was also used to evaluate the corrosion esistance of the samples. The X-ray diffraction confirmed the development of a Cr_2N, γ_N phase and magnetite (Fe3O4) phase under plasma nitrocarburizing and post-oxidizing processes. Additionally, it was found that the thickness and surface hardness increased with an increasing nitrocarburizing temperature. The experiment revealed that 316L stainless steel had better hardness and excellent corrosion resistance than the bare sample, when both the nitrocarburizing and oxidizing treatment temperatures were 400℃.

keyword : metals, surface modification, corrosion, hardness test, plasma nitrocarburizing

ASTM T23 steel (Fe-2.25Cr-1.6W-0.1Mo in wt%), T92 steel (Fe-9Cr-1.8W-0.5Mo in wt%), and T122 steel (Fe-11Cr-2W-1Cu-0.4Mo in wt%) alloys were corroded at 600 and 700℃ for up to 70 h in 1 atm of N2 gas containing 0.5% or 2.5%H_2S gas. The corrosion rates increased with a decrement in the Cr content and an increment of temperature and H_2S gas concentration. The T23 steel corroded fast, forming thick, fragile, and nonadherent scales that consisted primarily of an outer FeS layer and an inner (FeS, FeCr2O4, FeCr2S4)-mixed layer. Similar scales formed on the T92 and T122 steels, except that the scales became progressively thinner with an increment of the Cr content. Particularly, a large portion of the scales formed on the T122 steel was quite thin. During corrosion, mostly Fe with a small amount of W, Mn and Cu diffused outward to form the outer FeS layer, while oxygen and sulfur diffused inward to form the inner (FeS, FeCr2O4, FeCr2S4)-mixed layer.

keyword : metals, casting, corrosion, scanning electron microscopy, H_2S gas

Directional solidification experiments were carried out on a single crystal Ni base superalloy CMSX-10 over a wide range of growth velocity and temperature gradient conditions. Under the imposed temperature gradients, the primary dendrite arm spacings decreased proportionally with the growth velocity. Microstructural characteristics at the solid/liquid interface were found to depend strongly on the growth velocities and temperature gradients. Based on the experimental observations, a growth velocity versus temperature gradient map was selected for planar to cellular and cellular to dendritic transitions during solidification. As a result, the microstructure selection map for the alloy was established by comparing it to theoretical models. Good agreement was obtained from the results for planarcellular and cellular-dendritic transitions. The columnar-equiaxed transition, however, was calculated using Hunt`s model for castings. The results are discussed in terms of the combined effects of constitutional undercooling and nucleation undercooling ahead of the growing dendrite tips.

keyword : alloys, solidification, microstructure, undercooling

Directional solidification experiments were carried out on modified 9Cr1Mo steel and 10Cr1MoW steel to investigate their solidification and segregation characteristics. Under a given temperature gradient, both steels were solidified at the same growth rates of 1, 5, 50 and 100 μm/s, respectively. A detailed study of the solidification sequence for both steels revealed that delta ferrite was the leading phase and gamma austenite nucleated from the solid solidified earlier. A quick phase transformation of delta into gamma was made at growth rates faster than 50 μm/s. Therefore, almost all delta dendrites were identified as gamma phases to be transformed. On the other hand, compositional analyses performed on the solid/liquid interface confirmed that alloying elements in both steels were hardly segregated ahead of the solid/liquid interface. This appears to be responsible for a very narrow freezing range which suppresses elemental segregation.

keyword : alloys, solidification, segregation, undercooling

In this study, the dispersion behavior of a ferroalloy containing cerium in a steel melt and the concentration along the slab length were predicted by a fluid analysis and spreadsheet-based model development in the continuous casting process. The residence time distribution of the ferroalloy according to the flow pattern was calculated and used for calculating the ferroalloy discharge rate from the tundish and concentration in the slab. Effective operating conditions for homogeneous concentration distribution of the ferroalloy were determined by controlling the injection speed and time of the ferroalloy obtained from this analysis model.

keyword : alloys, casting, solidification, computer simulation, spread sheet model

For preparing spinel Li_4Ti_5O_12 nanofiber, a hydrogen titanate nanofiber precursor was mixed with LiOH·H2O and then the mixture was treated at 130℃ in an autoclave for 24 hrs. The hydrogen titanate nanofiber precursor was made using a TiO2 and NaOH solution as the starting material. As a result, the diameter of the Li4Ti5O12 nanofiber was 5-10nm and the length was over 100 nm longer fiber. The oleic acid (C17H33COOH) coated Li4Ti5O12 nanofiber with different oleic acid contents (5, 7.5, and 10 wt%) was obtained by a simple mixing method and heat treatment at 450℃ in a N_2 atmosphere. The results clearly revealed that the surface of the Li4Ti5O12 nanofiber was coated with an amorphous carbon layer (1 nm). The crystallinity of the samples was also enhanced. The oleic acid coated Li_4Ti_5O_12 nanofiber (5 wt% and 7.5 wt%) displayed a much lower impedance than the Li_4Ti_5O_12 nanofiber because of the decreased charge transfer resistance, therefore, it had an improved discharging/charging capacity, c-rate and cycle performance.

keyword : Energy storage materials, Chemical synthesis, Electrochemistry, scanning/transmission electron microscopy (STEM), Li_4Ti_5O_12 nanofiber

We report on the electrical properties of silicon nitride (SiNX) films deposited at 150℃ from highly diluted SiH4 in N2 by a plasma enhanced chemical vapor deposition (PECVD) method. The films were also prepared below 200℃ for comparison. Although the N2 dilution gas acted as a source of nitrogen atoms inside the SiNX film, it was necessary to introduce NH3 to obtain good dielectric quality in the low-temperature films. An amount of NH3 equal to the net SiH4 in the gas mixture was found to be adequate, and further addition of NH3 resulted in little improvement. For SiNX films deposited at 150℃, the NH3 addition decreased the C-V hysteresis (△Vth) from 15 V to 3 V, and increased the resistivity and the breakdown field strength from 109 Ωcm and 4 MV/cm to 1013 Ωcm and 7 MV/cm, respectively. When these films were applied as a gate dielectric layer, the resulting TFT prepared at 150℃ showed an on/off current ratio higher than 105, a threshold voltage of 1.1 V, a subthreshold slope of 1.2 V/dec, and a field effect mobility of 0.04 cm2/Vsec. Under a dc bias stress of VD = VG = 25 V, the on-current of this TFT was stable over a period of 5000 seconds.

keyword : dielectrics, plasma deposition, electrical properties, electrical resistivity, highly-diluted gas

In order to enhance the energy conversion efficiency by increasing the surface area of the counter electrode (CE), we employed different substrates with the flat glass, FTO (fluorine-doped tin oxide), and laser patterned FTO. Ruthenium (Ru) films with thicknesses of 34 and 46 nm were deposited by atomic layer deposition (ALD) on each substrate. In this way, the dye sensitized solar cell (DSSC) device with an area of 0.45 cm2 with a glass/FTO/blocking layer/TiO2/N719(dye)/electrolyte/Ru/substrate structure was prepared. The microstructure of the CE was investigated with FE-SEM, and the photovoltaic properties were characterized by cyclic voltammetry (CV), impedance spectroscopy (EIS), and current-voltage (I-V) measurement. When we assumed the surface morphology as the array of a pyramid (width 0.24, length 0.24, and height 0.14 μm), we determined the surface area of the substrate of the flat glass, FTO, and laser patterned FTO as 1.36×108, 2.32×108, and 2.56×108 μm2, respectively. CV and impedance results revealed an increase in catalytic activity and a decrease in interface resistance with increasing Ru thickness and surface area. When the Ru thickness was 34 nm (and 46 nm), the energy conversion efficiency of each substrate was 1.55% (1.96%), 2.62% (2.92%), and 2.95% (3.32%), respectively. These results suggest that increasing the Ru catalytic layer thickness and surface area of the CE contributed to increasing the efficiency. Moreover, increasing of surface area through laser patterning was more suitable for increasing the efficiency than the conventional flat glass, and FTO substrates.

keyword : dye sensitized solar cells, ALD, surface, FESEM, Ru