발간논문

Home > KJMM 논문 > 발간논문

In order to significantly extend the limit of the measurable strain range to obtain true stress-strain curves of materials using tensile testing, digital image correlation is used to measure local strains. In this study, a method to measure reliable true stress-strain curves in a wide range of strains covering the post-necking stage with reducing gage lengths has been investigated. In tensile testing, by decreasing the gage length for the strain measurements, not only accurate true strain but also reliable true stress can be obtained until sample fractures. This experimental method for accurate true stress-strain measurements is verified by comparing the experimental results and the finite element method simulated load-displacement curve, geometries, and strain distributions. The new method can be used to easily obtain the measurement of accurate and extended true stress-strain curves, which can assist in experimental and theoretical investigations of the mechanical behavior of materials. This simple method has shed new light on the measurement of the mechanical properties of materials for both experimentalists and simulation engineers, who need a wide range of true stress-strain curves.

keyword : metals, deformation, mechanical properties, tensile test, digital image correlation

Copper thin films were coated on polyethylene terephthalate(PET) substrates by electroless plating. The effects of the ultra-sonication, the initial roughness of the PET substrates, and the plating time on the thickness and roughness of the electroless-plated Cu deposits were studied. The thickness and roughness of the Cu deposits were measured by scanning electron microscopy(SEM) observations and a surface profiler. In the early stage of Cu plating, the ultra-sonication and high initial roughness of the PET substrates promoted an increase in the thickness of the Cu deposits, but they suppressed the increase in thickness of the Cu deposits when plated for a long time. The surface roughness of the Cu deposits plated on PET with a very low initial roughness increased with plating time due to the agglomeration of Cu atoms. However, the surface roughness of the Cu deposits on the PET with a high initial roughness decreased with increasing plating time, since the deep valleys of the rough PET surface were filled with Cu atoms preferentially.

keyword : thin films, plating, surface, ultrasonics, roughness

In this study, we have demonstrated that the Marc v.11 simulation program, based on heat transfer, was able to estimate the thermal distribution of a graphite mold and sintered compact, for sintering a Ruthenium (Ru) target material. The thermal distribution simulation analysis was conducted as a function of setting temperatures to obtain basic thermal behaviors, and to determine whether physical properties such as the density, grain size and compactness of the sintered Ru target material were influenced by temperature distribution in the graphite mold. It was found that a very small difference in temperature between the center and edge of the sintered compact could be observed at the simulation temperature of 1200 ℃. The highest relative density of 99.1% was achieved when the Ru target material was sintered at 1200 ℃ by spark plasma sintering(SPS). Also, it was confirmed that the grain size of the sintered Ru target was considerably increased with increasing sintering temperature, in spite of the fast heating rate and short dwell time. From these results, a very meaningful thermal characteristic simulation technique was confirmed that can predict the optimized conditions needed to obtain high quality sintered materials prior to SPS process.

keyword : metals, sintering, grain growth, scanning electron microscopy, SEM

The influence of preheat and microstructure on cold cracking for high strength and low hydrogen flux cored arc (FCA) weld metal was analyzed using Y-groove test. The weld microstructure consisted of acicular ferrite and martensite. As the preheat temperature increased, the volume fraction of acicular ferrite increased and that of martensite decreased, thereby decreasing the volume expansion during the transformation from austenite to martensite. This result was consequently associated with a reduced cold cracking ratio. Cold cracking progressed mainly along the prior austenite grain boundary of martensite, and the fractograph of cold cracking consisted of intergranular fractures and quasi-cleavage fractures. With increasing the preheat temperature and decreasing the carbon equivalent, the fraction of intergranular fractures decreased and quasi-cleavage fractures increased. The microstructure related with intergranular fractures was martensite; quasi-cleavage fractures were associated with acicular ferrite.

keyword : metals, welding, microstructure, transmission electron microscopy, TEM, cold cracking

The γ/γ´ two-phase growth commonly observed at γ interdendritic regions in Ni-base superalloys is known to be the eutectic microstructure. However, it is still unclear whether this is due to a eutectic or peritectic reaction. Directional solidification experiments of the Ni-base superalloy CMSX-10 were performed at low solidification rates in order to induce the coupled growth of γ/γ´ phases and to investigate their growth behavior. The γ and γ´ phases were found to grow simultaneously, maintaining an isothermal interface. Directional solidification experiments in a thin tube (0.8 mm ID) suggest that convection enhances the formation of two phase peritectic growth with a planar interface and the γ/γ´ might be the couped peritectic.

keyword : alloy, solidification, microstructure, scanning electron microscopy, SEM, eutectic

We investigated poly(methyl methacrylate) (PMMA) passivation of solution-processed indium zinc oxide (IZO) thin-film transistors (TFTs). PMMA provides solution-processability and good barrier characteristics against environmental elements such as water and oxygen. The PMMA passivation layers protect the IZO active layer of the TFTs without deteriorating their performance during gate bias stress measurements under ambient conditions, and improve their electrical properties by decreasing leakage current. Moreover, the potential to safely manipulate IZO-TFTs after PMMA passivation was proven by realizing a simple n-channel resistive-load inverter.

keyword : thin films, sol-gel, electrical properties, electrical, indium zinc oxide thin film transictors, TFTs

Rare-earth (RE) mono-nitrides can be used as magnetocaloric materials for the magnetic refrigerant for liquefying hydrogen. In this study, holmium nitride (HoN) particles were prepared by sol-gel method with microwave irradiation. HoN particles were successfully synthesized by 2-step heat treatment in N2 atmosphere form gel-type intermediate by microwave irradiation. X-ray diffraction patterns of the synthesized HoN particles exhibited the six sharp peaks in good agreement with each plane of crystallographic structure of HoN. Especially, the synthesized HoN particles reveal a high magnetic entropy change (△Sm) of 36.6 J/kgK in applied field of 5T at 14.2 K which is comparable with those of rare earth mono-nitrides by carbothermic reduction method or plasma arc discharge technique.

keyword : ceramics, chemical synthesis, magnetic properties, thermal analysis, microwave chemistry, magnetocaloric effect

ZnO thin films were spin-coated onto a Zn seed layer that was deposited on a quartz substrate by thermal evaporation. This was found to enhance the structural and optical properties of the resulting ZnO thin film, with a particular improvement in its optical transmittance. Furthermore, by varying the temperature at which the Zn seed layer was thermally annealed, a temperature of 100℃ was identified as being optimal to ensure a maximum increase in the crystallinity and optical properties of a ZnO thin film. On the basis of these results, it is proposed that the use of a thermally annealed Zn seed layer is an effective means of achieving high-quality ZnO thin films.

keyword : zinc oxide, optical materials, sol-gel, x-ray diffraction, optical properties

In order to mitigate the disadvantage of the Mg hydrides, several studies have been conducted that have used MgHx intermixed with carbon. Graphene is a kind of carbon allotrope that is easily subject to a desorption reaction at low temperatures because such a reaction is exothermic. In this work, an MgHx-graphene mixture has been prepared by reactive mechanical grinding. The synthesized powder was characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and simultaneous thermogravimetric (TG) and differential scanning calorimetric (DSC) analyses. The hydrogenation behaviors were evaluated using a Sievert’s type automatic pressure-composition-temperature (PCT) apparatus without activation treatment. From the characteristics of the absorption kinetics and the curves observed, the role of graphene as a catalyst in hydrogen absorption was determined. According to the results of the PCI curve, the available hydrogen storage amounts for MgHx-5 wt% graphene composites had maximum values of 3.69, 5.09, and 5.72 wt% at 423, 523, and 623 K, respectively. Those values for MgHx-10 wt% graphene were 5.08, 5.45, and 5.83 wt% at 423, 523, and 673K, respectively.

keyword : hydrogen absorbing materials, mechanical alloying/milling, hydrogen, X-ray diffraction, MgHx-graphene composites

The resistance of hydrogen induced cracking (HIC) in two different microstructures of API X65MS steel plates was investigated with plastic deformation. A banded type of polygonal ferrite/acicular ferrite (PF/AF) microstructure and a single phase acicular ferrite (AF) were produced by controlling the rolling and cooling conditions. As plastic deformation increased, dislocation density was increased, particularly in the banded AF within the two phase PF/AF microstructure. In contrast, the dislocation density was increased uniformly in the single phase AF microstructure. It was found that HIC resistance deteriorated with increasing plastic deformation. Between the two different types of microstructures, the uniform AF showed better HIC resistance and higher crack propagation resistance than PF/AF, which can be attributed to the more uniform distribution of dislocation density in the AF microstructure.

keyword : linepipe steel, plastic deformation, microstructure, hydrogen induced cracking , HIC, dislocation density