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The effects of the dissolved cementite on the mechanical properties have been experimentally investigated. The steel wires were fabricated depending on the carbon content of 0.82 and 1.02 wt.% and drawing strain from 4.12 to 4.32. The bending fatigue resistance and torsion ductility were measured by a hunter fatigue tester and torsion tester specially designed for thin-sized wires. The results showed that as the drawing strain and carbon content increased, the fatigue resistance and the torsional ductility of the steel wires decreased, while the tensile strength increased. In order to elucidate this behavior, the microstructure in terms of lamellar spacing (λP), cementite thickness (tC) and morphology of cementite was observed by advanced analysis techniques such as transmission electron microscope (TEM) and 3 dimensional atom probes (3-D AP).

keyword : cementite dissolution, mechanical properties, drawing strain, carbon content, Laser assisted 3-D AP

A minimum commitment method(MCM) was applied to predict the long-term creep rupture life for type 316LN stainless steel(SS). Lots of the creep-rupture data for the type 316LN SS were collected through world-wide literature surveys and the experimental data of KAERI. Using these data, the long-term creep rupture life above 105 hour was predicted by means of the MCM. In order to obtain the most appropriate value for the constant A being used in the MCM equation, trial and error method was used for the wide ranges from -0.12 to 0.12, and the best value was determined by using the coefficient of determination, R2 which is a statistical parameter. A suitable value for the A in type 316LN stainless steel was found to be at -0.02 ~ -0.05 ranges. It is considered that the MCM will be superior in creep-life prediction to commonly-used time temperature parametric method, because the P(T) and G(σ) functions are determined from the regression method based on experimental data.

keyword : minimum commitment method, type 316LN stainless steel, time-temperature parameter, TTP, creep, creep rupture, coefficient of determination

Conventional plasma carburizing or nitriding for austenitic stainless steels results in a degradation of corrosion resistance. However, a low temperature plasma surface treatment can improve surface hardness without deteriorating the corrosion resistance. The 2-step low temperature plasma processes (the combined carburizing and post nitriding) offers the increase of both surface hardness and thickness of hardened layer and corrosion resistance than the individually processed low temperature nitriding and low temperature carburizing techniques. In the present paper, attempts have been made to investigate the influence of the introduction of Ar gas (0~20%) in nitriding atmosphere during low temperature plasma nitriding at 370°C after low temperature plasma carburizing at 470°C. All treated specimens exhibited the increase of the surface hardness with increasing Ar level in the atmosphere and the surface hardness value reached up to 1050 HV0.1, greater than 750 HV0.1 in the carburized state. The expanded austenite phase (γN) was observed on the most of the treated surfaces. The thickness of the γN layer reached about 7 μm for the specimen treated in the nitriding atmosphere containing 20% Ar. In case of 10% Ar containing atmosphere, the corrosion resistance was significantly enhanced than untreated austenitic stainless steels, whilst 20% Ar level in the atmosphere caused to form CrN in the N-enriched layer (γN), which led to the degradation of corrosion resistance compared with untreated austenitic stainless steels.

keyword : austenitic stainless steel, plasma carburizing, plasma nitriding, expanded austenite

The magnetism of Fe3O4 nanoparticles was applied to magnetic bio-panning process for finding specific sequences against Fe3O4 crystal phase. Vibrating sample magnetometer (VSM) measurement showed that the coercivity of 30 Oe and the saturation magnetization of 55 emu/g were sufficient in controlling particle movement and magnetizing particles in the media, respectively. This ferrimagnetism of nanoparticles practically enhanced panning efficiency by exaggerating centrifuge step and preventing particle loss. Sequencing results showed that histidine which was commonly found in peptide sequences played an important role in the binding onto Fe3O4 nanoparticle surface. However, various possible motifs were also observed from several neighboring amino acids of histidine.

keyword : Fe3O4 nanoparticles, magnetic bio-panning, phage display, histidine, materials recognition

The inhomogeneous texture through the thickness direction can be developed during hot rolling deformation in aluminum alloy. In this study, the inhomogeneous texture evolution through the thickness direction during hot rolling deformation in Al-5 wt%Mg alloy produced by a new strip casting technology was measured experimentally. Macrotexture measurement was conducted using X-ray diffractometer. A finite element analysis with ABAQUS/StandardTM and rate sensitive polycrystal model were used to predict the evolution of hot rolling texture. The experimental results of Al-5 wt%Mg alloy were compared with calculated results. The shear texture components tend to be increased at the surface region of the hot-rolled specimen. It is found that triclinic sample symmetry is more accurate assumption for texture analysis and simulation in the surface region of hot-rolled aluminum alloy.

keyword : inhomogeneous texture, hot rolling, finite element analysis, rate sensitive polycrystal model

Equal channel angular pressing (ECAP) has been studied intensively over the decade as a typical top-down process to produce ultrafine/nano structured materials. ECAP has successfully been applied for a processing method of severe plastic deformation to achieve grain refinement of magnesium and to enhance its low ductility. However, difficult-to-work materials such as magnesium and titanium alloys were susceptible to shear localization during ECAP, leading to surface cracking. The front pressure, developed by Australian researchers, can impose hydrostatic pressure and increase the strain level in the material, preventing the surface defect on workpiece. In the present study, we investigated the deformation and fracture behavior of pure magnesium using experimental and numerical methods. The finite element method with different ductile fracture models was employed to simulate plastic deformation and fracture behavior of the workpiece.

keyword : magnesium, equal channel angular pressing, finite element method, ductile fracture simulation, front pressure effect

The temperature distribution of high speed tool steel rod has been studied during high speed hot rolling procedures. The tool steel rod shows severe temperature gradient during rolling procedures and the temperature at the center of rod are much higher than that at the surface of rod. This temperature gradient accumulated after every rolling procedure and the center of rolled rod could be remelt in some procedures to cause inside defects. In this study, the temperature distribution was simulated using finite element method and the processing parameters such as rolling speed, cooling condition, have been discussed to prevent the temperature increases at the center of rod.

keyword : high speed tool steel, rolling, FEM, temperature distribution

Thermally-evaporated 10 nm-Ni/1 nm-Ru/(30 nm or 70 nm-poly)Si structures were fabricated in order to investigate the thermal stability of Ru-inserted nickel monosilicide. The silicide samples underwent rapid thermal annealing at 300~1,100℃ for 40 seconds. Silicides suitable for the salicide process were formed on the top of the single crystal and polycrystalline silicon substrates mimicking actives and gates. The sheet resistance was measured using a four-point probe. High resolution X-ray diffraction and Auger depth profiling were used for phase and chemical composition analysis, respectively. Transmission electron microscope and scanning probe microscope(SPM) were used to determine the cross-sectional structure and surface roughness. The silicide, which formed on single crystal silicon and 30 nm polysilicon substrate, could defer the transformation of Ni2Si and NiSi2, and was stable at temperatures up to 1,100℃ and 1,000℃, respectively. Regarding microstructure, the nano-size NiSi preferred phase was observed on single crystalline Si substrate, and agglomerate phase was shown on 30 nm-thick polycrystalline Si substrate, respectively. The silicide, formed on 70 nm polysilicon substrate, showed high resistance at temperatures >700℃ caused by mixed microstructure. Through SPM analysis, we confirmed that the surface roughness increased abruptly on single crystal Si substrate while not changed on polycrystalline substrate. The Ru-inserted nickel monosilicide could maintain a low resistance in wide temperature range and is considered suitable for the nano-thick silicide process.

keyword : Ni silicide, Ru-inserted Ni silicide, salicide, nano-thick, thermal stability

Characteristics of etching and induced surface morphology variation by wet-etching of n-type GaN were investigated using phosphoric acidic solutions. Generally, the etch-rate was increased as the temperature of the etch solutions was increased, and the highest etch rate of about 300 A/min was achieved at the temperature of 180℃. The morphology variation of the etched surface was observed by optical microscopy and atomic force microscopy. Initially, high density of hexagonal holes or pits were formed on the etched surface at the time of 40 min with the bimodal size of 20 μm or 5 μm, respectively. However, as the etching time was increased further, the lateral size of the hexagonal holes or pits was increased, and finally, joined and merged together at the time of 100 min. This means that the etching of n-type GaN by phosphoric acidic solutions proceeded through the lateral widening and the merging of initial holes and pits.

keyword : wet etch, n-type GaN, morphology, phosphoric acid

The stability of the cathode materials for Li secondary battery is an important factor for its cyclability. The present paper focuses on the structural stability of LiNi0.5Mn1.5O4 during lithiation/delithiation of Li ions and compared to that of LiMn2O4. LiMn2O4 and LiNi0.5Mn1.5O4 powders are synthesized using a solgel method and their structural and electrochemical properties are investigated by XRD, SEM, and chargedischarge tests. LixMn2O4 and LixNi0.5Mn1.5O4(x=0.9,0.5,0.1) specimens are obtained after charge/discharge tests by controlling the cut-off voltage for XRD and TEM investigation. The charge-discharge tests shows that initial capacity of LiNi0.5Mn1.5O4 is 125 mAh/g and that of LiMn2O4 is around 100 mAh/g. The capacity of LiNi0.5Mn1.5O4 is maintained 95% of its initial capacity whereas the capacity of LiMn2O4 is maintained 65% of its initial capacity.

keyword : capacity fading, TEM, LiNi0.5Mn1.5O4, Lithium batteries