발간논문

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Severe Plastic Deformation (SPD) is a generic designation of a group of metal-working techniques involving very large shear strains which are imposed, in combination with hydrostatic pressure, without any significant change in the overall dimensions of a specimen or a workpiece. This technique can produce miraculously fine grained structures below submicron scale. Equal channel angular pressing (ECAP) is a unique and relatively simple metal forming process to the other metal forming processes, but still complex under coupled effects with multi-factors, such as geometric factors, material factors and processing conditions. Investigating the plastic deformation behavior in the deformation zone during ECAP is crucial for predicting the metal flow, microstructural evolution, controlling the quality of deformed workpiece and optimizing the ECAP process. In this review, modelling of ECAP using various numerical methods are reviewed. The papers from literature analyzing ECAP processing by the FEM are summarized and compared by software, mesh size, dimension, and analyzed results. It can be found that not only stress, strain, velocity, and fracture tendency but also their distributions in terms of the effects of the processing variables (speed, size, friction, die geometries, etc.) can be predicted. †(Received January 19, 2014)

keyword : equal channel angular pressing, severe plastic deformation, computer simulation

The effects of substituting a step quenching process for the traditional water quenching and of adding zirconium into an Al-Mg-Si alloy, were investigated using differential scanning calorimetry (DSC) and transmission electron microscope (TEM), together with a tensile and hardness tester. The results indicated that step quenching and zirconium addition affected the microstructure and mechanical properties significantly. Step quenching at 100 ℃ for the Al-Mg-Si alloy was the optimal choice because it promoted the precipitation of the β′′ phase, afforded the maximum age-hardening response and strength increment, and restrained the negative effect of natural aging. Zirconium addition improved the strength while retaining good plasticity for the precipitation of Al3Zr. Zr partially promoted the precipitation kinetics of the β′′ phase when step quenching was conducted at 50 or 100 ℃. †(Received January 15, 2013)

keyword : metals, aging, precipitation, thermal analysis, step-quenching

The main goal of the study is to obtain high strength and formability for automotive sheet steels such as wheel and chassis. Ferrite-based steels developed previously exhibited superior stretch-flangeability to 780 MPa and even to 980 MPa grade steels. However, it was difficult to achieve 980 MPa tensile strength sufficiently. To achieve a superior stretch-flangeability and to acquire a sufficiently high strength over 980 MPa, a composition of Fe-Cr-Ti-B was developed for hot rolling. The manufactured steel was coiled for 1 hour at 430, 450, and 470 ℃. Specifically, 998 MPa ultimate tensile strength, 11% total elongation, and a 59% excellent hole expansion ratio were obtained by microstructural constituents of a tempered martensite matrix and granular bainite when the coiling was conducted at 470 ℃. As the coiling temperature varied from 470 to 430 ℃, the fraction of granular bainite decreased and that of tempered martensite increased. Therefore, the deviation of phase fractions between two phases decreased at 470 ℃ coiling. Furthermore, as the hole expansion ratio (HER) increased, the hardness deviation of the two phases decreased. As the HER value increased, the crack propagation path dispersed on the second fracture surface and fracture was significantly delayed. Therefore, hot rolled steels using a tempered martensite matrix and granular bainite had the same behavior of stretch-flangeability with respect to the hardness deviation and crack propagation path as compared with the steels using a ferrite matrix. For high strength steels composed of the tempered martensite matrix and granular bainite, however, the minimal deviation between the two constituent phases was acknowledged as an important factor to increase stretch-flangeability. †(Received May 8, 2013)

keyword : metals, rolling, microstructure, precipitation, hardness test, stretch-flangeability

Ball-milled Ni-Al powder compacts were synthesized by the heat of molten aluminum at 1073 K and were coated on an aluminum casting. The effects of ball-milling on the microstructure of the intermetallic coatings are investigated. The experimental results showed that remelting the aluminum casting around the coating by the heat released during the intermetallic reaction could be prevented by the ball-milling for reactant powders. Further, the coating reaction was preferentially achieved by diffusion. This might be attributed to the fact that the heat released during the intermetallic reaction was dispersed over a prolonged reaction time by the ball-milling. †(Received December 6, 2012)

keyword : intermetallics, coating, microstructure, activation analysis, ball-milling

To development of anti-corrosive Fe-alloys with a high carbon content, we designed an amorphous alloy in the alloy system Fe-C-Si-B-Cr. In the (Fe0.79C0.11Si0.02B0.08)100-xCrx (x = 2, 4, 6, 8 at%) ribbon with a thickness of less than 25 μm, an amorphous phase was formed without Cr carbide formation. As the Cr content increased, the corrosion resistance in the 3.5 wt% NaCl and 1 N H2SO4 solutions monotonically increased, exhibiting an abrupt increase when the Cr content was higher than 6 at% (for the NaCl solution) and 8 at% (for the H2SO4 solution). From the corrosion test results for the fully amorphous and partially crystallized ribbons with high Cr content (i.e., 8 at%), we demonstrate that, even with a high Cr content, the homogeneous spatial distribution of Cr and the absence of microstructural defects in the ribbon are critical to significantly enhance the corrosion resistance of the high Cr ribbon. †(Received April 15, 2013)

keyword : amorphous materials, solidification, corrosion, scanning electron microscopy, immersion test

A sample with a composition of 90 wt% Mg-10 wt% TaF5 (named Mg-10TaF5) was prepared by reactive mechanical grinding, and its hydriding and dehydriding properties were examined. The activation of Mg-10TaF5 was not required. At n=1, the sample absorbed 3.63 wt% H for 5 min, 4.03 wt% H for 10 min, and 4.53 wt% H for 30 min at 593 K under 12 bar H2. At n=1, the sample desorbed 0.59 wt% H for 5 min, 1.46 wt% H for 10 min, 3.42 wt% H for 30 min, and 4.24 wt% H for 60 min at 593 K under 1.0 bar H2. Mg-10TaF5 after reactive mechanical grinding contained MgH2, Mg, and very small amounts of MgF2 and Ta2H. The XRD pattern of Mg-10TaF5 dehydrided at n=3 revealed Mg, MgH2, a small amount of MgO, and very small amounts of MgF2 and Ta2H phases. Mg-10Fe2O3 and Mg-10MnO were reported to have quite high hydriding rate and dehydriding rates, respectively. Mg-10TaF5 had a larger initial hydriding rate but a lower quantity of hydrogen absorbed for 60 min than Mg-10Fe2O3. However, Mg-10TaF5 had a higher initial dehydriding rate (after the incubation period) and a larger quantity of hydrogen desorbed for 60 min than Mg-10MnO. †(Received April 29, 2013)

keyword : hydrogen absorbing materials, mechanical alloying, milling, scanning electron microscopy, X-ray diffraction, TaF5 addition

The NiCr alloy has relatively large resistivity, good resistant to oxidation and low temperature coefficients of resistance (TCR) compared to other resistor metarials. These properties of NiCr thin films are dependent on the processing conditions including the deposition environment and subsequent annealing treatments. To establish optimizing conditions, Ni-60%Cr thin films were deposited by a sputtering method to control the resistivity and TCR. The experiments were carried out under various process pressures to determine the optimum conditions to achieve a high resistivity and low TCR. The thermal stability of Ni-60%Cr thin films at various heat treatment temperatures was also evaluated. The electrical properties of the sputtered Ni-60%Cr thin films were investigated by probe station and their crystal structures were observed by X-Ray Diffraction (XRD). The surface morphology was observed by field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM). As a result, a stable resistivity and TCR was respectively observed at 3-15 mTorr and 3-7 mTorr. The heat treatment results revealed an increase in thermal resistance with increasing temperature with a concomitant decrease in the TCR, and a near-zero TCR was obtained at 673 K. †(Received March 6, 2013)

keyword : thin film, sputtering, electrical properties, electrical resistivity

We report on characteristics of piezoelectric ceramic thick films of Pb(Zr0.54 Ti0.46)O3 + 1.0 wt% Nb2O5 + x wt% Cr2O3 (0.1 ≤ Cr2O3 ≤ 0.5 wt%) fabricated by a tape casting method. The effects of Cr2O3 were widely examined at the various sintering temperature of 1120-1150 ℃ to increase the piezoelectricity. As a result, when x = 0.2 wt% at a sintering temperature of 1130 ℃, the ceramics had a piezoelectric constant (d33) of 408 pC/N, an electro-mechanical coupling factor (Kp) of 62.9% and a dielectric constant (εr) of 1363. †(Received March 13, 2013)

keyword : ceramics, piezoelectric, casting, PZT, Cr2O3

We synthesized a gel-like phase of zinc oxide (ZnO) by employing a microwave-assisted technique and then used this gel-like phase to fabricate ZnO thin film transistors (TFTs). By utilizing this method, we were able to prepare the ZnO gel-like phase in a relatively short time and obtained polycrystalline ZnO thin films. This microwave-assisted technique also allows the use of low-temperature thermal processes in the fabrication of ZnO-TFTs. The bottom-gate TFTs with a ZnO layer as the active channel exhibited a field effect mobility of 0.6 cm2/V·s, a sub-threshold slope of 9 V/decade, and an on/off current ratio greater than 104. These results point to the possibility of using microwave-assisted techniques for transparent and flexible electronic devices based on ZnO. †(Received July 16, 2013)

keyword : ZnO, semiconductors, sol-gel, electrical properties, TEM

Some biomaterials bond with the surrounding living bone through a bone-like apatite layer which is formed on their surfaces in the human body. The existence of functional groups such as Si-OH and Ti-OH on the Ti surface accelerates the formation of a bone-like apatite layer in simulated body fluid (SBF) solution. In this study, Si-OH functional groups were formed on the Ti surface by water glass (WG, sodium silicate) coating and subsequent HCl-treatment. The thickness of the WG layers increased with an increasing concentration of WG solution and the number of WG coatings. The chemical composition of the WG coated layers changed from sodium silicate to pure silica by the HCl-treatment via ion-exchange. The content of hydroxyl groups on the HCl-treated samples was confirmed by TFAA treatment using XPS. The amount of hydroxyl groups on the Ti surface depended upon the vol% of WG and the number of WG coatings. The higher the concentration of WG and the greater the number of WG coatings, the more hydroxyl groups existed on the Ti surface. The cell (MSC) proliferation on the WG coated Ti was significantly improved compared to the non-WG coated sample due to the formation of hydroxyl groups on the WG coated Ti. The cell proliferation also depended on the WG coating thickness and the number of hydroxyl groups on the surface. That is, the increase in the number of hydroxyl groups on the Ti surface afforded by the WG coating resulted in better cell behavior. We conclude that Ti coated with WG can be employed for the early fixation of dental implants. †(Received April 29, 2013)

keyword : water glass, silanol functional groups, Si-OH, alkali treatment