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This study was carried out to investigate the effect of fatigue stress magnitude on the tensile properties of the damaged Fe - 22Mn - 12Cr - 3Ni - 2Si - 4Co damping alloy under various fatigue stress magnitudes. α and ε-martensite were formed by fatigue stress in the Fe - 22Mn - 12Cr - 3Ni - 2Si - 4Co damping alloy. The α and ε-martensite formed by fatigue stress in the fatigue damaged damping alloy was formed with a specific direction and surface relief, or crossing each other. With an increasing magnitude of fatigue stress, the volume fraction of α -martensite and ε-martensite was increased, and the increasing rate of volume fraction of α -martensite was more rapid than ε-martensite. At the same fatigue stress magnitude, a large amount of α -martensite was formed than ε-martensite. With the increasing of fatigue stress magnitude, the tensile strength slowly increased and elongation rapidly decreased, and then tensile strength slowly decreased by increasing the volume fraction of α -martensite. The tensile properties of the damaged Fe - 22Mn - 12Cr - 3Ni - 2Si - 4Co alloy under fatigue stress were strongly affected by the α -martensite formed by fatigue stress. This results were the same tendency for metal with the strain induced martensite transformation. (Received October 08, 2018; Accepted November 29, 2018)

keyword : fatigue stress, damping alloy, martensite, tensile properties, fatigue damage

Zn-Al-Mg alloy galvanized steel sheets have excellent corrosion resistance and improved cut-edge corrosion resistance. Most of the galvanized steel sheets used for outdoor applications are coated again on the galvanizing layer, or after a plastic film is laminated. As the corrosion products formed by the corrosion of Zn accumulate over time, the painting film blisters and peels, which causes a significant reduction in the value of the product. In this study, the coating adhesion and corrosion resistance of a Zn-Al alloy galvanized steel sheet with Mg added were evaluated by the salt spray test. The deterioraion of the coating film was also evaluated by Electrochemical Impedance spectroscopy(EIS) measurement. A quantitative evaluation of the coating film adhesion following water penetration between the plated layer and coating film under constant temperature and humidity condition was carried out using a Scanning Kelvin Probe(SKP). (Received October 10, 2018; Accepted December 19, 2018)

keyword : Zn-Al-Mg hot-dip galvanized steel sheets, paint coating film, corrosion, salt spray test, adhesion

Memristors have been extensively investigated as the fourth fundamental circuit element. Titanium oxide is a common material used to fabricate memristors. In this paper, we investigated the influence of the thickness of the oxide active layer on the Al/TiO_2-x/TiO₂/heavily doped electrode memristor architecture. An insulating TiO₂ thin-film was deposited using an atomic layer deposition system, and TiO_2-x active layers of 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, and 30 nm, respectively, were deposited using an RF sputtering technique. The conductivity and crystallinity of the oxygen vacancy based TiO_2-x/TiO₂ active layers were observed using atomic force microscopy. When the electrical characteristics of the memristor were evaluated, the 10 nm thick active layer memristor exhibited the most complete repeatable bipolar nonvolatile switching behavior, with an I-V curve of -3 to 3 V. The characteristics of the fabricated nonvolatile memory devices were confirmed by measuring the retention stability of the 10 nm thick active layer based device, which was judged to be the most suitable thickness ratio. Moreover, the endurance number of the ON and the OFF states of the fabricated device was maintained even when the measurement was performed for 1,000 seconds. This performance is indicative of potential applications in next-generation electronic memory devices. (Received November 26, 2018; Accepted January 9, 2019)

keyword : nonvolatile resistive random access memory, memristor, thin films, magnetron sputtering, surface, hysteresis, metal oxide semiconductor

The critical role of the thicknesses of the top and bottom oxide layers, as well as Ag layer, in ZnO/ Ag/ZnO transparent conductive electrodes were investigated. The Ag forms a nearly continuous layer at the thickness of 8 nm, at which the Ag sheet resistance of 8.0 Ω/Sq. is lower than the typical criteria of 10 Ω/ Sq. By making independent changes in the thickness of the top and bottom ZnO layers, which serve as antireflection layers, it was found that the top ZnO layer thickness has a dominant impact, with the bottom ZnO layer thickness contributing a less but still significant amount. The optimized thicknesses for the top and bottom ZnO layers were found to be 40 and 20-30 nm, respectively, resulting in a peak transmittance of 97.1% and average visible light transmittance of 90.8%. According to the Haccke figure of merit (Ф_H=T¹⁰_ave/R_S), the value for the optimized ZnO/Ag/ZnO electrode was 0.048, which is highly competitive for transparent conductive electrodes for future optoelectronic devices. (Received December 19, 2018; Accepted January 8, 2019)

keyword : transparent conductive electrodes, thin films, transmittance, sheet resistance

Among several candidates for nanoenergetic materials and systems, nanoscale reactive multilayer foils (NRMF) or bimetallic nano-laminates are highly promising candidates. They provide the highly desirable pyrotechnical properties of high energy density and reaction sensitivity as well as flexible tunability. They could be useful in various fields of explosive and propellant ignition devices, in alternative heat sources, and for the precision joining of metal components in miniaturized forms. NRMF consists of many alternating nanosized layers of two different metals. It exhibits novel energetic performances, including a considerably short ignition delay and superfast reaction propagation with high exothermic heat release. This numerical study presents the computational modeling of self-propagating intermetallic reaction waves in aluminum and nickel based multilayer NRMF microsystems. The existence of atomic pre-mixing at the bimetallic interface was also carefully considered in the numerical model. The computational results of reaction wave speed in NRMF with Al-Ni bilayer spacing from 10 to 180 nm were found to be in excellent agreement with the corresponding measurements, which validates the present numerical model and predictions. The fundamental physical mechanism of self-sustaining reaction waves was also closely investigated. It was found that increasing the thickness of interface pre-mixing leads to a clear observation of periodic wave unsteadiness and hot spots in intermetallic reaction wave propagation in NRMF systems. (Received November 12, 2018; Accepted December 10, 2018)

keyword : bimetallic reaction, nanoscale multilayers, reaction wave propagation, numerical modeling

As energy generation by recycling biomass has become increasingly attractive, research on biomass recycling has focused on improving operational efficiency. To develop a new material composition for the inner wall of a biomass boiler, we investigated the effect of Cr and Ni addition on the resistance of the α-iron (100) surface to Cl₂ gas corrosion using first principles theoretical calculations. We constructed model systems by partly substituting the outermost Fe atoms on the α-iron (100) surface with Cr and Ni atoms, and then calculated the adsorption energy of Cl and the escape energy of Fe on the surface. The adsorption energy results indicated that Ni added to the surface hindered the adsorption of Cl onto the surface, while Cr had little effect. The addition of both Cr and Ni greatly hindered the escape of Fe atoms from the surface. The mechanism governing the hindrance of the Cl adsorption differed between the Cr and Ni. The Cr hindered the escape of Fe atoms by increasing cohesive energy, while the Ni stabilized the α-iron (100) surface and in turn hindered the adsorption of Cl₂ gas. (Received November 30, 2018; Accepted January 14, 2019)

keyword : biomass, iron corrosion, theoretical calculation, adsorption energy, escape energy

The compression process is one of the more widely used industrial manufacturing methods for fabricating desired shape of specimens with various materials such as metals and ceramics. In the compaction process, the upper punch moves into the powder, and force is transmitted between particles, then achieving densification. In this process, the powder can be considered to be in a particulate state, which means that while the powder consists of solids, it has characteristics quite similar to the fluid. Therefore, particles in the process can be seen as responding to hydrostatic pressure, and it can be assumed that the pressure is constant. However, the forces acted on the inter-particle continue to change during the process. Many parameters affect the force change, including compaction speed and the contact angle between particles. However, it is very difficult to verify these effects through experiments because it is impossible to arrange the inter-particle angle. Therefore, in this study, the force transmission mechanism was simulated in the compaction process using FEM simulation. To examine the contact angle and force transmission between the particles, a green compact was modeled as individual particles rather than as a continuum green compact. Finally, it was confirmed through analysis that the pressure transmission between the particles remained constant during the compression process. (Received December 17, 2018; Accepted December 24, 2018)

keyword : compaction, FEM simulation, spherical powder, pressure transmission

In micro/nano electro-mechanical system, silicon nitride (SiN_x) membrane has been widely used in sensors, energy harvesting and optical filters because of its mechanical/chemical stability. However, it is necessary to verify mechanical and thermal properties of nanoscale SiN_x membranes to ensure the desirable reliability and durability of a device because the properties of nanoscale films vary with thickness which is severely depending on changes in density, grain size, and crystallinity. In this paper, SiN_x membranes were fabricated by low pressure chemical vapor deposition followed by reactive ion etching and KOH wet etching. The composition, surface roughness, thickness uniformity and residual stress of the deposited SiN_x films were measured to confirm the reliability of the deposition process. Plane-strain modulus, failure stress and emissivity were evaluated by bulge test and heat load test. As a result, the failure stress of the membrane was enhanced by decreasing SiN_x thickness while the plane-strain modulus was insensitive to the thickness variation. Through the UV laser heat-load test, it was found that the thermal durability of the thinner membrane deteriorated due to decreased emissivity. To investigate the emissivity depending on membrane thickness, a finite element method simulation was performed based on the experimental results. The calculated emissivity of each membrane coincided with the reported values within 8% difference. (Received December 26, 2018; Accepted December 27, 2018)

keyword : membrane, silicon nitride, mechanical properties, emissivity, chemical vapor deposition, stress-strain curve