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The corrosion and erosion-corrosion behaviors in the weld heat-affected zone (HAZ) of Hadfield steels with varying Cr contents (1, 2, and 3 wt%) were examined. Various experimental methods, including electrochemical polarization, impedance, and weight loss measurements, were utilized. Two types of isothermal heat treatments were conducted in a box furnace to simulate the intercritical HAZ, known to be the most vulnerable region in terms of mechanical properties and environmental stabilities, and large-scale samples for the erosion-corrosion experiment were fabricated. The results showed that increasing the Cr content improved the resistance to corrosion and erosion-corrosion, but there was an inflection point where adding more Cr had the opposite effect. Up to 2 wt%, a higher resistance was exhibited owing to the formation of a thin and protective oxide scale enriched with Cr that adhered to the steel surface. On the other hand, adding 3 wt% of Cr resulted in decreased resistance. This was due to the formation of coarse M₇C₃ (M: Cr) precipitated along the grain boundary, which caused the development of a thick and unstable oxide scale that detached locally. Based on these findings, it is essential to optimize the Cr content to ensure a high resistance to corrosion and erosion-corrosion in the HAZ of Hadfield steel. (Received 30 June, 2023; Accepted 22 September, 2023)

keyword : Hadfield steel, 13% Mn, Cr, Corrosion, Erosion-corrosion

This study examined the effects of minor alloying elements (Cr and Mo) on the corrosion behaviors of API grade steel in CO₂-saturated near-neutral aqueous solutions. Various experimental and analytical methods were applied to gain insights into the underlying CO₂ corrosion mechanism of the steels. The findings revealed that steel with a minute quantity of Mo (0.1 ~ 0.15 wt%) exhibited the lowest corrosion current density and the highest polarization resistance. This outcome can be primarily attributed to the formation of a thin layer of Mo-based oxides/hydroxide, covered with a fine FeCO₃ scale, consequently enhancing corrosion resistance in CO₂ environments. On the other hand, the addition of Cr (0.4 ~ 0.5 wt%) in combination with Mo resulted in degraded corrosion resistance due to the competitive precipitation of amorphous Cr(OH)₃ and crystalline FeCO₃. This led to an uneven interface and the growth of FeCO₃ particles, consequently reducing polarization resistance and increasing the corrosion rate. Based on the obtained results, it is recommended that cost-effective steels for CO₂ transportation can be developed by adding a small amount of Mo along with a higher quantity of Cr. This optimized combination of alloying elements is expected to significantly improve the anti-corrosion performance of the steel in near-neutral brine environments with CO₂. (Received 7 August, 2023; Accepted 7 September, 2023)

keyword : CO_{2 corrosion, steel, Cr, Mo, FeCO3, Cr, OH, 3, Mo, OH, 3}

In this study, a composition-gradient thin film was applied for the formation of intermediate layer of Ti seed layer for an stable electrode stack Ag metal layer. Various composition of Ag-Ti hetero metal layer were simultaneously deposited by using the sputtering process with Ti and Ag target, respectively. An intermediate layer was deposited at a gradient composition ratio such as 5:5 and 7:3. In addition, the optimal deposition conditions were evaluated by confirming the plasma codition such as density of plasma ion, plasma potential with the Langmuir Probe (Hiden ESPion). Flow rate, power, and composition ratio were optimized as variables for thin film structures of compositional gradient thin films. In addition, thin film samples were heat treated at 200 ℃, 300 ℃, and 400 ℃ to relieve the residual stress between the interface of laminated thin films. Under these conditions, a composition-gradient thin film was evaluated by XRD (X-Ray Diffraction, SmartLab Rigaku 9kW), SEM (Scanning Electron Microscope, Nova NanoSEM 450), and EDS (energy dispersive X-ray spectroscopy). As a result of the measurement, it was confirmed that interfacial diffusion occurred due to the composition gradient thin film. When the composition gradient intermediate layer was applied to thin film stack, the residual stress increased more than that of single thin film stack. However, after stress relief annealing, residual stress was dramatically decreased compared to single stack. (Received 26 June, 2022; Accepted 6 September, 2023)

keyword : Composition Gradient Thin Film, Sputtering process, Hetero metal layer, Langmuir Probe

SnTe has drawn much attention due to its Pb-free composition along with tunable electronic and lattice structures. However, its intrinsically high defect concentration and high lattice thermal conductivity (κ_l) have hindered its application in devices. Recently, Bi doping at Sn-sites in Sn_1-xBi_xTe (x = 0.0 - 0.08) has been demonstrated to be effective in improving the thermoelectric performance (zT) of SnTe. Bi doping was particularly effective in improving the Seebeck coefficient in a wide range of temperature while suppressing its κl. However, the effect of Bi doping on electronic band structure of SnTe has not been studied. Here, we applied the Single Parabolic Band (SPB) model to the room temperature electronic transport properties measurements (Seebeck coefficient, electrical conductivity, Hall carrier concentration) and analyzed how electronic band parameters like the density-of-states effective mass (m_d *), non-degenerate mobility (μ₀), weighted mobility (μ_w), and B-factor changes with a changing Bi doping content (x). As the x increases, the md * increases while μ0 decreases. As the μw depends both on md * and μ0, it peaks at x = 0.02. Lastly, the Bfactor is related to the ratio of μ_w to κ_l, due to significantly decreasing κ_l at high x, the B-factor also becomes the highest at x = 0.08. Based on the B-factor of x = 0.08 sample, the highest theoretical zT of 0.31 is predicted using the SPB model. This is approximately 2.2 times higher than the experimental zT (~0.139) reported in literature at 300 K. The SPB model also guides us that the highest theoretical zT of 0.31 can be achieved if its Hall carrier concentration is tuned to 9.06 × 1018 cm^-3. (Received 13 July, 2023; Accepted 6 September, 2023)

keyword : SnTe, Single Parabolic Band model, density-of-states effective mass, non-degenerate mobility, weighted mobility

ZnMn₂O₄ (ZMO) cathode possesses a high theoretical capacity of 224 mAh g^-1 and high operating voltage (1.9 V vs. Zn²⁺/Zn) for aqueous Zn-ion batteries. However, the disproportionation reaction of Mn³⁺ leads to Mn dissolution in the ZMO cathode, deteriorating lifespan. In this study, we attempted to reduce Mn dissolution by enlarging the particle size, thereby diminishing the electrode/electrolyte interfacial area. The ZMO particle grew with increasing the calcination temperatures of 400℃, 500℃, and 600℃. Higher calcination temperature created oxygen vacancies within the lattice, thereby increasing the contents of Mn3+ for charge neutrality. The rate capability decreased with the increase in particle size, which is presumed to be due to the lengthening of the diffusion path of Zn ions. After a long-cycle experiment of Zn-ion batteries assembled with ZMO cathode and Zn anode, the Mn deposit amount on the anode was measured to reveal the Mn dissolution from the ZMO cathode based on the disproportionation reaction. The ZMO particle synthesized at 600℃ with the largest particle size demonstrated the highest cyclability of 48.1% at 1.0 A g^-1 based on the lowest Mn deposit on the anode. Hence, the ZMO electrode with a larger particle size exhibited improved cycle stability by alleviating of the disproportionation reaction from the reduced electrode/electrolyte interfacial area. (Received 26 July, 2023; Accepted 21 September, 2023)

keyword : Zinc-ion battery, ZnMn_{2O4, Mn dissolution, Particle size, Cycle stability}

WC-Co cemented carbide has excellent mechanical properties and is widely used in many industrial applications including cold forging dies and cutting tools. The tensile and fatigue properties of WC-20wt%Co cemented carbides with microstructural variations were investigated. Microstructure parameters such as Co binder content, WC particle size, binder mean free path, and contiguity were obtained by linear intercept method using BSE microstructure images. Standard specimens of WC-20wt%Co cemented carbides were prepared for tensile and fatigue testing. Uniaxial tensile stress-strain curves and tensile-compression fatigue S-N curves were obtained. The 22Co-Cr alloy with higher Co content showed the largest binder mean free path and the lowest continuity. The 20Co-d_wc alloy with fine WC grains of submicron size showed the lowest binder mean free path due to fine WC grain distribution. The 20Co-d_wc alloy with fine WC grains showed the highest tensile strength and fatigue strength, compared to other alloys. The 22Co-Cr alloy with a higher FCC Co phase content, which has excellent plastic deformability, showed higher fatigue properties. The fatigue life of the 22Co-Cr alloy increased with increasing compressive mean stress level. Based on the axial tensile and fatigue properties, a reasonable fatigue life prediction of WC-20wt%Co cemented carbide dies for cold forging can be estimated. (Received 28 July, 2023; Accepted 5 September, 2023)

keyword : WC-20 wt%Co cemented carbide, microstructure parameter, tensile strength, stress-life curve, fatigue life

The friction properties of brake friction materials are investigated utilizing the different car cleaners. Water, wheel cleaner and tire dressing were used to designate driving conditions in the field. The friction materials were soaked in each solution before friction test. The Coefficient of friction (COF) was similar in the case of water and tire dressing due to a same menstruum. And, the effect of surfactants within wheel cleaner caused lowest the COF. Surface morphologies were measured using confocal microscope and wheel cleaners, showing significant effects on the formation of thick and even friction film. The hardness of friction film was measured through micro-Vickers test, and the specimen with wheel cleaner showed highest hardness due to the thick film. On the other hand, the specimens with water and tire dressing showed a lower hardness even though the film was thicker than specimen with dry condition. The components of wheel cleaners also affected the hydrophobicity of friction films for both disc and pad. Therefore, the contact angle was critically low compared to other specimens. The reduction of hydrophobicity caused by the solution effects increasing of rust occurring on the surface of the brake disc. This result suggests that the formation of friction film under different environments was deeply related to friction and corrosive properties of brake system. (Received 7 September, 2023; Accepted 10 October, 2023)

keyword : Brake disc, Brake friction materials, Friction film, Surface morphology, Contact angle, Corrosion

This study analyzed a recrystallization phenomenon that occurred simultaneously with hightemperature oxidation on the surface of a directionally solidified CM247LC creep specimen, using optical and scanning electron microscopy. After heat treatment, the surface of the specimen subjected to the creep test at 982oC was oxidized by exposure to high temperature and underwent microstructural changes due to high temperature stress. The outermost layer of the oxidized surface pits was found to consist of an oxide of the Cr/Co component, and the lower layer contained an oxide of the Al component. The area adjacent to the surface oxide layer is a precipitated free zone (PFZ) depleted of the γ’ precipitated phase. The PFZ is caused by the diffusion of the Al component from this area to the surface oxide layer, resulting in the depletion of γ'-Ni3Al as the main Al component. The area adjacent to the PFZ is a γ' coarsening layer, which is the result of increasing γ' phase fraction and coarsening as the Cr/Co component of this region diffuses into the PFZ of the γ phase composition. This γ' coarsening and γ' rafting occurs in the direction perpendicular to the creep stress. In the EBSD analysis, the PFZ and γ' coarsened layer were observed to be recrystallized regions, with the recrystallization composed of a single grain including the PFZ and γ' coarsened layer. It is presumed that this recrystallization is caused by the residual stress of one-way solidification or the residual stress of the specimen surface processing. Accordingly, high-temperature oxidation in DS CM247LC creep gauge caused surface pits associated with recrystallization. (Received 3 November, 2023; Accepted 5 November, 2023)

keyword : CM247LC Superalloy, creep oxidation, surface recrystallization, precipitate free zone, PFZ, γ’-coarsening

The phenomenon of mechanoluminescence (ML) refers to the emission of light induced by mechanical stimulation applied to mechano-optical materials for example SrAl2O3:Eu,Dy (SAO). Numerous technologies on the basis of ML have been presented to visualize the stress or strain in various structures for the applications including structural health monitoring. As a result, extensive attention has been devoted to the design, synthesis, characteristics, optimizations, and applications of ML materials. However, challenges still remain in the standardization of ML measurement and evaluation, thereby commercially viable ML applications are currently unavailable. To overcome these difficulties, present study proposes ML measurement and evaluation techniques employing the ML fracture mechanics, finite element method, and dual deep learnings. For the effective normalization of visualized ML images under fixed initial ML intensity condition, continuous UV irradiation above the critical ML power density has been subjected to tensile and compact tension (CT) specimens. Therefore, Plastic Stress Intensity Factor (SIF) as well as crack tip stress field have been extracted successfully from normalized ML images based on ML fracture mechanics. To complement and verify the ML analysis, numerical FEM simulation and analytical ASTM calculation have been also provided. Finally, a double deep learning consists of Generative Adversarial Networks (GAN) and Convolutional Neural Networks (CNN) has been trained and tested for the standard evaluation of in-situ ML images. (Received 31 August, 2023; Accepted 21 October, 2023)

keyword : Al 7075, mechanoluminescence, Plastic Stress Intensity Factor, Generative Adversarial Networks, Convolutional Neural Networks, Structural Health Monitoring