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The effect of hydrogen charging methods on the hydrogen embrittlement characteristics of tempered martensitic steels were discussed in terms of hydrogen diffusion behavior. Two tempered martensitic steels with different Si content were fabricated by quenching and tempering. The steel with high Si content had a lower cementite fraction because the addition of Si changed the morphology of cementite from a long film-like shape to a short-rod shape by suppressing the precipitation and growth of the cementite. To evaluate the hydrogen embrittlement resistance of the two tempered martensitic steels with different Si content, slow strain-rate tensile testing was employed after introducing hydrogen using three types of hydrogen charging methods (ex-situ electrochemical hydrogen charging, in-situ electrochemical hydrogen charging, and in-situ high-pressure gaseous hydrogen environment). For the hydrogen pre-charged tensile specimens using the ex-situ electrochemical charging method, the steel with high Si content had a better hydrogen embrittlement resistance, with a higher relative reduction in area. On the other hand, there was no significant difference in the relative notch tensile strength of the two tempered martensitic steels with different Si content, regardless of the hydrogen charging methods. In addition, the ex-situ hydrogen charging method exhibited higher relative notch tensile strength compared to the in-situ hydrogen charging method due to the release of hydrogen during the tensile test, after exsitu hydrogen charging. This implies that hydrogen embrittlement resistance can be differently estimated depending on the kind of hydrogen charging methods. (Received 17 July, 2023; Accepted 24 August, 2023)

keyword : tempered martensitic steel, hydrogen embrittlement, slow strain-rate test(SSRT), electrochemical, highpressure hydrogen

The microstructure, tensile and fatigue properties of a martensitic stainless steel (MSS) plate with a thickness of 0.152 mm are investigated in this study. The microstructural properties were initially studied using electron backscatter diffraction microscopy. High dislocation densities were observed in the microstructure, along with the rolling texture revealed by the pole figure, following an initial cold rolling process of the as-received MSS plate. Tensile tests were conducted in both the rolling and transverse directions to compare the strength of the material in both directions. The results showed that the tensile strength was relatively low in the rolling direction. This can be attributed to the microstructure and crystallographic orientation of the material along that direction. Additional tensile tests were performed at various temperatures within the operating temperature range in the rolling direction. The results indicate that the highest tensile strength and elastic modulus are observed at room temperature. A high-cycle fatigue test was performed to determine the fatigue limit of the MSS plate. Furthermore, the microstructure was analyzed by controlling the fatigue cycles within the same stress range. The results revealed a proportional relationship between the accumulated deformation within the grains and the fatigue cycles. This can be helpful in understanding the fatigue damage mechanisms of the MSS plate. (Received 30 June, 2023; Accepted 24 August, 2023)

keyword : Martensitic Stainless Steel Plate, Microstructure, Tensile properties, Fatigue properties

Cubic boron nitride (cBN) thin films were deposited using DC magnetron sputtering, and the effect of the ion flux on the deposition behavior and residual stress of the cBN thin films was investigated. To increase the ion flux, the magnetic force ratio of the central/peripheral permanent magnets inserted in the magnetron sputtering source was reduced. Due to the complementary relationship between ion flux and energy for cBN deposition, the critical bias voltage required for cBN nucleation decreased as the ion flux increased. The cBN content of the films was relatively higher under the deposition condition of the increased ion flux. This was interpreted to indicate the thinning of the intervening hexagonal boron nitride (hBN) layer formed prior to cBN nucleation. Comparing the compressive residual stress of the cBN films, the residual stress was relieved as the bias voltage decreased regardless of the ion flux. The increase in ion flux made it possible to deposit the cBN films at a low bias voltage, thereby depositing cBN films with lower residual stress. The results showed that reducing ion energy by increasing ion flux for cBN deposition is a promising method for depositing low-stress cBN thin film having thin intervening hBN layer. (Received 6 May, 2023; Accepted 17 July, 2023)

keyword : cubic boron nitride thin film, unbalanced magnetron sputtering, ion bombardment, ion flux variation, residual stress

This study examined the effects of sulfuric acid concentrations on the corrosion behaviors of three types of commercial steels, conventional carbon steel (SA192), sulfuric acid dew resistant steel (S-TEN), and duplex stainless steel (S32205), considered to be potential materials for the heat recovery steam generator (HRSG) in combined cycle power plants. Based on the results of electrochemical polarization, immersion and wet-dry cycle tests, when exposed to a high sulfuric acid concentration (50%), SA192 and S-TEN exhibited higher corrosion resistance than S32205, which was due to the formation of a stable FeSO₄ scale on the carbon steel materials. With prolonged exposure, S-TEN with slightly higher concentrations of Cu and Sb showed lower weight loss than SA192. Conversely, when subjected to low sulfuric acid concentrations (5 and 10%), S32205 exhibited passivation behavior, and showed significantly superior corrosion resistance (i.e., much smaller weight loss) than the other two steel samples. The examination of localized corrosion damages through cross-section observation using a scanning electron microscope revealed also that S32205 suffered less damages from the wet-dry cycling. The long-term superior corrosion resistance of duplex stainless steel in low sulfuric acid concentrations, compared to carbon steel and sulfuric acid due resistant steel, make it a promising candidate material for the HRSG in combined cycle power plants. (Received 9 June, 2023; Accepted 18 July, 2023)

keyword : combined cycle power plant, heat recovery steam generator (HRSG), duplex stainless steel, sulfuric acid corrosion, wet-dry

SnSe₂ alloys have been investigated in recent times as potential n-type thermoelectric materials. In this study, the thermoelectric transport properties of a series of Sb-doped SnSe₂, Sn(Se_1-xSb_x)₂ (x = 0, 0.015, 0.03, 0.045, 0.06) alloys are investigated. The electrical conductivity was generally enhanced with Sb doping owing to a large increase in electron concentration. However, the Seebeck coefficient largely decreased with doping. Consequently, the power factor was significantly lower at a low doping of x = 0.015, and then began rising as the doping was increased beyond x = 0.015. It was found that the density-of-states effective mass and weighted mobility decreased with Sb doping, implying that the electrical transport properties of SnSe2 were degraded by Sb doping. The total and lattice thermal conductivities gradually decreased due to additional point defect scattering. Thus, the thermoelectric figure of merit declined significantly, from 0.30 of the pristine sample with a low doping of Sb (x = 0.015) at 750 K, to 0.18, and then for x = 0.06 it gradually recovered to the value of the undoped sample. The thermoelectric quality factor decreased as the Sb doping was increased, implying that Sb doping did not enhance the thermoelectric transport properties, despite the large increase in electron concentration. (Received 3 July, 2023; Accepted 26 August, 2023)

keyword : thermoelectric, SnSe₂, Sb doping

With the rapid development of portable devices and Energy Storage Systems (ESS), secondary batteries with high energy density and high capacity are in great demand. Among various candidates, Lithium-sulfur (Li-S) batteries have been considered for next-generation energy devices given their high theoretical capacity (1675 mAh g^-1) and energy density (2500 Wh kg^-1). However, the commercialization of Li- S batteries faces challenges due to sulfur’s low electrical conductivity and the shuttle effect, caused by the dissolution of lithium polysulfide intermediates in the electrolyte during the charge-discharge process. Herein, to resolve these problems, we report the fabrication of a vanadium dioxide (VO₂) composite via a simple hydrothermal method and optimize the structure of VO₂ for constructing an effective Multi-Walled Carbon Nano Tube (MWCNT) and 3D flower-shaped VO₂ (MWCNT@VO₂) binary sulfur host by a simple melt diffusion method. In particular, the polar VO₂ composite not only physically absorbs the soluble lithium polysulfides but also has strong chemical bonds with a higher affinity for lithium polysulfides, which act as a catalyst, enhancing electrochemical reversibility. Additionally, MWCNT improves sulfur’s poor electrical conductivity and buffers volume expansion during cycling. The designed S-MWCNT@VO₂ electrode also exhibits better capacity retention and cycling performance than a bare S-MWCNT electrode as a lithium polysulfide reservoir. (Received 12 July, 2023; Accepted 7 August, 2023)

keyword : Lithium-Sulfur batteries, VO₂, MWCNT, Melt diffusion

SnSe is a promising thermoelectric material due to its low toxicity, low thermal conductivity, and multiple valence band structures, which are ideal for high electronic transport properties. The multiple valence band structure has attracted many attempts to engineer the carrier concentration of the SnSe via doping, to place its fermi level at a position where the maximum number of valence bands can participate in the electronic transport. Up until now, ~5 × 10¹⁹ cm^-3 was the highest carrier concentration achieved in SnSe via doping. Recently, introducing SnSe₂ into SnSe was found to effectively increase the carrier concentration as high as ~6.5 × 10¹⁹ cm^-3 (at 300 K) due to the generated Sn vacancies. This high carrier concentration at 300 K, combined with the reduction in lattice thermal conductivity due to SnSe₂ micro-domains formed within the SnSe lattice, improved the thermoelectric performance (zT) of SnSe - xSnSe₂ as high as ~2.2 at 773 K. Here, we analyzed the changes in the electronic band parameters of SnSe as a function of temperature with varying SnSe₂ content using the Single Parabolic Band (SPB) model. According to the SPB model, the calculated density-of-states effective mass and the fermi level are changed with temperature in such a way that the Hall carrier concentration (nH) of the SnSe - xSnSe₂ samples at 773 K coincides with the optimum nH where the theoretically maximum zT is predicted. To optimize the nH at high temperatures for the highest zT, it is essential to tune the 300 K nH and the rate of nH change with increasing temperature via doping. (Received 24 July, 2023; Accepted 6 September, 2023)

keyword : SnSe, Single Parabolic Band model, Carrier concentration, High-temperature zT, Power factor

Commercial AA1070 alloy for electrical wire is severely deformed by the drawing process when a rod with a diameter of 2 mm is greatly reduced to 0.4 mm by multi-pass. Changes in the microstructure, mechanical properties, and electrical properties of the Al alloy during the wire-drawing process were investigated in detail. The as-drawn Al wires showed a deformation structure in which the grains are greatly elongated in the drawing direction, even though recovery and/or partial recrystallization occurred more actively in the specimens which had more than 84% in reduction of cross-sectional area (R_A). In addition, the fraction of high angle grain boundaries tended to increase with the increase of R_A. For all drawn specimens, the fiber texture of the {110}<111> and {112}<111> components was mainly developed, and their maximum intensity tended to increase with increasing R_A. Recrystallization texture of (001)[100] and (110)[001] began to appear at an RA higher than 84%. The hardness tended to increase with increasing R_A due to work hardening. In particular, increasing R_A to 84% resulted in a great rise in hardness, accompanied by a distinct non-uniformity in hardness in the thickness direction. However, the average hardness hardly changed at R_A above 84%, even when R_A was increased to 96%. The strength also tended to increase stepwise as R_A increased, very similar to the change in hardness. The specimen with an R_A of 93% showed the highest tensile strength of 192 MPa, 2.8 times higher than that of the specimen before drawing. The electric conductivity did not decrease significantly, even with extreme increases in R_A, and remained at an average value of 61.6 %IACS. (Received 27 July, 2023; Accepted 8 September, 2023)

keyword : AA1070 alloy, wire drawing, microstructure, mechanical properties, electrical properties

In this study, the Vickers hardness of precipitation-strengthened Al-Sc-X (X = Zr, Si, and Fe) alloys were predicted using machine learning models, depending on the alloys’ compositions, solid-solution treatment and aging conditions. The data used for machine learning were collected from the literature. Among the models, tree-based ensemble models such as extreme gradient boosting and random forest performed well. Then the feature impact on the model output was analyzed with SHarpely Additive eXplanation (SHAP). Based on the SHAP analysis and prior domain knowledge, the process conditions were restricted to narrow down the inverse design search space. Candidate alloys suggested by the optimization using a genetic algorithm showed improved hardness values. The hardness prediction model and the inverse designsuggested candidates were then experimentally validated. The accuracy of the hardness prediction model was 0.994, when the predicted hardness was 85.4 Hv, and the experimentally measured hardness was 84.9 Hv. A specimen whose composition was close to the inverse-designed alloy was cast and heat treated according to the suggested conditions. The inverse design showed an accuracy of 0.965. Exploring the entire combination of possible feature space requires vast effort and time. An efficient search for materials with improved properties can be achieved using an appropriate configuration of well-performing machine learning models and explainable AI techniques guided by domain knowledge. (Received 31 July, 2023; Accepted 1 September, 2023)

keyword : machine learning, explainable AI, inverse design, aluminum alloys, heat treatment, hardness