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Materials with superior cryogenic strength and good ductility are increasingly in demand for resident friendly liquid-hydrogen (20K) storage tanks. Additionally, the space industry also requires materials that retain excellent mechanical properties at extremely low temperatures. However, mechanical testing at such low temperatures is highly limited due to the difficulties in achieving and maintaining such conditions, while also providing adequate thermal insulation to prevent heat transfer from the surrounding environment. In this study, we present a novel tensile testing technique for a Fe-15Mn-13Cr-3Si-3Ni-0.1C (wt.%) steel at the temperature of liquid helium. To minimize the use of expensive liquid helium, we adopted a method of injecting liquid helium vapor and set the temperature for tensile testing at 6 K. The present alloy has a single face-centered cubic (FCC) structure with a large amount of stacking faults after annealing treatment. The Fe-Mn-Cr steel exhibited a superior ultimate tensile strength of 1200 MPa and good ductility of 35% at 6K. Moreover, compared with room temperature tensile tests, discontinuous plastic flow, i.e. serrated flow, occurred at extremely low temperature. (Received 26 February, 2023; Accepted 13 March, 2023)

keyword : Fe-Mn-Cr steel, microstructure, mechanical properties, liquid helium, cryogenic strength

17-4 precipitation hardened stainless steel (17-4PH SS) has been reported to have excellent mechanical properties and excellent corrosion resistance, and is one of the materials used and studied with the powder bed fusion (PBF) method. Powder bed fusion (PBF) is a new manufacturing technology that has recently attracted attention in automotive, aerospace and other industries because of its ability to produce complex geometries for high-strength and lightweight applications. In the PBF process, each layer has a different laser scan length resulting from the application of laser rotation. The laser rotation could affect the laser scan length, which causes a difference in the peak temperature and cooling rate of the deposited layer, resulting in microstructure changes. This work aims to investigate how varying the laser scan pattern in the PBF process affects the microstructure and mechanical properties of 17-4PH SS. A decrease in cooling rate was observed after applying laser scan rotation, resulting in a higher austenite phase fraction. It was confirmed that a transformation induced plasticity (TRIP) phenomenon affects mechanical characteristics. These results could be suggested for fabricating thin wall shaped such as tire blow mold parts in the powder bed fusion process using the 17-4PH SS. (Received 5 Decemmber, 2022; Accepted 17 Fabruary, 2023)

keyword : powder bed fusion, 17-4 precipitation hardening stainless steel, laser scan rotation, retained austenite, transformation induced plasticity

The effect of Ni-based fillers on corrosion resistance of welded super austenitic stainless steel (SASS) was examined. The use of Ni-based fillers for gas tungsten arc welded SASS leads to the formation of fine Cr-Mo enriched phase in its weld unmixed zone, which makes it more susceptible to the localized corrosion. The metallographic observations showed that the pits were initiated around the fine Cr-Mo enriched phase precipitated in unmixed zone, and they were propagated along the base metal. The degree of corrosion damage among the welded samples using the three types of Ni-based fillers (Inconel 625, Inconel 622, Hastelloy 276) increased with the Mo contents in the fillers applied. The higher the Mo contents in the fillers, the higher the size/number ratio of Cr-Mo enriched phase with higher concentration of Mo, precipitated in unmixed zone. Based on the results, it is proposed that the Mo contents in Ni-based high alloyed fillers can have a great effect on the precipitation behavior of Cr-Mo enriched phase in unmixed zone, causing deterioration of corrosion resistance of SASS welds. Therefore, the Mo content in the filler should be optimized with the content similar to that of the base metal so as not to susceptible to localized corrosion. (Received 5 January, 2023; Accepted 6 March, 2023)

keyword : super austenitic stainless steel, welding, unmixed zone, sigma, corrosion

Dihydrogen sulfide (H₂S) gas has a flammable nature and is one of the most toxic and dangerous gases. Even small concentrations can be fatal to humans. Herein, we investigated the H₂S gas-sensing features of commercial pristine cerium oxide (CeO₂) and gadolinium (Gd)-doped CeO₂ (GDC) nanoparticles. First, the sensing materials were well-characterized using various methods including X-ray photoelectron spectroscopy, transmission electron microscopy and X-ray diffraction to gain insight into their chemical composition, morphology, phases, and crystallinity, respectively. In the next step, gas sensors were fabricated using a top electrode (Au/Ti) configuration. Preliminary H2S-gas-sensing studies revealed that GDC gas sensor had a superior gas response to H₂S gas than the pristine CeO₂ gas sensor at 350℃. The responses of the pristine CeO₂ gas sensor to 20 ppm H₂S gas was 1.542, while the response of the GDC gas sensor to the aforementioned H₂S concentration was 3.489. In addition, the GDC sensor exhibited good selectivity to H₂S gas among C₂H₅OH, C₇H₈ and NH₃ gases. Also, we investigated the response of the sensor in up to 60% relative humidity. The enhanced response of the GDC gas sensor to H₂S gas was mainly related to the formation of oxygen defects as a result of Gd-doping in CeO₂. Also, good selectivity to H₂S was related to the sensing temperature, the higher reactivity of H₂S relative to other gases and the small bond energy of H-SH. This study demonstrates the promising ability of Gd-doping to enhance the H₂S gas-sensing characteristics of CeO₂, which can be applied to other similar systems based on semiconducting metal oxides. (Received 6 February, 2023; Accepted 10 March, 2023)

keyword : H₂S, CeO₂, Gd-doped CeO₂ (GDC), gas sensor, sensing mechanism

Cu-Sb-Se ternary chalcogenide compounds composed of earth-abundant low-toxicity elements are attracting attention as economical and ecofriendly semiconductors. Among them, permingeatite (Cu₃SbSe₄) is a potential thermoelectric material with high Seebeck coefficient and low lattice thermal conductivity. However, it is necessary to improve its thermoelectric properties through doping, as it has low electrical conductivity due to its low intrinsic carrier concentration. In this study, samples of Cu₃Sb_1-x-ySn_xIn_ySe₄ (0.02 ≤ x ≤ 0.08 and 0.04 ≤ y ≤ 0.06) double-doped with In (group 13 element) and Sn (group 14 element) at the Sb (group 15 element) sites of permingeatite were synthesized and their thermoelectric performances were evaluated. All samples exhibited a single phase of permingeatite with tetragonal structure, and high relative densities of 97.4-98.9%. The lattice constants of the a- and c-axes were 0.5651-0.5654 and 1.1249-1.1257 nm, respectively, owing to the successful substitution of Sn and In at the Sb sites. As the doping concentrations of Sn and In increased, the carrier (hole) concentration increased. Thus, the Seebeck coefficient decreased, while the electrical and thermal conductivities increased. Sn doping was found to be more effective than In doping. Because Cu₃Sb_0.96-xSn_xIn_0.04Se₄ exhibits higher Seebeck coefficients than Cu₃Sb_0.94-xSn_xIn_0.06Se₄, larger power factors and higher dimensionless figures of merit (ZTs) were achieved for the Cu₃Sb_0.96-xSn_xIn_0.04Se₄ specimens. Cu₃Sb_0.92Sn_0.04In_0.04Se₄ achieved a maximum ZT of 0.59 at 623 K, based on its Seebeck coefficient of 161 μVK^-1, electrical conductivity of 4.69 × 10⁴ Sm^-1, thermal conductivity of 0.77 Wm^-1K^-1, and power factor of 1.22 mWm^-1K^-2. (Received 19 January, 2023; Accepted 27 February, 2023)

keyword : thermoelectric, permingeatite, double doping, mechanical alloying, hot pressing

The addition of Cu to layered Bi₂Te₃-based thermoelectric alloys has been studied as an effective way to enhance thermoelectric transport properties. In this study, the influence of adding Cu to Bi₂Se₃ alloys, which have the same rhombohedral crystal structure as Bi₂Te₃, was investigated by synthesizing a series of Cu_xBi₂Se₃ (x = 0, 0.004, 0.008, 0.012, and 0.016) alloys. The power factors of all the Cu-added samples were enhanced compared with that of the pristine Bi₂Se₃ sample, primarily because of the increase in electrical conductivity. The power factor for the Cu_0.016Bi₂Se₃ sample (x = 0.016) was 0.80 mW/mK², a 35% increase compared to 0.59 mW/mK² for the pristine sample at 520 K. A decrease in the total and lattice thermal conductivity was observed for the Cu-added samples, caused by additional point defect scattering after doping. The lattice thermal conductivity of the Cu_0.016Bi₂Se₃ sample (x = 0.016) was 0.56 W/mK, a 42% reduction. Consequently, the zT values of all the Cu-added samples were enhanced, and the maximum zT value was 0.38 for the Cu_0.016Bi₂Se₃ sample (x = 0.016) at 520 K, a 48% increase compared to that of pristine Bi₂Se₃. The phenomenological transport parameters, including density of state, effective mass, weighted mobility, and thermoelectric quality factor, were calculated to analyze the enhanced electronic transport properties of the Cu-added Bi₂Se₃. (Received 2 February, 2023; Accepted 28 February, 2023)

keyword : thermoelectric, Bi₂Se₃, Cu doping

Mg₃Sb₂-based materials are very promising for thermoelectric applications at low temperatures, and are strong candidates to replace n-type Bi₂Te₃ for cooling and power generation. Substituting Sb atoms with chalcogen elements (S, Se, Te) is a typical method of n-type doping, while doping the Mg site with Group 3 elements (Y, Sc) and Lanthanides has also been studied. Unique advantages have been recently reported. In this study, a La-containing compound, LaSb, was used to fabricate n-type Mg₃SbBi. The thermoelectric properties of polycrystalline Mg₃LaxSbBi (0 ≤ x ≤ 0.02) were investigated after synthesis by sequential processes of arc melting, ball milling, and spark plasma sintering. Undoped Mg₃SbBi is p-type with poor thermoelectric performance, and switched to n-type with La doping. The electron concentration of Mg₃LaxSbBi increased linearly with La content x, reaching up to 9.4 × 10¹⁹ cm^-3 at x = 0.02. The power factor and the figure of merit were also maximized in Mg₃La_0.02SbBi, reaching 1.8 mW m^-1K^-2 (573 K) and 0.89 (623 K), respectively. The lattice thermal conductivity decreased with increasing La content above ~500 K, and the minimum value of 0.73 W m^-1K^-1 was obtained in Mg₃La_0.02SbBi. This study shows that La doping using LaSb provides a reliable method for n-type doping of Mg₃Sb₂-based materials. (Received 21 February, 2023; Accepted 9 March, 2023)

keyword : thermoelectric, Mg₃SbBi, La doping, LaSb

MgO nanowires with a branched structure were fabricated using a thermal evaporation method in air at atmospheric pressure. The branched MgO nanowire was made up of two parts: a primary central nanowire trunk and lots of secondary nanowire branches. The branched MgO nanowires had a 4-fold symmetrical structure. The secondary nanowire branches grew perpendicular on the four side facets of the central nanowire trunks with square cross-sections. The nanowire branches also grew in a single row and were vertically well aligned in the same direction with each other. The scanning electron microscopy images of the branched nanowires grown at 1000℃ showed that the diameter of branches gradually decreased along the growth direction and no catalyst particle was found at the tips of the branches, indicating that the branches were grown by a vapor-solid process. For the branched nanowires grown at 1150℃, spherical particles which were shown to be catalysts were observed at the tips of the branches. The chemical analysis by energy dispersive spectroscopy showed that the spherical particles were composed of Mg and O elements. These results suggest that the branches’ growth resulted from a self-catalyzed vapor-liquid-solid process. The structural characterization by X-ray diffraction confirmed that the branched MgO nanowires had a cubic lattice structure. The room temperature cathodoluminescence spectra of the branched MgO nanowires exhibited a very strong visible emission which was associated with oxygen vacancies. (Received 10 January, 2023; Accepted 23 February, 2023)

keyword : magnesium oxide, branched nanowires, thermal evaporation, air, atmospheric pressure

High entropy alloys (HEAs) are defined as a multi-element alloy including more than 4 elements with near equi-atomic percentage. In general, the configurational entropy of the HEAs is known to be sufficient to stabilize a single solid solution, such as body-centered cubic (BCC), face-centered cubic (FCC) and hexagonal-closed pack (HCP). Compared to BCC single-phase alloys, FCC single-phase alloys draw extensive attention because they are advantageous in manufacturing and processing. FCC-based HEAs show excellent ductility but limited strength, so many research on improving strength has been conducted. Outstanding mechanical properties with a balance of strength and ductility are rarely achieved in single-phase FCC-based HEAs. This is why most alloys for structural applications exhibit a multi-phase microstructure. In this study, we aimed to develop multi-phase FCC-based HEA with superior mechanical properties than single-phase CoCrFeMnNi HEA, via Co substitution in CoCrFeMnNi HEA by Cu, which has a high mixing enthalpy. It was found that the CuCrFeMnNi HEA is composed of two FCC phases and one BCC phase. The CuCrFeMnNi HEA was cold-rolled, and subsequently aged at 500, 700, 900 ℃ for 1 hour. As the annealing temperature increased, the volume fraction of the FCC phase (FCC1 + FCC2) increased and the residual stress was gradually relieved by recrystallization. Furthermore, small amount of sigma phase was formed at 900 ℃. The effect of the microstructural evolution on the mechanical properties, such as hardness and tensile properties at room temperature, will be discussed. (Received 14 January, 2023; Accepted 3 March, 2023)

keyword : High entropy alloys, microstructure, cold rolling, recrystallization, mechanical property