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W-containing Ti-6Al-4V alloys (W=0, 1, and 5 wt%) were fabricated by the powder injection molding process, and the corresponding effects of tungsten content on the mechanical properties and microstructure of the alloys were investigated. The alloy powders were sintered at 1200 °C and then hot-isostatically-pressed at 900 °C. The fabricated alloys were subjected to microstructural and chemical analyses, and tensile and nano-indentation tests. The yield strength and tensile strength proportionally increased as the W content was increased from 0 wt% to 5 wt%. Ductility was not affected by the addition of up to 5 wt% W due to its complete dissolution in the matrix. Higher W addition induced finer α/β lamellar microstructures and increased the β to α phase ratio. Moreover, the added W dissolved preferentially in the β phase by solid solution hardening, increasing the hardness of the β phase, which originally was significantly softer than the α phase. For the alloys containing up to 5 wt% W, the strengthening without ductility loss was attributed to the finer α/β lamellae and the volume increase in the β phase hardened by W. These results suggest that adding W to Ti-6Al-4V alloy is a promising method for developing Ti alloys with both high strength and toughness. (Received January 6, 2021; Accepted March 23, 2021)

keyword : Ti alloy, mechanical properties, powder-metallurgy, tungsten element, microstructure

The effect of tailored microstructures in 0.5 wt% CaO added AZ31 on tensile, high-cycle fatigue, and fatigue crack growth properties was examined. By adding CaO, the average grain size (AGS) was significantly reduced from 4.25±2.32 μm (conventional AZ31) to 2.42±1.60 μm (CaO-AZ31). The fineprecipitates of CaO-AZ31 were more evenly distributed and their fraction was higher than those of conventional AZ31. The fine-precipitates were identified as Al₈Mn₄Ca and (Mg, Al)₂Ca in CaO-AZ31, meanwhile, were identified as Al₈Mn₅ and Mg₁₇Al₁₁ in conventional AZ31. The tensile test results showed that the yield strengths of CaO-AZ31 and conventional AZ31 were 238.0 MPa and 206.7 MPa, respectively. The elongation-to-failure also increased when CaO was added. The improved tensile properties of CaO-AZ31 could be explained by grain refinement and precipitation hardening. The high-cycle fatigue limit also increased about 15% with added CaO. The fatigue limits as a function of the tensile strengths of CaO-AZ31 and conventional AZ31 were 0.508 and 0.457, respectively. The origin of the improved fatigue resistance was attributed to inhibition of the formation of DTs, which acted as the fatigue crack source, in CaO-AZ31. In contrast, the fatigue crack growth property did not change when CaO was added. Based on the above findings, the relationships between microstructure, mechanical properties and deformation mechanisms are also discussed. (Received February 23 2021; Accepted March 30, 2021)

keyword : AZ31, magnesium alloy, CaO, microstructure, tensile, high cycle fatigue, fatigue crack propagation

The corrosion behaviors of a combined weld (plasma, gas tungsten arc) joint in a super austenitic stainless steel pipe were investigated using a range of experimental and analytical methods. To ensure superior corrosion resistance, a Ni-based super alloy (Inconel 625) was employed as the welding material only in the gas tungsten arc welding (GTAW). Nevertheless, pitting corrosion occurred preferentially around the sigma phase which had been precipitated in the interdendritic region of the GTAW. This indicated that the Inconel 625, which has a higher pitting resistance equivalent number (PREN), became even more susceptible to pitting corrosion than the base metal (BM). The higher Fe content in the Inconel 625 due to the dilution of Fe, supplied by the leading plasma arc welding, may increase the driving force for the precipitation of sigma phase. It was also revealed that the post weld heat treatment conducted at 1050~1150 ℃ effectively reduced the fraction of sigma phase precipitated in the weld. Even after such heat treatment, however, pitting corrosion occurred unexpectedly in the center region of the BM. This may be due to additional precipitation of the sigma phase in the BM, caused by inadequate control of the cooling rate during heat treatment at the industrial site. (Received February 10 2021; Accepted March 28, 2021)

keyword : corrosion, pitting, super austenitic stainless steel, sigma phase, PWHT

Tungsten carbides are very attractive because of their superior properties, e.g., high thermal and electrical conductivities, high melting point, high hardness, and relatively high chemical stability. Tungsten carbides with a binder metal, for example Ni or Co, are mainly used to fabricate nozzles, molds and cutting tools in the composite form. Al has been reported as an alternative binder in Tungsten carbide since Al shows a higher oxidation resistance than Ni or Co and is less expensive. Nanostructured WC-Graphene-Al composites were sintered rapidly using pulsed current activated sintering (PCAS). The mechanical properties (hardness and fracture toughness) and microstructure were investigated using scanning electron microscopy and Vickers hardness tester. The PCAS method successfully obstructed grain growth, resulting in nanostructured materials, and induced a very fast consolidation nearly at the level of theoretical density. The grain size of WC in WC-Graphene-Al composite decreased with the addition of Al content. The fracture toughness and hardness of the WC-5vol.% graphene-x vol.% Al (x=0, 5, 10, 15) were 4.7, 5.5, 5.9, 7.9 MPa·m^1/2 and 2008, 1961, 1883, 1731 kg/㎟, respectively. The fracture toughness was improved without remarkable decrease of hardness due to the small dimensions of the WC grain and the consolidation facilitated by adding Al to WC-Graphene matrix. (Received January 18, 2021; Accepted April 7, 2021)

keyword : sintering, composite, mechanical properties, tungsten carbide, nanomaterial

A novel magnesium (Mg) production process utilizing an electrolytic method was investigated for the direct reduction of magnesium oxide (MgO). Electrolysis of MgO was carried out with an applied voltage of 3.0 V using a copper (Cu), silver (Ag), or tin (Sn) cathode and carbon (C) or platinum (Pt) anode in magnesium fluoride (MgF₂)-calcium fluoride (CaF₂)-sodium fluoride (NaF) at 1273 K or MgF₂- lithium fluoride (LiF) at 1083-1093 K. After the electrolysis of MgO in MgF₂-CaF₂-NaF molten salt, Mg alloys such as Mg₂Cu, Cu₂Mg, or Mg₂Sn phases were produced with current efficiencies of 75.8-85.6% when the concentration of Mg in Mg alloys was 9.1-14.6 mass%. In addition, when the electrolysis of MgO was conducted in MgF₂-LiF molten salt, Mg alloys such as Mg₂Cu or AgMg phase were produced with current efficiencies of 76.2-81.7% when the concentration of Mg in the Mg alloys was 12.5-13.2 mass%. In addition, to produce high-purity Mg metal from Mg alloys, vacuum distillation was conducted. When vacuum distillation was conducted at 1100-1400 K for a duration of 5 h, the concentration of Mg in the Mg alloys feed decreased from 30.2-34.1 mass% to 0.64-1.75 mass%, and Mg metal with a purity of 99.998-99.999% was obtained under certain conditions. Therefore, the molten salt electrolysis using liquid metal cathode (MSELMC) process developed here is feasible for the direct reduction of MgO using an effective and environmentally sound method. (Received December 23, 2020; Accepted March 25, 2021)

keyword : magnesium, magnesium oxide, electrolytic process, liquid metal cathode, vacuum distillation

Extremely thin film high quality copper foil is required to ensure high performance in electronics and slimness in secondary batteries. During the electroplating of copper foil, Janus Green B(JGB) and Collagen were introduced as additives to the electrolytes to study their effects. The structural and electrical properties of the electroplated copper foil were evaluated. When each additive was added individually, the potential was increased. Specifically, the potential of the group with 30 ppm added collagen was about 27% higher rather than the non-additive group. When Cl- ions and MPSA(3-mercapto-1-propane sulfonic acid) were added to the electrolytes without collagen and JGB, the surface roughness(Rz) increased by about 136% to 2.24 μm compared to the non-additive group. This was the highest value among all groups. However, a uniform layer with a surface roughness value below 0.3 μm was formed when less than 30 ppm and 10 ppm collagen and JGB were added, respectively, to the electrolyte. The direction of crystal growth with the JGB additives tended to go forward to the (220) direction, and the crystal size was reduced by 10~27% compared to the non-additive group. The addition of Collagen is necessary to reduce the difference in resistivity of the shiny layer and right matte layer. JGB additives were required to reduce the deviation in grain size. The results confirmed that the accelerators, inhibitors and leveler need to be properly added to form a copper plating layer with low surface roughness and to reduce differences in crystal texture of the shiny layer and right matte layer. Copper foil can be safely and uniformly deposited from electrolytes with JGB below 10 ppm and collagen below 30 ppm. (Received December 4, 2020; Accepted December 14, 2020)

keyword : organic additive, surface roughness, electrical property, electroplating, copper foil

Intensive research has been conducted to develop flexible piezoelectric pressure sensors, since selfpowering devices are advantageous for wearable electronic applications. Recently, two types of piezoelectric devices, ceramic-PDMS composite film and PVDF nanofiber mats, have drawn attention in the research community. Piezoelectric ceramics such as BaTiO₃ (BTO) and PZT exhibit outstanding piezoelectric coefficients, while PDMS provides flexibility. In contrast, a PVDF nanofiber mat simultaneously exhibits piezoelectricity and flexibility. In the present study, a comparative analysis of BTO-PDMS composite film and a PVDF nanofiber mat for application to flexible pressure sensors was carried out. First, step-wise electric poling was conducted on these two types of pressure sensors, after which the open-circuit voltage (V_oc) was measured under compressive force. The 1.8 V peak-to-peak V_oc was measured in a BTO-PDMS composite with a 30 wt.% BTO content that was poled by 10 kV/mm electric field for 15 min. This peak-to-peak V_oc of the BTOPDMS composite increased further to ~ 4 V when it was poled for 24 hr. Unlike the BTO-PDMS composite films, the maximum V_oc (1.1 V) was measured in a PVDF nanofiber mat that did not undergo subsequent electric poling. A BTO-PDMS composite film and a PVDF nanofiber mat were fabricated, and the compressive force and strain-rate dependencies of V_oc and the short-circuit current (I_sc) were investigated. Overall, the V_oc and I_sc of the BTO-PDMS composite film exceeded those of the PVDF nanofiber mat in a force range of 1 - 25 N and frequency range of 0.5 - 2.0 Hz. However, the V_oc and I_sc signals from the PVDF nanofiber mat were more stable than those from the BTO-PDMS composite film due to the longer lifetime of the signals. (Received March 19, 2021; Accepted April 9, 2021)

keyword : piezoelectric, barium titanate, PVDF, PDMS, pressure sensor, nanofiber

Permingeatites Cu₃Sb_1-yGe_ySe₄ (0 ≤ y ≤ 0.14) were synthesized by mechanical alloying and hot pressing. The charge-transport parameters (Hall coefficient, carrier concentration, mobility, and Lorenz number) and thermoelectric properties (electrical conductivity, Seebeck coefficient, power factor, thermal conductivity, and figure of merit) were examined with respect to the Ge doping level. A single permingeatite phase with a tetragonal structure was obtained without subsequent heat treatment, but a small amount of the secondary phase Cu₂GeSe₃ was found for the specimens with y ≥ 0.08. All hot-pressed compacts exhibited a relative density of 97.5%-98.3%. The lattice constants of the a-axis and c-axis were decreased by the substitution of Ge at the Sb sites. As the Ge content increased, the carrier concentration increased from 5.2 × 10¹⁸ to 1.1 × 10²⁰ cm^-3, but the mobility decreased from 92 to 25 cm²·V^-1·s^-1. The Lorenz number of the undoped Cu₃SbSe₄ implied a non-degenerate semiconductor behavior, ranging from (1.57-1.56) × 10^-8 V²·K^-2 at 323-623 K. The thermoelectric figure of merit was 0.39 at 623 K, resulting from a power factor of 0.49 mW·m^-1·K^-2 and a thermal conductivity of 0.76 W·m^-1·K^-1. However, the Lorenz numbers of the Gedoped specimens indicated degenerate semiconductor characteristics, increasing to (1.63-1.94) × 10^-8 V²·K^-2 at 323-623 K. The highest thermoelectric figure of merit of 0.65 was at 623 K for Cu₃Sb_0.86Ge_0.14Se₄, resulting from the significantly improved power factor of 0.93 mW·m^-1·K^-2 and the thermal conductivity of 0.89W·m^-1·K^-1. As a result, the thermoelectric properties were remarkably enhanced by doping Ge into the Sb sites of the permingeatite. (Received March 23 2021; Accepted April 1, 2021)

keyword : thermoelectric, charge transport, permingeatite, mechanical alloying, hot pressing

We attempted to classify the microstructural images of spheroidal graphite cast iron and grey cast iron using a convolutional neural network (CNN) model. The CNN comprised four combinations of convolution and pooling layers followed by two fully-connected layers. Numerous microscopic images of each cast iron were prepared to train and verify the CNN model. After training the network, the accuracy of the model was validated using an additional set of microstructural images which were not included in the training data. The CNN model exhibited an accuracy of approximately 98% for classification of the cast irons. Typically, CNN does not provide bases for image classification to human users. We tried to visualize the images between the network layers, to find out how the CNN identified the microstructures of the cast irons. The microstructural images shrank as they passed the convolutional and pooling layers. During the processes, it seems that the CNN detected morphological characteristics including the edges and contrast of the graphite phases. The mid-layer images still retained their characteristic microstructural features, although the image sizes were shrunk. The final images just before connecting the fully-connected layers seemed to have minimalized the information about the microstructural features to classify the two kinds of cast irons. Matrix phases such as ferrite and pearlite did not show prominent effects on the classification accuracy. (Received February 23 2021; Accepted March 30, 2021)

keyword : machine learning, convolutional neural network, image recognition, microstructure, cast iron