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Cast A356(Al-Si-Mg) alloys are widely used in automotive and general applications because of their mechanical properties and castability. Al-Si-Mg-(Cu) alloys typically lose their strength above 170℃ due to coarsening of precipitates, which limits their application to components. To maintain their strength at elevated temperature, Al-Si-Mg-(Cu) alloys are modified by adding transitional metals. Several studies have been carried out to evaluate the effect of Zr addition on the high temperature mechanical properties of cast Al-Si alloys because Zr can form thermally stable phases such as Al₃Zr. Despite the relative studies on the influence of Cu and Zr on the mechanical properties of cast Al-Si-Mg-(Cu) alloys, investigations of the effect of Zr on the phase transformations and the mechanical properties during heat treatment remains limited. In this study, the effects of added Cu and Zr on the phase transformations and the mechanical performance during heat treatment of A356 cast alloy were investigated. Needle-like and block-like (Al,Si)₃(Ti,Zr) dispersoids formed as some Si and Ti replaced Al and Zr in Al₃Zr crystal structures were generally observed. Furthermore, with increasing solution treatment time, the size of Zr dispersoids was reduced, and smaller Zr particles were precipitated at the same time, which caused a decrease in the area fraction of the Zr dispersoids. In addition, the metastable L1₂ structures of Zr dispersoids in Al-Si-Mg-Cu-Zr alloys were transformed into stable D0₂₃ during solution heat treatment as the Cu addition accelerated the transformation. Tensile and low-cycle fatigue (LCF) tests were performed to reveal the effects of (Al,Si)₃(Ti,Zr) dispersoids on mechanical properties. As a result, elongation at elevated temperature was highly increased, while maintaining strength, according to the increase in solution heat treatment time, which improved low-cycle fatigue properties. (Received 4 January, 2023; Accepted 15 Fabruary, 2023)

keyword : Al-Si-Mg-(Cu)-(Zr) alloy, Heat treatment, Microstructure, Tensile properties, Low-cycle fatigue

TiB₂ is considered candidate materials for ultra-high temperature ceramics and cutting tools because of its a high thermal conductivity, a low coefficient of thermal expansion, a high hardness and high melting temperature. Despite these attractive properties, TiB2 applications are limited because it has a low fracture toughness below the brittle-ductile transition temperature. To improve on its mechanical properties, the approach universally utilized has been to add secondary materials to form a composite and to fabricate an ultra - fine material. A dense ultra - fine TiB₂- ZrO₂ composite was rapidly sintered using pulsed high current activated heating (PHCAH) methods within 3 min in one step from the mechanically synthesized the powders of TiB₂ and ZrO₂. Consolidation was reached using an effective combination of mechanical pressure and the pulsed high current. A highly dense TiB₂-ZrO₂ material with relative density of 97.2% was made by the simultaneous application of 75 MPa pressure and a pulsed 2500 A current. The grain sizes of TiB₂ and ZrO₂ in the composite were 135 nm and 84 nm, respectively. The fracture toughness and hardness of the TiB₂- ZrO₂ composite were 11.2 MPa.m^1/2 and 957 kg/mm², respectively. The fracture toughness of the TiB₂-ZrO₂ composite was three times higher than that of monolithic TiB₂. (Received 5 January, 2023; Accepted 31 January, 2023)

keyword : composite, TiB_{2-ZrO2, mechanical properties, synthesis, nanomaterials}

The effect of TiO₂ additive concentration and sintering temperatures on the densification of alumina (Al₂O₃) ceramics were investigated. Densified alumina specimens were prepared by uniaxial pressing at a pressure of 60 MPa followed by isothermal heating at either 1400, 1500, or 1600 ℃ for 2 h. The relative density, microstructure, crystallinity, and hardness were investigated, and the correlation between properties and structure was discussed in relation to TiO₂ addition and sintering temperature. The densities of the 0.05wt%-TiO₂ specimens sintered at 1500 and 1600 ℃ had higher values than the pure alumina specimens. Adding more than 5.0wt% TiO₂ lowered the relative density. The maximum achieved density was 99.7% at 0.05wt% TiO₂ addition. Shrinkage and hardness analyses confirmed that the sintering temperature for the Al₂O₃ insulator could be lowered by adding TiO₂. The optimal concentration to obtain a dense alumina with high hardness for insulator use was determined to be in the range of 0.05 to 1.0 wt% of TiO₂. These results provide fundamental composition and process optimization data for the development of high-dielectric alumina insulators. (Received 24 November, 2022; Accepted 1 Fabruary, 2023)

keyword : Al_{2O3 powder, densification, Microstructure, Insulator}

AA 2026 is an improved version of AA 2024, an alloy with added Zr to reduce Fe and Si content and inhibit recrystallization during hot working. Al 2026 alloy has high strength and high damage resistance, so it is widely used in aircraft parts. In this study, in order to investigate the hot workability of AA 2026 and to optimize the hot forming parameters, hot compression tests were conducted in the temperature range of 300 to 450 ℃, at a strain rate of 0.01 to 10 and in the 50% strain section. The true stress-true strain curve showed a dynamic softening phenomenon while the stress increased rapidly at a small strain and then remained steady. In order to evaluate its high temperature processability, the constitutive equations for flow stress, temperature, and strain were quantified based on the Arrhenius equation, and a process strain map was prepared. The peak stress accuracy of the constitutive equation was about 98.2%, which was consistent with the experimental data of AA 2026 under strain rate and temperature conditions. In addition, scanning electron microscopy (SEM) and backscattered electron diffraction pattern analyzer (EBSD) analyses were conducted to confirm the mechanism of the dynamic softening phenomenon. The CDRX phenomenon was confirmed under the high strain condition in the low temperature region and the DDRX phenomenon in the low strain condition in the high temperature region. (Received 18 November, 2022; Accepted 16 February, 2023)

keyword : hot compression test, flow behavior, constitutive equation, processing map, recrystallization

The amount and kind of additives in electrolytes play a very important role on the electroplating of Cu foil, which is used as a cathode materials for secondary batteries. As the use of Cu foil increased, various studies on the electroplated Cu foil are urgently needed. We studied surface characteristics and changes in structural properties due to the addition of inhibitors under electroplating conditions with high current density. The surface layer was observed, to analyze the effect of the amount of potassium carbonate as an inhibitor. When the potassium carbonate was added at 5~25 ppm without any other additives, the initial plating voltage was observed to decrease. To uniformly distribute crystals without agglomeration at the initial stage of electroplating, it is reasonable to add potassium carbonate within the range of 20~25 ppm. There was no significant difference in surface roughness and specific resistance due to the amount of potassium carbonate, when the additives were added complexly. There was an increase in the relative strength of the preferred growth orientation of (220) in a group with 20 ppm potassium carbonate, compared to the group with 10 ppm potassium carbonate. The grain size decreased with the amount of potassium carbonate added with other additives. However, there was no significant difference in tensile strength and elongation (p-value: 0.000). To ensure uniform properties with a combination of 4 kinds of additives it is reasonable to add potassium carbonate in the range of 10~20 ppm. (Received 14 November, 2022; Accepted 31 January, 2023)

keyword : potassium carbonate, copper foil, electroplating, surface roughness, additive

Famatinite (Cu₃SbS₄) is a promising p-type thermoelectric material because of its low lattice thermal conductivity and high Seebeck coefficient. In this study, famatinite powders, double-doped with Sn (a B^IV group element) and In (a B^III group element) to give Cu₃Sb_1-x-ySn_xIn_yS₄ (0.02 ≤ x ≤ 0.12 and 0.06 ≤ y ≤ 0.10), were synthesized by mechanical alloying and then consolidated by hot pressing. Phase analysis and microstructure observations were conducted over a range of doping levels, and the charge transport parameters and thermoelectric properties were evaluated. Except for the specimen with y = 0.10, in which the secondary phase CuInS₂ was found, all the specimens exhibited a tetragonal famatinite phase without secondary phases. The Sn/In double doping increased the unit cell a-axis to 0.5387-0.5389 nm and changed the c-axis to 1.0744-1.0752 nm. As the temperature increased, the electrical conductivity decreased while the Seebeck coefficient increased, which indicates that the Sn/In double-doped famatinites have degenerate semiconductor characteristics. With increasing Sn and In content, the carrier concentration increased, so that the electrical conductivity increased and the Seebeck coefficient decreased. Cu₃Sb_0.80Sn_0.12In_0.08S₄ exhibited the highest power factor, 0.87 mW m^-1 K^-2 at 623 K, with greatly increased thermal conductivity. Cu₃Sb_0.86Sn_0.08In_0.06S₄ showed the highest value for the dimensionless figure of merit, ZT = 0.53 at 623 K, with a power factor of 0.78 mW m^-1 K^-2 and thermal conductivity of 0.90 W m^-1 K^-1. (Received 14 December, 2022; Accepted 7 Fabruary, 2023)

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

Bytizite (Cu₃SbSe₃) has attracted interest as a promising thermoelectric material because of its ultralow thermal conductivity; however, there are few experimental studies. This study investigated the optimal processing conditions for the synthesis of Cu₃SbSe₃ using mechanical alloying (MA) and hot pressing (HP). The MA powder exhibited an orthorhombic Cu₃SbSe₃ phase, which remained even after HP. However, secondary phases of permingeatite (Cu₃SbSe₄) and berzelianite (Cu_1.78Se) were also identified in the X-ray diffraction patterns. Thermal analysis revealed that the MA powder and HP compacts exhibited a large endothermic peak near 727 K, which corresponds to the melting point of Cu₃SbSe₃. Dense compacts with a relative density higher than 99% were obtained at HP temperatures above 573 K. Microstructural and elemental analyses confirmed the presence of the secondary phase Cu₃SbSe₄ in the matrix of Cu₃SbSe₃. However, the Cu_1.78Se phase could not be observed. All specimens exhibited an electrical conductivity of (0.66- 1.06) × 10³ Sm^-1, a Seebeck coefficient of 324-376 μVK^-1, and a power factor of 0.09-0.11 mWm^-1K^-2 at 623 K. The thermal conductivity was lower than 0.7 Wm^-1K^-1 in the measured temperature range, mainly due to the phonon scattering caused by the lone-pair electrons of Sb. A dip in thermal conductivity was observed at 423 K, which was possibly caused by the order-disorder transition of bytizite. The dimensionless figure of merit ZT increased with increasing temperature, and the maximum ZT was 0.16 at 623 K. (Received 6 January, 2023; Accepted 16 Fabruary, 2023)

keyword : thermoelectric, bytizite, mechanical alloying, hot pressing

Compound materials have two or more unmixable parts that retain a shared surface with one another for engineering purposes. Such compound materials, like the Janus or core-shell configurations, create opportunities for relevant applications because they offer diverse combinations of complex impinging materials and complex surfaces. However, previous studies have only assumed spherical configurations, or focused on the bouncing dynamics, without considering the effect of the material size. The current work numerically studies the dynamic characteristics of Janus materials with ellipsoidal shapes for various impact speeds and viscosity ratios, to analyze the effect of the size and shape of the material on bouncing and separation behavior. The threshold Weber numbers at which separation starts after the collision are investigated as a function of the droplet size, ellipticity, and viscosity ratio. In addition, a regime map of the separation efficiency of the Janus droplets is established under various viscosity ratios and Weber numbers to investigate the effects of droplet shape on the asymmetric bouncing and separation behavior. It is found that the separation efficiency and mechanism of two prolate spheroids are different from each other at the same ellipticity. This study will provide an efficient strategy to control the bouncing of compound materials in applications, such as drug delivery, liquid purification, and bio- or multi-material printing. (Received 29 November, 2022; Accepted 25 January, 2023)

keyword : superhydrophobic surface, compound material, Janus material, interfacial dynamics

We constructed a convolutional neural network to estimate average grain size from microstructure images. In the previous study from our research group, the network was trained using GB-type images in which the grain matrix and grain boundary were represented in white and black, respectively. The model well estimated the same GB-type images, but did not properly predict CL-type images where grain boundaries were defined by color contrast between grains. In the present study, the convolutional neural network was trained using CL-type microstructure images, and evaluated the average grain size, for comparison with the previous GB-type model. The relationship between the microstructure image and the average grain size was determined using regression rather than classification. Then, the results were compared with the previous ones. Finally, the proposed approach was used for actual microstructural image analysis. Mid-layer images were extracted to examine how the network recognizes the characteristics of microstructures, such as grain color and grain boundary. Like the previous GB-type model, the present CL-type model seems to estimate the average grain size from the curvature of the grain boundary through edge detection of the grain boundaries. However, the GB- and CL-type models only properly predicted the grain size from the same kind of images as the training data, because the definitions of the grain boundaries of the two models were different. (Received 28 December, 2022; Accepted 7 Fabruary, 2023)

keyword : machine learning, convolutional neural network, image recognition, microstructure, grain size