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The primary boundary of the CANDU reactor consists of Zr-2.5%Nb alloy pressure tubes and SA106 alloy feeder pipes. The pressure tubes are exposed to fast neutron (E>1Mev) irradiation during reactor operation, and experience dimensional changes. The actual dimensional change in the pressure tube during in-service operation is greater than the design estimate. It is not fully understood what is responsible for the dimensional changes in the pressure tubes. In this study, to accelerate the aging effect on the pressure tube material, aging was performed at 300-400℃ for up to 20,000 hours and neutron diffraction was used to track variation in lattice spacing, thereby identifying the reason for the dimensional change. The analysis result showed that α-Zr, the main phase of Zr-2.5%Nb, had little dimensional change at 350℃ or lower. On the other hand, the aging treatment at 400℃ resulted in anisotropic expansion in the α-Zr of the HCP crystal, of 0.02% and 0.08% in the (1010) and (0002) directions, respectively. It was confirmed that above 350℃ the β-phase in the Zr-2.5%Nb alloy was decomposed to precipitate β-Nb . The driving force of the lattice expansion may be due to the diffusing out tendency of the super saturated Nb in α-Zr and due to the residual cold working effect applied during the manufacturing process. The enhancing effects of fast neutron irradiation on lattice diffusion are discussed in detail. (Received 6 October, 2021; Accepted 28 December, 2021)

keyword : Zr-2.5%Nb, aging, lattice variation, neutron diffraction, entropy

Recently, lightweight vehicle bodies are in increasing demand to satisfy exhaust gas and environmental regulations around the world. In particular, aluminum alloys are widely used to manufacture lightweight parts, because of their excellent properties including corrosion resistance and mechanical properties. After the forming process, the welding process is important for manufacturing aluminum alloy parts. Resistance welding of aluminum alloys has several problems, due to internal weld defects such as cracks, shrinkage cavity, or porosity, which can result from the Al₂O₃ oxide film on the surface of the aluminum alloy. This study investigated electrode-force type controls to improve the weldability of the aluminum alloy. It was found that a high electrode-force on squeeze time can collapse the Al₂O₃ oxide film on the surface. It can reduce defects in the nugget by about 42%, by reducing heat input energy, compared to the continuous electrode-force 4 kN (reference value). Also, with high electrode-force during the hold time, defects were reduced by about 80%, by increasing the cooling rate. The weld quality has a great influence on the electrode-force type control, and internal defects in the nugget are greatly affected by the electrode-force on hold time. (Received 22 November, 2021; Accepted 10 January 2022)

keyword : resistance spot welding (RSW), aluminum alloy, defects in nugget, microstructure, electrode-force control

This study investigated the effect of the electromagnetic stirring (EMS) process on the microstructure and mechanical properties of hyper-eutectic Al-15 wt.%Si alloy. The tensile and high-cycle fatigue properties of the EMS Al-15wt.%Si alloy were examined and compared with conventional direct chill (DC) cast alloy. The initial microstructure of the as-cast DC alloy showed a coarse primary Si of 70 μm, in contrast to the size (40 μm) of the as-cast EMS alloy. The DC extruded alloy exhibited a massive primary Si size of 40~55 μm. In contrast, the EMS extruded alloy had a spherical primary Si with 30~40 μm sizes. The average grain sizes were measured to be 8.8 μm (EMS extruded alloy) and 11.4 μm (DC extruded alloy), respectively. The EMS process was confirmed to contribute to the refinement of the grain size and the primary Si size of the hyper-eutectic Al-Si alloy. The yield strengths at room temperature were measured to be 388.0 MPa (EMS extruded alloy) and 375.0 MPa (DC extruded alloy), and the tensile strengths were 426.0 MPa (EMS extruded alloy) and 412.4MPa (DC extruded alloy), respectively. The elongations of both alloys were similar at room temperature. The fatigue limit of the EMS extruded alloy (130 MPa) was higher than that of the DC extruded alloy (120 MPa). The EMS extruded alloy exhibited superior fatigue resistance compared to the DC extruded alloy, regardless of the cyclic stress condition. This study also discussed the potential value of the EMS process as a method for improving the properties of hyper-eutectic Al-Si alloy, as well as the deformation and fracture mechanisms of the EMS Al-15wt.%Si alloy. (Received 21 August, 2021; Accepted 11 January 2022)

keyword : hyper-eutectic Al-Si alloy, EMS (electromagnetic stirring), primary Si refinement, tensile, high-cycle fatigue

Kerfless wafering is a beneficial technique that enhances the cost effectiveness of crystalline silicon (c-Si) solar cells, preventing silicon (Si) waste during diamond sawing. This study compared the advantages of three stress layers for kerfless wafering: aluminum (Al) paste, epoxy-, and electroplated nickel (Ni). These materials demonstrated the ability to exfoliate Si foil, with the electroplated Ni layer having the best result. To control crack propagation, a notch was created by laser scribing on the top and the side of the Si wafer with the Al paste and epoxy layers. The Al paste layer with silicon nitride (SiNx) exfoliated the 1 cm × 1 cm Si foil, and the epoxy layer exfoliated the 5 cm x 5 cm Si foil. However, the Si foils were fragmented after etching of the Al paste and epoxy layers. The thickness of the Si foil increased as the Al paste layer increased. The Al paste layer was etched completely but the epoxy was not removed completely. The Ni layer was electroplated on a titanium/nickel (Ti/Ni) seed layer. A 10 cm × 10 cm Si foil with a thickness of approximately 40 μm was exfoliated using an Ni electroplating method. This Si foil’s effective lifetime increased to 8.17us after Al₂O₃ passivation and annealing. (Received 27 August, 2021; Accepted 24 January 2022)

keyword : kerfless, Ni electroplating, epoxy, Al metal paste, exfoliation, stress-induced layer

Ge-doped tetrahedrites Cu₁₂Sb_4-yGe_yS₁₃ (y = 0.1-0.4) were prepared using mechanical alloying and hot pressing. An X-ray diffraction analysis after mechanical alloying showed a single tetrahedrite phase without secondary phases. The tetrahedrite phase was stable after hot pressing at 723 K under 70 MPa. As the Ge content increased, the lattice constant decreased from 1.0343 to 1.0334 nm, which confirms that Ge was successfully substituted at the Sb sites. Ge-doped tetrahedrites exhibited p-type semiconductor characteristics. When Ge⁴⁺ was substituted for Sb³⁺, additional electrons were generated. Thus, the electrical conductivity decreased and the Seebeck coefficient increased due to the decrease in carrier (hole) concentration. For the Ge-doped specimen with y = 0.1, a maximum power factor of 0.87 mWm^-1K^-2 was obtained at 723 K. As the Ge content increased, the power factor decreased. However, as the Ge content increased, the electronic and lattice thermal conductivities decreased. Therefore, the substitutions of Ge at the Sb sites intensified both ionization impurity scattering and phonon scattering, resulting in very low thermal conductivities of 0.4-1.0 Wm^-1K^-1 at 323-723 K for y = 0.1-0.4. As a result, the highest dimensionless figure of merit ZT = 0.74 was obtained at 723 K for Cu₁₂Sb_3.8Ge_0.2S₁₃. (Received 30 November, 2021; Accepted 12 January 2022)

keyword : thermoelectric, tetrahedrite, mechanical alloying, hot pressing, doping

Cu-based ternary chalcogenides composed of earth-abundant and nontoxic elements are considered promising thermoelectric materials, although there are few reports on them. In our previous studies, optimal mechanical alloying (MA) and hot pressing (HP) were used to synthesize Cu₃SbS₄ (famatinite) and Cu₃SbSe₄ (permingeatite). In this study, solid solutions of Cu₃SbS_4-ySe_y (y = 0-4) were prepared using MA and HP. Xray diffraction and Rietveld refinement confirmed the formation of Cu₃SbS₄-Cu₃SbSe₄ solid solutions. The variation in lattice constants with Se content followed Vegard’s law. As the Se content increased, the carrier concentration and mobility increased. All the specimens exhibited p-type conduction, showing positive Hall and Seebeck coefficients. As the Se content increased, the Seebeck coefficient decreased due to an increase in carrier concentration, while the power factor of the solid solutions was not enhanced compared with that of the end compounds. Cu₃SbS_2.4Se_1.6 exhibited the lowest lattice thermal conductivities of 0.84 Wm^-1K^-1 at 323 K and 0.56 Wm^-1K^-1 at 623 K because of phonon scattering (alloying scattering) caused by the formation of the solid solutions. In addition, the thermal conductivity was less than 0.8 Wm^-1K^-1 at 623 K. This was attributed to the phonon scattering (boundary scattering) caused by several grain boundaries introduced during MA. As the Se content increased, the dimensionless figure of merit gradually increased from 0.14 to 0.39 at 623 K. (Received 23 December, 2021; Accepted 19 January 2022)

keyword : thermoelectric, famatinite, permingeatite, solid solution, mechanical alloying

Superplastic forming (SPF) is a promising approach used for manufacturing parts with complex geometries, especially in the automotive, aerospace, and marine industries. However, the wider use of this method is limited by issues of low forming rate, high-temperature requirement, non-uniform thickness distribution, and expensive base materials. The two-stage hybrid forming (HF) method, in which hot-punch forming is executed before the SPF, was introduced to overcome these limitations. In this study, a conventional non-superplastic grade 5052 aluminum (AA5052) alloy with an average grain size of 70 μm was used to evaluate the applicability of two-stage HF for manufacturing parts with complex geometries from coarse grain alloys. Before implementing the two-stage HF, the optimal experimental conditions for hot drawing and hot blowing were first determined. The optimum HF conditions were identified, as follows: a hot-punching temperature of 400℃, punch depth of 35 mm, punching speed of 150 mm/min, blow forming temperature of 500℃, and gas pressure of 2.5 MPa. The HF results were also verified using the finite element method. The finite element analyses results for thickness distribution and optimal process condition were compared with the experimental results for one-stage and two-stage forming, and showed acceptable similarity. (Received 30 November, 2021; Accepted 18 January 2022)

keyword : AA5052, punching, superplasticity, compression test, two-stage hybrid forming

A comprehensive analysis of the corrosion characteristics of excavated bronze artifacts was conducted according to their alloy ratio and microstructure. Corroded by various factors, which resulted in different corrosion characteristics. Among these, corrosion may occur in the excavated bronze due to the difference in the alloy ratio, which is an internal factor. In this study, we tried to confirm the corrosion characteristics of the excavated bronze artifacts excavated from Songguk-ri site in Buyeo according to their alloy ratio and microstructure through a comprehensive analysis. The corrosion characteristics of the excavated bronze artifacts differed depending on the alloy ratio and microstructure, whereas bronze samples subjected to the corrosion tests were the same regardless of the alloy ratio and microstructure. Through this, the corrosion characteristics of excavated bronze artifacts can be determined based on the additional effects on pH, corrosion factors, and time in the burial environment which can affect metal ion movement, in addition to alloy ratio and microstructure. In the future, if a burial environment similar to the actual remains and a long-term corrosion test is conducted, it will be possible to confirm the corrosion mechanism as well as the corrosion characteristics of the excavated bronze artifacts. (Received 12 November, 2021; Accepted 17 January 2022)

keyword : goryeo dynasty, bronze artifacts, alloy ratio, manufacturing technique, corrosion product