Wide-bandgap power semiconductors have garnered attention for their potential use in electronic power control. For joining materials of these power semiconductors, sintering of metal nanoparticles as the bonding material has been focused. Silver nanoparticles have been extensively researched so far as these bonding materials; however, studies have shifted the focus to copper nanoparticles from the perspectives of cost and ion migration resistance. Nevertheless, copper nanoparticles are known for their low oxidation resistance, leading to a decline in sintering performance. Therefore, research has been conducted to tailor copper nanoparticle pastes to prevent their oxidation. Additionally, various techniques during the sintering process have been considered to enhance the sinter bonding. This paper introduces the paste design attempts and examples of the sintering process by using copper nanoparticle paste bonding materials.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 88 (2024) 270–280.

This work aims to investigate the effect of annealing temperature on the microstructure, the mechanical and corrosion properties of TiVNbZr_0.5Hf_0.5 light-weight refractory high entropy alloys. The designed alloys were first produced by the vacuum arc melting route and then annealed at different temperatures (400°C, 500°C, and 600°C) for 24 hours under high-purity Ar gas flow. The results showed that the as-cast alloy exhibited a single disorder BCC structure. Meanwhile, besides the main disorder BCC phase, the C15 Laves phase has been detected for the alloys after annealing at different temperatures, confirmed by X-ray diffraction patterns and selected area diffraction patterns. The as-cast and annealed samples at 400°C show their excellent compressive ductility at room temperature. Annealing at 500°C and 600°C reduced compressive ductility due to the more introduction of C15 Laves phase precipitated in the interdendritic areas. Generally, the Vickers hardness and strength of RHEAs increased with annealing temperature. Additionally, outstanding corrosion resistance of the RHEAs was obtained in acidic media with even better performance by a simple annealing process at 400°C, 500°C, and 600°C.

Hot-extruded Cu-Zn-Si alloy bars with various chemical compositions were cold-caliber rolled down to 91.7% reduction at maximum. Heterogeneous nano-structure, in which coarse initial grains were subdivided mainly by mechanical twins and shear bands, was gradually developed with increasing reduction. The as-rolled bars exhibited an extraordinarily high tensile strength of 988 MPa at best with a reasonable ductility of 5.9%. The tensile strength was further slightly raised to 995 MPa by low-temperature annealing. 3D atom-probe analyses revealed dense Si segregation at twin boundaries and increase in the amount of segregation after annealing. Multi-scale simulation indicated that strain field formed by the Si segregation at twin boundary obstructed dislocation glide to develop geometrically necessary dislocations, which resulted in higher yield stress and work-hardening rate to cause higher tensile strength. It was found that strengthening mechanisms of heterogeneous nano-structured Cu-Zn-Si alloy bars are, therefore, complicatedly combined ones of grain refinement, work hardening, solid-solution hardening and grain-boundary segregation.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 63 (2024) 56–64.

Rare earth borides (REB_x) have been the subject of many theoretical and experimental studies because of their excellent electron emission and magnetic properties. However, it is still challenging to obtain relatively pure rare earth borides with low cost by simple synthetic methods. Herein, an economical method for synthesizing RE borides was proposed, on the basis of reaction between rare earth oxides (RE_xO_y), boron carbide (B₄C) and aluminum (Al). 15 rare earth elements were investigated to identify their stable phases in the system of RE_xO_y-B₄C-Al, respectively. After high-temperature reaction at 1773 K and following alkaline leaching at 363 K, single phase YB₄, LaB₆, CeB₆, PrB₆, NdB₆, SmB₆, EuB₆, GdB₄, TbB₄, DyB₄, HoB₄, ErB₄, TmB₄, YbB₆, LuB₄ were successfully synthesized. Meanwhile, the harmless and resourceful treatment of the leaching solution was done to prepare nano γ-Al₂O₃ and NaCl. Thus, the zero discharge of the whole process was achieved.

Al-Si-Cu-Mg-(Ni) alloys are used for engine pistons or compressor scrolls due to their excellent heat and wear resistance. Although these components are typically produced using mold-casting or die-casting techniques, forging techniques have recently attracted attention because they allow for a lightweight design. However, wrought Al-Si-Cu-Mg-(Ni) alloys have rarely been investigated, and the effect of nickel on precipitate microstructures and strength remains unclear. Here, we investigated the precipitate microstructure and strength at 473 K of wrought Al-Si-Cu-Mg alloys with different nickel concentrations. The 0.2% proof stress and tensile strength decreased with increasing nickel concentration. The TEM observations showed that the decline was attributed to the decrease in the θ′ phase. The increase in the Al₇Cu₄Ni and Al-Fe-Cu-Si-Ni phases suggested that the nickel decreased the θ′ phase by consuming the copper in the matrix. In addition, the HRTEM observation revealed the difference in the orientation relationships of the Q phase between with and without nickel alloys. The Q phase observed in the 2%Ni alloy had a smaller lattice misfit with the matrix than that observed in the 0%Ni alloy, indicating excellent thermal stability.

Triethanolamine carboxylates, particularly triethanolamine decanedioate (TEA·C10), are widely employed as corrosion inhibitors for steel in aqueous solutions used in metalworking applications. This compound inhibits corrosion reactions at both the anode and cathode through physical and chemical adsorption onto the steel surface, forming a protective corrosion-resistant film. It is also a non-volatile organic compound with relatively low toxicity and is cost-effective. The corrosion inhibition mechanism and anticorrosion efficiency of TEA·C10 were investigated by analyzing the surface corrosion behavior of Q215 mild steel in 0.1 M NaCl solution compared with triethanolamine (TEA). The IP287 test results indicate that rust area of TEA·C10 on filter paper are significantly reduced compared to TEA. The results from potentiodynamic polarization testing show that TEA·C10 exhibits a lower corrosion rate compared to TEA, with inhibition efficiencies of 97.87% at a concentration of 2.4 M. Consistent with the EIS test results, it was also show that the presence of TEA·C10 reduces the double-layer capacitance during the corrosion process, leading to a more significant decrease in charge transfer resistance, resulting in a corrosion inhibition efficiency of up to 98.66% at a concentration of 2.4 M.

Understanding the leaching mechanism of chalcopyrite from natural resources is important for expanding Cu production via hydrometallurgical processes. This understanding could be effectively achieved by investigating the reaction phenomena using synthetic chalcopyrite with controlled compositions and morphologies. In this study, we prepared bulk samples of synthetic chalcopyrite and investigated the effects of thermal pretreatment on its oxidative dissolution behavior in acidic ferric sulfate solutions. Dense chalcopyrite pellets with a stoichiometric composition (Cu_0.25Fe_0.25S_0.50) were prepared by subjecting the synthetic chalcopyrite powders to pulse current pressure sintering followed by annealing. Thermal pretreatment at 580°C in a vacuum-sealed quartz ampoule resulted in the precipitation of a small amount of pyrite and a slight compositional change in the chalcopyrite phase. Leaching tests in 0.1 M Fe(SO₄)_1.5 − 10⁻⁴ M FeSO₄ − 0.18 M H₂SO₄ solution at 70°C revealed that thermal pretreatment increased the Cu extraction rate by approximately one order of magnitude and shifted the corrosion potential to a less noble value. Polarization measurements in 0.18 M H₂SO₄ solution at 70°C showed that the anodic dissolution rate significantly increased after thermal pretreatment. We also prepared a nonstoichiometric and pyrite-free chalcopyrite sample and confirmed its high dissolution rate in the acidic ferric sulfate solution. Therefore, we conclude that the enhanced dissolution of chalcopyrite observed after thermal pretreatment is caused not by its galvanic interaction with the precipitated pyrite phase but by the change in its reactivity owing to small compositional changes.

In community fitness venues, air quality is closely related to health status. Air filtration and monitoring of human respiratory signs are becoming increasingly important. Therefore, a polyvinyl alcohol-co-polyethylene is prepared by melt extrusion phase separation method, and its suspension is coated on a polypropylene melt blown non-woven fabric substrate to obtain a polyvinyl alcohol-co-polyethylene nanofiber membrane. Then, the silver-decorated carbon nanotube suspension is sprayed onto the nanofiber membrane to obtain a silver-decorated carbon nanotube nanofiber composite membrane. The results demonstrated that in the comparison of filtration efficiency, the filtration efficiencies of silver-decorated carbon nanotube nanofiber composite membrane-1, composite membrane-2, and composite membrane-4 reached 87.63%, 84.32%, and 84.01%, respectively. In the comparison of filtration efficiency cycle performance, with the increase of processing times, the filtration efficiency of the silver-decorated carbon nanotube air filter material mask stabilized at around 95%. In the resistance change rate curves under different humidity conditions, the silver-decorated carbon nanotubes increased the hydrophilicity of the nanofiber membrane, thereby reducing the resistance. Silver-decorated carbon nanotubes can respond promptly to changes in environmental humidity, effectively monitoring human movement and respiratory signs. Additionally, an appropriate amount of silver-decorated carbon nanotubes can enhance air filtration efficiency and reduce pressure drop. In community sports and fitness facilities, this method can significantly improve indoor environmental quality and enhance users’ health experience.

The excellent tensile strength and ductility of the CoCrFeMnNi high-entropy alloys (HEAs) at low temperatures have been recently reported. Regardless of the fabrication method, the existing CoCrFeMnNi alloys tend to exhibit lower hardness than conventional steel materials. To mitigate this issue, surface-modification treatments have been deployed as effective methods for improving the hardness of CoCeFrMnNi HEAs. Studies have demonstrated that the maximum thickness of the nitrided layer of the alloy was obtained by the treatment of its surface with a pure Ni screen at 873 K. In furtherance, this study was aimed at evaluating the properties of the nitrided layer by subjecting the pure-Ni-screen-sintered CoCrFeMnNi HEA to screen-assisted direct-current (DC) plasma nitriding (PN) treatments (S-DCPN) at varying gas pressures. The gas-atomized CoCrFeMnNi HEA powder was processed by ball-milling for 10 h to prepare a sintered body, which was subsequently subjected to S-DCPN treatments for 15 h at 873 K and varying gas pressures (200, 400, 600 Pa; gas compositions: 75% N₂ and 25% H₂). For the PN procedure, a pure Ni screen was installed as an auxiliary cathode to ensure uniform heating and enhanced nitrogen supply. Furthermore, the nitrided samples were characterized by X-ray diffraction analysis, cross-sectional microstructure observations, surface-morphology observations, glow discharge optical emission spectroscopy, and surface hardness, corrosion, and wear tests. A black modified layer was observed on the sample after nitriding, becoming darker as the gas pressure increased. Notably, the surface hardness of the HEA samples increased significantly after nitriding.

The hydraulic push-pull bending process is a method that is suitable to bend thin-walled tubes with a large bending radius. Wrinkling of tubes is difficult to predict due to the comprehensive influence of various factors such as geometric shape, process parameters, and boundary conditions. In this paper, artificial neural networks (ANNs) are used to predict wrinkling morphology in a tube during the bending process. Since neural network training involves many datasets, which are difficult to generate through experiments. A finite element (FE) model of hydraulic push-pull bending process is established, and the accuracy of simulation results is validated by experimental tests. The backpropagation neural network is established with five input parameters, including: relative bending radius (R/D), relative wall thickness (t/D), die clearance (c) and internal pressure (p). And three parameters of wrinkling number (N), maximum wrinkling height (Δh), and maximum wall thickness thickening rate (η) are used as the output. The results of FE simulations are used to train, test, and validate the ANN models. A simple and effective ANN model is established for the prediction of tube bending wrinkling of DC06 material. The results show that the hybrid method combining ANN and FE can predict wrinkling morphology produced by the hydraulic push-pull bending method with a high degree of accuracy.

We investigated the effects of temperature and sulfur adsorption on the surface tension of liquid copper using the oscillating droplet method in combination with electromagnetic levitation. At low temperatures, the surface tension of liquid copper decreased with increasing sulfur activity due to sulfur adsorption. However, at high temperatures, the surface tension approached that of pure state value of liquid copper regardless of the sulfur activity owing to the exothermic nature of sulfur adsorption. This led to an initial increase followed by a decrease in the surface tension with increasing temperature. We successfully described the surface tension of liquid copper as a function of temperature and sulfur activity using the Szyszkowski equation. Additionally, we estimated the enthalpy and entropy changes associated with sulfur adsorption on liquid copper.

This Paper was Originally Published in Japanese in Journal of Japan Institute of Copper 63 (2024) 184–188. The English of the abstract and the captions for tables and figures have been modified. Some minor errors in units, numerical values, and eq. (8) have been corrected.

The heat-affected zone (HAZ) formed on welding-repaired die-casting die features a hardness distribution that can potentially initiate thermal shock cracks. This study aimed to analyze the microstructure of a repair welding HAZ formed on Molybdenum-modified AISI H13 steel (Mo-modified H13 steel) used for the die-casting die in industry to identify the characteristics causing this hardness distribution. In a model weld-repaired die, the HAZ from 0–2.8 mm from the boundary of the weld metal was hardened by up to 140 HV compared with that of the original substrate, whereas the region from 2.8–5.0 mm was softened by up to 200 HV. Microstructural and crystallographic analyses reveal that the hardened region had a martensite structure, which corresponded to the origin of the hardening. Contrastingly, the softened region exhibited a typical annealed structure with granular Mo and V carbides. This structure results in the loss of the secondary hardening effect of the Mo-modified H13 steel substrate, which decreases hardness.

Springback of sheet metals consists of two components: springback caused by elastic recovery during unloading and time-dependent springback caused by viscoplastic phenomena over time after unloading. In this study, time-dependent springback in the L-bending of an advanced high-strength steel sheet was analytically investigated using unified constitutive equations based on kinematic hardening and overstress theory. The material parameters for the constitutive equations were determined by referring to the data from the stress relaxation tests in addition to the uniaxial tension and tension-compression tests. The calculated results for stress relaxation and time-dependent springback are compared with the corresponding experimental data. The calculated results captured the trend of the time-dependent springback over time reasonably well but overestimated the influence of the bend radius and underestimated the influence of bend holding. The obtained results suggest that unified constitutive equations based on kinematic hardening and overstress theory can qualitatively describe the time-dependent springback behavior through a stress relaxation-based approach.

The machining of SiC particle-reinforced aluminum alloy composites was performed using a lathe and PCD tools, and the effects of the particle size and the volume fraction of the SiC particles on the machinability was discussed. Composites were fabricated by squeeze casting. The range of the cutting force fluctuations during the cutting of the alloy became slightly narrower due to the reinforcement, while the average values increased due to the reinforcement. The resistance with larger volume fractions was higher. When comparing the composites with different SiC particle sizes within the same volume fraction, there was minimal variation in the resistance. The reinforcement decreased the surface roughness, achieving a finish closer to the theoretical surface roughness, indicating that the reinforcement reduces the formation of the built-up edge during the cutting. The roughness of the composites with 4 µm particles was lower than that of the composites with 25 µm particles. The flank wear of the tool after machining the composite with the 25 µm particles was more severe than that after machining the composites with the 4 µm particles. Larger particles in the composite would promote stress concentration leading to local fractures then the formation of partially agglomerated areas of the pulverized particles on the machined surface. Conversely, smaller particles would distribute the stress more evenly throughout the matrix, thus reducing the stress concentration. This would lead to a smoother surface and decrease in the tool wear.