A comprehensive research program on the creep of a die-cast Mg–5Al–2Ca (mass%) alloy, which has been conducted in the last two decades by our group, is overviewed. The obtained results are summarized into the following categories: microstructure and creep strength, creep parameters, dislocation analysis, and life assessment. The creep strength of the alloy is predominantly ascribed to the interconnected skeleton of C36–(Mg,Al)₂Ca phase, while the fine C15–Al₂Ca precipitates have limited effects on creep strength. The change in creep parameters, n and Q_c, results from the decreased creep strength driven by the divorce of the interconnected skeleton of C36 phase during creep. At stress levels below the yield stress, the dislocation climb in the primary α-Mg grains is inferred as the rate-controlling process for the alloy. Mostly 〈a〉 type dislocations are introduced within the primary α-Mg grains during die-casting, and the dislocation segments are mainly located on the basal plane. The basal segments of dislocations bow out and glide on the basal planes under stress, and the jogs follow the basal segments with the help of climb during creep. The minimum creep rate and creep rupture life follow the phenomenological Monkman–Grant relationship for the alloy. When the Larson–Miller constant is set at 20, the value of Larson–Miller parameter is uniquely described by the logarithm of the applied stress.

Heterostructured materials (HSMs) constitute heterogeneously distributed soft and hard zones with a mismatch in mechanical or physical properties of at least 100% between them. A synergistic effect resulting from the interactive coupling between the heterogeneous zones surpasses the properties predicted by the rule of mixtures. Therefore, the mechanical or physical properties of HSMs are not achievable by their homogeneous counterparts. HSM production commonly requires plastic deformation to refine the microstructure and subsequent partial recrystallization heat-treatments to obtain heterogeneous distributions of grain size, texture, or defect density. Other routes are by applying surface plastic deformation or by stacking layers with a high property mismatch between them. All of those routes can be achieved by severe plastic deformation (SPD) techniques. This overview focuses on describing the fundamentals of HSMs produced by SPD. A critical description of the physics of SPD and HSMs, as well as the factors influencing their microstructural evolution, perspectives, and outstanding issues, are included. A critical comparison of the strength–ductility relationship in HSMs produced by different SPD techniques is also included to guide upcoming research. This overview is intended to serve as a basis for understanding and designing future HSMs produced by SPD.

3D Additive Manufacturing is the heaven-sent child for Internet of Things (IoT) in the digital age, and is a process that can be used for customized design and production. In particular, metal Additive Manufacturing enables not only the fabrication of complex shapes but also the control of crystal orientation at the atomic level by locally melting and solidifying the metal material, thereby realizing high functionality of the product through simultaneous design of shape and materials properties. Therefore, it is expected to be applied in various social infrastructure fields including medical, energy-related, aerospace, and automotive, and also as a means of adding high value. In this review article, the new manufacturing concept that can be realized by metal Additive Manufacturing is introduced.

This Paper was Originally Published in Japanese in J. Japan Thermal Spray Soc. 58 (2021) 121–128. The title was changed due to the addition of “Review —”

Nanometric metal overlayer catalysts have been developed as novel catalyst structures with high three-way catalytic performance for practical applications. The metal overlayer is prepared via a dry process using pulsed arc plasma deposition, unlike wet coating processes for conventional powder catalysts containing nanoparticles. The key point to achieving high performance is the extremely high turnover frequencies for specific chemical reactions catalyzed by a large two-dimensional surface compared with metal nanoparticles. This overview study presents the preparation, structure, performance, reaction mechanism, and thermal stability of metal overlayer catalysts, focusing on rhodium (Rh) and catalytic conversion of nitric oxide (NO). Rh plays a pivotal role in the reduction of NO to N₂ in automotive three-way catalytic converters. The potential benefit of the overlayer structure is the minimum loading of precious metal, such as Rh, which is limited and expensive.

Alkaline water electrolysis (AWE) is a hydrogen manufacturing process that generates “green hydrogen” using electricity derived from renewable energies. Stainless steel (SS), specifically austenitic SS, has recently attracted attention as an anode material for the oxygen evolution reaction (OER) of the AWE. SS anode surfaces are generally activated by generating surface catalyst layers (SCL) for the OER through specific chemical pre-treatment, although the precise chemical compositions and microstructures of the SCL remain under debate. Furthermore, because fluctuations in the electrode potential derived from renewable energies cause remarkable elution of the constituent elements into the electrolyte, corrosion behaviors of the SS anodes should be clarified. This review introduces the recent progress of the SS anodes, particularly in the context of surface treatments to generate surface catalyst layers with high OER performances under simulated AWE conditions. In general, recent reports have clearly shown that surface-treated SS anodes are superior to the commonly employed Ni-based anodes for AWE applications.

High-entropy alloy (HEA) catalysts have attracted tremendous research interest owing to their versatile performances in various applications. However, research on HEA catalysts remains in the early stages of exploration, and the fabrication process, element selection, and application remain difficult. Herein, we summarize the current literature on HEA catalysts from the viewpoint of facile synthesis routes, tunable morphologies, attractive applications, and material discoveries related to machine learning and high-throughput experiments. Finally, perspectives and concepts are presented to design the desired multifunctionality HEA catalysts.

Zinc-Air Batteries (ZABs) are a promising solution for grid-scale storage. In this work, ZAB chemistry is reviewed and the role of catalysts at the cathode is explained. Transition metal oxides are an economical substitute for noble-metal based catalysts. However, their poor electrical conductivity requires additional efforts in catalyst activation. This is commonly done by reducing particle size and increasing electrical access which can be done simultaneously by hybridizing with carbon derivatives such as reduced graphene oxide.

This paper describes high-pressure synthetic studies on novel hydrides. High-pressure hydrogenation experiments are carried out using a cubic-type multi-anvil apparatus. In-situ synchrotron radiation X-ray diffraction measurements are effectively used to explore synthetic conditions, to investigate the reaction mechanisms, and to characterize the thermodynamic stabilities of the obtained hydrides. Theoretical calculations based on first-principles calculations enable us to predict the thermal stability and crystal structure of the target hydrides before the high-pressure experiments, which leads to the rapid discovery of the novel hydrides. Lithium-containing hydrides, YLiFeH₆, LiNiH₃, Li₄FeH₆, and Li₃AlFeH₈ are synthesized. Syntheses of aluminum-based alloys hydrides, Al₂CuH and Al₃FeH₄ under high-pressure are also described. These results demonstrate that the high-pressure technique is useful for discovering novel hydrides.

Metallic honeycomb catalysts are promising candidates for fuel cell and small-scale onsite hydrogen production applications. In this study, high-cell-density Ni honeycomb catalysts coated with a series of bimetallic surface layers were synthesized. Their catalytic performance for CH₄ steam reforming was investigated under a low steam-to-carbon ratio of 1.36 and a gas hourly space velocity of 6400 h⁻¹ in the temperature range of 400–700°C. The catalysts coated with Ni–Mg and Ni–Zr showed excellent catalytic performance, reaching a high CH₄ conversion and CO selectivity close to the equilibrium values within the test temperature range. The enhanced catalytic performance of the Ni–Mg and Ni–Zr coatings was attributed to the formation of oxide-supported fine Ni particles. In contrast, the catalysts coated with Ni–Fe and Ni–Sn exhibited an extremely low activity, which was lower than that of the catalyst coated with only Ni. The low activity of the Ni–Fe and Ni–Sn coatings is supposed to be due to the formation of aggregated Ni₃Fe, Ni₃Sn, and Ni₃Sn₂ phases.

Due to the rapid national development of various countries, organic dyes have been applied in manufacturing products such as leather, textiles, paper, and cosmetics. However, the wastewater produced by these industries is harmful to the environment and organisms. Moreover, organic dyes contain toxic carcinogens and cause the reduction of the oxygen content in water, which is harmful to nature and the water resources people use. Photodegradation is a low-cost, highly efficient, and low-energy way to remove these substances. Zinc-based materials were applied as a degradation catalyst in this study. ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposites were fabricated by the urea combustion method and used as photocatalysts for rhodamine B (RhB) degradation under ultraviolet A (UVA) light irradiation. ZnO–ZnCr₂O₄/g-C₃N₄ was investigated by XRD, FESEM, BET, UV-Vis, and TEM to confirm the crystalline microstructure. Based on the various annealing temperatures of ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposite, the specific surface area varied from 36.33 m²/g to 107.55 m²/g. In addition, the photocatalytic activities of ZnO–ZnCr₂O₄/g-C₃N₄ nanocrystals were investigated through the degradation of RhB under UV light for 12 hours. After 12 hours, 95.45% of the RhB was degraded under UV light irradiation. ZnO–ZnCr₂O₄/g-C₃N₄ nanocomposites annealed at 500°C exhibited the highest rate constant, up to 6.11 × 10⁻³ min⁻¹, and ZnO–ZnCr₂O₄/g-C₃N₄ revealed excellent stability based on the results of the cyclic test.

Quasicrystals and approximant crystals (ACs) have a unique complex structure with many crystallographically non-equivalent sites. In order to apply this characteristic potential to catalysts, we investigated catalytic properties of Pd-containing Tsai-type 1/1 ACs, i.e., Al–Pd–Sc and Ga–Pd–Sc, in the acetylene hydrogenation reaction and also performed density functional theory calculations of adsorption energies of reactants and products. The catalytic properties are found to significantly depend on the kind of the semimetal element such as Al and Ga, where the Al–Pd–Sc 1/1 AC shows higher catalytic activity and selectivity. The adsorption energy of reactant acetylene is smaller in the Al–Pd–Sc 1/1 AC whereas the amount of product ethylene are comparable for both ACs. Therefore, the adsorption rate of reactants is increased while the desorption rate of products remains almost the same in the Al–Pd–Sc 1/1 AC. Furthermore, the adsorption energies are found to differ significantly from site to site, implying a superior potential of ACs for designation of active sites using many non-equivalent crystallographic sites for high catalytic performance.

Suppressed hydrogen peroxide (H₂O₂) generation and practical H₂ oxidation reaction (HOR) activity of the anode catalyst surface is crucial to improve proton exchange membrane fuel cells (PEMFCs) performance. Here, the influence of surface modification on H₂O₂ generation and HOR activity by introducing tungsten suboxides (WO_x) was investigated for platinum catalyst surfaces. A Pt(111) single-crystal substrate surface was used as the model of Pt-nanoparticle anode catalyst surface and modified with WO_x through the reactive arc plasma deposition (APD) of W under an O₂ partial pressure (p(O₂) = 1 × 10⁻¹ or 10⁻³ Pa). The oxidation states of WO_x were estimated by X-ray photoelectron spectroscopy, and the resulting electrocatalytic properties of H₂O₂ generation and HOR activity were investigated using a scanning electrochemical microscope. The as-prepared oxidation states of WO_x were modified depending on p(O₂) during the APD. Contrarily, potential cycle (PC) loadings resulted in a similar oxidation state of WO_x: substoichiometric oxides containing W⁴⁺ or W⁵⁺, irrespective of the as-prepared oxidation states of the deposited tungsten. Regardless of the WO_x oxidation state, the WO_x/Pt(111) surfaces exhibited suppressed H₂O₂ generation, even accompanied by enhanced HOR activity compared with the clean Pt(111). Therefore, the WO_x surface modification can improve the properties of Pt-based anode catalysts and contribute to high-performance catalyst developments.

The thermal stability and CO oxidation activity of a non-equilibrium Pd–Ru alloy obtained by leaching Al–Pd–Ru alloy (3/2 approximant, P₄₀ phase: Al₇₂Pd_16.4Ru_11.6 (at%)) with 20 mass% NaOH aqueous solution were investigated. When the Pd–Ru alloy was annealed under a He or H₂ atmosphere, phase separation of the Pd–Ru alloy proceeded under H₂ at 500°C, whereas the non-equilibrium state was relatively stable under He at temperatures as high as 500°C. In addition, the Pd–Ru alloy sample annealed under He (at 300 or 500°C) showed substantially greater CO oxidation activity than those annealed under H₂ (at 300 or 500°C). The results suggested that there is a more suitable microstructure of Pd–Ru alloy in the nanocrystals for CO oxidation than the solid solution state or the phase-separated state.

Pd-loaded perovskite composite manganese oxide (Pd/AMnO₃, A = Ca, Sr, La) catalysts were prepared by coprecipitation method, in order to reveal metal-support interaction (MSI). Particulate PdO with sizes of a few ten nanometers were randomly formed on CaMnO₃. On the other hand, fibrous PdO with diameter approximately 20 nm was formed on LaMnO₃. Both shape of PdO were formed on SrMnO₃. HAXPES measurement showed a down shift of valence band of deposited PdO depending on the composite manganese oxide. Our characterization indicates that the MSI at Pd–AMnO₃ interface affects not only the shape but the electronic structure of deposited PdO on AMnO₃. The CO oxidation activity was in order of Pd/LaMnO₃ > Pd/SrMnO₃ > Pd/CaMnO₃, which corresponds to the order of the PdO valence band shift trend. We proposed that the observed correlation between the valence band shift and the CO oxidation activity for Pd/AMnO₃ can be understood in terms of the CO adsorption strength.

The combination of two dielectric-magnetic components in the same composite has been shown to significantly improve the effectiveness of electromagnetic (EM) shielding and microwave absorption (MWA) because they have both a combination of high dielectric and magnetic losses and good impedance matching. The novel Bi_1/2(Na_0.8K_0.2)_1/2TiO₃/Fe₃O₄ (BNKT/Fe₃O₄) composite has been successfully synthesized by a two-step method with wide effective absorption bandwidth (EAB = 16 GHz) in the high-frequency (2–18 GHz). It was evident that the MWA efficiency of the BNKT/Fe₃O₄ composite has been significantly improved compared with pure Bi_1/2(Na_0.8K_0.2)_1/2TiO₃ or Fe₃O₄ materials. In addition, the BNKT/Fe₃O₄ composite could achieve reflection loss (RL = −39.41 dB, ∼99.99% at 10.16 GHz) with a sample thickness optimal (d = 4.7 mm). This work shows that the novel BNKT/Fe₃O₄ composite has excellent MWA properties, all contributing to a potential candidate in the electromagnetic wave absorption and shielding fields.

Novel Ni–Ti–Zr–Hf–Nb and Ni–Ti–Zr–Hf–Ta high temperature shape memory alloys with multi-principal elements were developed, and differences in the effects of Nb and Ta on cold workability and shape memory properties were investigated. Constituent phases, microstructure, cold workability, transformation temperatures, shape memory properties were investigated in (Ni₅₀Ti₃₀Zr₁₀Hf₁₀)_100−xNb_x (x = 5, 10, 15) alloys and (Ni₅₀Ti₃₀Zr₁₀Hf₁₀)_100−yTa_y (y = 5, 10, 15) alloys. Although both of Nb and Ta were effective to improve cold workability of Ni₅₀Ti₃₀Zr₁₀Hf₁₀ alloy by forming a ductile β phase with a disordered body-centered cubic structure, it was found that Ta was more effective than Nb in improving cold workability. The addition of Ta was also effective to suppress the formation of Ti₂Ni-type intermetallic compound. Transformation temperatures were not significantly affected by the addition of Nb, while the transformation temperatures increased by the addition of Ta. According to thermal cycling tests, the (Ni₅₀Ti₃₀Zr₁₀Hf₁₀)₈₅Nb₁₅, (Ni₅₀Ti₃₀Zr₁₀Hf₁₀)₉₀Ta₁₀ and (Ni₅₀Ti₃₀Zr₁₀Hf₁₀)₈₅Ta₁₅ alloys exhibited almost full shape recovery under 200 MPa. These alloys are suggested as promising candidates for practical high temperature shape memory alloys that can be worked at room temperature.

The structural changes in electrodeposited palladium (Pd) films with desorption of co-deposited hydrogen were investigated, and the existing states of hydrogen in the Pd film were analyzed. The Pd films were electrodeposited from an alkaline bath consisted of palladium (II) chloride, ammonium chloride, and citric acid. Two pronounced desorption peaks at around 500 K and 880 K were observed in the thermal desorption spectrum of hydrogen from as-deposited Pd film. For the Pd films with lower hydrogen concentrations, the lattice contraction proceeded concurrently with hydrogen desorption at room temperature. The lattice parameter of Pd film decreased with increasing heat treatment temperature up to 500 K, and then increased with grain growth above 550 K. An exothermic peak corresponding to the hydrogen desorption at around 500 K was observed in the differential scanning thermal analysis curve. A large number of nano-voids were observed in the Pd film. These results suggested that the hydrogen desorption peaks observed at around 500 K and 880 K were ascribed to the break-up of vacancy-hydrogen clusters and the desorption of hydrogen molecules from nano-voids, respectively.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 86 (2022) 172–175. Figures 1 and 2 were slightly modified.

The validity of the Tsai-Hill criterion for a porous aluminum alloy with regularly aligned unidirectional pores was investigated experimentally and numerically. The Tsai-Hill criterion predicts failure in different directions in anisotropic composite material. Compression tests of porous aluminum alloy were performed with five different compression angles of 0, 30, 45, 60, and 90°. The compression angle is the angle between the loading direction and the longitudinal direction of the pore. A numerical analysis of a torsion test of the porous aluminum alloy was also performed to obtain shear strength. Compressive yield strength and equivalent shear strength of the specimen with 0 and 90° in compression angle were utilized in the Tsai-Hill criterion. As a result, the yield strength of the specimen with 30, 45, and 60° in compression angle was successfully predicted with a maximum relative error of 4%. The applicable strain range of the Tsai-Hill criterion was also investigated by altering the yield strength to various offset strengths. The resulted prediction showed a maximum relative error of 10% when the offset strain was 40% or less. Above that offset strain, densification of the porous structure caused a rapid increase in stress, leading to a drastic decrease in prediction accuracy.

Atomic stress, utilized in molecular mechanics and molecular dynamics, is valuable in analyzing complex phenomena such as heat transfer, crack propagation and void growth. However, traditional modeling techniques designed for large-scale systems may lack the precision achievable through first-principles calculations. To overcome this limitation, we propose an approach based on artificial neural network (ANN) potentials to compute atomic stress. A crucial aspect of this method is the use of central force decomposition to derive the atomic stress tensor of the ANN potential, ensuring compliance with the balance between linear and angular momentum. By comparing atomic stress calculations for surface systems in Fe and Al using the ANN and embedded-atom (EAM) potentials, we demonstrate that the ANN potential accurately reproduces the stress oscillations near the surface layer predicted by first-principles calculations. This scheme allows us to evaluate atomic stress with nearly the same accuracy as first-principles calculations, even in large-scale models with complex geometries and defect structures.

Resistance spot welding is used in the assembly of automobile bodies. The use of high tensile strength steel plates in automobile bodies is expanding, and there is a need to develop a welding process that improves the joint strength per resistance spot weld point. To improve the joint strength per resistance spot weld, resistance spot welding can be combined with other joining methods, such as the WeldBond method. As an additional joining method around the nugget, other than using adhesives as in the WeldBond method, brazing can be used to obtain a strong joint without melting the base metal. In this paper, it was proposed that the joint strength can be enhanced by using resistance spot welding and brazing or soldering based on the rule of mixture. And, based on this concept, effect of additional joint around a nugget, such as brazing or soldering, on rotational deformation of joint part and tensile shear strength of resistance spot welded lap joint was examined.

First, three types of lap joint specimens that were using only resistance spot welding, using only brazing and using both joining methods (called WeldBraze) were prepared. A high-strength steel sheet HT590 with a thickness of 1.0 mm was used. For the brazing filler metal, lead-free solder (composition: Sn–0.3Ag–0.7Cu) was used. Next, tensile shear tests were conducted using all the test specimens. In general, rotational deformation of the joint occurs due to the offset of the load axis for the tensile shear test of the lap joint. It is necessary to investigate the effect of the rotational deformation on joint strength. Therefore, a tensile shear test was performed using a test specimen in which the welded joint part was sandwiched between two steel blocks to suppress rotational deformation.

As a result, it was confirmed that the new joining method using both brazing and resistance spot welding increased the joint strength and the amount of displacement up to fracture as compared with the case of brazing or resistance spot welding only.

So, it was clarified that the factor of the strengthening of the joint was the rule of mixture, and the increase of displacement up to the fracture was affected by the suppression of rotational deformation.

In conclusion, it was found that the joining method using both brazing and resistance spot welding was effective in improving the joint strength.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 345–351.

The corrosion behavior and resistance of hot-dip galvanized steel were studied in a saturated Ca(OH)₂ aqueous solution containing chloride ions (Cl⁻) in concrete. In the saturated Ca(OH)₂ aqueous solution, which simulated water in the concrete pores, Ca(Zn(OH)₃)₂·2H₂O was formed on the surface of the hot-dip galvanized steel, acting as a protective film. However, the corrosion rate of zinc increased as the Cl⁻ concentration increased. This is presumed to be owing to the formation of Zn₅(OH)₈Cl₂·H₂O and CaCO₃ on the surface and the decrease in the coverage of the protective film Ca(Zn(OH)₃)₂·2H₂O. However, in concrete, the corrosion of hot-dip galvanized steel was not promoted by the cyclic corrosion test. This may be because the Cl⁻ that penetrated into the concrete did not reach the depth of the galvanized steel and the Ca(Zn(OH)₃)₂·2H₂O that was formed during the curing of the concrete remained after the cyclic corrosion test.

This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 71 (2022) 263–270. Abstract, captions of Figs. 1–13, and Figs. 2, 3, 12 were slightly modified.

The electrodeposition of bismuth–tellurium thermoelectric material was investigated by galvanostatic and potentiostatic electrolysis in ethylene glycol (EG)-BiCl₃–TeCl₄ non-aqueous solutions at 393 K. Controlling the molar ratio of BiCl₃ to TeCl₄ in the bath and the electrolysis conditions, the Bi–Te alloys with various compositions were electrodeposited. The composition of the electrodeposit obtained in the 97.50 mol%EG-2.45 mol%BiCl₃–0.05 mol%TeCl₄ bath with the BiCl₃ to TeCl₄ molar ratio of 50:1 at the current density of 20 A·m⁻² was 40.14 mol%Bi–59.86 mol%Te and close to that of Bi₂Te₃. This electrodeposit exhibited a n-type thermoelectric conversion for the given temperature difference and its Seebeck coefficient was −145 µV K⁻¹.

Leakage flux in rotor core bridges is a problem specific to interior permanent-magnet (IPM) motors. It is widely known that the partial non-magnetization of bridges reduces the magnetic flux leakage. In a previous study, a process was proposed whereby a part of the silicon steel sheet that bridges after pressing was non-magnetized by melting and mixing Ni–Cr alloy powder with a silicon steel sheet using a laser, and the rotor core was produced by laminating them. However, because the final solidification part had solidification defects, such as cracks and shrinkage cavity, the process was proposed to leave a homogenous part free of solidification defects. Therefore, the area of the improved portion increased. We focused on developing a new alloy for non-magnetic improvement to suppress solidification defects. The improved portion was melted and mixed using a laser with various B contents to obtain a composition of Fe–(15–20) mass%Ni–(15–20) mass%Cr–(2–3) mass%Si–(0–1.6) mass%B. Large cracks and large shrinkage cavity were observed in the boron-free alloy. The cracks and shrinkage cavity decreased with an increase in the B content. The minimization of the area of non-magnetic improvement is possible by suppressing solidification defects. Consequently, the laser processing speed per piece and the amount of expensive nickel were reduced. These new alloys show promise for practical applications in the partial non-magnetization process.

The effectiveness of a corrosion repair technique consisting of laser cleaning and cold spraying was investigated. The effect of laser pulse frequency on the removal of surface corrosion on steel specimens was analyzed. Subsequently, a zinc coating was cold-sprayed on specimens cleaned of surface corrosion using conventional disc grinder and laser methods. Furthermore, salt spray tests were conducted to compare the corrosion protection performance of the zinc coating on these specimens. The results showed that laser cleaning can effectively remove surface corrosion and that a laser pulse frequency of 15 kHz is more effective than that of 40 kHz for removing the surface oxide layer. A comparison of cold-sprayed zinc coatings on laser-cleaned and non-treated specimens indicated that surface oxidation during laser treatment may negatively affect zinc deposition efficiency. The zinc coating on the laser-cleaned specimen was more than twice as thick as that on the conventionally cleaned specimen, and the coating–specimen interface maintained good adhesion after a 168 h salt spray test. Although no corrosion was observed in both steel specimens after the salt spray test, cracking of the remaining corroded areas on the substrate and delamination of the coating occurred in the conventionally cleaned specimen.

This Paper was Originally Published in Japanese in J. Jpn. Thermal Spray Soc. 59 (2022) 137–143.

This work explores the effect of the Hf concentration on the constituent phase, microstructure, and room temperature mechanical properties of TiVZrNbHf_x (x = 0–1.0) light weight refractory high entropy alloys prepared by the arc melting technique. The results show that the TiVZrNbHf_x alloys possess a single disordered bcc phase and dendritic structure, with densities ranging from 6.47 ± 0.03 to 8.69 ± 0.02 g/cm³. Elemental scan mapping reveals a relatively homogeneous distribution of all elements, although local concentrations of elements can be observed in the microstructures. The addition of Hf enhances the Vickers hardness, ultimate strength, and the 0.2% proof stress, which could be attributed to solid-solution-like strengthening mechanism. The TiVZrNbHf alloy exhibits the highest values of Vickers hardness (449.0 ± 11.3 HV1), ultimate strength (1682.7 MPa), and the 0.2% proof stress (1291.8 MPa). However, the fracture strain decreases with the increase in Hf composition, from 39.9% (for TiVZrNb) to 18.1% (for TiVZrNbHf).

Pure copper, specifically oxygen-free copper (OFC), is widely used in superconductive and low-temperature refrigeration technologies owing to its superior electrical and thermal conductive properties at cryogenic temperatures. These properties, which can be expressed in terms of the residual resistivity ratio (RRR), are associated with the purity of copper or with its impurity concentration. High-purity copper with a low impurity concentration exhibits a high RRR. In addition to the impurity concentration, the existing form of impurities within the matrix affects the RRR, which is influenced by heat treatment. Specifically, for OFC, high-temperature heat treatment causes the impurities to dissolve into the matrix, resulting in a decrease in the RRR. Thus, to obtain a high RRR, the impurity concentration must be reduced, which often requires complex purification processes. In this study, we aimed to develop a pure copper material with a high RRR over a wide range of heat treatment temperatures using industrially feasible methods with OFC as the base material. For this, impurities with a negative effect on the RRR were investigated, and an additive element was selected to target the impurities and minimize the negative effect of these impurities without adversely affecting the RRR. We successfully developed a pure copper material using an extremely small amount of Ca as an additive element. This material not only exhibited an RRR comparable with that of high-purity copper but also maintained the high RRR value over a wide range of heat treatment temperatures, owing to the formation of CaS. The high RRR pure copper developed in this study is a promising material for use in components employed in superconductor and refrigeration applications such as in magnetic resonance imagining (MRI), in nuclear magnetic resonance (NMR), fusion reactors, maglevs, and particle accelerators.

This Paper was Originally Published in Japanese in Journal of Japan Institute of Copper 61 (2022) 329–333. Table 1, 2, Figs. 2, 4–6 and the caption of Table 1, Figs. 1–5 were slightly modified.

Multinary Cu₂ZnSnS₄ (CZTS)-based materials have attracted considerable attention for thermoelectric (TE) power generation owing to their cost-effectiveness and abundance. The device structure of the CZTS/Au diffusion barrier layer was effective in impeding chemical diffusion during operation; however, its interfacial contact resistance was relatively higher than that of the Bi₂Te₃ device. We report the discovery of p-type CZTS single crystals with a record-high dimensionless figure of merit (ZT) of 1.6 at 800 K and TE conversion efficiency of ∼4% at a temperature difference of 473 K, which is based on the Te-free concept. This study demonstrated the potential of CZTS-based TE materials for environment-friendly TE power generation.

“The International Symposium on Innovation in Materials Processing (ISIMP)” was held in Jeju, Korea, from 26th to 29th October 2021. The proceedings for the session on “Integrated Computer-Aided Process Engineering (ICAPE)” were published in October 2022 as a special issue of Materials Transactions (Vol. 63, No. 10). The primary purpose of the ICAPE session was to address the recent advances in scale-bridging simulations and characterization to understand, describe, and predict the microstructure–property relationship of newly developed materials in a lab to industrial-level processes. Among the papers presented at the symposium, this article briefly reviews the following topics: macroscale numerical analysis, such as finite element methods (FEM), microstructure simulations such as phase-field modelling (PFM) and molecular dynamics (MD), and optimization techniques such as machine learning (ML) and design of experiments (DOE).