This paper presents an overview of research studies for achieving high-strength Al alloys. Severe plastic deformation (SPD) is a major process to strengthen the alloys, which is capable of making simultaneous use of all strengthening mechanisms such as grain refinement, particle dispersion (precipitation), solid solution and dislocation accumulation, regardless of alloy type and/or alloy state. High-strengths may also be achieved by syntheses through consolidation of powder mixtures using the SPD process under high pressures such as high-pressure torsion and high-pressure sliding. Future prospect is made to produce Al alloys with the strength more than 1 GPa with a reasonable ductility.

This Paper was Originally Published in Japanese in J. JILM 73 (2023) 559–569.

Yamamoto’s rate equation was applied to two-step age hardening of an Al-6%Zn-0.75%Mg alloy. All aging processes at 160–200°C were described as the sum of three precipitation reactions, that is, spherical, plate-like, and rod-like precipitation independent of the holding time at room temperature (RT). These results agreed with the observation of TEM microstructures. Based on the activation energy of diffusion calculated from the rate equation, it was suggested that the diffusion of zinc controlled the spherical precipitation reaction, and the diffusion of magnesium did the rod-like one. The plate-like precipitation reaction which depended on the holding time at RT is controlled by the diffusion of zinc for a short time and the one of magnesium for a long time. These precipitation reactions were related to the meta-stable and stable phase particles. The comparison between electrical resistivity (ER) and Vickers hardness (HV) changes at 160°C aging showed that the HV change was faster than ER one in the long holding time at RT. On the other hand, the ER change became faster than HV change in the short one at RT. This difference is related to the formation of clusters and GP zones that are formed by holding at room temperature.

This Paper was Originally Published in Japanese in J. JILM 74 (2024) 188–197.

In this study, a carbon steel tube with a variable wall thickness was fabricated under various conditions by dieless mandrel drawing to evaluate the effect of the Zener-Hollomon parameter on its microstructure. STKM13C low carbon steel tubes with an outer diameter of 6.0 mm and inner diameter of 4.0 mm were drawn under various conditions of heating temperature, feeding speed, and the reduction in area. The feeding speed was set as constant, and the drawing speed was increased or decreased during dieless mandrel drawing to obtain tubes with a variable wall thickness. Thereafter, the wall thickness and microstructure of the drawn tubes were evaluated. Furthermore, the Zener-Hollomon parameter was calculated under various dieless mandrel drawing conditions. Consequently, a tube with variable wall thicknesses, with a wall thickness reduction range of 0–21%, was obtained by the dieless mandrel drawing. Furthermore, the Zener-Hollomon parameter increased as the heating temperature decreased, and as the feeding velocity and reduction in area increased. As a result of an increase in the Zener-Hollomon parameter, the average grain size decreased to less than 2 µm in a single pass of the dieless mandrel drawing.

Addressing the performance-cost trade-off dilemma in the selection of extruded billets and horizontal direct chill cast (HDC) billets for the manufacture of aluminum alloy control arms for automobiles, this study systematically reveals the microstructural inheritance effects and performance evolution mechanisms of the two base materials of 6082 aluminum alloy throughout the entire forging and heat treatment process. The results demonstrate that the gradient structure (surface fine-grain zone/mixed-grain zone/fibrous-grained core) in extruded billets triggers abnormal grain growth after heat treatment due to a drastic reduction in surface second-phase particle area fraction to 0.3% (caused by dissolution of Mg₂Si and coarsening of α-Al₁₂(Fe,Mn)₃Si). This results in inferior control arm properties with tensile strength of merely 335.2 MPa and elongation of 14.7%. In contrast, horizontal direct chill cast billets exhibit a homogeneous equiaxed grain structure. After forging-induced fragmentation of elongated α-Al₁₂(Fe,Mn)₃Si phases, the heat-treated material maintains a higher second-phase area fraction (0.7%). This effectively pins grain boundaries to inhibit coarsening, yielding refined and uniform microstructures. Consequently, the control arm achieves significantly enhanced tensile strength of 367.7 MPa and elongation of 15.8%.

In this paper, the effects of different additives Cr₃C₂, TaC, Re, and Ru on the microstructure and properties of WC-10Co cemented carbide were studied. 0.5%Cr₃C₂+1%TaC, 0.5%Cr₃C₂+1%TaC+1%Re, 0.5%Cr₃C₂+1%TaC+0.5%Ru were added to WC+10%Co fine crystal cemented carbide, and after ball milling for 60 h, sintered at 1410°C under argon atmosphere, The Rockwell hardness and transverse fracture strength of the alloy are increased, but the fracture toughness of the alloy is decreased by 0.5%Cr₃C₂+1%TaC. The introduction of 0.5%Cr₃C₂+1%TaC+1%Re or 0.5%Cr₃C₂+1%TaC+0.5%Ru can make the cemented carbide have high fracture toughness while maintaining high hardness and strength. The alloy containing WC+10%Co+0.5%Cr₃C₂+1%TaC+0.5%Ru has a hardness of 91.8HRA, a TRS of 3962 MPa, and a fracture toughness of 14.7 Mpa·m^1/2, and the comprehensive mechanical properties are the best.

Sintered Ag nanoparticles display brittle fatigue crack propagation behavior at 298 K because of their submicron-size crystals. On the other hand, at elevated test temperatures above 413 K, ductile fatigue crack propagation characteristics similar to those of soft metals such as solder appear owing to viscous creep behavior at the grain boundaries, and no temperature dependence of the fatigue crack propagation properties was observed. The fatigue crack initiation lives observed experimentally and the fatigue lives derived by a pseudo-fracture mechanics approach using numerical fatigue tests with the fatigue crack propagation law mostly coincided with each other. However, the fatigue crack initiation life was calculated on the excessively safe side because of the long incubation period for crack initiation due to the influence of continued sintering during the tests performed near the sintering temperature. The application of the pseudo-fracture mechanics approach made it possible to derive the fatigue crack initiation law for smooth specimens of sintered Ag nanoparticles with satisfactory accuracy if the test temperature is lower than the sintering temperature.

Developing efficient catalysts for the mature application of electrochemical CO₂ reduction reaction (CO₂RR) to produce value-added chemicals and fuels is crucial. Carbon-supported metals functioning as effective catalysts have attracted extensive attention in the field of CO₂RR. However, the understanding of carbon support effects on the electrocatalytic properties is insufficient. Herein, we conduct a comparative study of Ag nanoparticles (NPs) encapsulated in N-doped hollow carbon sphere (Ag@NHCS) and N-doped non-hollow carbon sphere (Ag@NCS) to uncover the structural effects of carbon support in CO₂RR. The unique hollow architecture of NHCS with a large surface area and high pore volume guarantees the high exposure of active sites and fast CO₂ diffusion. As a result, Ag@NHCS shows improved CO Faradaic efficiency and partial current density compared to Ag@NCS. Furthermore, characterizations and electrochemical results confirmed that poly(ethyleneimine) added as a dispersant agent contributes to the formation of additional active N sites, resulting in an enhanced CO₂RR activity. This work provides new insights into the rational design of metal-based carbon catalysts for electrochemical CO₂ conversions.

Magnetic pulse welding of aluminum and copper was performed by changing of type of aluminum alloy used as a flyer plate. Welding experiments showed that weldable charging energy-gap condition differed depending on the aluminum alloy type. Finite element method analysis of collision of aluminum flyer plate and copper parent plate indicated that when a relatively low-strength aluminum alloy was used for the flyer plate, the tip of the flyer plate was rounded and deformed by electromagnetic force. When the high-strength aluminum alloy was used for the flyer plate, the tip of the flyer plate collided with the copper parent plate while remaining flat. The collision velocity and collision angle depended on electrical resistance and tensile strength, respectively. Smooth particle hydrodynamic analysis revealed that the pressure on the order of GPa above Hugoniot elastic limit was generated at the welding interface. Regardless of the aluminum alloy used for the flyer plate, the pressure at welding interface exceeded the Hugoniot elastic limit, and a region above the melting point at high pressure was observed within a few µm from the welding interface at weldable condition.

In this study, the joint properties of dissimilar materials joined by friction welding between an Al-11%Zn-3%Mg-1.4%Cu alloy and SUS304 stainless steel was evaluated. Additionally, to compare the weldability, friction welding was also conducted on dissimilar materials involving the A7204 alloy, A7075 alloy, and SUS304 stainless steel. The A7204/SUS304 joints and the A7075/SUS304 joints both fractured in the aluminum alloy at tensile strengths of 316 MPa (joint efficiency of approximately 75%) and 353 MPa (joint efficiency of approximately 100%), respectively. It was found that by reducing both the friction pressure and the rotational speed, the interface could be maintained at a relatively low temperature for an extended period, thereby suppressing excessive growth of the intermetallic compound (IMC) layer while still achieving the joint. In Al-11%Zn-3%Mg-1.4%Cu alloy, the tensile strength increased to 377 MPa compared to the standard alloy, but the fracture occurred at the interface. FEM analysis revealed that the stress at the interface during tensile deformation increased more significantly at the edges than at the center, leading to stress localization. It was suggested that in such cases where localized stress occurs, interface fracture is more likely to occur.

This Paper was Originally Published in Japanese in J. JILM 75 (2025) 103–109.

Friction stir welding (FSW) is difficult to disperse the initial oxide scales in the stir zone when the joint interface is offset from the probe center or the area of weld interface increases by using insert metal. Increasing the pitch of the probe thread is effective to disperse oxide. In the present study, FSW of A6061 aluminum alloy plates supplying bulk build-up metal are performed by two tools with different thread pitch under the welding conditions of being difficult to disperse the initial oxide scales. The present study investigates the effect of the increased thread pitch on the weld microstructure, state of the oxide phase, tensile properties, and temperature history. The welding tool threaded a large thread pitch expands the stir zone in the joint. Increase in thread pitch promotes dispersion of the initial oxide scales in the stir zone. While the thread pitch had no significant effect on tensile properties, it greatly improved oxide dispersion. Therefore, this method is an effective in expanding a deformation area of the material without increasing the probe diameter.

As a fast developed magnesium alloy family, Mg-Nd-Zn alloys show great application prospect in varied fields. However, the grain refinement behaviors of this alloy family were still unclear. The present work investigated the process dependence of grain refinement effect by Zr refiner for the alloy. Gradually saturation of Zr dissolving as function of stirring time and melt temperature were revealed, giving out a saturate content of soluble Zr about 0.72 wt.%. The dissolve of Zr refiner undergoes three stages with increasing the added Zr refiner, respectively giving rise to grains microstructure without Zr-rich particles, single core Zr-rich particles and multi-shell particles. Most significant grain refinement effect occurred in the first stage without Zr-rich particles formation, due to refinement mechanism proved to be the grain growth restriction. The multi-shell Zr-rich particles were observed, for the first time, with soluble Zr beyond 0.68 wt.%. A limited Zr diffusion model was proposed to understand the formation of such multi-shell Zr-rich particles. Moreover, the Zr-rich particles evolution behaviors were revealed dependent on the melt holding time, building the relationship between Zr-rich particles and the grain refinement recession. The results shed new lights on the grain refinement process design of Mg-Nd-Zn alloys family.

The use of high-frequency microwaves in the 4G (0.7–3.5 GHz) and 5G (3.6–4.9 and 27.0–29.5 GHz) bands has increased with the development of the “Internet of Things (IoT)” communication technology. However, electromagnetic interference (EMI) has also become a serious problem, and microwave absorption materials (MAMs), which consists of magnetic powder and resin, can be attractive for preventing EMI.

In this paper, we investigated metastable Fe₃B and Fe₃B–Fe composites as microwave absorbers in MAMs for suppressing EMI. Although Fe₃B is metastable, it has relatively high magnetic anisotropy, and MAMs consisting Fe₃B were expected to show high magnetic loss in the gigahertz range. Furthermore, the band range showing high magnetic loss was controlled by using Fe₃B–α-Fe nanocomposite powder because the co-existing Fe₃B and α-Fe phases weakened the magnetic anisotropy.

The Fe₇₅B₂₅ as-spun ribbon was amorphous, and the Fe₃B phase appeared after heat treatment at 455°C. The matching frequency of resin and annealed Fe₇₅B₂₅ powder composites matched well in the 2.0–5.0 GHz range. The Fe₈₀B₂₀ and Fe₈₅B₁₅ melt-spun ribbons contained Fe₃B and α-Fe phases after heat treatment. The magnetic resonance frequency (f_r) and matching frequency decreased with increasing Fe content. f_r shifted from 6.5 to 3.2 GHz, and the matching region shifted to the lower side from 5.0 to 0.8 GHz. Furthermore, the Fe-B powder resin composites had smaller f_m · d_m values than that with other alloys. Consequently, resonance frequency of the Fe-B alloy can be tuned by changing Fe content and the alloy may be suitable for high performance microwave absorption materials in the 4G (0.8–3.5 GHz) and 5G (3.6–4.9 GHz) bands.

Recent investigations have demonstrated that potassium titanate whiskers (PTW) as reinforcement can significantly improve the mechanical properties of aluminum alloys. Building on these findings, 7xxx alloy-5.0 vol% PTW composites were fabricated via rheological processing, with characterization conducted on microstructural evolution during solidification. With prolonged processing duration, PTW exhibits progressively improved dispersibility in the alloy matrix. Specifically, a 59.3% reduction in the average whisker length was observed following 30 s of semi-solid slurrying, attributed to the shear-induced fragmentation during the treatment. Meanwhile, the composite’s tensile strength reached 363.0 ± 3.7 MPa, gaining ca. 29.0% increase. Findings from this study demonstrate that high-performance Al-PTW composites can be fabricated via semi-solid rheological processing, concurrently enabling a reduction in PTW reinforcement content. Additionally, this research deepens the comprehension of PTW breakage and distribution mechanisms during the solidification of semi-solid aluminum melts.