Recent discoveries of long-range magnetic order in Tsai-type hypermaterials have opened a new avenue for rationally designing magnetism in quasicrystals (QCs) and their approximant crystals (ACs). This review establishes a unified design framework based on three key principles: (1) electronic tuning via the valence-electron concentration (e/a), (2) control of spin anisotropy through crystal electric field effects, and (3) the use of structural degrees of freedom to relieve magnetic frustration. Together, these principles explain the emergence of ferromagnetic, antiferromagnetic, and spin-glass magnetic states in both ACs and QCs. Furthermore, by integrating experimental results, theoretical insights, and case studies, this review identifies key open problems —such as the microscopic mechanisms of magnetic ordering, universality classes in quasiperiodic systems, and the development of stable magnetic quasicrystals —and charts future directions for the field.

Recent discoveries of long-range magnetic order in Tsai-type quasicrystals and approximant crystals (ACs) have, for the first time, enabled systematic evaluation of the magnetocaloric effect (MCE) in these hypermaterials. With magnetic transitions now clearly identifiable, pioneering works are revealing magnetocaloric responses unique to hypermaterials. This review summarizes recent progress, focusing on Tsai-type ACs, and highlights that the electron-per-atom (e/a) ratio is the central tuning parameter governing both the magnetic ground state and the resulting MCE, which offers an unconventional MCE control method. Notably, tuning the system toward a magnetic phase boundary yields MCE values comparable to conventional materials promising for low-temperature applications. Furthermore, the recently proposed double hetero-valent elemental substitution expands the compositional stability range of hypermaterials, thereby enabling broader e/a tuning and enhanced MCE in newly synthesized compounds. These advances demonstrate that hypermaterials—including potentially quasicrystals—constitute a promising platform for future exploration of cryogenic magnetocaloric materials.

Tsai-type phases, built from concentric polyhedral shells, represent one of the most structurally prominent families of quasicrystal-related materials. Despite their prevalence, many reported Au-based 1/1 approximants have structural features that may not be fully refined, due in part to unaccounted atomic occupancy disorders and/or incomplete treatment of cubic interstices and because of the large atomic scattering factor of Au compared to the other constituent elements in the various crystalline approximant systems, which obscure subtle spatial and structural disorders inherent to these compounds. In this work, we review and analyse the crystallographic details of Au-containing Tsai-type 1/1 approximants, with particular emphasis on the nature of atomic occupancy disorders that lead to deviations from integer stoichiometric chemical compositions, and the role of interstitial occupancies in conjunction with electronic structure theory. A comparison against several reported structures is presented, and a unified model is proposed to describe the crystallographic disorders among these phases. Furthermore, a reinvestigation of the Gd-Au-Ge 1/1 approximant reveals that subtle differences in the concentric clustering and partial atomic occupancies may lead to compositional and symmetry changes.

Quasicrystals consist of three-dimensional aperiodic arrangement of atoms with a long-range structural order, which are distinct from periodic crystals and random systems. Here, we construct the classical Heisenberg model on Yb sites of the icosahedral quasicrystal (i-QC) Cd_5.7Yb, including interactions up to the seventh nearest-neighbors, based on the RKKY interaction estimated for the i-QC Au₆₅Ga₂₀Gd₁₅. By performing Monte Carlo simulation for the model with ferromagnetic (FM) interactions, we show that FM transition takes place as a continuous transition. By finite-size scaling of the Monte Carlo data, we have identified the FM transition temperature with the critical exponents ν = 0.714(39), β = 0.407(30), and γ = 1.330(58). The results satisfy the hyper scaling relation and we estimate the critical exponents α = −0.142(12), δ = 4.27(28), and η = 0.138(81). We discuss contribution from the Yb sites located at vertices of regular icosahedrons to the criticality by comparing our recent theoretical study as well as the present study with experimental identification of critical exponents in the Gd-based approximant crystals.

We investigated the electronic structure, magnetic properties, and thermoelectric characteristics of a newly discovered icosahedral quasicrystal (QC) Zn–Au–Yb. High-energy-resolution x-ray absorption spectroscopy confirmed that the Yb ions are in an intermediate-valence (IV) state with ν ≈ 2.48. The uniform magnetic susceptibility follows a power-law temperature dependence but saturates at the lowest temperatures without divergence, indicating the absence of non-Fermi-liquid (NFL) behavior. The Seebeck coefficient also exhibits a linear temperature dependence at low temperatures. Under applied pressures up to 10 GPa, the Yb valence increases smoothly without anomalies such as a valence singularity. These results establish Zn–Au–Yb as an IV-QC that exhibits Fermi-liquid (FL) behavior, providing a valuable reference system for elucidating the mechanisms governing NFL behavior and quantum criticality in IV-QCs such as the Au–Al–Yb QC.

The unoccupied electronic states of Al-Pd-Ru quasicrystal (QC) have been investigated by X-ray absorption spectroscopy (XAS). In addition, the core-level peak binding energies have been verified by hard X-ray photoemission spectroscopy. From the comparison of the XAS spectra of the Al-Pd-Ru QC with those of single-element metals, the shift of the Al K edge can be explained by a shift in the Al 1s core level. In contrast, the shifts of the Pd and Ru L₃ edges cannot be explained solely by the shifts of the core levels. These differences are due to differences in the orbital-dependent partial density of states near the Fermi level.

We have investigated the electronic states of Au-Al-Tb quasicrystal approximants (ACs), which exhibit different magnetic ground states at low temperatures, by high-energy electron spectroscopies in the paramagnetic phase. The Tb M_4,5-edge x-ray absorption and the Tb 3d-edge resonant photoemission spectroscopy (RPES) have revealed the dominant Tb³⁺ state with the localized 4f electrons in all samples. In addition, the hybridization between Tb 4f and 5d orbitals just below the Fermi level has been observed in the Tb 3d-edge RPES. By using the Tb L₃-edge high-energy-resolution fluorescence-detected x-ray absorption spectroscopy, the different Tb 5d electronic states have been verified among the Au-Al-Tb ACs. This variation of Tb 5d states would affect the difference in the magnetic ground states of the Au-Al-Tb ACs.

High-quality single-crystal X-ray diffraction data are used to determine the crystal structures of two types of 3/2 periodic approximants (PAs) to face-centred icosahedral quasicrystals, which were previously reported to form in the quaternary Al-Pd-W-Fe system. Both structures are composed of two types of atomic clusters, referred to as mini-Bergman clusters and pseudo-Mackay clusters, which are densely packed without incorporating glue atoms. A reliable structural refinement has been succeeded for one of the structures, demonstrating that its cluster-packing geometry can be described using the canonical-cell tiling model. This structure is identified as the cubic 2³ · 3/2 PA, closely resembling those previously reported in the Al-Pd-TM-Fe (TM = Cr, Mo) and Al-Pd-Ru systems. Furthermore, the correlation between the local geometry of each atomic site and the atomic species occupying it is found to be comparable to that observed in the corresponding Al-Pd-Cr-Fe compound. The second type of 3/2 PA, which the data suggest to be a twinned trigonal crystal, has so far only allowed for preliminary structural refinement. The results indicate a distinct cluster arrangement from the cubic 2³ · 3/2 PA, characterized by the inclusion of a significantly shorter inter-cluster linkage of 4.3 Å. This observation implies that an extension of the conventional canonical-cell tiling model is necessary to fully describe the structure of the trigonal 3/2 PA.

In this study, a two-phase alloy with high hardness and low added element content was fabricated by preparing rapidly solidified Au–SM (Si, Ge)–RE (Gd, La) approximant crystal dispersed Au alloy powders through gas atomization and subsequently sintering them using spark plasma sintering (SPS).

Atomized powders with particle sizes below 100 µm exhibited a microstructure in which the Au phase grew along the grain boundaries of the 1/1 approximant crystal phase. Furthermore, the sintered samples exhibited a two-phase structure comprising Au and approximant crystal phases and possessed a finer microstructure than the as-melted samples obtained by arc melting. In addition, a comparison of hardness revealed that the sintered samples were generally harder than the as-melted samples of the same composition. This hardness enhancement is attributed to the combined effects of fine dispersion of approximant crystal phases and grain refinement achieved through rapid solidification and SPS. Overall, these findings indicate that sintered Au alloy powders containing dispersed approximant crystal phases show significant promise for use as high-strength Au alloys.

The chemical states of Al₇₁Pd₁₉Ru₁₀, an icosahedral Al-Pd-Ru quasicrystal, and of the approximants Al₇₂Pd_16.4Ru_11.6 and Al_70.4Pd_14.7Ru_14.9 were investigated using hard X-ray photoemission spectroscopy. Their catalytic properties for the hydrogenation of acetylene were also evaluated. Each of these Al-Pd-Ru alloys comprised mini-Bergman and pseudo-Mackay clusters but had varying structures as a consequence of differences in the arrangement of these clusters. The chemical states of the constituent elements and the acetylene hydrogenation characteristics were similar among these alloys. However, all three materials were significantly different from pure Pd, especially with regard to their chemical shift and performance during the semi-hydrogenation of acetylene. The catalytic activity of such alloys was found to be determined by the local surface structure rather than the crystal structure and/or the arrangement of the constituent clusters.

The thermoelectric properties of icosahedral Al–Pd–Ru quasicrystals were systematically investigated. A relatively wide single-phase region of the icosahedral phase was identified at 1273 K. Bulk samples were prepared by arc-melting, followed by annealing and spark plasma sintering. The thermoelectric properties, including the Seebeck coefficient, electrical conductivity, and thermal conductivity, were measured from room temperature to 873 K. All samples exhibited p-type conduction with a non-metallic temperature dependence of σ. A subsequent annealing after the SPS process was found to significantly enhance Seebeck coefficient, reaching a high value of approximately 110 µV K⁻¹, while maintaining a relatively high electrical conductivity. This resulted in a large power factor of 780 µW m⁻¹ K⁻² at 573 K. Combined with a low thermal conductivity of 1.2 W m⁻¹ K⁻¹, a maximum dimensionless figure of merit of 0.24 was achieved at 573 K, which is one of the highest values reported for undoped quasicrystals. This study demonstrates the high potential of Al–Pd–Ru quasicrystals as p-type thermoelectric materials.

The Ta–Te dodecagonal quasicrystal (dd-QC) is the only known van der Waals (vdW) layered quasicrystal and exhibits bulk superconductivity at ∼1 K. In this study, ternary and quaternary compounds were synthesized by adding a third element (X = Cu, In, Sb, Ag, Au, Ti, Nb) to the Ta–Te dd-QC system. Powder X-ray diffraction revealed that single-phase dd-QCs were obtained for several compositions, with improved quasicrystallinity indicated by a reduced full width at half maximum of the quasiperiodic peaks. Electrical resistivity measurements demonstrated superconducting transitions at approximately 1 K, with T_c varying systematically depending on the added element. The Ta–Ti–Nb–Te quaternary system exhibited the highest T_c of 1.14 K. These results demonstrate that compositional modification effectively tunes both the structural and superconducting properties of vdW layered quasicrystals.

With the aim of applying it to molecular adsorption on quasicrystalline surfaces, we investigated the applicability of the cluster model in density functional theory calculations by comparing it with the slab model. The adsorption structure and stability of pentacene on the Cu(111) surface were evaluated using two van der Waals (vdW) functionals (vdW-DF and rev-vdW-DF2). The rev-vdW-DF2 functional, which yields a larger adsorption energy than the vdW-DF functional, is clearly superior to the vdW-DF functional for the adsorption height. The potential energy surfaces (PES) obtained using cluster models exhibited an artificial jagged structure around the PES maximum when using a smaller cluster, but otherwise agreed well with those from slab models, validating the cluster approach for predicting stable adsorption structures. These findings provide a foundation for the accurate modeling of adsorption on quasicrystalline surfaces.

Thermal conductivity in solids is generally derived from electrons and the lattice, while the difficulty in dividing the experimental results into two contributions is widely known. Recently, we suggested an effective method to divide into two contributions based on the neural network method [T. Hirosawa, et al., J. Phys. Soc. Jpn. 91 (2022) 114603.], which can be used in not only the periodic systems but also the aperiodic systems. Here, applying this method to the Al-Pd-Re quasicrystal, we evaluate the thermal conductivity due to electrons. In this method, the microscopic transport theory of electrons is adopted, and it is suggested that it gives an accurate magnitude of thermal conductivity of electrons beyond the Wiedemann-Franz law.

Motivated by recent realizations of Truchet-tiling structures in DNA molecular networks and metal-organic frameworks, we study a metal-insulator transition driven by disorder following the random Truchet tiling. We uncover a hidden hyperuniform organization encoded in the coordination-number distribution of the random Truchet tiling. This structural hyperuniformity suppresses long-wavelength fluctuations in local connectivity in a strongly anisotropic way, giving rise to correlated disorder that crucially affects the electronic states. Using a tight-binding model on the random Truchet tiling and inverse participation ratio, we show that the electron system exhibits an extended-to-localized transition accompanied by a mobility edge at a finite disorder strength. Our results indicate that the hidden hyperuniformity provides a possible structural origin of the appearance of delocalized states and mobility edge.

The growth and structural evolution of ultra-thin TiO_x and Yb-Ti-O films on Rh(111) were investigated using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), Auger electron spectroscopy, and photoelectron spectroscopy (PES). After Ti deposition and subsequent oxidation–vacuum annealing cycles, well-ordered ultra-thin TiO_x films exhibited a TiO_x-(9 × 9) superstructure commensurate with the Rh(111)-(10 × 10) lattice. Upon Yb deposition, no LEED patterns were observed, and the STM images revealed an uneven surface morphology, indicating the formation of disordered Yb species. After the simultaneous deposition of Ti and Yb atoms, followed by oxidation and reduction steps, an ordered ultra-thin Yb-Ti-O film was formed, exhibiting a Yb-Ti-O-(3 × 3) superstructure commensurate with the Rh(111)-(4 × 4) lattice, together with a fundamental Yb-Ti-O-(1 × 1) lattice. The surface phase diagram summarizes the coverage-dependent evolution from binary TiO_x and YbO_x to a ternary Yb-Ti-O ordering phase. These results provide an experimentally verified framework for understanding how rare-earth incorporation modifies ultrathin Ti-O networks on metal surfaces and may guide future exploration of more complex oxide tiling in rare-earth oxide quasicrystals.

Cu-4Cr-2Nb (at%) alloy developed by NASA (National Aeronautics and Space Administration) is a promising Cu alloy that combines high strength and high conductivity. Recently, the Cu-4Cr-2Nb alloy manufactured by Laser powder bed fusion (L-PBF) process has been attracted attention because a large amount of alloying elements are dissolved in Cu-matrix phase due to ultra-rapid solidification, resulting in significant age hardening. This study investigated the microstructural changes of Cu-4Cr-2Nb alloy that occur before and after L-PBF process and during post-aging treatment. The L-PBFed alloy was aged at 200–700°C for 2 h in an Ar atmosphere. XRD results suggested that Cr₂Nb phase, which existed in the alloy powder, was melted by L-PBF process and then formed again during aging. TEM observation revealed that the aging treatment causes the precipitation of fcc-Cr phase in the fcc-Cu matrix. The observation also suggested that bcc-Nb phase existed in the as-built sample changes to Cr₂Nb phase by aging. The obtained results indicated that the mechanism of precipitation strengthening in L-PBF Cu-4Cr-2Nb alloy is mainly due to the formation of metastable fcc-Cr phase during aging.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 89 (2025) 313–321.

Displacement-plated Zn on Mg powder forms a eutectic liquid during sintering, wetting interparticle contacts, and promoting capillary densification. In simple powder mixtures (5 mass% Zn), the diffusion rate imbalance created Kirkendall voids on the Zn side, leaving pores that acted as stress concentrators and reduced the transverse rupture strength. For plated powders, the persistence of the liquid determines their final microstructure. At low Zn loadings (60 s, 4 mass%), the liquid was largely depleted during the 700 K hold (transient liquid-phase sintering), producing a nearly single-phase Mg matrix containing only discontinuous intermetallic compounds. At high Zn loadings (120 s, 14 mass%), the liquid persisted at lower temperatures. During cooling, the liquid phase formed a continuous Mg₇Zn₃ morphology. This leads to embrittlement of the compact and reduced bending strength. Overall, Zn coating is an effective method for improving the sinterability of Mg powders; however, controlling the Zn content is essential to avoid a continuous brittle intermetallic network and to optimize the mechanical performance.

Calculated phase diagram of C-Co-V-W quaternary system was attempted to investigate constitution phases of WC-VC-Co cemented carbide during liquid phase sintering. It was found that VC phase could not exist stably for low addition region of VC over liquidus temperature of Co phase in (VC-20Co)-(WC-20Co) pseudobinary system. It was considered that the formation of (V,W)C phases was classified into three types depending on the additive amount of VC. For the additive amount of less than solubility limit of VC in solid Co at solidus temperature, (V,W)C phase precipitates on WC/Co interfaces from solid Co on low temperature side during cooling. In the range of additive amount between solubility limit in solid Co at solidus temperature and solubility limit in liquid Co at liquidus temperature, (V,W)C phase crystallizes out of liquid phase in temperature during solidification of Co due to the difference between solubility limit in liquid Co and that in solid Co. Above additive amount of solubility limit in liquid Co at liquidus temperature, (V,W)C phase exists in equilibrium with liquid Co above liquidus temperature. It was concluded that inhibition mechanism of grain growth was to differ at solubility limit of VC in Co liquid phase.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 72 (2025) 279–285: https://doi.org/10.2497/jjspm.25-00010

Nowadays, α/β layered microstructure is attracting attention as one of the methods which improve mechanical properties in Ti alloys. In our previous studies, it has been revealed that compositional modulated α_Zr-rich/α_Zr-lean layered microstructure is formed in the slow-cooled Ti–35Zr (at%) binary alloy. So, we focused on the possibility of forming α/β layered microstructure with compositional modulation by adding β-stabilizer. In this study, we chose V and Fe which have different β phase stabilization effects. (Ti–35Zr)–xV and (Ti–35Zr)–yFe alloys (x, y = 0.5–5 at%) fabricated by vacuum arc melting were annealed at 1000°C (β phase field) for 2 h and subsequently slow-cooled to room temperature at 50°C/h. Scanning electron microscope observation revealed that the layered microstructure were formed at x = 0.5–1 and y = 0.5–4. To investigate crystalline phases and orientation relationship in layered microstructure, electron backscatter diffraction measurement and energy dispersive X-ray spectroscopy analysis were conducted. These measurements showed that the enrichment of V and Fe resulted in the retention of β phase in the Zr-rich layer, leading to the formation of an α/β layered microstructure with compositional and structural modulation. In addition, it was found that the state of microstructure (α_Zr-rich/α_Zr-lean layered microstructure, α/β layered microstructure, and non-layered α/β) can be organized by using [Mo]_eq value. The Vickers hardness increases with increasing concentration of β stabilizer, but its value does not differ between layered and non-layered microstructures. These results indicates that the addition of V and Fe is an effective approach to introduce α/β layered microstructure into Ti–Zr alloys.

This study investigates the effects of long-period stacking ordered (LPSO) nanoplate precipitation on the microstructure evolution, tensile properties, and fracture toughness of extruded Mg_95.5Y_3.5Al_1.0 (at%) alloys subjected to different pre-extrusion heat treatment conditions. The α-Mg matrices of three extruded alloys with different heat treatment conditions, namely GC-Ex (GC: gravity casting), T6-Ex (T6: T6 heat treatment), and CT6-Ex (CT6: continuous T6 heat treatment), exhibited similar bimodal grain structures composed of dynamically recrystallized (DRXed) and worked α-Mg grains. The volume fraction and grain size of DRXed grains were nearly identical in all extruded materials. Meanwhile, the morphology of the worked α-Mg grains varied depending on the precipitation morphology of the LPSO nanoplates that was controlled by pre-extrusion heat-treatment. The GC-Ex alloy contained a certain amount (approximately 8%) of brittle Mg₂₄Y₅ phase within the α-Mg matrices, resulting in low strength, poor ductility, and the lowest fracture toughness. In contrast, the CT6-Ex alloy, which had worked grains with highly dispersed LPSO nanoplates and disappearance of the brittle Mg₂₄Y₅ phase, exhibited the highest tensile strength and ductility among all the alloys, as well as high fracture toughness. The effective dispersion and arrangement of worked grains, including LPSO nanoplates, facilitated formation of secondary cracks and consume the energy for crack propagation, thereby contributing to further improvement in fracture toughness.

Binders affect both the mechanical properties and electrochemical performance of electrode materials for lithium-ion batteries. This study has developed and applied a mechanical model of electrode materials on negative electrodes made of polyvinylidene fluoride (PVDF) and styrene-butadiene rubber (SBR) based binders. A series of tensile tests was performed on carbon-based negative electrodes with several concentration ratios of the binders, and the stress, strain and dissipated strain energy of the binders inside the electrodes were estimated using the mechanical model. The results were summarized as follows. (1) Both the PVDF-based and SBR-based specimens fractured microscopically due to the rupture of the binder inside the specimen. This indicates that the structure of the negative electrode is supported by the binder. (2) The mechanical model was valid for the PVDF-based specimen in that the constituent of the PVDF-based binder formed inside the specimen did not change with respect to the binder concentration. (3) The tensile strength and permanent strain of the SBR-based specimen showed the same tendency as those of the PVDF-based specimen, and the mechanical model was valid for the SBR-based specimen as well. The dissipated strain energy showed a bipolarized tendency between the low and middle binder concentrations. This tendency would be caused by the adhesion mechanism between carbon particles, SBR and CMC, and the dispersibility of carbon particles.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 74 (2025) 661–668.

Composites comprising calcined fly ash and scrap aluminum were prepared using the stir casting technique. The microstructure of the samples was analyzed by optical microscope and SEM, while mechanical properties were tested by Brinell hardness machine and tensile testing machine. Phase analysis of fly ash was conducted with XRD. The properties of composite materials largely depend on the amount of calcined fly ash. The composite with 6% fly ash showed the best mechanical performance with a hardness of 107 HB and ultimate strength of 209 MPa. Calcination of fly ash removed excess impurities and significantly improved the effectiveness of reinforcement. Automobile scrap aluminum and power plant fly ash can be effective for producing an aluminum matrix composite.

This study presents tensile properties of Al-Mg-Si alloys after processing by severe plastic deformation through high-pressure torsion (HPT). The alloys were fabricated so as to include excess Fe and Si with different additions of Cu as model alloys for recycling purpose. The tensile strength well exceeded 500 MPa with the total elongation more than 15% after HPT-processing under 2 GPa for 1 turn in all the model alloys. The strength further increases to more than 700 MPa with increasing addition of Cu while maintaining reasonable ductility (∼8%). Transmission electron microscopy confirmed that the grain size was reduced to 180 nm and further reduced to 160 nm with increasing Cu addition. Microstructural analyses using scanning transmission electron microscopy and atom probe tomography revealed that Cu was segregated at grain boundaries, contributing to the increase in the tensile strength. The high strength with enhanced ductility is discussed in terms of strain rate sensitivities measured from strain rate change tests.

Fatigue tests with three different stress ratios were carried out on shot-peened SUS316L steel, and in situ X-ray stress measurements were performed to analyze the behavior of the residual stress change and related factors. The compressive residual stress decreased during tensile loading and increased during compressive loading. The reduced compressive residual stress was caused by the tensile yielding of the core region, whereas the increase in compressive residual stress was caused by the compressive yielding of the core region. This behavior was observed when the yield strength of the surface layer was significantly increased by shot peening, and the compressive yielding of that layer and the resulting relaxation of compressive residual stress were prevented. In this case, the behavior of the residual stress change during cyclic loading was contingent on the stress ratio. It is because the compressive residual stress, which decreased during tensile loading, increased under subsequent compressive loading, and the amount of increase is based on the magnitude of the applied compressive loading.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 74 (2025) 771–778.

Cu-Zn-Si alloy bars were caliber rolled using three different groove shapes of conventional rhombus, flattened hexagon, and combination of oval and circle. In all the rolled bars, heterogeneous nanostructure comprising mechanical twins and shear bands was uniformly developed. The hardness increased with increasing reduction. Among the groove shapes, the combination of grooves of oval and circle produced the most homogeneous hardness distribution. Up to reduction of 70%, the combination of grooves of oval and circle derived also highest tensile properties of 801 MPa tensile strength and 11% ductility. Nevertheless, these differences in the mechanical properties attained by using three groove shapes became gradually smaller with increasing reduction over 70%. The finite element calculation, which simulated change in the plastic strain distribution during caliber rolling, revealed that the combination of oval and circular grooves produced the most homogeneous distribution of plastic strain with the highest average. These simulation results were consistent with the experimentally observed microstructural evolution and enhancements of mechanical properties.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 64 (2025) 32–37.

We previously proposed a novel method for fabricating NiTi coatings on pure Ti surfaces using electron-beam multi-track scanning to improve wear resistance. However, electron-beam scanning coatings exhibit spatial inhomogeneity in phase and elemental distributions, which is suggested to decrease wear resistance. In the present study, we investigated the effects of heat treatment on reducing inhomogeneity and improving the performance of the coatings. As a result, the homogeneity of the electron beam (EB) scanned coatings was significantly enhanced after annealing (900°C, 10 min), and then the Ni₄Ti₃ phase precipitated in the annealed EB-scanned coatings after aging (400°C, 60 min). The results of nanoindentation show that the hardness/indentation modulus (H/E) ratio and depth recovery ratio (η_d,h) of the Ti-rich EB-scanned coatings increased by 27% and 21%, respectively, after annealing and aging. The H/E ratio and η_d,h of the Ni-rich EB-scanned coating improved by 8% and 14%, respectively, after annealing and aging. The wear test results demonstrate that the wear rates of the two coatings decreased by 52% and 26% after annealing and aging, respectively, approaching the wear performance of bulk Nitinol.

Multi-component white cast iron (MWCI) possesses superior anti-erosion performance compared to high-chromium cast iron (HCCI). However, MWCI is not economically viable due to the high cost of transition metals. To address this issue, two types of high-chromium multi-component cast alloys (Hi-Cr MCAs) have been developed by substituting expensive transition metals with cost-effective Cr. Therefore, this study investigated erosion performance of Hi-Cr MCAs at 1173 K by reducing the contents of V, Mo, W, and Co from 5 mass% to 3 mass% and increasing the Cr content from 5 mass% to 18 mass% and 27 mass%. A comparison is made with two typical heat-treated MWCI materials (5V-5Co and 5Nb-5Co). The evaluations carried out included investigations of microstructure (matrix and carbides), hardness, heat oxidation, and wear mechanism conditions. Prior to testing, the material was heat treated to increase the hardness of the matrix and form secondary carbides. The microstructure and high temperature corrosion behavior of the material were evaluated using optical microscopy, SEM-EDS and XRD. The hardness of the material was measured using a high-temperature Vickers hardness tester. Meanwhile, a high-temperature erosion testing machine was used to understand the erosion wear characteristics of the material at 1173 K with three different impact angles (30°, 60°, and 90°). The results show the presence of M₇C₃, M₂C, and MC carbides precipitated in the microstructure of MWCI. In contrast, MC carbides were not observed in either Hi-Cr MCA, possibly due to the reduction of V, Mo, W, and Co from 5 mass% to 3 mass%. Both Hi-Cr MCAs exhibited higher carbide volume fraction (CVF) and hardness than MWCI, attributable to the increased Cr content, which in turn enhanced their high-temperature erosion wear resistance. Moreover, the superior high-temperature oxidation resistance of 27Cr-MCA contributed to its highest overall wear resistance among all specimens.

The strain-induced melt activation (SIMA) process is used to obtain fine spherical primary crystals by introducing strain into dendritic primary crystals and then heating them to a semi-solid state. In-situ observations using a laser scanning microscope elucidated the refinement and spheroidization processes during heating to the semi-solid state, which had previously only been inferred. The inverse pole figure map obtained by electron backscatter diffraction shows that, after strain introduction, the dendritic primary crystals were divided into multiple regions with different crystal orientations. During heating to the semi-solid state, linear gaps due to thermal grooving were observed in the primary crystal from about 500°C onward. Linear gaps appear at various locations in the primary crystals. When the eutectic temperature was reached, the liquid phase flowed into the gaps. The results indicate the following: In the SIMA process, a difference in crystal orientation occurs within the primary crystal when strain is induced, which is thought to function as a grain boundary. This is considered the main factor contributing to the refinement of the primary crystals in the SIMA process. Furthermore, it was confirmed that refinement occurred as the liquid phase flowed into the grain boundaries within the primary crystals.

Nine Co-Ni-Al alloys with the γ+β dual phases were designed by varying two electronic alloying parameters, Bot and Mdt, and the ingots were prepared by cold crucible melting. The phase boundaries of the γ, β, and γ+β regions in the Co-Ni-Al system were specified using the Bot-Mdt diagram. The microstructures of the nine Co-Ni-Al alloys consisted of primary γ or β phases and the eutectic of γ+β, and their volume fractions and phase stabilities were evaluated on the basis of the Bot-Mdt diagrams. The as-cast application possibility of Co-40Ni-28Al could be suggested in the view of the tensile and hardness properties as macro properties, and the segregation degree of solute elements as micro one. Similar tensile behaviors were shown on the as-cast experimental alloys with similar microstructural characteristics. The Hv and Young’s modulus values depended on the mass ratios and degree solid solution strengthening in both the γ and β phases, which resulted from the Bot and Mdt values. The maximum n value and minimum yield ratio were observed for both the Co-23Ni-23Al and Co-23Ni-18Al alloys, which indicated an inverse relationship between the Hv or Young’s modulus behavior and n or yield ratio behavior. Their alloys consisting of γ or β, and eutectic γ+β were on the equivalent vector of Ni and Al in the Bot-Mdt diagram. The oxidation curves consisted of an initial acceleration stage followed by a subsequent arrest stage, and similar oxidation behavior was observed for all experimental alloys. The high-performance Co-Ni-Al alloys with the γ+β dual phases can thus be designed within the Bot-Mdt diagram, which can be used to predict phase stability, mechanical and oxidation properties.

Titanium diboride (TiB₂) is a candidate of dispersion materials for dispersion-strengthened alloys due to its exceptional hardness and high melting point. This study focuses on the processing condition of TiB₂-Fe composite powders synthesized by using a reaction between ferrotitanium (FeTi) and ferroboron (FeB), aiming to a starting material of TiB₂-reinforced steels fabricated by the powder metallurgy method. The effects of heat-treatment conditions, B/Ti molar ratios, and scaling up production were investigated. The research demonstrates that TiB₂-Fe composite powders with fine TiB₂ particles can be fabricated by optimizing heat-treatment at 1200°C and controlling the B/Ti ratio to suppress the formation of undesirable liquid phases. The TiB₂ particles were observed to have a homogeneous dispersion in the TiB₂-Fe composite powders after crushing and milling, making them suitable for powder metallurgy processes. In addition, scale-up experiments confirmed the feasibility of industrial production, ensuring consistent quality and uniform particle distribution.

Fe was added to Al–25%Si to make a model alloy of recycled Al–25%Si. The effects of Fe addition on flow length, the formability of thin fins, casting cracks, and heat dissipation were investigated. The addition of Fe did not significantly affect flow length, the formability of thin fins, casting cracks, and heat dissipation. To produce an economical aluminum alloy with a Si content of 25%, Si was added to ADC12 aluminum alloy for die casting. ADC12, which contains about 10% Si, was made from recycled aluminum alloy and was economical. A heat sink with thin fins, where the fin tip was 1 mm and the fin height was 50 mm, could be made using Si-added ADC12 with a Si content of 25%. The addition of Si to ADC12 up to 25% was effective in creating an economical heat sink with thin and tall fins.

This Paper was Originally Published in Japanese in J. JSTP 66 (2025) 151–156.

The effects of die temperature, die gap, plunger speed and molten metal temperature on the flow length of A1070 pure aluminum in a thin die gap were investigated. The die gaps were 0.5, 0.8 and 1.0 mm. The plunger speeds were 0.2, 0.4, 0.6 and 0.8 m/s. The die temperatures were 30, 70, 110 and 150°C. The molten metal temperatures were 680, 730 and 780°C. When the die gap was 0.5 mm, the flow length was largest when the die temperature was 30°C and the plunger speed was 0.2 m/s, which is contrary to conventional expectations. However, when the die gap was 1.0 mm, the flow length increased as the die temperature and plunger speed increased, which aligns with previously reported results. It became clear that when the die gap was 0.5 mm, the effects of die temperature and plunger speed on flow length were opposite to those for a die gap wider than 0.5 mm. The cause of this phenomenon is discussed in terms of the adhesion and peeling of the solidification layer.

This Paper was Originally Published in Japanese in J. JSTP 66 (2025) 205–211.

CoCrFeMnNi high-entropy alloys (HEAs) have recently attracted considerable attention owing to their excellent tensile strength and ductility at low temperatures. However, regardless of the fabrication method, conventional CoCrFeMnNi HEAs generally exhibit lower hardness compared with that of traditional steels. To address this limitation, surface-modification techniques have been employed as an effective approach to enhance the surface hardness of CoCrFeMnNi HEAs. In this study, CoCrFeMnNi HEAs were sintered and subsequently treated by screen-assisted direct current plasma nitriding (S-DCPN) using a Ni-containing multi-screen; the properties of the nitrided samples were then systematically evaluated. During S-DCPN, three types of auxiliary cathode screens, namely stainless steel (SUS316L), Ni-based superalloy (Alloy 600), and pure Ni, were employed to ensure uniform heating and enhanced nitrogen activation. The nitrided samples were characterized by X-ray diffraction, scanning electron microscopy, glow discharge optical emission spectroscopy, and surface hardness and wear tests. Compared with the untreated specimens, all nitrided samples exhibited a significant increase in surface hardness and wear resistance, confirming the effectiveness of the S-DCPN process for strengthening CoCrFeMnNi HEAs.

In Mg₂(Si,Sn)-based alloys, compositional gradient layered microstructure (CGLM) has been reported as an effective factor in enhancing thermoelectric performance, primarily due to reduced lattice thermal conductivity. However, it is quite difficult to control CGLM, since Mg loss via evaporation and oxidation during the melting and solidification processes tends to gradually shift the composition of the liquid phase away from the nominal composition of Mg₂(Si,Sn) toward the Mg-poor side of the phase diagram. This leads to the formation of Si and Sn phases as the final products of non-equilibrium solidification. Hence, compensation for Mg loss is necessary to facilitate solidification being completed within the Mg₂Si–Mg₂Sn pseudo-binary system under a proper condition of solidification. Consequently, an alloy consisting almost entirely of CGLM (hereafter denoted as Full CGLM), achieved by 10 at% excess Mg compensation in this work, comprises six layers, starting from the primary solidification of Mg₂Si phase as a core part and extending to the outer shell layer of Mg₂Sn phase formed via solidification after the peritectic reaction. The core of the primary phase is considered to be a single crystal grain, while each surrounding layer of CGLM is composed of Mg₂(Si,Sn) polycrystalline solid solutions exhibiting widely varied Si/Sn ratios following the solidus line of the Mg₂Si–Mg₂Sn phase diagram. When the solidification route, presented by a monovariant line on the liquidus surface projection of the Mg–Si–Sn ternary phase diagram, deviates from the pseudo-binary edge toward the Mg-rich side, corresponding to overcompensation for Mg loss, solidification terminates with the formation of eutectic Mg and Mg₂Sn phases as the final products. The single colony size of CGLM decreases as the cooling rate of solidification becomes higher, since the nucleation of the core Mg₂Si is enhanced. Ultimately, the Full CGLM alloy exhibits a significantly low lattice thermal conductivity of 1.22 W m⁻¹ K⁻¹ at 573 K, which is attributed not only to the solid solution effect of Mg₂(Si,Sn) phases with a large volume fraction of CGLM but also to a larger interfacial area with a high density of misfit dislocations induced by the large differences in lattice parameters between Mg₂(Si,Sn) grains according to the widely spread Si/Sn ratios.

The effect of Nb content on the oxidation behavior of Al₂O₃-forming ferritic stainless steels with different Al contents was investigated. The Al₂O₃ scales formed on all alloys exhibited a duplex structure, with the outer layer containing Fe and Cr and the inner layer consisting of relatively pure Al₂O₃ oxide. No apparent influence of Nb on the growth rate of the outer oxide scale was observed among alloys with the same Al content, but the growth rate of the inner oxide layer increased with increasing Nb content. This effect of Nb was more pronounced in alloys with lower Al content. In this study, it was also observed that the growth rates of both the inner and outer oxide scales increased with increasing Al content. Fe and/or Nb segregation was confirmed in the Al₂O₃ scale formed on 1Al and 3Al alloys with Nb addition. However, neither Fe nor Nb segregation was detected in the Al₂O₃ scales formed on 5Al alloys, with or without Nb addition. EELS analysis suggested that Nb⁴⁺ segregated along the grain boundaries of the inner Al₂O₃ layer formed on 1Al and 3Al alloys, which could explain the enhanced inward diffusion of oxygen, resulting in the faster growth of the inner Al₂O₃ layer.

Recovery of precious metals, Au, Pt, and Pd, from hydrochloric solution using sulfur-impregnated carbonaceous adsorbent was examined. Sulfur-impregnated adsorbent with adsorption abilities of precious metals can be prepared from activated carbon using pyrolysis at 600°C under 10% H₂S gas flow, and the adsorbed Au and Pd were precipitated as metal phase on the surface of the adsorbent. The adsorbents after precious metals adsorption were burned at 700°C to obtain the residue mainly containing precious metals. The adsorbent was found to be highly efficient and selective for the uptake of precious metals, especially Au, in the presence of excess base metal ions from simulated spent automotive catalyst eluted solution using a column pack test.

The dissolution of platinum-group metals (PGMs: Pt, Pd, and Rh) from spent automotive catalysts using aqua regia or HCl/Cl₂ is significantly inhibited by the low solubility of ZrO₂, CeO₂, and PGM oxides (PtO₂, PdO, and Rh₂O₃), as well as by the formation of solid solutions between these oxides and the coating layer. In this study, a NH₄HSO₄ molten salt treatment was investigated as a pretreatment to improve the recovery of PGMs. The molten salt reduced the PtO₂ and PdO to metallic Pt and Pd, which are soluble in aqua regia, and decomposed the coating layer containing ZrO₂, CeO₂, and Rh₂O₃, thereby releasing the PGMs as ultrafine particles. These results demonstrate that the NH₄HSO₄ molten salt treatment enhances the recovery of PGMs from spent automotive catalysts.

In this study, selective fluorine adsorption behavior of zirconium silicate in the presence of coexisting anions was evaluated using a high concentration of iodine-containing liquid waste with a small amount of fluorine for the purpose of recycling iodine. Fluoride solutions with iodine, bromine, chlorine, and sulfate ions as coexisting anions were prepared, and the fluorine adsorption behavior was evaluated in terms of adsorption amount and selectivity coefficient. The results showed that fluorine was selectively adsorbed even in solutions containing 1000 times more coexisting anions than fluorine, and that the concentration of residual fluorine could be reduced below 8 mg/L, the target value for fluorine removal, by adding 30 g/L of ZrSiO₄ to the simulated effluent. This indicates that selective fluorine removal from iodine-containing liquid waste with ZrSiO₄ is feasible.

Zeolite products, including zeolites A and X, were successfully synthesized from two types of industrial waste, crushed stone cake and aluminum dross, using alkali fusion treatment, with a high removal ability for Ba²⁺ and Sr²⁺. Waste stone cake and aluminum dross are industrial waste materials, and their effective utilization is highly anticipated. Because the main components of the two waste materials are Si, Al, and O, they can be used as starting materials for the synthesis of zeolites. In this study, industrial waste was converted into crystalline zeolite products using alkali fusion. The stone cake, dross, and a mixture of these materials were transformed into a soluble phase via alkali fusion and then agitated in distilled water at room temperature to obtain an intermediate gel-like solid, followed by synthesis at 80°C to obtain the final product. The zeolites were successfully synthesized via the alkali fusion process, and selective synthesis of zeolites A and X was achieved by controlling the mixing ratio of aluminum dross to stone cake, with different cation exchange capacities (CECs). The adsorption of Ba²⁺ and Sr²⁺ from aqueous solutions using zeolite products synthesized from industrial waste, waste stone cake, and aluminum dross was examined. The zeolite products from the mixture of stone powder and dross (3:1) exhibited higher adsorption abilities for Ba and Sr ions from aqueous solution than commercial zeolite-X and zeolite-A.

The magnetostrictive properties of Cu_0.5Co_0.5Fe_2.0O₄ were controlled via partial substitution with various trivalent ions. A single-phase cubic spinel structure was successfully obtained for the Cu_0.5Co_0.5X_0.1Fe_1.9O₄ (X = Al, Cr, Mn, Ga, and In) samples, indicating that Fe³⁺ ions in Cu_0.5Co_0.5Fe_2.0O₄ can be partially substituted by Al³⁺, Cr³⁺, Mn³⁺, Ga³⁺, and In³⁺ ions. In the magnetic field dependence of the strain, the significant enhancement in magnetic field sensitivity of the strain at low magnetic fields was observed for the X = Mn and Ga samples. When the magnetization direction changed from parallel to perpendicular (or vice versa) to the measurement direction, the X = Mn and Ga samples demonstrated a larger strain magnitude at approximately half the magnetic field of CoFe₂O₄. Therefore, the partial substitution of trivalent ions, particularly Mn³⁺ and Ga³⁺ ions for Fe³⁺ ions, provides an effective strategy to develop superior magnetostrictive materials.

A 0.1 M NaCl solution film containing bromothymol blue and having a thickness of 0.4 mm was formed on a re-melted die-cast ADC12 aluminum alloy (Al–12Si–Cu). An optical microscope was used to identify cathodic reaction sites by observing the color changes caused by local alkalization. In the alloy, Al₂Cu phase, Fe-containing phase, Si-rich phase, and a phase rich in Mg, Al, Si, and Cu were present in addition to the Al-matrix. A color transition from yellow to blue, indicating alkalization, first occurred in the Al₂Cu phase and then spread to the Fe-containing phase. Based on the surface morphology of each phase after immersion, the difference in cathodic activities between the Fe-containing and Al₂Cu phases were analyzed and discussed.