Some high-entropy alloys (HEAs) with the face-centered cubic structure have an excellent strength-ductility balance. While unique deformation modes such as fine twinning patterns other than dislocation glide contribute to the mechanical properties, it is still unclear what properties and features of HEAs cause such unique deformation process. In the present study, fundamental properties responsible for the excellent mechanical properties of CoCrFeNiMn and its subsystems were explored comprehensively by the first-principles calculations. The local lattice distortion reaches almost 2% of the Burgers vector, which contributes to improving strength in HEAs. Furthermore, the stacking fault energy (SFE) was significantly low in random solid solution, while it increases around some domains where the short-range order (SRO) is formed. The increase in the SFE is caused by the disturbance of the chemical SRO and the spin order due to the SF formation. Our calculations suggest that low and high SFE domains distributed in a solid solution region unique to HEAs lead to successive activation of various deformation modes (Plaston), which achieves excellent strength-ductility balance.

Neutron total-scattering experiments were conducted to search for signatures of the short-range ordering in medium-entropy alloys Tr-Co-Ni (Tr = Cr, Mn, and Fe). The reduced pair distribution function of the Mn-Co-Ni sample clearly deviates from that of the fully random face-centered cubic structure, whereas those of the as-quenched Cr-Co-Ni and Fe-Co-Ni samples can be explained with an almost random face-centered cubic structure model. The results of neutron thermal analyses and reverse Monte Carlo structure modeling indicate a possible compositional and/or magnetic short-range ordering in the Mn-Co-Ni system.

The effective atomic radii of the constituent elements and their temperature dependence in quaternary and ternary subset alloys derived from the CrMnFeCoNi high-entropy alloy have been experimentally determined by a combination of lattice parameter measurements at room temperature and thermal expansion measurements down to the liquid helium temperature. The determined relative order of the effective atomic radii at 0 K is Co ≪ Cr < Ni < Fe ≪ Mn. The temperature dependence of the effective atomic radii differs from each other, but the difference is not large enough to change the relative order of the effective atomic radii at 0 K and room temperature. Absolute values and relative order among the elements do not agree with those calculated using the ab-initio calculations.

The current trend in research on the physical properties of high-entropy alloys has been progressively increasing as there are many unknown possibilities for developing high-entropy alloys for advanced applications. This study investigated the effect of microstructures of rolled high-entropy alloy from the viewpoint of crystal orientation and dislocation density distribution to reproduce mechanical anisotropy using crystal plasticity finite element simulation. The crystal orientation and the geometrically necessary dislocation density of the rolled material were quantitatively estimated from experimental data of electron backscatter diffraction. Microstructural observation showed that particular textures were preferably oriented like in typical FCC metals. Even though the simulation results where only the preferred crystal orientation was considered did not show the expected mechanical anisotropy as in the experiment, the computational model with the dislocation density distribution and the preferred orientation showed the same tendency as the experiment.

In this study, we focused on the effects of various deformation amounts and subsequent heat treatment on σ-phase precipitation in CoCrFeMnNi high entropy alloy, known as Cantor alloy. Homogenized specimens with an initial FCC single-phase structure were deformed to various shear strains (1.1–25.8) using high-pressure torsion (HPT). This process resulted in a variety of deformation microstructures, with low to medium shear strains leading to the formation of twin-matrix lamellae intersected by shear bands, while high shear strains resulted in nanocrystalline structures. After deformation, the specimens were heat-treated at 700°C for up to 5 h, which led to recrystallization of the FCC matrix accompanied by precipitation of σ phase. The kinetics of recrystallization and precipitation and their interactions during the heat treatment were greatly different among the specimens with different degrees of pre-deformation. Notably, the precipitation of σ phase was accelerated in the specimens subjected to higher shear strains, particularly in those with nanocrystalline structures. The increased rate of precipitation was beneficial for grain refinement since the presence of numerous precipitates within the recrystallized microstructures inhibited their grain growth. Tensile testing of the heat-treated specimens displayed various combinations of strength and ductility, with specimens subjected to higher pre-deformation exhibiting enhanced strength due to finer recrystallized grain sizes and larger fractions of precipitates. Our findings offer valuable insights into fabrication processing of HEAs, aiming to optimize microstructure and mechanical properties for potential engineering applications.

In this study, thermodynamic quantities were investigated using cluster expansion and the variational method for high-entropy and medium-entropy alloys (HEAs and MEAs, respectively). In the alloy systems combined with elements that tend to form HEAs, the absolute enthalpy value was small, and the entropy term was close to the value of an ideal solution. The tendency of ordering and the accompanying change in mean square atomic displacement (MSAD) were investigated in detail for four MEAs; namely, the FeCoNi, MnFeNi, MnCoNi, and CrCoNi systems. For these alloys, the ordering tendency and the phase-separation tendency in the low-temperature region were investigated. In addition, the presence of short-range order (SRO) in the high-temperature range was confirmed from the analysis using the Warren–Cowley order parameter. Thermal equilibrium structures were used to evaluate the MSADs at various temperatures. Long-range ordering developing at low temperature tended to reduce MSAD, although in the high temperature range the effect on MSAD from the SRO was very small.

Chemically ordered face-centered cubic (fcc) phases consisting mainly of Cr, Co, and Ni were synthesized by annealing a Cr_26.7Co_26.7Ni_26.7Si₁₀B₅P₅ amorphous alloy. The annealed Cr_26.7Co_26.7Ni_26.7Si₁₀B₅P₅ sample was composed of Cr_4.5BP₂, Cr₃Co₅Si₂, and fcc phases. Synchrotron radiation anomalous X-ray scattering (AXS) measurements confirmed that the annealed Cr_26.7Co_26.7Ni_26.7Si₁₀B₅P₅ sample had superlattice reflections from an L1₀-, and/or L1₂-, and D0₂₂-type ordered structure. The diffraction intensities of the superlattice reflections observed in the AXS measurements indicated strong dependence on the incident X-ray energy. The diffraction intensity of the superlattice reflection increased at the X-ray energy near the K-edge of Cr, suggesting that Cr is mainly ordered in the L1₀- and D0₂₂-type structures. Microstructural analysis using scanning transmission electron microscopy (STEM) revealed crystal grains with diameters of approximately 20 nm, showing diffraction spots corresponding to superlattice reflections of 001 and 110, in addition to a two-dimensional diffraction pattern corresponding to the fcc structure. Simultaneous STEM–energy dispersive X-ray spectroscopy analysis indicated that the grains had a higher Cr concentration than the mother alloy composition of Cr_26.7Co_26.7Ni_26.7Si₁₀B₅P₅.

TiZrHfAgV_0.2 (Ti_23.8Zr_23.8Hf_23.8Ag_23.8V_4.8, at%) high entropy alloys (HEAs) with a dual bcc structure were developed. Fine lamella structure was observed in the arc-melted ingots and melt-spun ribbons. The TiZrHfAgV_0.2 HEAs with a dual bcc phase structure were designed by exploiting the concept of immiscibility of the constituent elements. The immiscibility of the constituent elements in the multi-component alloys was discussed using the mixing enthalpy (ΔH_i-j) matrix of the i-j elements, binary phase diagrams with liquid miscibility gap, and the thermodynamic calculations.

Formation of stacking fault tetrahedra and diffuse streaks along 〈111〉 in the equiatomic Cr-Co-Ni medium-entropy alloy subjected to different heat-treatments and different specimen preparation methods are examined by transmission electron microscopy. Neither stacking faults nor stacking fault tetrahedra are formed simply by quenching from a high temperature due to the high energy barrier for the formation and migration of vacancies. These defects, however, are found to form by Ar⁺-ion irradiation, as abundant vacancies are continuously introduced during ion-irradiation so as to bypass the high energy barrier for the formation. Diffuse streaks along 〈111〉 are usually observed in the SAED pattern with the 〈110〉 incidence regardless of heat-treatments and specimen preparation methods, indicating the occurrence of diffuse streaks is nothing to do with the formation of stacking faults and stacking fault tetrahedra.

The strain-rate dependence of slip persistence in TiZrNbHfTa was investigated using microcantilever bending tests instead of conventional single-crystal tensile tests. These tests were performed using micrometre-sized cantilevers fabricated within a grain of the polycrystalline material, and the ψ–χ relationship was obtained in detail. Bending tests were conducted in two strain rates to determine the ψ–χ relationship. The results showed that the slip plane macroscopically persisted on the {112} plane in the high-strain-rate test, while in the low-strain-rate test, the apparent slip plane roughly coincided with the plane where the maximum shear stress was applied. Detailed tracing of the slip bands, using atomic force microscopy, on the specimen surface showed that the slip plane was microscopically persisted on the {112} plane in high-strain-rate tests, while in low-strain-rate tests, the slip plane often cross-slipped between the {112} and {110} planes.

This study explores the conduction mechanism of NbTe₄, a novel phase-change material (PCM) for phase-change random access memory (PCRAM), and addresses the limitations of the widely used Ge₂Sb₂Te₅ (GST). Unlike traditional PCMs, NbTe₄ in its amorphous state demonstrates low resistance, which indicates semiconductor behavior. However, the Hall and Seebeck coefficient measurements reveal an intriguing anomaly—amorphous NbTe₄ displays N-type conduction with Hall voltage and P-type conduction in the positive Seebeck coefficient. This Hall effect anomaly, which is typically associated with highly resistive chalcogenide materials, raises questions about the conduction mechanism in amorphous NbTe₄. This study delves into the electrical transport properties of NbTe₄ and provides insights into the unique characteristics of this PCM.

We investigated the conditions under which extremely thin films of CuCl are formed by molecular-beam epitaxy (MBE) on Si substrates, which are expected to grow epitaxially due to the small lattice mismatch of 0.44% with CuCl. Atomic force microscopy (AFM) observations of samples fabricated under various growth conditions revealed that the lower the substrate temperature, the thinner films could be formed. Experimental results also showed that suppression of the sublimation effect is effective for thinner film formation in CuCl, which has a low melting point, and a thin film of around 10 atomic layers is successfully fabricated by growth with the lowest substrate temperature.

In this study, FDEM (ECZM), which is the combined finite-discrete element method (FDEM) based on an extrinsic cohesive zone model (ECZM), was improved by incorporating a novel algorithm that makes it easier to insert the cohesive elements than the conventional adaptive remeshing and a scheme to avoid the generation of dormant cohesive element, which is generated as a result of inserting a single cohesive element within the FDEM mesh. The improved FDEM (ECZM) was implemented in a self-developed 2-dimensional code, and it was first validated against a numerical experiment assuming a virtual rock. Subsequently, the proposed FDEM was applied to model the uniaxial compression and the Brazilian tests of siliceous mudstones under quasi-static loading. The results of the proposed FDEM simulations reasonably captured the trends of fracturing and stress-strain curves observed in the experiments well. The proposed scheme can also be extended to parallel computation based on general-purpose-graphic-processing-unit (GPGPU) and 3-dimensional FDEM (ECZM) with minimal efforts.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 71 (2022) 206–213. References were modified due to duplications.

This study investigated the basal and prismatic slip systems in the 10H, 14H, and 18H long-period stacking-ordered (LPSO) structures using first-principles calculations based on density functional theory to understand the plastic deformation behavior of Mg–Zn–Y alloy with an LPSO structure from the atomistic scale. The generalized stacking fault energies (GSFEs) of the basal plane at positions not crossing the solute clusters, along the 〈a〉 direction of LPSO structures are 14%–27% higher than that of hexagonal close-packed (hcp)-Mg, thus causing the basal slip in LPSO structures slightly more difficult than hcp-Mg. The GSFEs of the prismatic plane along the 〈a〉 direction were over 50% higher than that of hcp-Mg due to the influence of solute clusters. These results suggest that the LPSO structures in Mg–Zn–Y alloys have a higher resistance to dislocation motion than hcp-Mg in terms of the basal and prismatic slips, and the anisotropy is more emphasized in LPSO structures. A linear relationship was found between the GSFE and the solute cluster density on the prismatic plane of the hexagonal-type LPSO structures, i.e., GSFE increases as Mg layers sandwiched between solute cluster layers in the unit cell decrease. We proposed an equation to estimate the stacking fault energy of the solute cluster regions on the prismatic planes with LPSO structures. The estimated stacking fault energy in the part of solute cluster layers on the prismatic plane was almost the same regardless of the hexagonal-type LPSO structure.

The stress corrosion cracking (SCC) behavior of an extruded AZ31 alloy plate with a large planar anisotropy was investigated using constant load test in 0.01 mol/l NaCl solution at 35°C. Due to the texture with a large spread of (0001) orientation in the transverse direction (TD), the extruded AZ31 alloy plate exhibited a much higher yield strength (169 vs. 70 MPa) along the extrusion direction (ED) compared with the TD. The threshold stress for SCC was slightly higher for the ED specimen compared with the TD specimen (90 vs. 70 MPa) due to the much higher yield strength. However, the ratio between threshold stress and yield strength was much larger for the TD specimen (100% vs. 53%), indicating its low susceptibility to SCC. Corrosion grooves formed perpendicular to the tensile direction and along the interstices of Mg(OH)₂ corrosion product film. Corrosion grooves acting as fracture initiation sites tended to propagate along non-basal planes, particularly, prismatic plane, due to lower corrosion resistance compared with basal plane. The higher resistance to SCC for the TD specimen may be attributed to the higher probability of basal plane on the exposed surfaces of corrosion grooves.

This Paper was Originally Published in Japanese in J. JILM 74 (2024) 83–90.

To predict the failure of mechanical parts, it is necessary to understand the residual stress and its source, i.e., inherent strain. In this study, the distribution of three-directional residual stress components in a carburized 18NiCrMo14-6 cylindrical roller test piece with a diameter of 80 mm and length of 240 mm, was measured using the extended contour method (XCM), which combines the contour method and X-ray diffraction. Thereafter, an inverse analysis was applied to the measured residual stress to reproduce the associated inherent strain distribution. First, it was shown through numerical experiments of a carburized cylindrical specimen that the distribution of the three-directional residual stress components can be accurately reproduced using XCM. Next, it was demonstrated that the distribution of the three-directional residual stress components could be obtained using general-purpose equipment by physically measuring the same type of specimen. The inherent strain distributions were evaluated. Compressive residual stress and corresponding tensile inherent strain were detected in the carburized layer. By contrast, tensile stress and inherent contraction strain were determined in the non-carburized layer just before the carburized crust. Finally, the mechanism of inherent strain generation was investigated using a thermal–elastic–plastic analysis. Possible explanations include (i) increase in transformation strain due to a change in carbon content, (ii) delay in martensitic transformation, and (iii) decrease in the martensitic transformation rate due to a decrease in the cooling rate at the core.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 72 (2023) 550–557. The abstract, the captions of Figs. 2, 4–9, 11–14, and the section 4.3 are slightly modified.

In magnetic pulse welding, the deformation phenomenon of a moving sheet is divided into the deformation and continuous collision processes, then, the deformation analysis of the sheet is performed by using a magnetic pressure delivered to the sheet. When applying the pressure provided by neglecting an electric field generated during the deformation of the sheet, the calculated values accord only with one point for many measured values of both deformation height and deformation velocity, and errors greatly increase as the calculated values deviate from the point according. The new magnetic pressure corrected using a pressure drop coefficient is introduced because the deforming sheet leads to a drop in the pressure. If a successive approximation is applied to the deformation analysis, many calculated values approach the standard measured values. The calculated values are within a small error range for the measured values at 17 out of 18 points. It has been clarified that the deformation of the moving sheet is estimated fairly accurately by using the successive approximation.

This Paper was Originally Published in Japanese in J. JSTP 64 (2023) 159–164.

Single crystal elastic constants are vital parameters associated with the mechanical properties of polycrystals. To accurately invert these constants, precise experimental techniques must be employed. However, traditional ultrasonic measurement methods can introduce errors when dealing with complex microstructures. To address this issue, we propose an approach that combines metallographic measurements, homogenization techniques, and intelligent algorithms to invert the single crystal elastic constants of polycrystals. First, we developed a modified single scattering response (SSR) model based on metallographic measurements and the best-fitted spatial correlation function (SCF). In addition, different homogenization techniques were compared to enhance the accuracy of the model. We then propose an Adaptive Probability Differential Evolution Quantum Particle Swarm Optimization (AP-DE-QPSO) algorithm to enhance the accuracy of optimization results with an adaptive probability (AP) operator in the optimization process. After verification with three standard test functions, we demonstrated that the algorithm achieved higher accuracy. Finally, by utilizing the AP-DE-QPSO algorithm and fitting the spatial variance of the modified SSR model to the ultrasonic backscattering experimental data, we successfully inverted the global optimal solution of the single crystal elastic constants. The Self-Consistent averaging technique yielded the best results, with relative errors of approximately 3% for C₁₁ and C₄₄, and a relative error of approximately 10% for C₁₂. Our study confirms the accuracy and reliability of the modified SSR model and AP-DE-QPSO algorithm in inverting the elastic constants of single crystals.

The adsorption structure of atomic oxygen on the PdZn(111) surface is studied using the first-principles calculation. At lower coverages up to 0.5 ML, oxygen atoms prefer to adsorb at threefold hollow sites surrounded by two Zn atoms and a Pd atom. Surface Zn atoms bonded with adsorbed oxygen atoms move toward the vacuum region, and the maximum displacement has reached 0.1 nm at 0.375 ML. At higher coverages from 0.625 ML to 1.0 ML, some oxygen atoms intruded into the surface layer and formed bonds with the Zn atoms in the second layer. At 0.75 ML, we observed the formation of the Zn-O hexagonal rings on the surface layer. It seems that the formation of the hexagonal rings largely contributes to the stability of the surface layer. However, the surface DOS and H₂O adsorption indicate no essential difference owing to the hexagonal Zn-O rings, and the hexagonal Zn-O rings disappeared at higher coverages. We discussed the relation of the adsorption energy, coverage, and the number of atoms located in the vicinity of the adsorbed oxygen atoms.

This paper focuses on the effect of tungsten addition on the steam oxidation behavior of the Fe-20Cr-35Ni (at%) alloy at 1073 K. Steam oxidation tests of the W added Fe-20Cr-35Ni (at%) alloy were carried out at 1073 K in an Ar-15%H₂O gas mixture for up to 604.8 ks. The oxidation resistance of the alloys was improved by adding W into the alloy. There are two effects of W on the steam oxidation behavior: the effect of solid solution and the effect of precipitation. Tungsten in the alloy is enriched as a Ni-W alloy in the matrix of the internal oxidation zone (IOZ) during oxidation. The Ni-W alloy in the IOZ may act as a diffusion barrier for oxygen and promote a continuous Cr₂O₃ layer. On the other hand, Fe₂W precipitated in the alloy decomposes during oxidation and provides alloying elements for forming FeCr₂O₄ and CrWO₄ in the IOZ, resulting in a continuous Cr₂O₃ layer formation.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 86 (2022) 121–130.

The effect of structure of organic additives on the electrodeposition behavior of Zn from alkaline zincate solution and its crystal morphology was investigated. Zn was electrodeposited on an Fe electrode at 20–1000 A·m⁻², 2.4 × 10⁴ C·m⁻², 300 K from unagitated zincate solutions containing the various organic additives as a leveling agent. The suppression effect of additives on the charge transfer and diffusion of ZnO₂²⁻ ions in Zn electrodeposition corresponded to the number of adsorption site per a straight chain molecule of polymer. The effect of polymer alone on the decrease in size of Zn platelets crystals was small, but the crystal size significantly decreased with coexistence of low molecular additive. The crystal size of deposited Zn decreased in spite of small suppression effect on Zn deposition, showing that the crystal size of deposited Zn doesn’t depend on the overpotential for deposition. With coexistence of low molecular additive with polymer, the crystal of deposited Zn was fine regardless of kind of polymer even though Zn deposited at the diffusion control of ZnO₂²⁻ ions.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 88 (2024) 58–67.

Perovskite solar cells (PSCs) can produce solar energy that is both affordable and highly effective. Still, they currently face challenges in achieving peak performance in important areas, including sustainability, stability, and efficiency. Recent studies examine tandem perovskite solar cells based on CsPbI₃ in great detail, analyzing their photovoltaic characteristics with SCAPS 1D software. This work examines the effects of multiple parameters on performance metrics, including power conversion efficiency (PCE), fill factor (FF), open-circuit voltage (Voc), and short-circuit current (Jsc), with a focus on a multi-layered design. The photoactive layer thickness, defect densities, electrode contact quality, and operation temperatures are the factors. Compared to conventional lead-based perovskites, CsPbI₃ offers advantages in terms of long-term stability, reduced moisture susceptibility, and reduced lead toxicity. However, there is an issue with achieving efficiency levels comparable to MAPbI₃ and FAPbI₃. The research reveals correlations between material properties and device performance by applying advanced diagnostic techniques like quantum efficiency (QE), carrier concentration, and recombination rate analysis. This information has the potential to result in material enhancements and device optimization. With a particular focus on CsPbI₃, the work offers crucial insights into tandem perovskite solar cells that will advance the creation of more reliable, effective, and sustainable solar energy systems.

The influence of sample geometry on various measured physical properties (including the magneto-Seebeck effect, Nernst effect, magnetoresistance effect, Hall effect, and thermal conductivity) in the presence of a magnetic field was examined using a polycrystalline Bi-Sb sample. The sample, consisting of polycrystalline Bi₈₈Sb₁₂, was prepared through spark plasma sintering and subsequent annealing. Measurements of the physical properties were conducted under a magnetic field of 5 T, and the obtained values were compared with simulated results derived using the finite element method for different sample geometries. Distinct shapes were found to be necessary for accurate measurements, with each physical property requiring a specific aspect ratio of sample length (l) to width (w). These ratios were determined to be l/w > 0.57, 2.9, 4.2, 1.2, and 3.1 for the magnetoresistance, Hall, two-wire magneto-Seebeck, four-wire magneto-Seebeck, and Nernst effects, respectively. Additionally, to achieve a minimal error of less than 2% in thermal conductivity measurement, a thermal conductance ratio of K_s/K_w > 27 was necessary, where K_s and K_w denote the thermal conductance of the sample and lead wire, respectively.

This Paper was Originally Published in Japanese in J. Thermoelec. Soc. Jpn. 20 (2023) 67–74. The vertical axis label of the graph in Fig. 9 is modified.

In ultra low-carbon Fe-18%Ni alloy with a lath martensitic structure, the effects of microstructure and dislocation density on the yielding and deformation behavior were investigated. The microstructure of the matrix became finer with decreasing austenite grain size but no difference was found in the yielding behavior up to 2% deformation. Dislocation density was constant independent of the microstructure and was estimated to be about 1.5 × 10¹⁵/m². The dislocations that had been introduced by martensitic transformation formed a tangled dislocation cell structure within a lath and the amount of dislocation strengthening was determined by a critical stress that was required to make the tangled dislocations bow out. As a result, it was confirmed that the 2% proof stress of the steel used can be evaluated by adding the amount of dislocation strengthening to the friction stress.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 73 (2024) 306–313. In section 4.3, a minor correction is added.

This study introduces research trends in the electronics materials, such as joining materials and methods, reliability evaluations and conductive materials by reviewing the special issue published in Materials Transactions (Vol. 63, No. 6) entitled Frontier Researches Related to Interconnection, Packaging and Microjoining Materials and Microprocessing -Part III-. In the recent development of bonding materials and reliability evaluation methods for electronic components, in order to improve the bonding reliability bonding materials and bonding methods for not only dissimilar metals but also metals and resins by focusing on elements and processes that have received little attention are proposed. On the other hand, in the field of conductive materials, research that is not limited by conventional methods is being proposed in order to improve the efficiency of heat/electrical conduction and thermoelectric conversion.

The 18th International Conference on Aluminium Alloys (ICAA18) was held in Toyama, Japan from September 4 to 8, 2022, and the special issue entitled “Aluminium and Its Alloys for Zero Carbon Society” was published on Materials Transactions in February 2023 (Vol. 64, No. 2). The biennial conference covered a wide range of current trends in aluminium research; e.g. “modeling and simulation”, “casting, solidification, recycling and refining”, “additive manufacturing”, “foams and composite materials”, “mechanical properties and advanced processing”, “thermomechanical processing, texture and recrystallization”, “heat treatment, phase transformation and precipitation”, “corrosion and surface treatments”, “joining, emerging processes and multi material” and “advanced characterization”. This article briefly reviews selected papers from the conference with significant experimental outcome and discussion on aluminium alloys. The author hopes that these papers are useful for all the researchers who develop next-generation technologies and materials concerning aluminium alloys.