This review delineates the mechanisms of pit nucleation and growth, establishes the foundational principles of etching technology, and presents the findings from investigations on the behavior of anodic dissolution and anodic film formation on high-purity aluminum foils for electrolytic capacitors based on electrochemical analyses and surface electron microscopic observations of etched surfaces. To elucidate pit nucleation and growth mechanisms, the effects of crystalline oxide and small amounts of lead on etching behavior were investigated. Pits initiate at cracks surrounding MgAl₂O₄ spinel or γ-Al₂O₃, resulting from the crystallization of the oxide film at metal ridges on the aluminum substrate. Using ultra-high-resolution field-emission scanning electron microscope (FE-SEM), high-angle backscattered electron (BSE) images revealed the presence of lead as the bright nanoparticles, approximately 10 nm in size, at the surface oxidation layer along rolling lines attributable to pick-up inclusions during hot rolling.

Pitting attacks predominantly occur in the oxidation layer owing to the less noble potential for tunnel dissolution in the initial DC etching phase. Increased titanium content within the aluminum foil accelerated hydrogen evolution in the pit and hydrous oxide formation during the cathodic half-cycle of alternating current etching. The crystallization of anodic oxide films around MgAl₂O₄ spinel crystals, formed in a boric acid solution, was observed using transmission electron microscopy (TEM). Round-shaped γ′-Al₂O₃ formed around the MgAl₂O₄ crystals and expanded across the surface as the formation voltage increased.

Analytic expressions for the absorption coefficient (AC) of a weak electromagnetic wave (EMW) in 2D Graphene under influence of strong laser radiation are calculated using the quantum kinetic equation (QKE) in the case of electron-optical phonon scattering in both the absence and presence of a magnetic field perpendicular to the graphene sheet. The dependence of the AC on the intensity E₀₂ and the frequency Ω₂ of a weak EMW, on the intensity E₀₁ and the frequency Ω₁ of a strong laser radiation, on the temperature T of the system is obtained. These results are investigated from low temperature to high temperature. These results are obtained from the QKE method, which broke the limit of the Boltzmann kinetic equations (only investigated in the high-temperature domain). Besides, the numerical results show that the AC of a weak EMW in 2D Graphene can have negative values. This demonstrates the possibility of increasing weak EMW by strong laser radiation in 2D Graphene. This is different from a similar problem in bulk semiconductors and the case without strong laser radiation. In the case of the presence of an external magnetic field, the numerical calculation results also show the appearance of the peak spectral lines that obey the magneto-phonon resonance conditions. The appearance of these resonance peaks provides a model illustrating the dependence of the Half-Width at Half Maximum (HWHM) on the external magnetic field. This is an important criterion for the fabrication of graphene-related electronic components and orientation for future experiments.

The porosity of rocks is one of the most fundamental physical properties and is required to quantitatively evaluate the characteristics of rocks in drilling projects in fault zones. In the drilling project of the Futagawa fault, which ruptured during the 2016 Kumamoto earthquake mainshock, although the porosities of intact rock core samples were measured, there was no continuous porosity profile because core samples could not be obtained in fractured zones. Therefore, we estimated a vertical, continuous porosity profile for a depth interval of approximately 300–660 m, except for 383–399 m in borehole FDB-1 of the project, using sonic log data. First, we tested several different empirical equations proposed in previous studies for both sedimentary and volcanic rocks and proposed a new equation considering the effects of compaction and lithology on sedimentary rocks. Second, we compared the estimated porosities with the core porosities at the depths of the measured core samples. As a result, our new equation provided better estimates for sedimentary rocks, but a previous equation called Li et al.’s equation provided closer estimates for volcanic rocks. The porosities estimated by our new equation for sedimentary rocks were approximately 50% at depths of approximately 300–330 m and approximately 20–40% at approximately 330–350 m and 510–660 m. The porosities estimated by Li et al.’s equation were approximately 15% for volcanic rocks (massive lava) at depths of approximately 380–460 m and approximately 30–40% for volcanic rocks (autobrecciated lava) at approximately 350–380 m and 460–510 m. Obviously, the porosities derived from the sonic logs of volcanic rocks were greater than those measured using intact core samples due to existing fracture porosity and alteration. Therefore, the derived porosity profile might reflect a reasonable in situ state in the borehole of the Futagawa fault drilling project.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 71 (2022) 243–250. The abstract is slightly modified.

Breaking trade-off relationship between strength and ductility in steels has been a perpetual topic, since researchers have been pursuing more energy and resource-efficient manner to use steels. In this study, rolling, tempering and inter-critical annealing were innovatively combined to obtain high strength-high ductility combination in Fe-4Mn-4Ni-3Al-0.1C steel. The process referred as multi-step inter-critical annealing (MIA) was designed to accomplish the following objectives: (a) a stretched lamellar-shaped grain structure resulting from rolling at non-recrystallization region of austenite, (b) Mn/C-heterogeneous austenite inherited from carbides formed during tempering. Therein, Mn/C-depleted austenite areas originated from ferrite; meanwhile, Mn/C-enriched austenite areas originated from the carbide dissolution, (c) transformation hardening of Mn/C-depleted austenite area during cooling in the inter-critical annealing, (d) stress relaxation of Mn-enriched austenite area retained as retained austenite at room temperature. The specimen achieved excellent mechanical response through the optimized MIA process (tempering at 400°C for 3 hours, then inter-critical annealing at 765°C for 40 minutes). The obtained microstructure was composed of alternating lamellae of bainite and a special annealed martensite on microscale, with the latter consisting of secondary martensite and retained austenite films on nanoscale. The yield strength and total elongation of the steel were increased from 858 MPa and 12% in direct inter-critical annealing process to 1011 MPa and 32% in MIA respectively. The modified Crussard-Jaoul analyses were used to evaluate the deformation behavior of the steels. Due to lots of bainite formation, a relatively high initial strain hardening exponent (0.72 m⁻¹) was observed for the DIA steels. While lower initial strain hardening rate (0.32 m⁻¹) and more stages of work hardening because of increasing stable austenite and secondary martensite in the MIA steels ensured the excellent strength–ductility combinations.

Mechanical properties of Li_10.35Ge_1.35P_1.65S₁₂ (LGPS) solid electrolytes with different grain sizes were investigated via indentation tests. Hand-milled LGPS (HM LGPS, d₅₀: 1.32 µm) and wet-milled LGPS (WM LGPS, d₅₀: 0.46 µm) powders were uniaxially pressed to obtain pellet samples. The HM LGPS and WM LGPS pellets had a similar bulk density of 1.63 g cm⁻³. At the initial loading/unloading, the WM LPGS pellet showed a large deformation owing to a decrease in cavities compared with the HM LGPS. In the subsequent cycles, both pellets exhibited elastic deformation behavior in the pressure range up to 20 N. The elastic modulus and relative residual depth of HM LGPS and WM LGPS were 21.7 GPa and 15.0 GPa and 0.75 and 0.71, respectively. This result revealed that WM LGPS is more elastically deformable and less plastically deformable than HM LGPS, which could be associated with the grain boundary strengthening interpreted by the Hall-Petch relation. Based on these mechanical properties, the superior cycle stability at the In-Li electrode/WM LGPS electrolyte interface during charging/discharging was discussed. Controlling the mechanical properties of sulfide solid electrolytes by the grain size is important for suppressing physical degradation in all-solid-state batteries.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder. Powder. Metall. 71 (2024) 92–97.

Y₂O₃/Hf co-doping has been used to design novel alumina-forming Co-Cr-Al-Ni oxide dispersion strengthened (ODS) superalloys. In order to gain a deep understanding of the effect of Y₂O₃/Hf co-doping on the microstructure and mechanical properties of the superalloys, a comparative study of Co-20Cr-10Ni-15Al (at%) superalloys with and without the Y₂O₃/Hf co-doping fabricated by mechanical alloying (MA) and spark plasma sintering (SPS) has been carried out. The microstructure and the elemental distribution of the constituent phases of the two superalloys, in particular the distribution of fine oxide particles, were studied in detail by electron microscopy. Compared with a large grain size and coarse Al₂O₃ dispersoids in Y₂O₃/Hf-free alloy, Y₂O₃/Hf co-doping effectively refined the grains and inhibited the generation of Al₂O₃ particles through the preferential formation of fine Y₂Hf₂O₇ oxide particles. Y₂O₃/Hf co-doped alloy exhibited an ultrahigh ultimate tensile strength (UTS) of 1816 MPa, which is 240 MPa higher than that of the undoped Y₂O₃/Hf one owing to the dispersion strengthening of the dense Y₂Hf₂O₇ oxide particles as well as fine-grain strengthening. The fracture mechanism of Y₂O₃/Hf doped alloy during tensile loading was illustrated through transmission electron microscopy (TEM) observations and the optimization based on Y₂O₃/Hf doping content was proposed.

Among Al-Zn-Mg alloys, A7003 alloy is considered to be an excellent alloy from the viewpoint of weldability because it is an alloy with relatively low Zn and low Mg composition. In this paper, changes in mechanical properties and microstructure during aging treatment are investigated by the addition of Si in the Al-5.6 mass%Zn-0.75 mass%Mg alloy containing Cu, Mn, Zr and Fe. Cast billets of alloys with different Si contents (Si: 0.05 mass%, 0.15 mass%, and 0.30 mass%) were prepared, and these cast billets were homogenized and extruded. After extrusion, four aging treatments were performed: one step aging at 423 K for 8 hours, and two step aging at 373 K for 3 hours, 6 hours, and 9 hours, followed by 423 K for 8 hours. The higher the amount of Si, the smaller the thickness of recrystallization layer near the inner surface and near the outer surface, and the smaller the existence rate of recrystallized grains in the cross section. After one step aging at 423 K for 8 hours, Si: 0.05 mass% showed lower strength than Si: 0.15 mass% and Si: 0.30 mass%. On the other hand, two-step aging resulted in Si: 0.05 mass%, Si: 0.15 mass%, and Si: 0.30 mass% with similar strength. The low strength of Si: 0.05 mass% after one step aging at 423 K for 8 hours is thought to be due to the coarse η phase, which was precipitated in the grain boundaries and grains. In Si: 0.15 mass% and Si: 0.30 mass%, Al(Mn,Fe)Si which formed during homogenization process is present in the grain boundaries and grains after extrusion. It is thought to suppress the generation of the precipitate of the coarse η phase in the aging at 423 K for 8 hours, which is a condition that the coarse η phase is easy to generate.

This Paper was Originally Published in Japanese in J. JILM 74 (2024) 167–172.

Using the tin alloy module in the material phase diagram and thermodynamic simulation software JMatPro, the phase compositions of different components of Sn-Bi-Pb low melting point alloys with the same Sn content were simulated based on the phase diagrams of Sn-Bi, Sn-Pb, and Pb-Bi binary alloys, and the influence of alloying element content on the melting characteristics of the alloys was investigated, which was used for the optimization of the components and the obtaining of low melting point alloys with excellent melting characteristics. The microstructure and melting characteristics of the optimized alloys were characterized by Scanning electron microscope (SEM), Thermogravimetry Analysis-Differential Scanning Calorimetry (TG-DSC), X-ray diffractometer (XRD), etc., and the influence of phase content on the mechanical properties was investigated. The results show that the Sn-Bi-Pb alloy possesses Sn-(Bi, Pb) phase, Bi-(Pb) phase and Pb₇Bi₃ intermetallic compound phase, and the simultaneous increase of the Bi element and decrease of the Pb element content to the Sn-Bi-Pb alloy obviously improves its tensile strength, but the elongation rate shows a decreasing trend, among which Sn50Bi30Pb has the optimal comprehensive performance, with a tensile strength of 38.60 MPa, an elongation of 61.13%, and a melting range of 94.0°C∼100.6°C.

To investigate the effects of laser peening (LP) on the surface and bending fatigue properties of carburized steel (JIS SCM420), surface roughness measurements, residual stress measurements, retained austenite measurements, and four-point bending fatigue tests were conducted. After LP, the compressive residual stress was applied on the surface, with the effect extending to a depth of more than 1 mm. Due to the high compressive residual stress, the bending fatigue strength of LP specimen was 1.46 times higher than that of the gas carburized specimen even though the surface roughness was increased by LP. Notably, the amount of retained austenite was 10.9% at the outermost surface of LP specimen, and high-density twins were observed in the lath martensite structure. This can be attributed to high strain rate deformation by LP.

This Paper was Originally Published in Japanese in J. Japan Inst. Metals 87 (2023) 205–210.

In Al-Zn-Mg alloy, hydrogen (H) leads remarkably to the degradation of mechanical properties. It is indispensable to suppress this phenomenon called hydrogen embrittlement (HE) for developing the high-strength Al-Zn-Mg alloy. Because intergranular fracture (IGF) is mainly observed when HE occurs in the alloy, we need to understand the initiation behavior of IGF in order to suppress HE. Heterogeneous distribution of stress, strain and H concentration usually influence the IGF initiation in polycrystalline material. In the present study, we investigated distribution of stress, strain, and H concentration in actual fractured regions by simulation employing a crystal plasticity finite element method and H diffusion analysis in a 3D image-based model, which was created based on 3D polycrystalline microstructure data obtained from X-ray imaging technique. Combining the simulation and in-situ observation of the tensile test sample by X-ray CT, we examined the distribution of stress, strain, and H concentration in actual crack initiation behavior. Based on this, the condition for intergranular crack initiation were discussed. As a result, it is revealed that stress normal to grain boundary induced by crystal plasticity dominates intergranular crack initiation. In contrast, accumulation of internal H due to the stress has little impact on crack initiation.

This Paper was Originally Published in Japanese in J. JILM 73 (2023) 530–536. Abstract was slightly modified.

The prediction accuracy of the deformation behavior of materials by finite element (FE) simulation depends on the parameters in selected material models. Although the parameters are conventionally identified from standard material tests (e.g., uniaxial tensile and multiaxial material tests) to characterize the deformation behavior, the identification process requires a large number of experiments. We develop a novel inverse methodology for estimating the material model parameters by combining digital image correlation (DIC) measurement and FE simulation coupled with an ensemble-based four-dimensional variational method (En4DVar). En4DVar incorporates the experimental data obtained from a material test into the FE simulation that reproduces the test and inversely estimates the parameters such that the simulation results follow the experimental data, allowing for the reduction of experimental effort. We use the proposed method to estimate the parameters of a strain-hardening law and anisotropic yield function from the results of uniaxial tensile test of a round bar of aluminum alloy. DIC measurement is conducted to obtain experimental data of the three-dimensional displacement and strain field over the surface of the specimen, including the post-necking range. The results demonstrate that En4DVar is a promising method for inversely estimating the parameters and characterizing the deformation behavior of a material from the results of a small number of tests.

This Paper was Originally Published in Japanese in J. JSTP 64 (2023) 195–201.

To improve estimation of sea salt deposition distributions on structural surfaces such as that of an airborne sea salt and corrosion sensor, we numerically simulated approaching flows with particles around a vertical flat plate. This is a typical object that mimics a sensor with a support plate. We used a computational fluid dynamics (CFD) model based on the unsteady Reynolds averaged Navier–Stokes equation. After validating the results by comparison with existing studies for flows with particles around a cylinder, we examined the changes in particle impaction efficiency on the plate with different approaching flow directions (0, 45 deg) and particle diameters (5 × 10⁻⁶–1.6 × 10⁻⁴ m). The impaction efficiency increases rapidly with particle diameter, whereas the influence of flow direction is small. Such increases in impaction efficiency are due to contributions from inertial impaction, and thus the variation in Stokes number with wind speed and the plate size can be used to predict the flow and particle conditions required for increases in impaction efficiency. The efficiencies for small particles on the front surface of the plate are higher than those on a cylinder. The impactions of small particles on the plate are locally activated by flow separations around a bluff body, whereas those on a cylinder are caused by intercepts without flow separations.

This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 71 (2022) 253–262. The abstract and the captions of Table 1 and Figs. 1–12 were slightly modified.

The purpose of this research is to experiment with different weight percentages of nanoB₄C (5%, 10%, and 15%) in order to develop nano B₄C/graphite-reinforced AA7075 alloy composites. An analysis of the morphological and mechanical behavior of the material was carried out using SEM in accordance with ASTM E8, E9, E23, and D790. All of the reinforcement particles are distributed uniformly throughout the matrix alloy, and there are no porous portions left over. Composite materials have a hardness of 30.18 percent, an ultimate tensile strength that is 40.93 percent higher, and a compressive strength that is 19.73 percent higher than the base material. The flexural strength of the material is improved by 26.75% when the matrix dislocation density and elastic modulus fluctuations are higher. There was a 66.21% increase in impact strength as a result of reduced porosity and grain refinement.

It is well known that carbon addition enhances the strength of steel, but it also increases the sensitivity to delayed fracture. The origin of the increased sensitivity to delayed fracture at higher carbon content is not clarified yet. In this study, high-strength steels with different carbon contents were fabricated, and their delayed fracture properties were evaluated. As the amount of carbon in the material increased, the carbon concentration at the grain boundary (GB) and the dislocation density also increased, and the material became more sensitive to delayed fracture. Subsequently, a GB model was developed on the basis of the samples used in the experiments, and the effect of hydrogen on the GB cohesive energy was evaluated using first-principles calculations. The cohesive energy without carbon atoms was 3.47 J/m² in the presence of hydrogen, whereas that with one additional carbon atom was 3.70 J/m². This result is inconsistent with the experimental results. Therefore, based on the dislocation density measurements, we assumed that more vacancies are generated in materials with higher carbon content. By adding a vacancy at the GB, the cohesive energy decreased to 1.79 J/m² and exhibited the same tendency as in the experiment. This suggests that not only hydrogen and carbon atoms but also vacancies are responsible for a decrease in delayed fracture resistance.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 73 (2024) 109–114. The captions of Fig. 1, Fig. 6 and Figs. 8–15 are slightly modified.

In high-strength steel (HSS), shear edge failure due to stretch flangeability is a serious problem. Shear edge failure is strongly affected by the cutting methods. As countermeasures, cut-off punching and double punching of the entire circumference have conventionally been considered. However, it has not been shown whether these processes can be applied to the stretch-flange-forming part of the outer circumference of automotive parts. In this study, the effect of the punching allowance and the method of improving the double punching process on the outer circumferences of automotive parts were investigated using hot-rolled HSS sheets. The results of hole expansion tests and observations of shear edge faces and metal flow show that partial double punching reduces the rigidity of the cut-off side and increases the collapse probability. Also, the crack propagation caused by high tensile stress near the upper cutting edge is accelerated. In comparison with double punching of the entire circumference of a 590 MPa-class or 780 MPa-class HSS sheet, in partial double punching, the stretch flangeability is improved and the hole expansion test value is 1.5 times that of double punching when the cut off amount is greater than the thickness. However, it is considered that the stiffness of the cut-off side varies depending on the curvature and the thickness of the plane, and the effect of such variations will be confirmed in a wide range of curvature and thickness in the future.

This Paper was Originally Published in Japanese in J. JSTP 64 (2023) 109–114. Abstract, the caption of Figs. 3–12, and the axis notation of Figs. 4, 5, 10 were slightly modified.

For alumina (Al₂O₃) powder produced by corrosion of aluminum, we investigated the effect of the solvent composition (C₂H₅OH/H₂O of 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100 v/v) on the particle size distribution. The powders were calcined at 1473 K and identified as α-Al₂O₃. The particle size of alumina powder obtained with H₂O as the solvent was 0.3–100 µm. As the volume fraction of H₂O in the solvent mixture increased, the particle size of the alumina powder also increased. With the increase in the H₂O volume fraction, agglomeration of the obtained alumina particles was observed by scanning electron microscopy.

Copper-based conductive ink has received attention to fabricate thin, flexible, and lightweight devices through printing techniques. In particular, the copper formate based conductive inks, called metal organic decomposition (MOD) inks, which are fabricated by using amine ligand coordinated copper formate have been well studied because of higher oxidative resistant against air than that of metallic copper before thermal annealing. The copper formate moiety of amine ligand coordinated copper formate complexes varies by changing the coordinated amine species, however, for the thermal decomposition temperature of such ink is not well understand. Here, we analyzed the influence of the amine ligand on the thermal decomposition temperature drop of copper formate moiety. Amine–copper formate complexes bearing several amine ligands containing primary amine, pyridine, and imidazole groups were fabricated. The relationship between the evaluated decomposition temperature and the differences between the wavenumbers of symmetric and antisymmetric vibration mode peaks in Fourier transform infrared (FTIR) spectra was found to be nearly linear. This finding demonstrates that the thermal decomposition temperature is governed by the structural modification of formate ions by the coordination of amine groups. Based on this finding, the order of thermal decomposition temperature of the copper moiety is predictable by using FTIR measurement.

In this study, we examined a method for whitening an aluminum surface by anodizing it in a sulfuric acid solution to industrialize the process. Anodic oxidation in a sulfuric acid solution at 2.5 Adm⁻² for 120 s resulted in the whitening of the aluminum surface, but localized burning occurred. When anodization was performed at a low current density for a short time (2.5 Adm⁻² for 4.5 s) after a high current density, a white anodic film was successfully obtained without burning. Cross-sectional scanning electron microscopy (SEM) observation of the white film revealed that etching by pretreatment created an uneven surface, and pore branching created an uneven metal-oxide interface. The fluctuating value of the film thickness obtained by length measurements using SEM was 0.2–0.3 µm. This value is sufficient to whiten the anodic film because of increase in diffusion reflection at the film surface and the metal-oxide interface. These results indicate that the difference in the peak voltage between the first and second anodizing resulted in the formation of a branched pore structure and film whitening.

This Paper was Originally Published in Japanese in J. JILM 73 (2023) 273–278. Abstract, Table 1 and the caption of Fig. 6 were slightly modified.

One of the main causes of atmospheric pollution comes from mobile sources that emit noxious gases from internal combustion engines. For the suppression of toxic exhaust gases, a catalytic converter is used as an anti-pollution device, because it catalyzes or accelerates the degradation of emissions making them less harmful. In the catalytic converter, platinum (Pt), palladium (Pd) and rhodium (Rh) catalysts are normally used for their excellent performances. However, these metals are expensive, rare, and scarce in the earth’s crust. These precious metals thus emphasized the importance of developing efficient recycling practices. For the recycling of these precious metals, a very fast, easy, economical, environmentally friendly and high-safety electric pulse discharge method was used to separate the Pd and Rh from the spent metal-honeycomb (MH) catalytic converter consisting of p as the catalyst carrier. To standardize the precious metals recycling process and attain the highest level of separation of Pd and Rh, electric pulse shots ranging from 120–240 and standoff distances (gap width between the positive electrode and the sample surface) ranging from 5–10 mm were applied. During this process, an electrical explosion occurred within the honeycomb structures through the electric shock. After the electrical explosion, the particles are collected, then sieved and physical characterizations are performed. Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDX) analyses revealed that the most separated particles are highly pure, and dispersed homogeneously without destroying particle structures. In this article, we first report about 87.91% of Pd and 90.77% of Rh are separated from each catalytic converter using the electric pulse discharge method that can overcome challenges and achieve ambitious recycling targets in the recycling industry. The effects of the discharge energies and the shock energy determined by varying electric pulse shots and standoff distance, respectively, are discussed. These findings provide insight into the recovery of precious metals for electric pulse discharge and play a role in developing a very effective recycling method not only for the catalytic converter but also for other devices.

Zinc (Zn) is expected to be a biodegradable material because of its high biocompatibility and moderate biodegradability. However, its mechanical properties are poor and must be improved for practical use. In this study, pure Zn was prepared using an electrodeposition process that can produce alloys with fine grains and excellent strength and ductility balance. With the possibility of Zn being expanded to other applications such as alloying, pyrophosphate and chloride baths with simple bath compositions and high current efficiencies were used as basic baths. We succeeded in producing dense specimens using a chloride bath with high current efficiency, where the complexing agent affected the mechanical properties. Pure Zn prepared in an ammonium chloride bath showed the maximum elongation of 44.5%, yield stress of 109 MPa, and tensile strength of 132 MPa. The strength and ductility balance were comparable to those of strongly processed pure Zn.

TiC films were prepared by a process that combines microwave plasma CVD with a sputtering method using titanium electrode to improve film deposition speed. The film depositions were carried out while changing the substrate bias voltage to 0, −200, and −400 V. The flow rate of CH₄ gas was performed under three conditions of 2.0, 1.0, and 0 sccm. By applying the substrate bias voltage, titanium oxide was easily deposited and the film hardness was lowered. The film hardness was high in the specimens without the substrate bias voltage, and the maximum hardness was 22 GPa. The maximum film deposition speed was 24 µm/h.

At present, almost 50% of electrical power is consumed by motors. Thus, increasing the efficiency of motors is an important issue. To achieve more efficient operation, it is vital to improve the accuracy of input data for motor loss design. In this study, we focused on the iron loss of electromagnetic steels, which is assumed to account for a large proportion of motor losses, and examined whether the effect of stress on the iron loss and its frequency dependence could be predicted with high accuracy by machine learning. First, experimental iron loss data are obtained at flux densities of 0.1–1.7 T, frequencies of 50–3000 Hz, and applied stresses from −200 to 200 MPa. No significant deterioration in iron loss behavior is observed in specimens subjected to 3% and 10% pre-strain by tensile loading. These data show that the effect of stress on iron loss varies significantly depending on the excitation conditions. The complex iron loss behaviors are the result of interplay between magnetic wall movement and magnetic domain rotation during the magnetization process. As simple regression of the magnetization process is difficult, we apply three machine learning algorithms to the experimental dataset. The results show that the LightGBM algorithm produces the most accurate predictions of the experimental iron loss values. The contributions of the explanatory variables are found to be consistent with empirical knowledge. This study demonstrates the potential for machine learning to enable improve the accuracy of iron loss data input to motor loss design.