A number of studies have been carried out to identify the mechanism of grain refinement in cast aluminum alloys by addition of grain refiners containing potent heterogeneous nucleation site particles. Although some mechanisms and theories have been proposed, in this review, the role of heterogeneous nucleation site particles on grain refining performance will be focused. The content of this article is as follows. First, the morphology of Al₃Ti particles in the Al–Ti refiners is presented, since the grain refining performance of the Al–Ti refiners is affected by the morphology of the Al₃Ti particles. Evaluation methods of lattice matching between heterogeneous nucleation site and crystallized phase are, then, discussed. Since we conclude that M value is potentially applicable to predict the effective heterogeneous nucleation site, M value at elevated temperatures is described. Next, fragmentation behavior of Al₃Ti particles caused by deformation of the refiners is presented, because the number density of the Al₃Ti heterogeneous nucleation site particles is increased by the fragmentation of Al₃Ti particles. At the same time, the morphology of the Al₃Ti particles, which affects the grain refining performance, can be changed. The concept of grain refiners with high-symmetry L1₂ modified (Al_1−xMe_x)₃Ti intermetallic compound particles, fabrication method and some results on grain refining performance are also given. Finally, application of heterogeneous nucleation for other fields are described.

The optimization of processing parameters is indispensable for the laser powder bed fusion (L-PBF) process. The deposited energy density (DED) is one of the process indexes for the L-PBF process and has a simplified formula of P·v^−0.5, where P is the laser power, and v is the scan speed. This parameter describes the change in the relative density and the melt pool morphology with laser power and scan speed well, whereas it does not include the effect of other processing parameters, e.g., hatch spacing (S). In the present study, an attempt was made to incorporate the effect of S into DED. Al–12Si (mass%) alloy cube samples were fabricated by L-PBF under various P, v, and S, for evaluating the relative density and the melt pool morphology. The melt pool depth and width of L-PBF-manufactured Al–12Si alloy increased linearly with P·v^−0.5 and did not exhibit a clear correlation with S. Based on the experimental observation, the effect of hatch spacing on DED was estimated to be S^−0.5, and a new index of P·v^−0.5·S^−0.5 was proposed. This index described the change in the relative density of the L-PBF-manufactured Al–12Si alloy with laser conditions (P, v, and S) well when the thermal conduction mode melting was dominant. This study also indicated the limitation of the applicability of P·v^−0.5·S^−0.5 under the keyhole or transition mode melting.

A thick film scintillator with a few tens of micrometers thickness is expected to improve sensitivity and spatial resolution in radiation detection and imaging, while its production method is limited to a costly process of thinning a melt-grown single crystal ingot. Here, we demonstrated the high-speed epitaxial growth of terbium- and europium-doped yttrium aluminum perovskite (Tb³⁺:YAP and Eu³⁺:YAP) transparent thick film phosphor using laser-assisted chemical vapor deposition (CVD). The (110)-oriented YAP thick film was epitaxially grown on a (100) SrTiO₃ (STO) substrate. The deposition rate was 53 µm h⁻¹, which was 50–90 times faster than those reported for conventional thermal CVD. Under UV and X-ray irradiation, the films emitted green and orange lights originating from 4f → 4f transitions of Tb³⁺ and Eu³⁺ centers, respectively. The fluorescence decay curves of the films were fitted to 1.96 and 1.89 ms for Tb³⁺ and Eu³⁺ centers. The 9 µm-thick Eu³⁺:YAP thick film phosphor can be used as a scintillation screen of X-ray imaging test to see though a semiconductor storage device.

The morphology of microstructure and tensile properties of Ti–48Al–2Cr–2Nb (at%) alloy rods fabricated by the electron beam powder bed fusion (EB-PBF) process were investigated with a particular focus on the influence of scan speed of the electron beam. Homogeneous near lamellar structure composed of the α₂ and γ phases can be obtained in the rod fabricated under the slowest scan speed. The fine lamellar spacing which contributes to the high strength of the alloy is derived from the fast-cooling rate of EB-PBF. On the contrary, a layered microstructure comprising a duplex-like region and an equiaxed γ grain layer (γ band) perpendicular to the building direction is obtained when increasing scan speeds. We observed for the first time that an increase in the scan speed results in a narrow width of the γ band. We also found that these microstructural changes have a significant influence on the mechanical properties of the rods. The near lamellar structure exhibits higher strength compared to the layered microstructure. Whereas, the rods with the layered microstructure show large ductility at room temperature. The elongation of each rod strongly depends on the width of the γ band owing to the preferential deformation of the γ band.

Freeze-dry pulsated orifice ejection method (FD-POEM) is a promising approach of fabricating spherical refractory alloy or composite powders for laser powder bed fusion (L-PBF). However, due to the weak particle strength induced by their intrinsic pore characters, the FD-POEM powders were likely crushed during the powder recoating process. Taking MoSiBTiC alloys as an example, in this work, plasma spheroidization (PS) treatment was utilized to strengthen the FD-POEM powders. The MoSiBTiC powders were completely melted and rapidly solidified during PS, leading to the evolution of mesh-pore to a fully dense structure. Correspondingly, the PS powders displayed an increased sphericity of 0.98, while showing a significantly decreased particle size of 53.1 µm. Moreover, the laser absorptivity of PS powder was reduced because of the decreased multiple reflections via the smooth powder surface. Microstructure evaluations illustrated that the PS MoSiBTiC powders mainly consisted of the elemental (Ti, B, and C)-supersaturated Mo phase, as well as small amounts of Mo₅SiB₂, TiC, and Mo₂B, indicating the occurrence of in-situ alloying during PS. Furthermore, the internal and inter-particle microstructures of PS powders were highly homogeneous, attributing to the uniformly dispersed elemental powders of FD-POEM powders. The results of this work suggest that the combination of FD-POEM and PS is an effective approach of fabricating refractory powders for L-PBF.

Aluminum matrix composites reinforced with ceramic particles have recently received considerable research interest owing to their light weight, high modulus, and high strength. This study fabricated SiC reinforced Al–10 wt%Si–0.35 wt% Mg (AlSi10Mg) alloy matrix composites with high relative density using a laser-based powder bed fusion (LPBF) process. The reaction phase of Al₄SiC₄ from the interface of the SiC particles occurred under LPBF and its fraction increased with increasing energy density. A high compressive stress of >600 MPa was obtained for the AlSi10Mg/SiC composite produced via LPBF at an optimum energy density (250 J/mm³). This study analyzed the mechanical properties of the LPBF composite hierarchically from micro to macro levels. A higher Young’s modulus (evaluated using a compression test) was obtained for the AlSi10Mg/SiC composite fabricated using LPBF than the AlSi10Mg alloy. From the micro level analysis on mechanical properties, the increased Al₄SiC₄ formation with an increase in energy density improved the hardness and Young’s modulus compared to those of the AlSi10Mg alloy. However, SiC in the composite is more effective for being high strength and Young’s modulus than Al₄SiC₄.

We first fabricated the single crystal of 316L stainless steel by laser powder bed fusion (L-PBF), focusing on the applicability of the µ-Helix scanning strategy with narrow pitch as a method for obtaining a single crystal. Various combinations of laser power and laser scanning speed were examined. The cubic block samples were orientated to 〈100〉 in the X-laser scanning direction, to 〈110〉 in the Y-laser scanning direction, and to 〈110〉 in the building direction, which is contrary to that obtained by the µ-Helix scanning strategy in electron beam melting (EBM), in which the X- and Y-laser scanning directions are orientated to 〈110〉, and Z-direction is oriented to 〈100〉 direction. Also, it has been demonstrated that melt pool monitoring by on-axis and off-axis dual photodiodes can detect the nonequivalence of ±X-scanning and ±Y-scanning, which is responsible for the unexpected crystal orientation.

Metal additive manufacturing enables producing complex geometric structures with high accuracy and breaks the design constraints of traditional manufacturing methods. Laser powder bed fusion, a typical additive manufacturing process, presents a challenge in experimentally understanding the nano-scaled microstructure-process relationship regarding the wide range of process parameters. In this study, we aim to reveal the novel nanoscale structural features by advanced scanning transmission electron microscopy to clarify the formation mechanisms in 316L stainless steel by laser powder bed fusion. Here we show that the slender columnar grains were confined to the centreline of the melt pool along the build direction, and the columnar cell structure at the side branching of the melt pool grew along orthogonal directions to follow drastic changes in thermal gradient across adjacent melt pools. Novel nano-scaled modulated structures have been observed in the dislocation cells parallel to the laser scan direction, which were mainly caused by the elastic strain involving the thermal gradient inside the melt pool and across adjacent melt pools as well as the effective strain field in the dislocation cell interiors. An in-depth understanding of microstructure developments is worthy of fabricating high-performance materials by controlling the additive manufacturing process.

In this study, a multi-phase-field (MPF) framework for predicting epitaxial grain growth in selective laser melting (SLM) additive manufacturing (AM) with multi-track and multi-layer scanning was developed. The spatiotemporal change in temperature was approximated using the Rosenthal equation, which provides a theoretical solution for the temperature distribution due to a moving point heat source. The powder bed was modeled as a polycrystalline layer. Large-scale MPF simulations for SLM-AM were performed using parallel computing with multiple graphics processing units. Using the MPF framework developed herein, we simulated SLM-AM with four tracks and four layers for 316L stainless steel. By observing the epitaxial grain growth process on two-dimensional cross-sections and in three dimensions, we clarified a typical growth procedure of grains with characteristic 3D shapes. The MPF framework will potentially enable a systematic estimation of the material microstructures formed during SLM-AM.

Additive manufacturing has attracted much attention as a new technology for producing lightweight and high-strength materials. The multi-phase-field method has been used in powerful numerical simulations to predict solidification microstructure formation in additive manufacturing. To verify the non-equilibrium multi-phase-field (NEMPF) model that can consider strong out-of-equilibrium solid/liquid interfacial conditions, the NEMPF model coupled with the CALPHAD-based thermodynamic database was used to simulate the solidification process in 316L stainless steel (SS) under a rapid cooling condition. The results show that interstitial carbon atoms tend to segregate at the solid/liquid interface, whereas no concentration gradient is observed at the grain boundary between the solid phases. Conversely, substitutional Cr, Mo, and Mn atoms were segregated in the solid grain boundaries. The effects of interfacial mobility and interfacial permeability on the solidification behavior were investigated. The results show that these parameters strongly influence the solidification rate and distribution of the solute concentration at the solid/liquid interface.

Laser powder bed fusion (L-PBF) has been utilized to prepare a high-strength titanium alloy builds using 0.3 mass% MXene-decorated Ti–6Al–4V alloy particles produced by an electrostatic self-assembly. The MXene/Ti–6Al–4V powder mixture had similar spherical morphology and powder size, while showing higher laser absorptivity as compared with the Ti–6Al–4V powders. Under high-energy laser irradiation, MXene was fully decomposed and uniformly dissolved into the Ti matrix. Microstructure observations illustrated that the MXene/Ti–6Al–4V build was completely consisted of ultrafine carbon-saturated martensite structures. Consequently, the MXene/Ti–6Al–4V build exhibited a high tensile strength of ∼1.4 GPa, attributing to the refinement of martensitic structure and the solid solution strengthening of carbon. These findings of this study may broaden the potential application of MXene and pave up the way towards the fabrication of advanced titanium parts via L-PBF.

Powder bed fusion using a laser beam (PBF-LB) was performed for Ti–6Al–4Nb–4Zr (mass%) developed by our group to improve the oxidation resistance at temperatures greater than 600°C by adding Nb and Zr to near-α alloys. Microstructure evolution of the PBF-LB samples by heat treatment was investigated, especially for heat treatment duration in the α + β phase, cooling rate, and heat treatment in the β phase. The equiaxed α phase formed during heat treatment along the melting-pool boundaries. The high volume fraction of the α phase and high Nb contents in the β phase was obtained by slow cooling (furnace cooling) compared with fast cooling (air cooling). The α/β lamellar structure formed in the melting pool boundaries with 100 µm in size and no equiaxed α phase formed along the boundaries by heat treatment in the β phase regime. Creep life at 600°C and 137 MPa was similar for the air-cooled and furnace-cooled samples, but the slightly slower deformation was obtained in the furnace-cooled sample. Creep life of the sample in the β phase region drastically increased due to the absence of the equiaxed α phase. Dominant deformation mechanism of creep was grain boundary sliding. The small equiaxed α phase accelerated grain boundary sliding.

Pellets made of α-alumina were sintered by fiber laser irradiation (1070 nm) for 1 min. The optical measurements showed that the laser selectively applied heat around the voids and gaps between grains in the green pellets. Even though the laser can penetrate deep inside the pellets, the heating efficiency is low because alumina poorly absorbs the laser energy. The graphite layer effectively improved the heating efficiency on the irradiated surface at the low-temperature region to “ignite” laser sintering. The microstructure of the sintered pellets was controlled by the microstructure of the green bodies. Porous pellets were obtained from coarse powders and transparent dense pellets from mixtures of coarse and fine powders. The porous pellets showed excellent bending strength owing to the joining of grains by the selective heating of gaps between them. The transparent pellets comprised large alumina single crystals anisotropically formed by spontaneous melt growth under laser irradiation.

Laser irradiation of β-NaFeO₂ shows local melting and crystal growth from the melt pool and sodium ion conductivity. Local melting, densification, and crystal growth were confirmed on the β-NaFeO₂ surface through laser irradiation with a wavelength of 1 µm. Scanning electron microscopy and electron backscatter diffraction analysis confirmed anisotropic grain growth and grain boundary shrinkage in the laser-irradiated area. X-ray diffraction results suggested that the laser-irradiated area was in a low crystalline state. In addition, the precipitation of Fe₃O₄ in laser-irradiated β-NaFeO₂ indicates a reduction reaction, suggesting that the laser-irradiated area is heated to 1500°C or higher. We confirmed that the structure change region could be controlled down to a thickness of about 10 µm by laser irradiation with varying laser power to β-NaFeO₂.

We investigated the electronic structure of the Mg_99.2Zn_0.2Y_0.6 alloy using hard and soft X-ray photoemission spectroscopy and electronic band structure calculations to understand the mechanism of the phase stability of this material. Electronic structure of the Mg_99.2Zn_0.2Y_0.6 alloy showed a semi-metallic electronic structure with a pseudo-gap at the Fermi level. The observed electronic structure of the Mg_99.2Zn_0.2Y_0.6 alloy suggests that the presence of a pseudogap structure is responsible for phase stability.

The reaction process of ZrC–2ZrB₂-based Cu cermets from the (1 − x wt.%)(1B₄C–3Zr)–x wt.%Cu system (Zr/B₄C = 3 in molar ratio) was explored. Results showed that ZrC and ZrB₂ were mostly produced through the dissolution of B₄C into a preformed Zr–Cu liquid. With an increase in x, the synthetic ZrC–2ZrB₂ and generated heat were reduced. This effect decreased an ability for Zr–Cu liquid to propagate in the reactants, and restrained the dissolution Zr. As a result, the complete synthesis of ZrC–2ZrB₂ failed in the systems with a higher Cu content (e.g., 50 wt.%). Furthermore, after the precipitation of ZrC and ZrB₂, the liquid surrounding would prevent ZrC and ZrB₂ from growing. Increasing Cu content enhanced the amount of Zr–Cu melt. This behavior contributed to a decline in ZrC–2ZrB₂ particle sizes, and the production of fine ceramic particles (∼200 nm). It is also revealed that the formation of ZrC–2ZrB₂ is a multistep process, which results in the inhomogeneity of ZrC–2ZrB₂ particle sizes. A valuable approach was proposed to explore the relationship between reaction process and synthesized products of combustion synthesis-related technique.

Since the die-casting mold repeats rapid heating and cooling during operation, the defects such as heat checks generate. Therefore, the quality and the life of the mold are degraded. To improve these demerits, the improvement of the thermal conductivity of the mold material is effective. In this study, carbon fibers were added to SKD61(40CrMoV5) tool steel to fabricate the composites with high thermal conductivity. Since carbon fiber has a chemical reaction with steel, electroless copper plating was applied to carbon fiber to suppress the reaction. The composites were fabricated by the unidirectional arrangement of carbon fibers and spark plasma sintering. The results obtained are as follows. 1) Carbon fiber/SKD61(40CrMoV5) composites with a high relative density were obtained. It is considered that the copper plated on the carbon fiber acted as a sintering accelerator. Furthermore. the plated copper remained around the carbon fibers in the composite, and it seems copper prevented the direct reaction between carbon and steel. 2) The composites had higher thermal conductivity than the monolithic SKD61(40CrMoV5) block. As increasing the carbon fiber content, the thermal conductivity increases. 5 vol% copper-plated carbon fiber/SKD61(40CrMoV5) composites have a thermal conductivity of 42 W/mK.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 61 (2022) 295–299.

The cold spray technique is expected to effectively form a metallic coating with fine crystal grains originating from the microstructure of the original powder. We previously reported that a pure iron coating with fine crystal grains can be formed by the cold spray technique by using mechanically-milled pure iron powder. In this study, the tensile strength of the pure iron coating was investigated. The as-sprayed coating showed significantly low Young’s modulus, tensile strength, and ductility owing to the low cohesion strength between particles. For tensile strength improvement, the coating was subjected to spark plasma sintering (SPS) treatment. As a result, the Young’s modulus was considerably improved by the SPS treatments at 740 and 786°C; moreover, the tensile strength of the SPS-treated coating was approximately four times higher than that of the bulk material. In contrast, the ductility was not improved by the SPS treatment. The low ductility was likely attributed to the presence of Fe oxides at the particle–particle interface.

This Paper was Originally Published in Japanese in J. Japan Thermal Spray Society 59 (2022) 84–90.

To clarify the effects of vanadium additions on the strengthening mechanisms of tempered martensitic steel, the microstructures, precipitates, dislocation densities, and tensile properties of water-quenched and tempered Fe–0.2C–0.5Si–2.5Mn–xV (mass%; x = 0–0.82) steels were analyzed. The vanadium carbide precipitates were plate-shaped and had a Baker–Nutting orientation relationship with the ferrite matrix. The size and shape of the vanadium carbide precipitates on the slip plane were considered when evaluating the contribution of precipitation strengthening. The increase in yield strength upon adding 0.82 mass% vanadium to the tempered steel was mainly caused by precipitation strengthening owing to the vanadium carbide precipitates and dislocation strengthening owing to the high dislocation density. This study demonstrates that the contribution of precipitation strengthening might be overestimated if it is assumed that the precipitates all hinder dislocation motion by the Orowan mechanism.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Heat Treatment 61 (2021) 5–12. Abstract was modified and Ref. 8) was added.

Effect of pH and precipitations on copper–molybdenum ore rougher flotation has been investigated in seawater with flotation experiments followed by precipitation estimation with thermodynamic calculations, turbidity measurements and XRD analysis. Results of flotation experiments in seawater indicated that the effect of pH on copper and molybdenum flotation maximum recovery seems limited. On the other hand, pH drastically influences copper and molybdenum flotation kinetic constant, it decreased a lot with increasing pH, and results of pH 8.5 and 9.0 is quite similar. From thermodynamic calculation, precipitation effect on seawater less than pH 9 seems limited since CaCO₃ and Mg(OH)₂ does not exist on this pH region. To estimate precipitation on pH in seawater, turbidity measurements were carried out with controlled pH and results indicated even pH is less than 9, noticeable turbidity can be seen. To confirm precipitation in seawater, precipitation was collected from controlled pH seawater solution. XRD analysis of precipitation indicated that obtained precipitation at pH 8.7 is CaCO₃, CaSO₄ and Mg(OH)₂. This result is not same as the result of thermodynamic calculation and it might be due to the activity coefficient and ionic strength effect. Although flotation kinetic is influenced with turbidity, turbidity influence on maximum recovery is limited. Effect of kinetic might be due to that precipitation exist as suspension and it prevent bubble and mineral attachments. Small effect of precipitation on maximum recovery might be due to exist of few precipitation on the surface of copper and molybdenum mineral since most of mineral in rougher flotation is gangue minerals and also most of precipitations might be on gangue minerals. These results indicated that seawater flotation have to take into account of precipitation effect more for flotation kinetic than maximum flotation recovery.

Increasing dissolved oxygen (DO) concentration of saturated Ca(OH)₂ solution accelerated formation of corrosion resistant film, calcium hydroxy zincate (Ca(Zn(OH)₃)₂·2H₂O, CHZ) on zinc and increased the density of the CHZ layer. The increase in the DO concentration enhanced the oxygen reduction reaction, which is the rate-determining step of the CHZ formation. The higher the DO concentration of the saturated Ca(OH)₂ solution, the shorter the period for the improvement of corrosion resistance of zinc. This is because the accelerated formation of insulating CHZ rapidly reduced the exposed metal surface of zinc.

This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 71 (2022) 21–29. The captions of all Figures and Table 1 have been slightly modified from the original paper.

Molecular dynamics (MD) simulation is a powerful tool to estimate materials properties from atomistic viewpoint. However, the scope of application of MD simulations is limited to problems where the Newton’s equation of motion for atoms is dominant. Therefore, it is inherently insufficient to estimate thermal conductivity of metallic materials, which consists of phononic and electronic components. In this study, machine learning (ML)-based regression model is employed to predict thermal conductivity of metals with high accuracy using deficient results from MD simulations. A regression analysis with the least absolute shrinkage and selection operator (Lasso) including electrical conductivity as predictor variables successfully predict the thermal conductivity of metals with negative temperature dependence, which indicates a significant contribution of electrons to thermal conduction in metals. It should be stressed that our prediction is better than the conventional estimation from the Wiedemann–Franz law. This study shows us new possibilities of new ML approach for improving the accuracy of physical properties obtained from MD simulations.

In this paper, Fe–20Mn–xNb alloys were prepared by ball milling and spark plasma sintering techniques. Phase constituents were characterized by X-ray diffractometer and Microstructure were characterized by scanning electron microscopy, and the tensile properties, hardness, compactness and damping properties were tested as well. The results show that after ball milling, the metal powders of Fe–20Mn–3Nb first exhibited an irregular plate-like or flake-like morphology, then large irregular particles were broken into fine particles. The addition of Nb element significantly increases the content of γ and ε phase, promoting the formation of γ and ε, and Fe2(Nb, Mn) phase precipitates in Fe–20Mn–3Nb alloy. And mechanical properties of the alloy are significantly improved by adding Nb element. The UTS and YS of Fe–20Mn–3Nb alloy are the highest, which are 818 MPa and 628.7 Mpa respectively, while the elongation of Fe–20Mn–0.5Nb alloy is the highest, which is 31.7% (20.2% higher than that of 0Nb alloy). The hardness and density of the alloy can be increased by adding Nb. The fracture mechanism of Fe–20Mn–xNb (x = 0.5, 1.5, 3) alloys is typical ductile fracture, while the fracture mechanism of Fe–20Mn alloy is quasi-xcleavage fracture. The damping property of Fe–20Mn alloys increases with increasing strain. The addition of Nb can significantly improve the damping properties of Fe–Mn alloys.

Here, we investigated the microstructure and mechanical properties of dissimilar joints between grade 2 commercially pure titanium (CP-Ti-2) and Ti–6Al–4V alloy welded via friction welding. Accordingly, specimens with 15 mm diameter and 50 mm length were prepared, and friction welding was performed at a constant rotation speed (1600 rpm) and different upset lengths: 1, 2, and 3 mm. Electron backscattered diffraction (EBSD) method was introduced to systematically analyze grain boundary characteristic distributions (GBCDs), such as grain size, grain orientation, phase distribution, and misorientation angle distribution. In addition, mechanical properties of dissimilar joints were evaluated using the micro-Vickers hardness and tensile test machine. The microstructures at both sides of the CP-Ti-2 and Ti–6Al–4V alloy were refined materials, compared with each base material. These grain refinements were determined via dynamic recrystallization accompanied by frictional heat and severe plastic deformation. In addition, the increase in the upset length distinctively affected the change trend of the grain size of the CP-Ti-2 and Ti–6Al–4V alloy. The hardness values in regions around the weld interface were higher than those of base materials. Furthermore, yield and tensile strengths were maintained at a similar level to those of CP-Ti-2, and the tensile specimens were fractured at the base material zone of CP-Ti-2, not at the weld interface and its periphery. Consequently, friction welding to dissimilar joints between the CP-Ti-2 and Ti–6Al–4V alloy was successfully conducted and sound welded materials without welding defects were obtained.

Pulsed anodization allows to form a Ni-free TiO₂ layer on an almost equiatomic NiTi alloy. The surface is then hydrophilized and impedes the release of Ni ions from the alloy. This study aimed to assess the suitability of electrolytes used in the pulsed anodization process with the view of application as a biomaterial. Toward this end, the NiTi alloy was pulsed-anodized in four different aqueous electrolytes, HNO₃, NH₄NO₃, H₂SO₄, and (NH₄)₂SO₄, after which the endothelium cell behavior was compared. The use of H₂SO₄ as the electrolyte resulted in the formation of a TiO₂ layer with a groove-like structure of several tens of nanometers wide, and this surface inhibited the activity of endothelium cells. The ability to prevent the release of Ni ions was diminished when using the HNO₃ electrolyte, resulted in inferior cell proliferation. The other two electrolytes, NH₄NO₃ and (NH₄)₂SO₄, formed a Ni-free TiO₂ layer with a comparatively smooth surface, which more effectively suppressed the release of Ni ions; thus, the cell attachment as well as proliferation were superior to that of the other two electrolytes. The surface smoothness and Ni suppression were thus concluded to be the factors governing the selection of an electrolyte.