Co–Ni spinel ferrite particles were synthesized via chemical co-precipitation and subsequently performed hydrothermal treatment at different temperatures. Fine particles of size with a few nanometers and spherical or cubic particles of size approximately 30 nm, which are responsible for magnetic properties, were obtained. The crystallite size obtained using the Scherrer equation was 24–27 nm and showed no dependency on the hydrothermal treatment temperature unlike the apparent particle size observed using transmission electron microscopy. The coercive force showed a remarkable increase with the decrease in the hydrothermal treatment temperature from 141 kA/m at 240°C to 400 kA/m at 100–120°C, in contrast to the decrease in magnetization from 60.0 Am²/kg at 240°C to 37.2 Am²/kg at 100°C. The specific composition of the Co–Ni spinel ferrite particles is expected to affect the high coercive force and the remarkable dependency of the coercive force on the hydrothermal treatment temperature.

It is of practical significance to improve the recycling value of hard-to-deform titanium wastes by means of melting-free technologies. One such technology, severe plastic deformation in the form of equal channel angular pressing (ECAP) was used to consolidate chips of commercial purity titanium. Electron backscatter diffraction (EBSD) was used to resolve the microstructure into high angle grain boundaries (HAGBs, with misorientation ≥15°) and low angle grain boundaries (LAGBs, <15°). The strain energy stored in HAGBs and LAGBs were considered to be the primary driving force of dynamic recovery (DRV) and dynamic recrystallization (DRX). As a result of DRV and DRX, the formation of microstructure was analyzed by evaluating the stored energy for various ECAP conditions. The ductility of the recycled titanium was analyzed by verifying the failure mode according to the inspection of the fracture surface of the tensioned specimen with scanning electron microscopy.

Quantitative mechanical analyses by nano indentation were performed for two types of ferritic heat-resisting steel that contained 12 mass% chromium and 2 mass% tungsten (12Cr–2W). One of the 12Cr–2W with a superior creep-fatigue property showed transgranular fractures, whereas the other steel with an inferior creep-fatigue property did intergranular fractures. In the inferior steel, coarse subgrains in the coarse blocks neighboring the prior austenite grain boundaries were formed during the creep-fatigue testing. Nano-scale hardness of the coarse subgrains or blocks neighboring the grain boundaries were markedly lower than those of the fine blocks far from the grain boundaries after the creep-fatigue test. Moreover, a pop-in behavior (in the relationship between force and penetration depth) occurred at only the coarse blocks neighboring the prior austenite grain boundaries. The pop-in behavior indicates that the dislocation density in the coarse blocks should be extremely lower by the recovery or rearrangement of dislocation by the creep-fatigue process at high-temperature. Therefore local deformations were assisted in the coarse blocks neighboring to grain boundaries and introduce the intergranular fractures.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 66 (2017) 887–892.

The high-temperature creep mechanism in an extruded Mg alloy comprising an α-Mg matrix and a long-period stacking ordered (LPSO) phase was investigated by theoretical analyses, indentation creep tests, and finite-element (FE) simulations. The creep behaviors of the Mg alloy with the LPSO phase (a dual-ductile-phase alloy), as a potential next-generation lightweight material, were robustly predicted using the characteristic creep parameters, volume fractions, and creep strengths of the two constituent phases. The results of FE analysis showed that highly effective bridging phenomenon may occur at certain geometrical arrangements of the reinforcing phases, where a high reinforcement efficiency close to 1 could be achieved. The experimental results suggested that the creep strength of the dual-ductile-phase alloy closely followed the rule of mixtures and the isostrain rate conditions. The stress exponent n for creep of the dual-ductile-phase alloy was expressed by the harmonic mean weighted by the effective volume fractions of the constituent phases, which strongly depended on the deformation rate. In addition, n consistently fell between the corresponding values for the two constituent phases in the power-law creep region. A similar trend was observed for the deformation rate dependence of the creep activation energy Q, which was expressed by the weighted arithmetic mean value. Thus, the newly derived equations of n and Q were shown to quantitatively capture the mechanical contribution of the reinforcing phase to the creep strength of the overall dual-ductile-phase alloy.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 82 (2018) 108–116. In order to more accurately describe the characteristic phenomenon in the dual-phase structure model, a part of Fig. 13 was modified. The Refs. 26), 35), and 39) were also added.

In order to improve the mechanical properties of pure titanium, a series of pure titanium was processed by cryogenic rolling (CR) and annealing. The annealing was carried out in the temperature range of 420°C to 530°C, and the annealing time was 5 min to 15 min, respectively. It showed that CR at the liquid nitrogen temperature could significantly enhance the strength, but the ductility simultaneously decreased. Subsequent annealing could recover some of the ductility while the strength lowered. Compared with other samples, the sample annealed at 450°C for 10 min had the best combination of strength and ductility because of the bimodal distribution of grain sizes.

The effect of anodizing time on the multiscale porous structure of the inner wall of the microchannel produced in a titanium alloy body has been investigated. The microchannel was produced by a powder-metallurgical process in which a titanium-powder compact containing thin aluminum wire was sintered at a temperature above the melting point of aluminum. During sintering, microscopic infiltration of molten aluminum into the porosity of the compacted titanium powder and subsequent diffusion of aluminum into the titanium powder particles brought about the formation of a microchannel lined with a Ti–Al alloy layer in the sintered body. The inner walls of the microchannels with uniform composition, Ti–18.0(±1.8) mol%Al, were provided for anodization experiments. When the anodizing time was in the range from 1.8 to 28.8 ks, the structure of the anodic oxide film was nanotube array. Each specimen had a microchannel several hundreds of micrometers in diameter, inner-wall asperities of several tens of micrometers in size, and a nanotube array structure of the anodic oxide film. In the specimen anodized for 59.6 ks, on the other hand, the nanotube array had changed to a different structure resembling that of nanoporous metals produced by dealloying.

This Paper was Originally Published in Japanese in J. Japan Inst. Light Metals 67 (2017) 589–594.

A high-purity palladium film with a three-dimensional nanoporous structure was fabricated from a reactive sputtered Al–N–Pd alloy film by a dealloying method that used citric acid chelation. Its characteristic porous structure could be controlled by the concentration of nitrogen gas in the Ar sputtering gas. The added nitrogen gas inhibited the formation of the intermetallic Al₄Pd phase in the as-deposited film, thereby improving the purity of Pd in the dealloyed nanoporous Pd film up to 99 at%. Furthermore, the formed structure of the dealloyed film changed with the nitrogen gas concentration during initial sputtering, i.e., the structure of the film could be controlled from a three-dimensional nano-network to an aggregated nanoparticle-like structure with increasing N₂ gas concentration.

In this study, pH measurement was performed in a thin electrolyte droplet with a thickness <1000 µm by the measurement of the equilibrium electrode potential of an Sb/Sb_xO_y electrode used as a pH sensor. The equilibrium potential of the Sb/Sb_xO_y electrode was evaluated by using the Kelvin probe (KP) technique. To investigate the potential response of the Sb electrode in a thin electrolyte droplet, the dependency of the Volta potential difference between the Sb and a gold wire as a KP on electrolyte droplet thickness was measured. The Volta potential difference had a linear response with respect to the buffer solution pH, independent of the droplet thickness. This result indicates that the KP technique, combined with an Sb electrode, is sensitive to the pH of a thin electrolyte droplet of thickness ≥50 µm. This pH measurement technique was also applied to measure pH in a corrosion model of steel. The corrosion model consisted of two steel plates in the same plane as the anode and cathode, with a constant current between them. During the corrosion process, the pH value decreased from 6 to 5 near the anode and increased from 6 to 12 at the cathode. The changes in pH measured in the thin electrolyte droplet were in good agreement with the color changes of the solution containing pH indicators.

The purpose of this research is the development of technology to make complex-shape closed-section parts directly from sheet blanks (direct sheet forming). It is expected to form closed-section parts with large expansion of the circumferential length by direct sheet forming. In this paper, the deformation type of horn tubes, which consists of circular, conical and transient portions, is studied. The horn tube is one of the typical shapes for automotive parts. With reference to deformation types in the conventional stamping process, those in the forming process of horn tubes are discussed on the basis of the results of FEM analysis. The validity of FEM analysis is confirmed by comparison with experimental results. It is clarified that the forming process can be broken down into several stages and deformation types (uniform bending of sheet, stretch flanging, axial bending of U-section, deformation into double curved surface and plane-strain compression).

This Paper was Originally Published in Japanese in J. JSTP 59 (2018) 27–31.

“Rod-dipping Process” was developed to simultaneously fabricate and strengthen porous metals with pore with unidirectionally aligned throughout their matrix. Carbon rods were dipped into a molten A6061 alloy, quenched, and processed by equal-channel angular extrusion (ECAE). The rods were subsequently removed, producing the porous metal throughout a metallic matrix. To determine the possibility of using our method to fabricate various porous metals, a theoretical equation was formulated for calculating the volume of hydrostatic pressure required for a molten metal to infiltrate the spaces between rods of various diameters and spaced at various intervals. The primary crystal was isotropic, and no reaction products were formed on the pore surfaces. Therefore, the crystal and pore growth directions could be independently controlled. By introducing an equivalent plastic strain of 1.2, the Vickers hardness values of the samples homogeneously increased to 1.5 times higher than that of the as-cast samples. Consequently, this method enabled us to fabricate porous metals with pore with unidirectionally aligned throughout their matrices and whose pore sizes, positions and volume fractions could be arbitrarily controlled. Furthermore, the metallic matrix could be simultaneously hardened by plastic deformation.

The solidification microstructure and magnetic properties of rapidly solidified melt-spun ribbons in Cu_43.2Ag_32.0La_4.8Fe₂₀ (at%) alloy, which was designed as the combination of the Cu–Ag-based Cu₅₄Ag₄₀La₆ alloy with high glass-forming ability and Fe, was investigated. The composite of an amorphous matrix and dispersed crystalline phases were obtained in the melt-spun ribbons. The spherical BCC-Fe nanocrystals, which were surrounded by Cu-rich crystalline phases, were embedded in a Cu–Ag–La-based amorphous matrix. The particular solidification structure in melt-spun ribbons can be explained by liquid-phase separation to form major Cu–Ag–La-rich and minor Fe-rich liquids, an amorphous phase formation in major Cu–Ag–La-rich liquid, and the crystallization of the separated Fe-rich liquid globules during the cooling of the thermal melt. The melt-spun ribbon shows typical ferromagnetic magnetic properties caused by the 10 nm-ordered spherical BCC-Fe nanocrystals.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 65 (2017) 45–51. To explain more precisely the background, the purpose of the study, experimental procedures, and results, some parts of the contents were slightly revised. Reference 10) was added as the reference for the Fe–Ag-based liquid-phase-separation-type amorphous alloys. References 11–13) were added to explain the liquid-phase-separation behavior in the Fe–Cu-based alloy system in more detail. Reference 14) was added to explain the relationship between an amorphous-phase formation and the mixing enthalpy of constituent elements in more detail. Reference 27) was added to explain the cooling rate during the arc-melting process in more detail. References 31–33) were added as references for the Co–Cu-based liquid-phase-separation-type amorphous alloys.

The selective laser melting could be employed in fabrication of near-net shape products for airplane and biomedical applications from Ti–6Al–4V alloy, which is difficult-to-process material. In this method, the localized laser irradiation forms the unique Ti–6Al–4V microstructures which correspond to the laser scanning patterns and local thermal history as it could be observed from sample cross-sections with OM or SEM. In this study, the effects of heat treatments on mechanical properties of Ti–6Al–4V samples produced by selective laser melting are discussed based on quantitative analysis of microstructures with image processing and machine learning tools. It was found that microstructures of heat-treated samples retained their original morphologies and secondary α phase precipitated regularly at β grain boundaries with increased treatment time. These microstructures were appropriately segmented and classified. Each α particle geometrical characteristics were successfully extracted and evaluated by image analysis. Importantly, the hardness of the heat-treated samples was lower compared to that of as-built ones and it tended to increase with the area fraction of α phase, the α particle width, and the nearest neighbor distance between α particles.

Ag nanoparticles were prepared by the arc plasma method under various conditions. The particle properties, such as size, size distribution, shape, and purity, were investigated. It was revealed that the average size of the Ag nanoparticles decreased with increasing arc current during the application of the arc plasma method. Moreover, the crystallite size measured using X-ray diffraction (XRD) was smaller than the average particle size, regardless of the arc current. Thus, it was concluded that the Ag nanoparticles prepared by this method are polycrystalline particles. No Ag oxides were observed in the nanoparticles by either field emission scanning electron microscopy observations or XRD analysis. Moreover, the solid solution of oxygen in Ag was not detected by XRD or wavelength dispersive X-ray spectrometry analysis.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) doi:10.2320/jinstmet.JBW201801.

In four probe sensing for measurement of voltage-current curve of superconducting layer-coated tapes, it has been reported that, when a large accidental defect is included in the region between the voltage probes, the critical current value estimated with the one micro-volt/cm criterion is low when the voltage probe spacing is small but it becomes higher when the spacing is larger. In the present work, a simulation study was carried out to reproduce the feature stated above and to describe the dependence of critical current value on the voltage probe spacing under the co-existence of a large crack and multiple small cracks. In simulation, the current shunting model at cracks and a Monte Carlo simulation method were applied to a model superconducting tape, which was composed of one local section with a large crack and multiple local sections with small cracks of different size from each other. The experimentally observed increase in critical current with increasing voltage probe spacing in the large defect included tape was reproduced well by the present simulation. Then, it was shown that the increase in critical current of the large crack-included region with increasing voltage probe spacing is caused by the increase in the ligament part-transported current and shunting current at the large crack due to the increase in critical voltage for determination of critical current. Furthermore, it was shown that the upper and lower bounds of the critical current of the region, in which a large crack and multiple small cracks exist, can be derived from the voltage-current curve of the section with a large crack, and the critical current of the region is given by the upper bound for a given size of the largest crack when the difference in size between the large crack and multiple small cracks is large.

Zn-based biodegradable materials have aroused wide attention due to promising biodegradability and adaptability to tissue regeneration in recent years. Zn–0.5%Li alloys was cast and hot extruded in this study. The microstructure, electrochemical properties and cytocompatibility of the as-extruded Zn–0.5%Li alloy were investigated. Four different simulated body fluids (Ringer’s solution, simulated body fluid (SBF), Dulbecco’s Modified Eagle Medium (DMEM) and DMEM-supplementing with 10% fetal bovine serum (DMEMp)) were employed to study the corrosion behavior of the alloy. The result indicated that Zn–0.5%Li alloy was composed of the matrix Zn and the second phase LiZn₄. It showed a higher corrosion resistance in the buffer solutions than that in Ringer’s solution, the corrosion rate can be ranked as Ringer’s > DMEMp > DMEM > SBF. What’s more, the cytotoxicity test shows that the alloy is not toxic to BMSCs. Zn–0.5%Li alloy has the potential to be the biodegradable metal.

A new process of internal activation of carbon fiber reinforced thermoplastic polymer (CFRTP) of polypropylene (PP) by applying electron beam irradiation (EBI) under oxygen (O₂)–rich nitrogen gas (N₂) atmosphere to CF chopped strand matt (CSM) layers prior to assembly and hot press to strengthen the typically weak CF/thermoplastic polymers (TPs) adhesion was proposed. Samples were interlayered composite with layup of alternating PP and CF plies, [PP]₄[CF]₃. Composite fabrication was performed by one directional hot-press under constant pressure of 4.0 MPa at 473 K for 1 min. Results showed applying an optimum 0.22 MGy-EBI under protective N₂ gas with O₂ concentrations between 200 ppm and 200,000 ppm mostly improved the bending strength (σ_b) while reducing strain at the bending strength (σ_b) apparently increasing the elasticity. The method appears to work well for the weakest samples in the data sets: at low accumulative probability P_f = 0.06 by median rank method, σ_b was apparently improved by the 200 ppm and 2,000 ppm O₂ atmospheres. Namely, 0.22 MGy-EBI under N₂ gas atmosphere with 200 ppm to 2,000 ppm-O₂ improved σ_b at P_f = 0.06 (57 MPa) about 21%, over that of untreated (47 MPa). Strength increase could be explained by mutual entangling of both sizing epoxy film on CF and PP with strong covalent bonding, which formation of direct induced by EBI and oxygen assisted by concentrating the O₂ gas molecules from 200 ppm to 2,000 ppm-O₂ in N₂ atmosphere, rather than weak molecular bonding CF-(H₂O, N₂, O₂)-PP for the untreated samples. Moreover, the action of the EBI apparently acts to clean residual H₂O, N₂, and O₂ to purify and activate the CF surface increasing polar group and active site density. They most likely contributed to bending strength enhancement. The 0.22 MGy-EBI in O₂-rich N₂ atmosphere appears to be a viable method to increase carbon fiber-thermoplastic polypropylene adhesion enhancing reliability and safety of the PP-CFRTP.

The morphology of γ′ precipitates for the wrought Ni-based superalloy Udimet 720Li was quantitatively evaluated by applying the absolute moment invariant technique. The diameter of the secondary γ′ precipitates, d, increased continuously with the decrease of the cooling rate, v, after solution treatment at 1473 K for 1 h along the following equation; d ∝ v^−0.4. In addition, with decreasing cooling rate, the shape of the secondary γ′ precipitates changed from spherical to octodendritic. For the oil-quenched alloy after solution treatment, d continuously increased with the increase of the aging time at 1173 K, and the spherical shape remained unchanged during the process. On the contrary, for the furnace-cooled alloy, the shape evolved from octodendritic to spherical with the aging time, exhibiting an almost constant d.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 82 (2018) 375–383.

The cold spray method is a coating process that should improve the wettability of solder to aluminum surfaces. In this study, Cu powder particles were deposited on commercially pure Al via cold spraying, and the characteristics and microstructural developments of the deposited layer, interface, and substrate were investigated. Cu coatings, with an electronic resistivity similar to that of commercial solder, could be formed on the Al substrate. Solid-state bonding was partially achieved between the Cu particles. Interdiffusion occurred at the interface between the cold sprayed Cu particles and the Al substrate, forming the reaction phases, and the Cu coating was bonded to Al substrate along almost the entire bond interface. The impact of high-velocity particles induced dynamic recrystallization and grain refinement of Al, resulting in increased hardness near the surface of the Al substrate. The thickness of the hardened region was ∼10 µm. Tin-based solder paste exhibited good wettability to the cold-sprayed Cu coating.

Ternary Nb–Re–Si alloy samples were melted and pulled simultaneously in a tetra-arc furnace and rapidly water-quenched after heat treatment at 1150°C for 1200 h. The microstructures of the samples that had been water-quenched after heat treatment were observed and the Nb, Re and Si concentrations in the constituent phases were measured using an electron probe microanalyzer (EPMA). In the cross-sectional structure of the obtained alloy samples, the lower portion of the pulled part consists of Nb₅Si₃ and Nb–Re–Si ternary compound phases. By contrast, the upper portion consists of Nb₅Si₃ and Nb solid solution phases. The Nb solid-solution phase, Nb₅Si₃ and Nb–Re–Si ternary compound are in equilibrium. It was found that the solubility limit of Si in the Nb solid-solution phase ranges from 0.6 to 1.7 at%, while the solubility limit of Re in the Nb₅Si₃ phase is 1.3–2.0 at%. The maximum amount of Re in the Nb₅Si₃ phase in equilibrium with the Nb solid solution is about 0.9 at%.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 82 (2018) 409–414.

Dynamic differential scanning calorimetry (DSC) was used to investigate the synergistic effect of graphene oxides (GO) and hydroxyl functionalized multi-walled carbon nanotubes (OH-MWCNTs) on the cure behavior of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin E-51 cured with methylhexahydrophthalic anhydride (MHHPA). The dynamic DSC results showed that with the introduction of nano-fillers, the initial reaction temperature, exothermal peak temperature, and finishing reaction temperature increased; while total heat of reaction decreased. The model-free Friedman method and model-fitting methods (Sestak-Berggren autocatalytic model and Kamal model) were employed to quantify the cure kinetics of the neat epoxy resin and nanocomposite respectively. The results indicate that the oxygenic functionalities on the surface of nano-fillers act as both a catalyst and initiator; and facilitated the early curing reaction. Moreover, the nano-fillers also facilitated the curing reaction at the diffusion-controlled stage due to their favorable thermal conductivity. Finally, the steric-hinerance effect of three-dimensional OH-MWCNTs caused a rise in the activation energy value.