Cold spray (CS) technology is a “solid-phase particle deposition process without melting”; however, it has been established as an additive manufacturing technology that can be applied beyond the framework of one field of thermal spraying. The scope of application of this technology has expanded to include ceramics and polymers. There are other solid particle deposition processes besides CS, such as aerosol deposition (AD), which differ in the material type, size, impact speed, and temperature of the target particles. We can expect that there is a common intrinsic mechanism through which solid-phase particles are joined and deposited in the solid phase. This review summarizes previous studies on the mechanism of cold-spray deposition and bonding, which can be understood as a mechanochemical phenomenon in part, and it is driven by the deformation of the particles and the resulting change in the chemical state of the particle surface, and stabilization by contact in a short time. When we understand these issues correctly, the optimal mechanical conditions (material size and collision conditions) for joining particles of various materials will be systematically understood, and it will be possible to perform different fabrication processes from thin films to additive manufacturing without melting various materials.
This Paper was Originally Published in Japanese in J. Japan Thermal Spray Soc. 57 (2020) 58–71.
The effect of porosity on coercive force in iron powder cores with different porosities was analyzed quantitatively. The coercive force of the iron powder cores decreased 11.0 A m−1 with a decrease in porosity of 0.01. From in-situ observation by Kerr effect microscopy, nucleation of the reverse domain was observed in local areas along narrow gaps such as the contact interface between particles and fine pores among particles, and nucleation of the reverse domain did not occur at coarse pores. This indicates that the local decrease in the diamagnetic field with a decrease in porosity may be reduced in these areas, resulting in a decrease in coercive force. This result suggests that densification of iron powder cores can be an effective method for reducing coercive force.
This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 68 (2021) 20–27.
The description of the short-range ordering (SRO) for binary systems was expanded to multi-component systems in the framework of the CALPHAD method. The present formulation estimates the effect of SRO in high entropy alloys (HEAs) based on the Gibbs energy functions from the CALPHAD-type thermodynamic assessments. It was applied to the BCC- and FCC-based HEAs and their subsystems. The excess Gibbs energy and excess entropy due to SRO in the FCC solutions were higher than those in the BCC solutions because of the difference in their coordination numbers. In the present calculation, the total contribution of the SRO to the Gibbs energy was approximately −1 kJ mol−1 and −0.2 kJ mol−1 in the BCC- and FCC-based HEAs at 1000 K, respectively. The variations in the excess Gibbs energy and excess entropy due to SRO in lower-order systems can disappear in the HEAs because of the mixing of multiple elements, as formally suggested in the previous work for the excess Gibbs energy in Cantor alloy.
Fig. 3 (a) WC-SRO and (b) excess Gibbs energies due to SRO in the BCC-based HEA (Mo–Nb–Ta–V–W) as a function of temperature.
The classification of the microstructures of Al–Si–Mg casting alloy in different cooling rates was accomplished by using our originally developed methods and machine learning techniques. The mechanical properties of the samples were slightly increased with the increase of cooling rates. The microstructures of the samples were similar because of the approximate cooling rates. High classification rates of about 80% to 90% were obtained using the software with machine learning techniques developed by us. The classification rate change with the number of images in training data was tested and a suitable number of images for training in the machine learning process was found.
Cu–Al2O3 composite coatings were prepared in the use of pulse jet electrodeposition technology. Pulse current parameters including duty cycle and frequency were researched about their effects on the microstructure variation and Al2O3 nanoparticles content of the composite coating. The mechanical properties and anti-corrosion resistance of the composite coating were examined. The results show that properly decreasing the duty cycle and raising the pulse frequency are able to generate a compact and fine nanocrystalline microstructure and increase the incorporation of Al2O3 nanoparticles in the composite coating, which is beneficial to strengthen the coating properties. With the optimal parameter combination as duty cycle 30% and pulse frequency 11000 Hz, the co-deposited Al2O3 content in the composite coating reaches a maximum of 14.4 at%, with the highest hardness of 308 HV and tensile strength of 755 MPa. The composite coating with optimized pulse parameters also displays improved corrosion resistance.
Nanocrystalline microstructure of the coating prepared at pulse duty cycle 30%.
AgCuS is believed to be an ideal material for diodes or transistors. Because of its relatively low lattice thermal conductivity and narrow band gap energy, also, AgCuS is a strong candidate for light- and heat-energy harvesting material. Since AgCuS exhibits superionic conductivity like other I–VI group semiconductors, its application to secondary batteries and nanogap switches has been investigated in recent years. In this study, we demonstrated the synthesis of AgCuS nanoparticles (NPs) with a narrow size distribution. First, we investigated elemental sulfur and thioacetamide (TAA), for sulfur sources to reduce the size distribution of the AgCuS NPs. When elemental sulfur was used as the sulfur source, NPs with a relatively wide size distribution were obtained. As elemental sulfur oxidized the 1-dodecanethiol absorbed onto the surface of the NPs to disulfide, the passivation layer of the AgCuS NPs, which prevents coagulation growth, was destructed. Conversely, the use of TAA as the sulfur source enabled the fabrication of nearly monodispersed AgCuS NPs. Ar gas flow plays an important role for a synthesis of monodispersed AgCuS NP with a regular shape. The Ag/Cu molar ratio in the NPs increased with reaction temperature and duration time. We confirmed that β–α and α–δ phase transitions occurred in the AgCuS NP system. The β–α phase transition temperature of 6 nm AgCuS NPs was lower than that of bulk crystals. Moreover, cation migration in the AgCuS NPs was confirmed via high-resolution transmission electron microscopy. Owing to the potential gradient induced by focused electron beam (EB) irradiation, the metal ions in AgCuS lattice moved to the EB irradiation area and were reduced to the metal state. This phenomenon, which resembles EB-lithography, could be exploited for the fabrication of metal/semiconductor nano-heterostructures. Moreover, as AgCuS NPs can store electrons by the reduction of metal ions according to Ag+(in AgCuS) + e− = Ag(on AgCuS), they could serve as cathode materials for rechargeable batteries.
Fig. 4 Lattice images of the Ag/AgCuS NPs (a) before and (b) after focused electron beam irradiation. (c) and (d) are the FFT patterns of the areas marked with squares in (a) and (b), respectively. (Bar = 10 nm)
Lock-in infrared thermography was used to estimate the fatigue limit for Ti–6Al–4V alloy at room temperature. The method detected infrared emitted from specimen during cyclic loading, i.e., temperature change related to frequency (f) of the loading. The temperature change contained reversible component and irreversible component, which related to thermos-elastic effect and plastic deformation, respectively. The latter component was divided from the former one by lock-in analysis to estimate fatigue limit. Estimated fatigue limits corresponded to those obtained from conventional fatigue tests at several stress ratios. A new assessment line was identified as σa = σw(1 − σm/σB)1/n, where σa represents the stress amplitude, σw signifies the fatigue limit, σm denotes mean stress, σB expresses tensile strength, and the n exponent for the alloy is about 2.3. The non-failure region of the new diagram was smaller than that of the modified Goodman diagram because of a reduced fatigue limit at near-zero stress ratios in the titanium alloy.
In our previous study, a thin wire of a Cu–0.29 mass%Zr alloy was produced by annealing after rolling and two subsequent intermediate annealing steps during wire drawing (IA wire). The produced IA wire had an ultimate tensile strength, σu, of 610 MPa and a small total elongation, εt, of 1.2% despite a small grain size, D, of 240 nm. In this study, a thin wire of the same alloy was produced by annealing after rolling and subsequent wire drawing (S wire). Even though the S wire had almost the same values of σu = 620 MPa and D = 230 nm as the IA wire, the S wire exhibited a larger εt = 2.9%. This study investigated the cause of this smaller value of εt for the IA wire. In both the IA and S wires, voids were formed by decohesion of the interface between the Cu matrix and particles consisting of a eutectic (Cu + Cu5Zr) during wire drawing; however, the fraction of eutectic particles with voids was larger in the IA wire than the S wire. The IA wire exhibited lower ductility as a result of easier coalescence of the voids during tensile testing. In addition, the larger fraction of eutectic particles with voids in the IA wire is attributed to the larger size of recrystallized grains generated by the intermediate annealing during wire drawing than that of the grains before intermediate annealing. As a result, dislocations accumulated significantly around the eutectic particles during the subsequent wire drawing, resulting in a high stress concentration.
This Paper was Originally Published in Japanese in J. Japan Inst. Copper 59 (2020) 76–79. The captions of all figures and table have been modified slightly.
Fig. 5 Fraction of eutectic particles with voids (fe) as a function of the inverse of the diameter (d−1) for the IA, S, and ECAP wires.
The application of high-density pulsed electric current (HDPEC) is one of the effective methods for the modification of material properties in metals. To evaluate fracture behavior modified by HDPEC, critical fracture parameters such as fracture strength, fracture toughness, and fracture profile of crack tip are important criteria. This work investigates the finite element analysis (FEA) based evaluation of improved fracture characteristics by the application of HDPEC in a SUS 316 austenite stainless steel. Tensile tests were first conducted to deduce the modified material properties with different conditions of HDPEC. A series of theoretical considerations was employed to estimate the modified fracture toughness. The relationship between critical fracture strength and critical crack length was numerically determined based on the estimated fracture toughness. The results in FEA showed that critical von Mises stress on the singularity at the crack tip increases as the effect of HDPEC increases. The evolution of increased fracture toughness with respect to conditions of HDPEC was specified. Crack opening profiles were simulated to assist the explanation. The evaluation of fracture parameters in this study proposes that the modified material properties by HDPEC play a positive role to resist crack propagation.
This study systematically investigated the effect of aging temperature on microstructure, impact toughness and pitting behavior of flux-cored arc welded 2205 duplex stainless steel joint. 600°C–900°C aging treatments were carried out with 1-hour insulation. The results showed that the ferrite (α) underwent serial metallurgical transformations with increasing the aging temperature. The formation of the sigma (σ) phase was detected in the weld zone (WZ) under all aging temperatures due to eutectoid reaction of the α phase, while the peak content of the σ phase was reached at 800°C aging. A similar content variation of the σ phase was detected in the heat-affected zone (HAZ). Secondary austenite (γ2) and chi (χ) phases were only observed in the WZ and HAZ below 800°C aging. The impact toughness and the pitting resistance of the welded joints were strongly related to the σ phase content, that the 800°C aging specimen possessed the lowest impact toughness and the worst corrosion resistance due to the maximum content of the σ content in both the WZ and the HAZ.
Aging temperature were strongly related to sigma (σ) phase precipitation, which produced negative effects on impact toughness and pitting behavior of flux-cored arc welded 2205 duplex stainless steel joint.
In this study, the iron rust layer formed on the low alloy steel under air-solution alternating conditions was investigated by cross-sectional observation and analysis, and the mechanism of the accelerated corrosion of the steel under alternating conditions was clarified. The observations and analysis showed that the multilayered iron rust layer composed of the red rust layer (γ-FeOOH), rust crust layer (Fe3O4), inner crystal (Fe3O4), and inner rust layer was formed on the low alloy steel. It can be considered that the multilayered iron rust layer accelerated the cathodic reaction rate of dissolved oxygen under alternating conditions. This acceleration is the reason why the corrosion rate of the low alloy steel under alternating conditions was accelerated.
This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 68 (2019) 205–211.
Fig. 11 Schematic representation of the mechanism of accelerated corrosion of low alloy steel in air-solution alternating condition.
Self-sustaining Ce0.9Gd0.1O1.95 (GDC) film was fabricated by aqueous tape casting method. An aqueous tape casting of slurry was performed using poly acrylic acid (PAA) as dispersant, poly carboxylate ammonium (PCA) as binder, poly ethylene glycol (PEG) as plasticize, and deionized water as solvent. The rheology of the slurries was evaluated with cone-plate viscometer. The conditions for preparing stable slurries were studied, regarding with solid concentration and ball-milling period, and were optimized by viscosity measurements and aging experiments. Green tapes with thickness in the range of 65 to 130 µm were prepared by conditions; solid concentration of 25–26 vol% and gate height of 0.2–0.3 mm with moving rate of 45 cm min−1. Conventional sintering techniques were available for densification, and semi-transparent GDC film of 47–95 µm thickness was successfully fabricated by heating a green sheet at 1500°C in air.
This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 68 (2019) 543–548. The caption is slightly changed.
The prevention rate (PR: the index calculated from the corrosion rate of the non-protection and protection time) is generally employed as the evaluation index of the effect of the cathodic protection of the port steel structures in Japan. When designing the cathodic protection, the prevention rate has been employed as 90% since ancient times. However, if the corrosion rate of the steel is sufficiently close to zero, it is considered that the prevention rate is theoretically close to 100%.
In this paper, firstly, we investigated aging changes of the cathodic prevention effect for 4 years by using test pieces installed in Japanese 4 ports. We arranged aging changes of the evaluation index which showed as the effect of the cathodic protection based on the result. Next, based on the above survey results, by using a large number of test pieces (total of 650 pairs) installed at port facilities in the whole country, we grasped the actual condition of the effect of the cathodic protection and verified while comparing evaluation index which shows the effect of the cathodic protection.
On the result, it was considered that it was desirable to evaluate the effect of the cathodic protection as “corrosion rate during the cathodic protection” which is a state at the time of the cathodic protection and includes factors of time as the index of the effect of the cathodic protection. In addition, it was suggested that the value of “corrosion rate at the cathodic protection” as an evaluation index of the effect of the cathodic protection was about 0.01 mm/y.
This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 68 (2019) 220–226.
Corrosion resistance of carbon steel covered with resin coating containing nickel sulfate has been evaluated under chloride and sulfuric acid mist environment. The structure of corrosion products formed on steel surface was investigated by XRD and XAFS analyses using synchrotron radiation. Nickel sulfate promoted the formation of goethite and akaganeite. It was considered that this akaganeite was not tetragonal β-FeOOH but monoclinic akaganeite containing nickel.
This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 69 (2020) 148–153.
An electrochemical surface treatment technique was developed in this study to improve the localized corrosion resistance of zirconium in a chloride ion environment. A combination of anodic and cathodic polarization cycles was applied to induce the selective dissolution of the inclusions that could potentially initiate the localized corrosion of zirconium. Shallow dips were observed on the specimen surface after the treatment, thereby indicating the dissolution of the inclusions. The electrochemical treatments via galvanostatic anodic polarization and potentiostatic cathodic polarization in concentrated phosphate-buffered saline resulted in a high pitting potential of greater than 2 V in a simulated body fluid. This indicated that the devised technique realized a significant increase in the localized corrosion resistance of the treated Zr.
This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 69 (2020) 307–314. The captions of Figs. 2–14 and Table 1 are slightly modified.
A time-resolved measurement system, which combined a commercially available solution-flow cell and an inductively coupled plasma mass spectrometer, was established for determining the dissolution rate of platinum (Pt) and palladium (Pd) in this study. The detection limit of the system was successfully improved by thinning the cell channel, and the limits for Pt and Pd were 0.13 pg cm−2 s−1 and 0.39 pg cm−2 s−1, respectively. When the system was applied to Pt and Pd under potential cycling that mimicked the start/stop conditions of a polymer electrolyte fuel cells (PEFCs), it was revealed that the dissolution of Pt and Pd started from a more negative potential than that reported in previous studies. In addition, we succeeded in obtaining time-resolved measurements of the Pt and Pd dissolution rates below the open circuit potential of the PEFCs (approximately 1.0 V) and clarified that the dissolution mechanisms of Pt and Pd were different.
This Paper was Originally Published in Zairyo-to-Kankyo 69 (2020) 221–230. Abstract and captions of figures, and Fig. 6 are slightly modified.
Zn was electrodeposited on an Fe electrode at a current density of 50–5000 A·m−2, charge of 4 × 104 C·m−2, and temperature of 313 K in an unagitated zincate solution containing 0.62 mol·dm−3 of ZnO, 4.0 mol·dm−3 of KOH or NaOH, and organic additives. The effects of KOH and NaOH on the deposition behavior of Zn in the solution containing the organic additives and on the microstructure of the deposits were investigated. In a solution containing a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA), the current efficiency for Zn deposition in a high-current-density region (1000–5000 A·m−2) to produce glossy films was higher with KOH than that with NaOH. At high current densities above 1000 A·m−2, the Zn deposition approached the diffusion limitation of ZnO22− ions. With the addition of PQ and QA, the diffusion of ZnO22− ions was significantly suppressed, and the degree of suppression was smaller with KOH than that with NaOH. The polarization resistance at 200 A·m−2, which was investigated through alternating current impedance, revealed that the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH. Since the suppression effect of the additives on the Zn deposition was smaller with KOH than that with NaOH, the current efficiency for Zn deposition in the high-current-density region was larger with KOH. The upper limit of the current density needed to produce glossy films was smaller with KOH than that with NaOH, and spongy thin films were partially observed on platelet crystals obtained at high current densities in the KOH solution. The C content resulting from the additives in the deposited Zn was smaller with KOH because the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 59–66. Caption of Fig 12 is slightly changed.
Fig. 6 Current efficiency for Zn deposition from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]
The synergistic effects of metal cations in a solution on the ability of sodium gluconate to inhibit the corrosion of mild steel were investigated by immersion and electrochemical tests. The effects of metal cations on the inhibition ability of sodium gluconate was investigated quantitatively, with particular focus on the parameter Y, which represents the “corrosion inhibitory effect of cations.” The results of the immersion and electrochemical tests showed that the inhibition ability of sodium gluconate improved with increasing Y value of the metal cations in model freshwater. The electrochemical and surface analyses indicated that gluconate ligands and large-Y metal cations formed a protective layer with few defects on the mild steel.
Fig. 11 Corrosion rates of specimens in a solution as a function of the Y value of the metal cation in the solution.
The standard Gibbs energies of formation, ΔfGo, of CrB4, CrB2, Cr3B4, Cr5B3, and CrBO3 existing along the oxidation path of Cr–B binary alloy were determined using the electromotive forces of galvanic cell composed of the ZrO2–Y2O3 solid electrolyte. The electromotive force showed plateaus and decreases in the process that CrB4–CrB2 two-phase alloy used for the cell materials were oxidized via two- and three-phase regions along its oxidation path. From the plateaus of electromotive forces corresponding to the cell materials in the three-phase regions, the values of ΔfGo(CrB4), ΔfGo(CrB2), ΔfGo(Cr3B4), ΔfGo(Cr5B3), and ΔfGo(CrBO3) in the temperature range from 1273 to 1346 K were determined as follows:
ΔfGo(CrB4)/J (mol of compd.)−1 = 167500 − 261.2 T ± 7200
ΔfGo(CrB2)/J (mol of compd.)−1 = −21020 − 79.22 T ± 2100
ΔfGo(Cr3B4)/J (mol of compd.)−1 = −173400 − 95.47 T ± 2500
ΔfGo(Cr5B3)/J (mol of compd.)−1 = −318900 + 20.64 T ± 5800
ΔfGo(CrBO3)/J (mol of compd.)−1 = −958800 + 59.24 T ± 4100
The ΔfGo values determined in the present study satisfied the phase equilibria in the Cr–B binary system. Using the determined ΔfGo values, the composition-oxygen partial pressure diagram of the Cr–B–O system was constructed under the conditions at 1300 K and a total pressure of 1 bar (100 kPa). It is useful to understand the oxidation property of the Cr–B binary alloys.
Fig. 8 Composition-oxygen partial pressure diagram of the Cr–B–O ternary system at 1300 K. The horizontal lines show the following three-phase equilibria: (1) CrB2–CrB4–B2O3; (2) Cr3B4–CrB2–B2O3; (3) CrB–Cr3B4–B2O3; (4) Cr5B3–CrB–B2O3; (5) Cr5B3–CrBO3–B2O3; (6) Cr2B–Cr5B3–CrBO3; (7) Cr2B–Cr2O3–CrBO3; (8) Cr–Cr2B–Cr2O3; (9) CrB4–B–B2O3.
Unsupervised machine learning (ML) is examined for the result of molecular dynamics (MD) simulation to extract characteristics of catalytic reaction. O–H bond dissociation of ethanol on Fe–Co nanoparticle in ab initio MD simulation [S. Fukuhara et al., Chem. Phys. Lett. 731 (2019) 136619] is employed as an example. Hierarchical clustering of radial distribution function successfully classifies coordinates on reaction in the dendrogram. Moreover, receiver operating characteristic curve reveals the distance to the farthest-neighbor atom from the target atom is a dominant descriptor for the clustering. An optimum structure of catalytic nanoparticle is predicted based on these automated analyses. This study shows a new way of post-process of results of MD simulations based on the unsupervised learning technique and it paves the way for a new possibility of ML-based materials design.
Schematic image of hierarchical clustering of structural (radial distribution function, RDF) and electronic (density of states, DOS) characteristics obtained from molecular dynamics simulation of catalytic reaction on metal nanoparticle.
Highly functional and multifunctional material surfaces may be created through fabrication of unusual surface structures. We fabricated mushroom-like nanostructures on a stainless steel surface by simple processes: heat treatment under an atmosphere of low partial pressure of oxygen, followed by electrochemical etching. Island-like titanium oxide was formed on the Ti-containing stainless steel surface by selective oxidation during heat treatment. The titanium oxide became the cap part of the mushroom-like nanostructure and the stem part was fabricated by selective dissolution during electrochemical etching. Detailed analysis revealed that the cap part of the mushroom-like nanostructure was mainly composed of γ-Ti3O5, and the stem part was composed of the stainless steel substrate. The contact resistance was significantly reduced by the fabrication of the mushroom-like nanostructures. The protrusions of electrically conductive γ-Ti3O5 can contribute to the reduction of the contact resistance. The mushroom-like nanostructures improved the hydrophilicity of the surface without fluorine coating, but improved the hydrophobicity of the fluorine-coated surface. The contact between the stainless steel surface and the water droplet was in the Wenzel state.
Peen forming is a method of bending a metal plate by generating plastic strain near its surface when colliding with steel shots. In this study, the effect of coverage on the curvature after single-sided and double-sided peen forming of a high-strength aluminum alloy plate was investigated by experiments and finite element method (FEM) analysis. In the experiment, the curvature increased as the coverage increased in single-sided peen forming. In double-sided peen forming under the same peening conditions for both sides, the curvature is smaller than that after single-sided peening. FEM analysis was performed as follows. In step 1, multiple shot collisions were analyzed by the dynamic explicit FEM. In step 2, the plastic strain distributions analyzed in step 1 were input to the specimen to analyze the deformation by the static implicit FEM. FEM analysis results agreed with the experimental results in single-sided peen forming. In double-sided peen forming, the plastic strain distribution was calculated considering the residual stress distribution near the second surface after single-sided peen forming, and the analysis results agreed with the experimental results. A method of approximate calculation of the curvature was proposed, in which the plastic strain distribution was expressed as a function of coverage and initial stress. The curvature after single-sided and double-sided peen forming could be easily predicted by this method under various peening conditions.
This Paper was Originally Published in Japanese in J. JSTP 61 (2020) 115–123.
The ingots of CuxZnMnNi (x = 1, 2) medium-entropy (ME) brasses were fabricated using metallic mold-casting process without a vacuum chamber. The molten metal was obtained by high-frequency melting of the mixture of pure Cu, pure Ni, and pre-alloy ingots of Mn–Cu and Zn–Ni using silica-based crucible in Ar flow. The metallic mold-casting ingots were obtained using centrifugal casting in air atmosphere. The composite of body-centered-cubic (BCC) and face-centered-cubic (FCC) phases were obtained in the ingots of equiatomic CuZnMnNi ME brass, while a near-single FCC phase was obtained in the ingots of non-equiatomic Cu2ZnMnNi ME brass, where the identification of the constituent phases was mainly performed by XRD analysis. The ingots showed superior deformability and high 0.2% proof stress during compression test conducted at room temperature.
This Paper was Originally Published in Japanese in J. Japan Institute of Copper 59 (2020) 24–31. Minor corrections in abstract, main text, Figure and Table captions were performed with the translation from Japanese to English and proofreading by native speakers. Reference 43) was updated. An appendix containing figures based on the original and new experimental results has been added to the paper.
Metal additive manufacturing (AM) technologies are attracting attentions not only as a fabrication process of complicated three-dimensional parts but also as microstructure controlling processes. In powder bed fusion (PBF)-type AM, crystallographic texture can be controlled by scanning strategies of energy beam. To optimize microstructures, computer simulations for predicting microstructures play very important roles. In this work, we have developed simulation programs to explain the mechanism of the crystal orientation control. First, we simulated the shape of melt pool by analyzing the heat transfer using apparent heat conductivity when the penetration of laser beam through keyholes was taken into consideration because of the evaporation and accompanying convections. It was assumed that the primary crystal growth direction can be determined by the temperature gradient, and the crystals grow keeping the growth direction as generally recognized. The shapes of simulated melt pools agree well with experimental observations. The modified cellular automaton simulations successfully reproduced two typical textures with different preferential orientations along the building directions of 〈100〉 and 〈110〉 when the bidirectional scanning with and without a rotation of 90°, respectively, was accomplished between the layers.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 103–109.
Fig. 8 Comparison of analysis results (lower side) with experimental results (upper side: after Ishimoto et al.1)) of scan strategy X.
Improving the oxidation resistance of Mg2Si is important for its practical use in thermoelectric devices. The oxidation behavior of Sb-doped Mg2Si with and without added Al2O3 or Al under heating from 293 K to 1023 K in a 200 Pa water vapor atmosphere was observed by using an environmental scanning electron microscope (E-SEM). Compositional analysis before and after in situ oxidation in the E-SEM was performed by energy-dispersive X-ray analysis (EDX). The depth profile of the samples after thermal oxidation in the E-SEM was evaluated by X-ray photoelectron spectroscopy (XPS). The dimensionless figure of merit of Sb-doped Mg2Si with added Al2O3 or Al was also evaluated. The oxidation onset temperature was 603 K for Mg2Si, and 747 K to 793 K for Mg2Si with 0.8 to 4.5 mol% Al2O3 added or 4.0 at% Al added. The O concentration after oxidation at 873 K as measured by EDX (accelerating voltage: 3 kV) was 35.85 at% without Al2O3 or Al addition, but 11.55 at% to 13.80 at% with Al2O3 or Al addition. The Si concentration after oxidation at 873 K as measured by EDX (accelerating voltage: 3 kV) was 0.30 at% when no Al2O3 or Al was added to the Mg2Si, but 15.32 to 20.97 at% with Al2O3 or Al addition. Evaluation by XPS revealed a layer with a relatively high concentration of aluminum oxide or aluminum at a depth of about 20 nm from the surface of the Mg2Si sample with added Al2O3 or Al, respectively. This layer appears to suppress Mg2Si oxidation. The addition of Al2O3 or Al had a slightly positive effect on the thermoelectric properties of Mg2Si.
Rapid formation of magnesium hydroxide passivation layer during hydrolysis of MgH2 seriously blocks its application as a hydrogen generation material. MgH2 is also susceptible to oxidation. MgH2 with Nb2O5 and/or CeO2 doping has been prepared via high energy ball milling in this work with the aim of enhancing the hydrolysis performance and oxidation resistance. The phase compositions, microstructure and hydrogen evolution properties have been systematically investigated. Nb2O5 and CeO2 particles with particle size in the range of ∼20–200 nm are uniformly dotted on the surface of MgH2 matrix after ball milling. It is found that Nb2O5 shows better facilitating effects than CeO2 on the hydrolysis performance of MgH2. As-milled MgH2+10%Nb2O5 and MgH2+10%CeO2 samples generate ∼705 and 474 mL g−1 of hydrogen in distilled water within 60 min, respectively. Introducing MgCl2 in water can also significantly enhance the hydrolysis reaction kinetics and conversion rate. As-milled MgH2+10%Nb2O5 sample shows a hydrogen yield of 1222 mL g−1 in 5% MgCl2 solution, corresponding to a conversion rate of 74.6%. However, air exposed MgH2+10%Nb2O5 sample hardly generates any hydrogen in MgCl2 solution. CeO2 can modify the oxidation resistance to some extent. MgH2 with CeO2 doping can still produce ∼250 mL g−1 of hydrogen after 24 h air exposure.
Fig. 5 Kinetic curves of hydrogen production from hydrolysis of as-received MgH2 and as-milled samples in (a) distilled water and (b) 5% MgCl2 solution at 20°C. (c) is the corresponding conversion ratios in (a) and (b).
Sm–Fe–M (M = Ti, V) sintered magnets with the ThMn12-type structure which consist of higher Sm content than the stoichiometric composition of ThMn12-type compound, were produced by the powder-metallurgy. Sm–Fe–Ti sintered magnets had a low level of coercivities of about 10 kAm−1. In contrast, Sm–Fe–V anisotropic sintered permanent magnets, particularly, Sm10.5Fe74.1V15.4 magnet exhibited magnetic properties with a coercivity of 648.7 kAm−1, a remanence of 0.73 T and a (BH)max of 97.6 kJm−3. Through microstructure observation of the grain boundary between the adjacent ThMn12-type grains using STEM, no grain boundary phases were observed in Sm10.0Fe81.7Ti8.3 magnet, whereas two types were observed in Sm10.5Fe74.1V15.4 magnet: an amorphous structure with a thickness of about 1 nm and an incomplete periodic structure with a thickness of about 2 nm. EDS analysis indicated that the composition ratios of Sm to the sum of Fe and V were about 1 to 1 and 1 to 1.7, respectively. It suggested that the existence of a grain boundary phase is important to raise coercivity.
Fig. 3 Demagnetization curves of Sm–Fe–V sintered magnets.
A shaking table is used in gravity separation utilizing the difference in density of solid particles and has been widely employed in mineral processing and recycling processes. Although the principle of the table separation is simple, there is still limited knowledge about the segregation and separation mechanisms. To better understand these mechanisms, in this study, the discrete element method was applied to simulations of density segregation associated with the stratification of particles between riffles on a shaking table. The behavior of binary particles having the same diameter and different densities was simulated under various operating conditions. The mixing index was employed to evaluate the segregation state on the shaking table and the progress of segregation was investigated from the viewpoint of the vertical velocity difference between binary particles. Simulation results showed that although a larger amplitude and longer frequency of the shaking table promoted density segregation with time, an excessive amplitude and frequency of vibration was ineffective in promoting density segregation on the shaking table. It was also shown that the time variation of the average vertical velocity difference within one period of the shaking table can explain the progress of density segregation under conditions of various shaking table amplitudes and frequencies. In addition, the velocity difference between particles became larger near the wall than near the center, independent of the amplitude and frequency conditions. Consequently, the discrete element simulation newly revealed density segregation due to the vertical velocity difference between heavy and light particles, especially near the wall. This result will contribute to the optimization of shaking tables.
CO2 tolerance of hydrogen storage alloys of AB2-type (C14 Laves phase) Ti0.515Zr0.485Mn1.2Cr0.8M0.1 (AB2-M, M = none, Fe, Co, Ni, and Cu) depends on dopant M. Since our goal is to clarify this mechanism, we determined the elemental analysis using X-ray absorption spectroscopy (XAS), scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), and neutron powder diffraction (NPD) with Rietveld refinement in this study. As a result of XAS analysis, a strong evidence of all doped elements occupying B site in AB2 was obtained. SEM-EDX showed inhomogeneous composition with vacancy in B site and linear correlation of Ti/Zr and Mn/Cr ratio. The peak width in XRD patterns of AB2-M depends on the magnitude of homogeneity, therefore the Rietveld analysis using NPD patterns could not be well refined. Thus, homogeneity is not important but element of B site would be important for CO2 tolerance as well as AB5 type alloys.
Additive metal M occupies B site in AB2-type hydrogen storage alloys. Therefore, CO2 tolerance depends on B element.
Titanium oxide can be converted directly to its metallic state. This extraordinary technology is realized in a CaCl2-based molten salt that can extract oxygen ions from the cathode. Several discussions have been exchanged in the field of electrochemistry and metallurgy in the last two decades. Recent papers on titanium refining are overviewed by selecting popular papers, especially those published in this journal, Materials Transactions. Basic and scientific studies on titanium and its refining are briefly analyzed, and some practical applications are introduced in related oxide reductions and the corresponding molten salts.