In metal additive manufacturing, it is possible to control the microstructure and the associated mechanical and chemical properties of metal products over a wide range. For example, in laser powder bed fusion (L-PBF), the laser process parameters are considered critical for the fabrication of functional parts. However, the effect of atmospheric gas on L-PBF has not yet been comprehensively documented. In L-PBF, gas flow is used to remove spatter and fumes, preventing degradation of quality due to laser attenuation and spatter contamination of the fabricated product. Thus, the use of atmospheric gas is inevitable in fabrication via L-PBF. In this review, we focus on the use of atmospheric gas in L-PBF, explain the effects of atmospheric gas on the microstructure and mechanical properties of fabricated products, and describe the importance of selecting the right atmospheric gas.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 220–226. The title was changed due to the addition of “Review—”.

Additive manufacturing (AM) has been attracting a great deal of attention in both academia and industry in recent years as a technology that could bring innovation to manufacturing. AM was originally developed as a method specialized in fabricating three-dimensional structures by the additive manner. However, in reality, a huge number of parameters involved in AM has a significant effect on the microstructure and the resulting physicochemical properties of the metallic material. Therefore, in very recent years, metal AM is being recognized as a technology for controlling the microstructure of metals rather than its shape. In addition, AM can even customize the microstructure of each site by applying locally controlled heat energy. The ability to simultaneously control complex shapes and microstructures will add even higher value to light-weight metal materials. This paper describes the potential of metal AM to control material and shape properties that dictates the essential mechanical properties of the product with introducing latest results.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 327–333. The title was changed due to the addition of “Review—”.

Additively manufactured metal parts often have a high level of residual stress and can exhibit complex crystalline phase properties due to the rapid cooling nature of their fabrication process. X-ray diffraction (XRD) is a non-destructive technique that can characterize both the residual stress and the crystalline phase properties in detail. However, XRD is an ex-situ measurement and provides only the final state of the manufactured parts. In this article, a method that combines the XRD analyses and numerical simulation of the thermal history during the manufacturing process is reviewed with two examples of titanium alloys fabricated by laser and electron beam powder fusion techniques.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 227–233.

Metal additive manufacturing is a powerful tool for providing the desired functional performance of hard tissue biomaterials through a three-dimensional structural design. It is essential to use the interactions between living organisms and materials for functional hard tissue reconstruction. In particular, anisotropic high-performance materials that imitate bone tissue properties are required for regaining bone functionality based on collagen/apatite microstructure. This review article describes the current development of controlling hard tissue compatibility by additive manufacturing of titanium alloys, including our recent findings on the bone medical devices for guiding the anisotropic bone microstructure.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 339–343. The title was changed due to the addition of “Review—”.

High entropy alloys (HEAs) have been developed as a new class of structural materials that consist of multicomponent elements with an approximately equiatomic ratio for increasing the mixing entropy to stabilize the solid solution phase. HEA for biomedical applications (BioHEA) was first developed in Japan; HEA comprising nonbiotoxic elements was specifically designed, demonstrating excellent mechanical properties and biocompatibility. However, elemental segregation, often observed in BioHEAs, hinders the inherent functions derived from high entropy effects and solid solution hardening. In this review article, elemental homogenization and functionalization of BioHEAs utilized by ultra-rapid cooling via laser-powder bed fusion and the characteristics of these BioHEAs, especially focusing on their excellent properties for biomedical applications, are introduced.

This Paper was Originally Published in Japanese in JILM 72 (2022) 334–338. The title was changed due to the addition of “Review—”.

In order to avoid the formation of defects in additive manufacturing (AM) by powder bed fusion (PBF) process, it is crucial to understand the relationship between the quality of powder bed and a powder spreading process. In this study, the influences of conditions of powder raking process on the densities and homogeneity of powder bed have been examined by computer simulation using Discrete Element Method (DEM) and experiment of powder bed formation using a blade-type spreader for Ti–6Al–4V powder by way of example. The results were analyzed with a special focus on the effects of the relative size of powder particles with respect to the gap between the blade and the build platform. It has been clearly shown that the gap needs to be larger than the upper bound of the distribution of powder particles diameter to obtain a high-density powder bed. The DEM simulation indicated that the blade can sweep even powder particles that are not in direct contact with the blade when the powder particles are in a close-packed tetragonal configuration. This is the probable reason for the experimental fact that powder particles smaller than half of the gap are suitable for PBF.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 291–297.

Microstructure and tensile properties of Ti–48Al–2Cr–2Nb (at%) rods fabricated by electron beam powder bed fusion (EB-PBF) process were investigated by changing input energy density (ED) which is one of the important factors affecting formation of the melt pool. We found that unique layered microstructure consisting of an equiaxed γ grain layer (γ band) and a duplex region can be formed by EB-PBF with ED in the range of 13 to 31 J/mm³. It is interesting to note that the width of the γ band and the volume fraction of the γ phase in the duplex region decrease with increasing ED. On the other hand, the α₂/γ lamellar grain in the duplex region increases with increasing ED. These morphological changes in the layered microstructure are attributed to variation of temperature distribution from melt pool caused by increasing ED. Moreover, we also found for the first time the strength of the alloys can be improved by decreasing width of the γ band and increasing of the α₂/γ lamellar grain in the duplex region. Whereas, the width of the γ band and the fraction of the equiaxed γ grain in the duplex region should be increased to enhance ductility of the alloys.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 298–303.

One of the important physico-chemical properties of materials under metal additive manufacturing process is the interfacial properties of liquid/solid such as the wettability between a liquid and its own solid. However, the data of the wettability between a liquid and its own solid for metal systems is limited. In this study, we propose the evaluation method for the interfacial properties between liquid alloy and its solid alloy based on the morphology of as built surface in powder bed fusion. The liquid/solid contact angle for Ti–6 mass%Al–4 mass%V is measured based on the sample built by electron-beam additive manufacturing, and its interfacial tension is evaluated by Young’s equation. The contact angle and interfacial tension between Ti–6 mass%Al–4 mass%V liquid alloy and its solid alloy are found to be 12° and 376 mN/m, respectively.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 304–307.

The densification process plays a critical role in determining the microstructure and performance of Ti matrix composites (TMCs). Herein, a comparative study was performed on a graphene oxide (GO)/Ti–6Al–4V composite fabricated by laser powder bed fusion (L-PBF) and spark plasma sintering (SPS). The flexible GO sheets were homogeneously decorated onto the Ti–6Al–4V powders via an electrostatic self-assembly without significantly changing the particle size or sphericity. Under high-energy laser irradiation, the GO sheets were completely dissolved into the matrix. The L-PBF-produced composite was composed of fine α′ martensite structures due to the rapid solidification and the solute carbon atoms. In contrast, the GO was reacted with Ti matrix and completely transformed into submicron TiC particles during SPS; the composite consisted of α + β phases with randomly dispersed TiC. Moreover, the L-PBF-produced composite exhibited a higher hardness of 481 HV as compared with the SPS-produced one of 367 HV, attributing to the fine α′ microstructures and high residual stresses. The present work offers deep understanding on the structural evolution of GO during high-temperature densifications, and shows new insights for fabrication of high-performance TMCs with tailored microstructures.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 314–320.

To promote the recycling of Ti scrap, it is essential to develop new technologies that can efficiently remove oxygen impurities from the Ti scrap. However, the direct removal of oxygen dissolved in solid Ti is extremely difficult, and currently, there are no effective deoxidation methods that can be used industrially. In this study, we experimentally verified a new deoxidation technique for Ti using the vapor of rare earth metals with high vapor pressures, such as Sm, Eu, Tm, and Yb. It was confirmed that Eu did not decrease the oxygen concentration in the Ti samples below 1000 mass ppm. On the other hand, it was shown that the vapor of Sm, Tm, and Yb decreased the oxygen concentration in the Ti samples through their oxide formation reactions. In particular, it was demonstrated that Tm vapor can deoxidize Ti at or below the oxygen concentration of the Ti sponge produced by the Kroll process (∼500 mass ppm O).

The effect of oxygen addition on the microstructure formation during cooling from the β phase in the Ti–Nb alloy was investigated. The alloy ingots of Ti–(13∼20) at%Nb–(0∼3) at%O were arc-melted. They were homogenized at 1200°C for 3.6 ks and then hot-rolled at 850°C into 1.5 mm thick sheets. The disk specimen of 3 mm diameter was heated up to β phase field at 1000°C in the differential thermal analysis apparatus. And then specimen was cooled to room temperature at a cooling rate of 20°C/min. The β → α transformation was accelerated by the oxygen addition in Ti–(13∼15) at%Nb alloys. On the other hand, the oxygen addition promoted β → isothermal ω_i transformation in Ti–(16∼20) at%Nb alloys. The promoting effect increased up to 1.5% oxygen, but the effect weakened by adding more than 1.5 at% oxygen in Ti–(18, 20) at%Nb alloys. The addition of oxygen over 3.0 at%O might suppress the β → ω_i transformation in Ti–20 at%Nb alloy.

The metastable-phase characteristics of Ti–Nb alloys can be exploited to improve their functional properties such as damping. In this study, we investigated the structural changes in the metastable quenched martensite structure of Ti–Nb alloys subjected to heating and tensile strain. We examined the differential scanning calorimetry (DSC) heating curves in the reduction state, X-ray diffraction (XRD) profiles under loading/unloading, and material properties such as Young’s modulus and internal friction upon heating. In the DSC heating curve of the 10%-cold-rolled Ti15Nb specimen, an exothermic peak was observed, and for Ti18Nb and Ti20Nb, the exothermic peak exhibited broadening. We speculate that the underlying reason is the biphasic formation resulting from specimen deformation. From the XRD measurements, we found that the lattice tended to shrink upon stress application and recover upon unloading. Significant changes in Young’s modulus and internal friction were observed in the α′′ structures of Ti18Nb and Ti20Nb during initial heating up to 373 K. We posit that the material properties changed owing to structural changes, such as lattice-constant changes, biphasic formation, and crystal orientation changes, resulting from heating or plastic deformation.

Yield phenomena during the tensile testing of pure titanium sheets of 0.2 mm and 0.4 mm in thickness were investigated in detail focusing on the effects of grain size (d), thickness (t), t/d ratio and tensile direction. With decreasing grain size, the yielding behavior changed from continuous yielding to one accompanied with yield point drop. Upper and lower yield stresses and 0.2%-proof stress followed the Hall–Petch relationship; however, coarse-grained specimens (d ≧ 20 µm) showed larger scatter in 0.2%-proof stress than the others. Consequently, the Hall–Petch coefficient (k) and friction stress (σ₀) derived from 0.2%-proof stress are not accurate enough. The values of k and σ₀ derived from various yield stresses and tensile directions were in the range of 250–600 MPa·µm^0.5 and 30–180 MPa, respectively. Therefore, the validity of stress for yield stress was a concern, and the combination of the lower yield stress in the fine-grain range (d ≦ 20 µm) and 0.2%-proof stress excluding work-hardening in the coarse-grain range (d > 20 µm) was suggested to obtain reliable values of k and σ₀, resulting in values of 370–460 MPa·µm^0.5 and σ₀ 65–140 MPa, respectively. Moreover, it was revealed that k decreased and σ₀ increased with the increasing angle of tensile direction to the rolling direction, regardless of thickness. The anisotropy of k is presumed to be affected by the grain boundary character rather than the Schmid factor, and neither rigidity nor the length of the Burgers vector are responsible. Meanwhile, the anisotropy of σ₀ is verified to be affected by Schmid factors. Furthermore, it was clarified that the t/d ratio hardly affects upper and lower yield stresses (t/d > 14) nor 0.2%-proof stress (1.5 ≦ t/d ≦ 14).

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 86 (2022) 53–61. The captions of Table 1 and Figs. 2–16 except for Fig. 5 were slightly modified.

Ti–6Al–4Nb–4Zr (mass%) was prepared by selective laser melting (SLM) under various conditions, and the microstructure evolution resulting from SLM processing and subsequent heat treatments was investigated. The effects of the unique SLM-induced microstructure on the high-temperature compressive strength and creep properties of the samples were then elucidated. Under rapid cooling conditions, the martensitic structure formed in a scale-like pattern, with a 100 µm in size, consistent with the laser scanning pattern. By contrast, under slow cooling conditions, the α/β lamellar structure formed in β grains with a 300 µm grain size instead of in a scale-like pattern. The martensitic structure drastically changed to a Widmanstätten structure during heat treatment. The equiaxed α phase also formed at the interface of the scale-like patterns. By contrast, the α/β lamellar structure did not exhibit a change in response to heat treatment. The compressive strength of the SLM samples was governed by the martensite α size and the grain size, both of which depended on the cooling rate. The dominant creep deformation mechanism at 600°C and under a loading stress of 137 MPa was grain boundary sliding. The creep life depended on the grain size. The HIP treatment improved the creep life because it eliminated pores introduced by the SLM process.

Titanium aluminide (TiAl) alloy is attracting attention in the automobile and aviation industries as a promising lightweight heat-resistant alloy material. Parts used in high-temperature environments must have ductility and toughness at room temperature in addition to high strength at high temperatures, however, some alloys, such as TiAl alloys, are known to be difficult materials for casting and machining. Recently, manufacturing processes using additive manufacturing (AM) have been studied to solve these problems. In the present research, the effect of energy density during the AM process on surface roughness and on the strength of the built parts was studied by changing build parameters. As the energy density increased, the porosity and surface roughness decreased. Additionally, higher energy density tended to increase the fraction of lamellar structure. It was found, however, that an increase in lamellar structure does not necessarily lead to any improvement in tensile or creep strength at 750°C. We found that a constant energy density of 15 J/mm³ showed better tensile and creep properties than those of the standard parameter. Our results suggest that it is important to optimize the parameters according to the required properties of the parts.

This Paper was Originally Published in Japanese in J. JILM 72 (2022) 308–313. Abstract is slightly modified. The captions of Table 2, 3, 5 and 6 are slightly modified. The captions of Fig. 1, 2, 3, 4, 7 and 10 are slightly modified.

This research focuses on the systematic study of a Ti–6Al–2Sn–4Zr–2Mo–Si titanium alloy and the characterization of α + β (equiaxed and bimodal) and α + α′ (duplex) microstructures. It provides more insights on the outstanding advantages of the duplex (α + α′) microstructure, especially on its exceptional work hardening and strength-ductility balance. The heat treatment conditions required to form equiaxed, bimodal and duplex microstructures and their effects on the grain size and the phase proportion are discussed. It shows how the microstructural parameters can be controlled thanks to the heat treatment temperatures, the holding times and possible aging processes. The influence of such microstructural factors on the tensile properties of each alloy is investigated, especially on strength (proof stress, ultimate tensile strength), ductility (plastic elongation) and work hardening properties. The duplex (α + α′) microstructure is compared with the equiaxed and bimodal microstructures and its advantages are displayed, highlighting the better strength-ductility balance and superior work hardening properties of the duplex microstructure. Indeed, the deformed microstructure of the duplex (α + α′) microstructure reveals more homogeneous strain partitioning than that of the bimodal (α + β) microstructure. Thus, this work proved the potential of an optimized duplex (α + α′) microstructure for the enhanced tensile properties at room temperature. Finally, a machine learning model using gradient boosting regression trees is used to quantify the importance of the microstructural factors (type of microstructure, grain size and phase ratio) on the mechanical properties.

The lightweight and high strength Ti–27.5Al–13Nb intermetallic alloy, based on the orthorhombic Ti₂AlNb phase (O phase) and the α₂ phase incorporated, was previously developed by the authors. This alloy would seem to have good potential for applications where fatigue behavior is a main concern, such as automobile and aircraft engine parts. The minor addition of boron (B) is known to refine the ingot grain size and thus to improve the subsequent mechanical properties. For these reasons, the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) properties of B-free and 0.1 pct B-modified Ti–27.5Al–13Nb alloy were examined in the present study. HCF tests were performed at room temperature (RT) in tension-tension mode at an R of 0.1 and a frequency of 10 Hz, while VHCF tests were performed using an ultrasonic resonance fatigue test machine at an R of −1 and a frequency of 20 kHz. In both fatigue tests, hourglass-shaped specimens were used. With the addition of 0.1 pct B, the prior B2 grain size of an ingot was reduced drastically, from 600∼1000 µm for the B-free alloy to 100∼250 µm. The 0.1 pct B-modified Ti–27.5Al–13Nb alloy with a duplex microstructure consisting of a globular α₂ phase and a lamellar microstructure exhibited superior elongation of 6.1 pct at RT. The HCF curve for this alloy with a duplex microstructure was almost the same as that for a Ti–6Al–4V alloy with a fully lamellar microstructure. Although prolonged fatigue life was previously reported in the HCF region in the 0.1 pct B-modified Ti–6Al–4V alloy, the addition of 0.1 pct B to the Ti–27.5Al–13Nb, Ti–6Al–4V and Ti–4Al–2.5V–1.5Fe alloys had no such effect in the VHCF region. VHCF strength for a lamellar microstructure was ranked in the order of Ti–4Al–2.5V–1.5Fe, Ti–6Al–4V and Ti–27.5Al–13Nb from the highest. Well-defined striations were observed at the propagation stage area of the fatigue fracture surface of B-free Ti–27.5Al–13Nb, and the measured striation spacing kept a constant of 0.29 µm through the propagation distance of 300 µm. The calculation based on this observation showed that the fatigue life spent in the propagation stage was very short and thus almost 100 pct of HCF life was thought to be spent in the fatigue initiation stage. For the B-free Ti–6Al–4V alloy with an equiaxed microstructure, the striation spacing increased from 0.06 µm to 4 µm as the fatigue crack propagated for a distance of 1,000 µm. Calculation based on the striation spacing revealed that, similar to the case with the Ti–27.5Al–13Nb alloy, the fatigue initiation stage consumed almost 100 pct of fatigue life regardless of the B addition.

In orthopedics, occasionally, different types of metals are used in applications in which they are in contact with each other. However, few studies have electrochemically investigated the galvanic corrosion of orthopedic implants formed of different metals. In this study, galvanic corrosion of Ti–6Al–4V ELI alloy, Co–Cr–Mo alloy, and 316L type stainless steel, which are used in orthopedics, and a newly developed Zr–1Mo alloy as a low-magnetic susceptibility material was evaluated in saline. Coupling of the Ti–6Al–4V ELI and Co–Cr–Mo alloys did not exhibit localized corrosion and maintained highly stable passive films. Coupling of the 316L type stainless steel and Co–Cr–Mo alloy, temporary localized corrosion occurred. Similarly, coupling of the Zr–1Mo and Co–Cr–Mo alloys, temporary localized corrosion occurred. However, both of 316L type stainless steel and Zr–1Mo alloy were finally repassivated spontaneously with the immersion time. The degree of the localized corrosion of the Zr–1Mo alloy was smaller than that of 316L type stainless steel. No galvanic current was observed when the Ti–6Al–4V ELI and Co–Cr–Mo alloys were coupled. A slight galvanic current flowed when 316L type stainless steel or the Zr–1Mo alloy was coupled with the other alloys; however, the galvanic current with the Zr–1Mo alloy coupling recovered to zero after a certain period owing to repassivation. No metal ions were detected from the couplings with Zr–1Mo.

The microstructures, mechanical properties, and biocompatibilities of α+β-type Ti–Nb–O alloys with a wide range of Nb and oxygen contents were investigated. Ingots of Ti–(5–25)Nb–(0.5–1)O (mass%) alloys were prepared by arc melting and hot forged. The forged bars were heat-treated at 873–1373 K for 3.6 ks followed by quenching. The volume fraction of equiaxed α-phase (f_α) in the alloys was determined experimentally. The Ti–5Nb–yO and Ti–(15–20)Nb–yO alloys contained α′- and α′′-martensite, respectively, while the Ti–10Nb–yO alloys contained both α′- and α′′-martensite. Therefore, changing the Nb content alters the α′/α′′ ratio. The addition of oxygen increased the distribution coefficient of Nb and accelerated Nb distribution in the β-phase, which increased the boundary temperature for the formation of α′- and α′′-martensite in the Ti–5Nb–yO alloys. The Ti–5Nb–(0.5–0.75)O alloys with f_α value of 0.5 had higher elongation and strength and a lower elastic modulus than Ti–6Al–4V alloy. Ion elution from the Ti–5Nb–(0.5–0.75)O alloys into 0.1 M NaCl–0.1 M lactic acid solution was comparable to that of a Ti–6Al–4V extra low interstitial (ELI) alloy. The Ti–5Nb–(0.5–0.75)O alloys did not exhibit cytotoxicity, indicating their excellent biocompatibility. Thus, we propose that Ti–5Nb–(0.5–0.75)O alloys are suitable for use as low-cost α+β-type Ti alloys for biomedical applications.

This paper proposes the concept and fabrication process of titanium alloy rods for spinal fixation. A part of rod for fixing the lower side of the lumbar vertebra is strengthened, while the other part for fixing the upper side has low stiffness. The results obtained by finite element analysis reveal that a rod with partially lowered Young’s modulus has higher flexibility and fixity compared with a rod possessing high Young’s modulus throughout. Using Ti–29Nb–13Ta–4.6Zr alloys with oxygen contents of 0.2 and 0.4% as the model alloys, rods with partially different Young’s moduli were fabricated by aging treatment at 723 K, followed by partial heating up to above the β-transus temperature and quenching by high-frequency induction heating (IH-treatment). A single β-phase, which has low Young’s modulus, is obtained by IH-treatment and has lower strength. With regard to the as-aged parts, the precipitated condition of the α-phase can be changed by varying the aging time. The obtained Young’s modulus and strength reflect this change. Near the boundary between the as-aged and IH-treated parts, the hardness is gradually changed, and it is possible to gradually soften the material from the as-aged part to the IH-treated part.

To prevent infection in dental implants using photocatalytic activity under visible-light irradiation, the fabrication of Au-added TiO₂ layers on Ti substrates and their antibacterial properties were studied. Pure Au and Ti–(60, 40) mol%Au alloy films with thicknesses of 10–47 nm were sputtered onto Ti, followed by thermal oxidation in air at 873 K for 1.8 ks to form TiO₂ layers. The antibacterial properties against Escherichia coli, cytotoxicity, and bonding strength to Ti substrates were evaluated. The highest antibacterial activity under visible-light irradiation was obtained when the sputtered film was pure Au and its thickness was 38 nm. Compared with as-polished commercially pure Ti, the number of viable mouse osteoblast-like cells and human gingival fibroblasts on Au-added TiO₂ layers increased after placement in the dark but decreased after visible-light irradiation. The best antibacterial property-bonding strength balance was achieved when the Ti–40 mol%Au sputtered film with a thickness of 42 nm was formed on Ti. To the best of our knowledge, this study is the first to report the formation of TiO₂ layers with antibacterial activity under visible-light irradiation by combining Au-sputtering and thermal oxidation of Ti.

Shunt-type potential-induced degradation (PID) is one of the degradation phenomena of photovoltaic (PV) modules which degrade PV performance drastically in short time compared to other degradation modes. In this paper, a new suppression technique of the PID was developed by coating a glass layer (GL) on the top or bottom surface of cover glass using a chemical solution known as liquid glass. PID tests were conducted using PV modules prepared with and without GL. A clear suppression effects of the PID were observed by forming GL, and the occurrence of the PID was delayed about 4 times by the formation of GL on the bottom side of cover glass in PV modules.

The Al–Si–Mg casting alloy was modified by strontium, and the microstructural classification of unmodified and Sr modified samples was accomplished by using our originally developed methods and machine learning techniques. The classification rates of unmodified and modified samples were at high levels and the highest rate reached 97.5% accuracy when using statistical data and the support vector machine as the classifier. The additive of Sr caused the distribution of eutectic-Si particles to change from a random distribution to a clustering arrangement, and decreased the particle size of eutectic-Si. The tensile properties of the alloy were significantly increased after modification due to the refinement of the eutectic-Si phase.

The combined impacts of copper content, cold working and homogenization in aluminium–silicon alloys were examined using microstructure study, thermodynamic modelling, image analysis and hardness measurement. Two aluminium alloys of varying copper contents were cast in a permanent mould using induction furnace. Microstructural observation revealed the formation of varying amount of intermetallic phases due to the presence of copper. Image analysis confirms the relative changes in percent phase fractions owing to presence of copper as intermetallic phases increases. Al₂Cu phase was most favoured with increasing copper. Cold working was performed by reduction of 5%, 10% and 15% of the cast alloys by equal amounts of strain. Hardness measurements indicated increase in hardness values attributed to both higher amounts of copper and cold working. Al₂Cu phase was predicted contribute compared to Al₇Cu₄Ni and Alpha phases. Homogenization at 300°C for 4 hr would suffice to obtain a uniform structure. The reasons for changes in microstructure and mechanical properties for different copper content were established by thermodynamic phase fraction calculation. Higher amounts of copper leads to greater volume fractions of intermetallic phases. Using solidification calculations, it is found that copper in liquid encourages dendrite formation which was also evident from microstructure.

Amorphous thin film thinned for transmission electron microscopy observation is reported to crystallize by electron beam (EB) irradiation of TEM. If thin film deposited on substrates can be crystallized by EB irradiation of scanning electron microscopy (SEM), new nanoscale devices can be fabricated such as electronic nanoscale circuits in amorphous thin films. Therefore, an amorphous AlO_x (am-AlO_x) thin film is deposited onto a Si substrate by atomic layer deposition (ALD) and point and area scan irradiation of EBs were performed by SEM. Then, structure and elemental analyses are carried out by TEM. Only the point EB irradiated area crystallized from the surface to the interface between am-AlO_x and the Si substrate, and γ-Al₂O₃ appeared. Moreover, the oxygen content of the not irradiated area close to the irradiated area was lower than that of the not irradiated area far from the irradiated area. These results show that the EB irradiated area was positively charged by the emission of secondary and Auger electrons under point irradiation conditions, and that oxygen was supplied by attracting its negative charge to the EB irradiated area. Also, the electrostatic repulsion between positively charged atoms and the structure relaxation due to electron excitation were additional driving forces for the crystallization.

Demand for lead-free, free-cutting brass is increasing in response to lead regulations in Europe. The authors of this paper have attempted to develop a lead-free brass with an α + β two-phase microstructure, which is different from existing C6932 alloy. As a result of basic investigations, it was found that the addition of Si to β-brass dramatically reduces the cutting resistance due to an effect that appears to be a reduction in the stacking defect fault energy, and that if phosphorus compounds are dispersed in such β-brass, the compounds make stress concentration sources improving chip fragmentation during cutting. Based on these findings, we have succeeded in developing a new free-cutting α + β brass (62.5Cu–1Si–0.07P–Zn). The developed alloy contains about 55% alpha phase and has excellent machinability and 1.3 times higher strength than C3604.

This Paper was Originally Published in Japanese in J. Japan Inst. Copper 61 (2022) 323–328.

In order to establish new technology for the storage of natural gas hydrates in underground rock tanks, it is necessary to precisely and quantitatively evaluate and understand the gas/liquid permeability characteristics as well as the mechanical and deformational characteristics of the rock mass at low temperatures. Moreover, based on these characteristics, it is essential to evaluate the behavior, robustness, and tightness of the rock tanks.

In this study, gas permeability experiments were conducted on Berea sandstone at low temperatures, which have not been done before, to evaluate the effect of ice saturation on the permeability. As a result, it was confirmed that the permeability tended to decrease with an increase pore fluid pressure. Compared to the initial permeability measured at 20°C, the permeability measured at −20°C showed a smaller value. Furthermore, it was observed that the permeability decreased with increasing ice saturation. The water permeability was estimated by taking into account the Klinkenberg effect, and the change in permeability was examined using the Kozeny-Carman equation. As a result, the permeability at a pore occupation ratio of 0, estimated from the regression curve, tends to overestimate the permeability obtained from the water permeability experiments.

The relationship between the P-wave velocity and ice saturation was found by measuring the elastic wave velocity, and the validity of the experimental results was discussed using estimation equations based on the cementation theory. The P-wave velocity at low temperatures tended to increase with increasing ice saturation and was 2.0 times higher when the pore occupation ratio was greater than 70%. In the range of porosity between 0.1 and 0.2, the experimental values were within the range of the estimated values, while in the range of porosity below 0.1, the experimental values exceeded the estimated values.

Acoustic emission (AE) method enables real-time monitoring of damage initiation and progression. Recently, AE analysis using machine learning has become widely popular; however, the AE source location is often located manually to ensure reliability and accuracy. Therefore, it is desirable that the detection of AE source location is fully automated with a high accuracy. This study proposes a novel arrival time identification method for AE source location that can accurately and automatically locate AE sources. First, a wavelet transform is applied to an AE signal to extract the wavelet coefficient of a specific frequency. Subsequently, the Akaike information criterion is applied to the time transient of wavelet coefficient to identify the initial wave arrival time. The localized AE source accuracy is compared with conventional arrival time identification methods: visual initial wave arrival time identification, visual S₀ mode arrival time identification at the time transient of the wavelet coefficients, automatic peak identification at the time transient of the wavelet coefficients, and automatic AIC wave arrival time identification. The proposed method reduces the number of events with incorrect initial arrival detections. Moreover, the correct detection rate increases by 1.5 times compared to a normal AIC. In addition, the proposed method is approximately 30 times faster than the conventional visual method and is an excellent AE wave arrival time identification in terms of both accuracy and speed of analysis. Lastly, we verify that the proposed method can be effectively applied to anisotropic materials.

The Cr effects on hydrogen embrittlement behaviors in pure Ni, Ni–20Cr, and Ni–44Cr alloys with similar grain sizes were investigated using tensile tests under electrochemical hydrogen charging and microstructure observations. The relative elongation (defined as elongation under hydrogen charging divided by elongation in air) in the pure Ni was higher and lower than those of the Ni–20Cr and Ni–44Cr alloys, respectively. The behaviors of the hydrogen-charged specimens were as follows. The hydrogen embrittlement susceptibility nonmonotonically varied with an increase in the amount of Cr substitution. The fracture surfaces of the pure Ni and Ni–20Cr alloy showed intergranular fractures, and that of the Ni–44Cr alloy showed a fully ductile feature. Post-mortem electron backscattered diffraction analyses revealed that grain reference orientation deviation (GROD) values, which correspond to local plasticity-induced lattice distortions, were high around grain boundary triple junctions, and the maximum value in the pure Ni at the grain boundary triple junction (18°) was nearly the same as that of the Ni–20Cr alloy (17°), although the total elongation of the pure Ni was twice that of the Ni–20Cr alloy. This result indicated that Cr addition promoted the plasticity-induced local stress evolution associated with dislocation pile-up. Moreover, the maximum GROD value of the pure Ni near the fracture surface was 56°, which is considered to be the critical level for plasticity-induced cracking in pure Ni. Interestingly, the Ni–44Cr alloy showed a similar GROD value (55°) even in the uniformly deformed portion of the fractured specimen, while showing ductile fracture and no cracks in the fractured specimen. This result indicated that the Ni–44Cr alloy had higher grain boundary strength than that of the pure Ni even after hydrogen charging.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 86 (2022) 77–84.

If a rectangular wave excitation current is used for eddy current testing (ECT) instead of a sinusoidal wave, multifrequency testing can be performed directly because rectangular waves contain a fundamental wave as well as harmonic waves. Furthermore, considering the skin effect, it is possible to determine whether the specimen has a corrosion flaw on the front surface, back surface, or flaws on both surfaces. In this study, we first evaluate a method to classify these flaws on non-magnetic metal plates using rectangular wave ECT (RECT) systems with a linear amplifier. Here, we present an indicator in which each harmonic amplitude is divided by its fundamental amplitude. The results indicate that the amount of attenuation of the indicator in relation to the frequency depends on the type of flaw; therefore, we can classify the type of flaw using the indicator. Subsequently, we developed a RECT system with an inverter and conducted the same experiment. The results indicate that the tendency is the same as that when using the RECT system with a linear amplifier, which contributes to high efficiency, low cost, and weight reduction.

This Paper was Originally Published in Japanese in J. JSNDI 71 (2022) 215–221.

Duplex stainless steel has excellent strength and corrosion resistance, and it is widely used such, for example, in plant applications. The microduplex structure affects not only the physical properties but also the deformation behavior. The fracture considered to be caused by the peeling duplex interface has been confirmed in various experiments and parts manufacturing factories, but there are few detailed references to the morphology of the microduplex structure. In this study, we examined the relationship between the morphology of the microduplex structure and the deformation behavior by the shearing test with load, horizontal or vertical to the stretching direction of the austenite phase. The latter load divided the duplex interface and delayed the fracture. The results showed that the microduplex structure acts as resistance to the load orthogonal to itself.

This Paper was Originally Published in Japanese in J. JSTP 63 (2022) 9–14.

The homogenization method is effective for analyzing macroscopic mechanical properties from the substructure of a material. In perforated sheet metal, macroscopic mechanical properties depend on the pattern of hole arrangement. In this study, the macro–plastic properties of perforated sheets with 60° standard staggered and 90° square arrangements are modeled. Biaxial stress is applied to the unit cells given the periodic boundary condition, and the contours of equal plastic work are obtained. The yield surfaces of the perforated sheets on the tension–compression combined stress state are strongly affected by the hole arrangement. To model the yield surface, a yield function for the rotational symmetry peculiar to the perforated sheet is proposed. The yield surfaces are modeled using the CPB2006 yield function that takes into consideration tension–compression asymmetry. Also, a surface–interpolated differential hardening model is applied to model the changes of yield surfaces. The analysis results obtained using the modeled yield surfaces are compared with the experimental ones obtained on the uniaxial tensile and deep drawing tests. The results of both tests show good agreement.

This Paper was Originally Published in Japanese in J. JSTP 62 (2021) 118–125.

Cu electrorefining using a low-grade copper anode is desirable from the standpoint of electric power savings. Cu electrolysis was carried out in an unagitated sulfate solution with a low-grade copper anode, and the effect of impurity ions and additives in the solution on the passivation of the anode was investigated. The time when anode passivation firstly occurs shortened significantly in a solution containing 0.596 mol·dm⁻³ of Ni²⁺ ions as impurity and shortened somewhat in a solution containing As⁵⁺(0.053 mol·dm⁻³) or Bi³⁺(0.0005 mol·dm⁻³) ions. Sn²⁺(0.0004 mol·dm⁻³) and As³⁺(0.053 mol·dm⁻³) ions slightly decreased the time to passivation, but Sb³⁺(0.004 mol·dm⁻³) ions did not. The viscosity coefficient of the solution increased when the 0.596 mol·dm⁻³ of Ni²⁺ ions were added to the solution, while the diffusion coefficient of Cu²⁺ ions decreased. The compound of the As–Sb–O or As–Bi–O system was formed in anode slime when the As⁵⁺(0.053 mol·dm⁻³) or Bi³⁺(0.0005 mol·dm⁻³) ions were added to the solution, which seemed to increase the compactness of slime. The time to passivation was slightly longer in thiourea-free solution but shortened when the concentration of thiourea was increased from 0.525 to 2.24 mmol·dm⁻³. The time to passivation was constant in solutions containing 0 to 1.13 mmol·dm⁻³ of Cl⁻ ions, but significantly decreased as the concentration of Cl⁻ ions increased above 1.13 mmol·dm⁻³. Cl⁻ ions formed Cu–Cl at the upper area of anode slime, which increased the compactness of slime and promoted the passivation.

This Paper was Originally Published in Japanese in J. Japan Inst. Metals 86 (2022) 97–106.

Coating layers having different ratios of a Γ phase to an Fe–Zn solid solution phase were produced on galvannealed steel sheets by heat treatment. The corrosion prevention characteristics of these layers were then investigated using combined cyclic corrosion test, electrochemical measurements and analyses of corrosion products. A coating layer with a larger proportion of the Γ phase showed better corrosion prevention properties along with a smaller corrosion mass loss. The Γ phase exhibited a sacrificial corrosion protection effect with regard to both the Fe–Zn solid solution and the steel substrate while the Fe–Zn solid solution showed a sacrificial corrosion protection effect on the steel substrate. The corrosion products formed from the Γ phase comprised Zn₅(OH)₈Cl₂·H₂O, Zn₅(CO₃)₂(OH)₆, ZnO and various amorphous compounds, while the Fe–Zn solid solution generated Zn-containing ferrihydrite and amorphous compounds. The presence of Zn evidently stabilized the ferrihydrite and delayed the formation of crystalline iron oxyhydroxide based on comparisons with corrosion products formed on cold-rolled steel sheets. These effects improved the corrosion prevention performance of the Fe–Zn solid solution. The zinc-based corrosion products formed from the Γ phase provided even better corrosion prevention performance by delaying the formation of crystalline iron oxyhydroxide and were also themselves resistant to reduction.

The increasing demand and exorbitant use of river bed sand in foundries over the years have led to its depletion at an alarming rate. Consequently, many states in India have imposed severe restrictions on the mining of riverbed sand. Therefore, an effort is made to explore the potential use of sea sand for cast iron foundry applications. Experiments were performed for optimizing the composition of molding sand with respect to functional properties namely, strength, mold hardness, and permeability. The optimum composition of molding sand comprised of 8.33 wt% bentonite and 3.12 wt% moisture, and 88.54 wt% sea sand. Trials were conducted in a local foundry to ascertain the suitability of using a mixture of sea sand, bentonite, and water for producing defect-free gray iron casting. The trial was successful and parts were accepted for production purposes.

The effects of temperature and phosphorus addition on the surface tension of molten Cu–P alloys were measured by the oscillating droplet method using the electromagnetic levitation technique. We successfully measured the accurate surface tension of molten samples that were sufficiently above the liquidus temperature. The surface tension of liquid copper was determined in its pure state, which is free from any contamination, such as oxygen adsorption and chemical reaction with the supporting material. The surface tension of the molten sample was decreased as phosphorus addition was increased. The surface tension of molten Cu–P alloys was well expressed as functions of temperature and phosphorus activity from the measurement results using the Szyszkowski model. Furthermore, the enthalpy and entropy changes in phosphorus adsorption on liquid copper were also estimated.

This Paper was Originally Published in Japanese in Journal of Japan Institute of Copper 61 (2022) 135–139.

To reduce the processing time for in-line carburizing, we performed ultra-rapid carburizing and quenching using induction heating. Conventional gas carburizing and vacuum carburizing (low-pressure carburizing) are treated at 1203–1323 K, which is lower than the eutectic temperature. In contrast, in this study, the carburizing temperature of 1523 K was higher than the eutectic temperature for speeding up this process. The SCM420 sample was rapidly heated to the treatment temperature by induction heating. CH₄ and N₂ were mixed and treated at atmospheric pressure at 5 vol% CH₄ and 10 vol% CH₄. For ultra-rapid carburizing, the total carburizing depth was proportional to the square root of the carburizing time, following the parabolic law, regardless of the CH₄ concentration. We obtained that the amount of carbon penetrating the steel from the atmosphere was proportional to the carburizing time. In other words, the carbon penetration rate was approximately constant during the carburizing process. The processing time for the ultra-rapid carburizing and quenching at 1523 K in this study was only 5% of that for the conventional gas carburizing at 1203 K.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Heat Treatment 61 (2021) 183–190.

Microbiologically influenced corrosion (MIC) is the rapid deterioration of structural materials induced by the action of microorganisms in an environment. Microbial adhesion and proliferation on the material surface are precursors to corrosion initiation, and if the material is stainless steel, its open-circuit potential can be ennobled. Therefore, extensive biofilm formation on metal surfaces and the ennoblement of the open-circuit potential of corrosion-resistant steels are recognized as indicators of MIC.

Numerous laboratory-scale studies have been conducted on the correlation between microbial adhesion on materials and initiation of MIC. However, only few studies have been conducted on the correlation between metallurgical factors of structural materials and the amount of microbes or flora on the material surface in actual environments.

We conducted systematic research focused on material types or alloying elements to investigate how such metallurgical factors affect microbial activity in the field. The corrosion behavior was examined using corrosion engineering methods such as potential measurement and weight loss evaluation. Thereafter, the amounts of microbes and flora adhering to copper, carbon steel, and stainless steel coupons in freshwater were determined using a genetic analysis method. Material analysis indicated that the no rapid change in potential over time was observed for the copper specimen, which ranged from −10 to 60 mV. Moreover, minimal surface stains such as slime was observed on the surface of the copper specimen when compared to the carbon steel surface, which exhibited significant amounts of rust within one month of exposure. Microbial analysis also showed a remarkable decrease in the concentration of bacteria on the surface of the copper specimen over time. These results indicate the considerable potential for the use of copper in practical environments. Thus, this study has captured the growth of microorganisms and the transition of microbial community structure reflecting nutrient requirements in parallel with the process of corrosion induction in an actual environment.

This Paper was Originally Published in Japanese in J. Japan Institute of Copper 60 (2021) 150–156. The captions of Table 1, Table 2, Fig. 3, Fig. 4 and Fig. 5 are slightly modified.

A machine learning method was developed, which predicts ionic conductivity based on chemical composition alone, aiming to develop an efficient method to search for lithium conductive oxides. Under the obtained guideline, the material search was focused on the Li₂O–SiO₂–MoO₃ pseudo-ternary phase diagram, which is predicted to have high ionic conductivity (>10⁻⁴ S·cm⁻¹). We investigated the formation range, ionic conductivity, and crystal structure of the lithium superionic conductor (LISICON) solid solution on the Li₄SiO₄–Li₂MoO₄ tie line. The ionic conductivity of the LISICON phases is about 10⁻⁷ S·cm⁻¹, which is higher than that of the end members; however, two orders of magnitude lower than that of the analogous LISICON materials. In addition, the experimental values were two or three orders of magnitude lower than the predicted conductivity values by machine learning. However, the developed prediction model can be used as an initial guideline for material exploration since the predicted values follow the trend of practical conductivity in the phase diagram. The crystal structure analysis indicated that the distance between the lithium sites and the occupancy of each lithium site in the crystal structure contributed to the decrease in ionic conductivity. This strong correlation between crystal structure and ionic conductivity was one of the reasons for the discrepancy between the predicted ionic conductivity based on chemical composition alone and the experimental value.

This Paper was Originally Published in Japanese in Japan Soc. Powder Powder Metallurgy 69 (2022) 108–116.

Laser-wire-based directed energy deposition (DED) is a type of additive manufacturing. We investigated the chemical composition and fatigue performance to verify the usefulness of Ti–6Al–4V fabricated by laser-wire-based DED which deposited dot-shaped beads on a base material and conducted in preventing atmospheric intrusion with local shielding only. We compared the fatigue performance to a wrought material equivalent to AMS4967M and evaluated the deposited material in the as-fabricated condition. The chemical composition of the deposited material was within the specifications for the alloy. The fatigue strength at 1 × 10⁷ cycles of the deposited material was more than 100 MPa higher than that of 682 MPa of the wrought material. Perhaps the high fatigue strength of the deposited material was caused by the low porosity and the acicular α′ martensitic phase. From these results, we showed the usefulness of Ti–6Al–4V deposited by a laser-wire-based DED to which local shielding was applied.

The electrodeposition of Si metal in molten CaCl₂ containing various types of calcium silicate has been studied at 1373∼1673 K. It was shown that the influence of the molar ratio of CaO to SiO₂ on Si electrodeposition was not remarkable in contrast with our previous results that the molar ratio of CaO to TiO₂ strongly affected the Ti metal deposition in the similar system. Silicon metal was obtained on MoSi₂ cathode in the melt by potentio-static electrolysis. The cathodic current efficiency was about 30% regardless of the added types of calcium silicate. The current efficiency tended to be better as the electrolysis potential became negative, but the electrolysis below the border potential lead to lower current efficiency due to Ca co-deposition.