The requirements for electronic devices in high-temperature environment such as avionics and automotive have promoted the development of high-temperature solders. The Au–20Sn solder, which is one of the hot topics of the current research in the field of electronic packaging, is widely used in flip-chip, light-emitting diode and hermetic package fields because of its good creep resistance, corrosion resistance and flux-free soldering. Recent research about the microstructure, wettability, interfacial intermetallic compounds, and mechanical properties of Au–20Sn solder were reviewed. This paper focuses on the interfacial reaction of Au–20Sn with different substrates and the mechanical properties of Au–20Sn solder joints. In addition, the current research shortages of Au–20Sn solders and the development directions are presented.

A new approximate solution has been proposed for diffusional growth and dissolution of cylindrical precipitates, for which practical exact solutions and useful approximate solutions are not available. The present approximation uses the solutions of the invariant field (steady state) approximation and the mass conservation conditions. The present approximation is compared with the numerical simulations based on the finite difference equation considering the moving boundary. It is shown that the agreement between the present approximation and the numerical simulation results is good for growth and dissolution of cylindrical precipitates.

In order to study the microstructure control of continuous casting slab of grain oriented silicon steel, the method of field trial was adopted to analyze the influence of different factors on the microstructure of slabs by adjusting the process parameters that affect the formation and transformation of equiaxed crystals and columnar crystals. Studies have shown that as the superheat increases, the proportion of equiaxed crystals in the grain oriented silicon steel slab decreases. The use of electromagnetic stirring can achieve the casting of grain oriented silicon steel under the condition of the superheat exceeds 40°C and the proportion of equiaxed crystals >50%. The box-type of electromagnetic stirring in secondary cooling zone can be used to effectively control the proportion of equiaxed crystals of the slab from 10% to 75%, and increase the electromagnetic stirring current, which contribute to the linear increasing of the proportion of equiaxed crystals. The increasing of the casting speed and width range is conducive to the proportion of equiaxed crystals in slabs. Meanwhile, moderate electromagnetic stirring intensity (400 A, 3 HZ) is beneficial to the uniformity of the slab composition with the optimal effect of improving the segregation along the thickness direction of the slab. The deviation of the measured and predicted value of the equiaxed crystal proportion of the slab is less, the effective control of the equiaxed crystal ratio can be achieved by adjusting the process parameters.

High thermal conductivity, high strength and non-flammability have successfully been achieved concurrently for the first time in Mg–4.5Al–2.5Ca, Mg–5Al–3Ca, Mg–6Al–4Ca, and Mg–4Al–2Ca–0.03Be (at%) alloys, which are produced by hot extrusion of the heat-treated cast alloys. These alloys consist of α-Mg, and C36, C14 and C15 compounds, and exhibit a high thermal conductivity of 111–119 Wm⁻¹K⁻¹, a high ignition temperature of 1343–1408 K and a high tensile yield strength of 318–363 MPa.

The physical properties desired for various applications of aluminum alloys are usually modified by tailoring their precipitation behavior during heat treatment. In this study, the precipitation behavior of a cast Al–Mn alloy annealed with two heating methods (electric resistance heating and radiative furnace heating) at different temperature has been studied through the electrical conductivity (EC) test, and scanning electron microscope. The impact of the two heating methods on the mechanical properties of the treated Al–Mn samples was evaluated through hardness and EC analysis. The results clearly show that under the conditions of electric resistance heating, the precipitation kinetics of the alloy is promoted. The mechanisms behind this phenomenon are discussed in detail.

To clarify the effect of constraint conditions on the kink formation, fabrication process of the texture and porosity controlled Ti₃SiC₂ polycrystals was investigated and microstructural evolution during high temperature deformation was examined in it under high temperature uniaxial compression tests at 1200°C. Dense textured Ti₃SiC₂ sintered body was fabricated by slip casting in the high magnetic field of 12 T and following pressureless sintering at 1400°C for 1 h. The porosity of the textured Ti₃SiC₂ was controlled by dispersing polymethyl methacrylate (PMMA) particles into the textured Ti₃SiC₂ as a spacer media. The highly textured Ti₃SiC₂ polycrystals with porosity of 8.4 vol% and 16.7 vol%, respectively, were successfully fabricated by the slip casting in the high magnetic field. After the high temperature uniaxial compression perpendicular to the c-axis of the textured structure, both the porous and dense Ti₃SiC₂ showed kink formation, which is a common deformation mode for anisotropic layered materials. However, the average rotation angles of the kink boundaries were higher in the porous specimen than in the dense specimen. Since the crystal rotation is necessary for the kink formation, kink bands would be preferably developed in the porous area due to its weaker constraint than in the dense area. It can be concluded from the microstructural analysis that the constrain factor caused by the neighbor grains affects the crystalline rotation, resulting in the kink boundary formation with different rotation angles.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 256–263.

We investigated the mechanical alloying (MA) processes of Al and Al₂O₃ powders via X-ray diffraction analysis, transmission electron microscopy (TEM), and Vickers microhardness. The yield strengths and microstructures of consolidated compacts were evaluated using tensile tests and TEM, respectively. When the amount of oxygen in the MA atmosphere was higher than a certain threshold, Al₂O₃ particles of about 7 nm were precipitated in the Al grain interior from a supersaturated solid solution (SSS) of Al–O due to significant precipitation driving force. Two types of alloyed powders were obtained: Al₂O₃ particle-dispersed Al nanocomposite and SSS. The consolidated compact of the former exhibited smaller Al grains and higher yield strength than the latter due to the enhancement of the pinning effect on Al grain boundary migration during consolidation. The difference in the nature of the two kinds of powders affected the Al grain sizes and yield strengths of the consolidated compacts.

The relationship between the mechanism of high temperature deformation and the evolution behavior of microstructure and the texture during deformation, which has been found in various solid solution alloys, is clarified. It is shown that the preferential dynamic grain growth (PDGG) mechanism proposed by the authors can explain the behavior of microstructure change as well as texture change of all studied alloys without contradiction. The essential aspect of the PDGG mechanism is the preferential growth of crystal grains with the orientation stable for deformation and with low Taylor factor in the given deformation mode. It is concluded that the Taylor factor corresponds to dislocation density and stored energy during the high temperature deformation of solid solution alloys when viscous glide of dislocations is the rate controlling process. The possibility of the occurrence of the PDGG mechanism in materials other than solid solution alloys is also discussed.

Long-term creep tests on the solid-solute-strengthened zirconium alloy, i.e., Zircaloy-4, revealed the presence of double steady-state creep behavior at 623–673 K. The first steady state manifested as a faster creep strain rate at a strain (ε) of ∼0.05, and the second appeared with a slower creep strain rate at ε > 0.1. Several electron microscopy revealed a change in the dislocation structure during creep from the motion of individual dislocations to the generation of cell structure. Along with this change, the stress exponent differed in each steady state, increasing from 6.7 in the first steady state to 10 in the second steady state. The temperature dependency also changed because the apparent activation energies were 201 and 298 kJ/mol for the first and second steady states, respectively. These results show that pipe diffusion and self-diffusion are the rate-controlling processes in the first and second steady state, respectively. Because such changes in the deformation mechanism during one creep curve are highly unusual, the double steady state is a characteristic feature of high-temperature deformation in Zircaloy-4. For engineering, observation of the second steady state is necessary in estimating the time-to-rupture by means of the Monkman–Grant relationship because creep strain rates at the first steady state deviate from the relationship.

Age hardening in stable austenitic stainless steel wires with a chemical composition of Fe–18%Cr–12%Ni and different N contents was investigated to clarify the role of N. Age hardening could be enhanced by increasing the drawing ratio and N content. The highest age-hardening effect was observed at 800 K in the N-bearing specimens. In addition, severe drawing induced low-temperature age hardening at 450–600 K. Differential scanning calorimetry (DSC) analysis revealed that age hardening at 800 K may be attributed to particle dispersion strengthening by Cr₂N. Meanwhile, the exothermic peaks observed at 450–600 K were controlled by the pipe diffusion of N. However, nitride precipitates and clusters could not be detected by 3D atom probe analysis, which implies the formation of atomic-scale N products, such as I-S pairs, at the dislocations.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Heat Treat. 61 (2021) 112–118. Result and Discussion are slightly modified.

The present study shows that effect of impact load on microstructural feature in 27Cr white cast iron alloy, which is destabilized at 1150°C for 3 hour holding. The specimen is received with a microstructure consisting of ductile matrix (57% retained austenite) and amount of dispersed secondary. The residual stresses distribution in the alloy as a function of the impact load, they localized around the eutectic carbides and eutectic carbides, at defect positions. This distribution tends to decrease in the impact loading direction. The dislocations are formed in alloy as a result of the concentration stress, which caused by impact load. High stress concentration promotes the interaction between dislocations and these dislocations tend to pile-up at grain boundaries. In this study, the helical dislocations are observed in the stress concentration areas. The voids are formed by the pile-up of dislocations. The diameter of voids can be from several tens nanometers to 500 nanometers and depending on the applied load. When the voids expand (exceed 500 nm diameter), the ligament around void is thinner and leads to an increase in the deformation region between voids. As a consequence, the voids coalesce by formation the necking of the ligament between two voids to form a micro-crack.

This work examined the behaviors of densification and microstructural formation of the Al–10 mass%Si–0.35 mass%Mg alloy fabricated by Selective Laser Melting (SLM) method on the basis of experimental work and machine learning. Additionally, the effect of scanning repeated twice in each layer (double scanning) in the SLM process was also investigated. The SLM-ed Al–10 mass%Si–0.35 mass%Mg alloy exhibited the columnar grained microstructure with a (α-Al–Si) eutectic cell structure. Refined microstructures were produced at an increasing scanning speed with a decreasing the energy density (J/mm³). Relative density tended to increase with an increasing of energy density for scan pitch conditions of 0.1 mm and 0.05 mm. And a scattering was obviously exhibited at a higher relative density more than 95%. The analysis based on machine learning revealed that a scanning pitch of 0.2 mm was just a condition to achieve a high relative density. Except for the condition at a scanning pitch of 0.2 mm, a scan speed was the most important factor in affecting the relative density. Thus, a machine learning approach enabled to identify the important processing factor for affecting the behavior quantitatively. Additionally, compared to a conventional single scanning process, it was found in this work that the double scanning resulted in a higher relative density with keeping the fine microstructural formation.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 365–373. Figure 2 and Fig. 6 were slightly modified. Caption of Fig. 7 was slightly modified.

Oxidation of the Nd-rich phase in Nd–Fe–B magnet alloys is known to deteriorate their coercivity. Therefore, it is important to prevent the dissolution of oxygen into the Nd–Fe–B magnet alloy during the manufacturing process. This requires the knowledge of the properties of oxygen in Nd–Fe–B magnet alloys, which has not been investigated yet. In this study, the oxygen solubility of Nd–Fe–B alloy at 1673 K was measured, and the effect of boron concentration on the affinity between oxygen and Nd–Fe–B alloy was analyzed. The standard Gibbs energy for the dissolution of oxygen into the molten Nd–Fe–B alloy at 1673 K was derived from experimental results, which indicated that the addition of boron into the molten Nd–Fe alloy decreased the affinity between oxygen and the alloy. The thermodynamic cause of this effect of boron is discussed, and a method to suppress oxygen dissolution into Nd–Fe–B alloy during the melt manufacturing process is suggested.

For the application of Nd–Fe–B magnets at high temperatures, it is imperative to enhance their coercivity. Grain boundary diffusion using small quantities of dysprosium (Dy) is a significant method to enhance the coercivity of Nd–Fe–B magnets. However, the high oxygen content in the Nd–Fe–B magnet oxidizes the introduced Dy in the Nd-rich phase, which decreases the availability of Dy to generate the (Nd, Dy)₂Fe₁₄B phase and, therefore, requiring the addition of increasing amounts of precious Dy. In the present study, the thermodynamic property of oxygen in the Nd-rich phase was estimated by measuring the oxygen solubility in the molten Nd–Dy alloy (at 1673 K) and molten Dy metal (between 1773 and 1873 K). Additionally, the standard Gibbs energies for the oxygen dissolution into the molten Nd–Dy alloy and Dy metal were derived from experimental results. The results indicated that the affinity between oxygen and the molten Nd–Dy alloy gradually increased with the increasing Dy concentration in the Nd–Dy alloy. Thus, the oxygen solubility in the Nd-rich phase was estimated based on experimental results.

This paper reports the first attempt to synthesize un-doped spherical Zn₂GeO₄ stoichiometry and Mn²⁺ doped Zn₂GeO₄ nanoparticles to achieve a controlled luminescence, particularly, by changing Zn:Ge ratio and Mn²⁺ concentration during the hydrothermal process. The morphology of the un-doped Zn₂GeO₄ nanoparticle was depended on the Zn:Ge ratio that was observed to have a nanorod structure to sphere-like morphology; however, the morphology of Mn²⁺ doped Zn₂GeO₄ was not changed as the use of Mn²⁺ as a dopant. The un-doped Zn₂GeO₄ showed a broad emission at ∼510 nm and its intensities were functioned of Zn:Ge ratio. On the other hand, the Mn²⁺ doped Zn₂GeO₄ showed emission peaks at ∼532 nm and they can be attributed to the ⁴T₁ → ⁶A₁ transition in Mn²⁺ ions doped in Zn₂GeO₄. These results suggest the effectiveness use of Zn:Ge ratio and Mn²⁺ doped Zn₂GeO₄ in tuning the luminescence of Zn₂GeO₄ nanoparticles, which are of potential applications as phosphors in wide range of engineered fields, in particular optoelectronics.

Fine particle peening (FPP) at room temperature was introduced to create finer grains at the surfaces of Fe–Cr alloys that were then treated by atmospheric controlled induction heating FPP (AIH-FPP). The effect of the proposed treatment on the grain refinement behavior of the alloys was investigated using optical microscopy and micro-Vickers hardness testing. The FPP pretreatment at room temperature increased the effectiveness of the grain refinement via dynamic recrystallization during subsequent AIH-FPP. Grain refinement at the surface of Fe–Cr alloys progressed with an increase in the cycles of the treatment. The addition of Cr in Fe–Cr alloys promotes the formation of fine grains during the treatment. Specimens with fine grains created via static recrystallization were also prepared and compared to grains formed via dynamic recrystallization by the room temperature FPP pretreatment and subsequent AIH-FPP to evaluate the effectiveness of the proposed treatment. The grains formed through dynamic recrystallization by the proposed treatment were finer than those formed through static recrystallization. These results indicate that the proposed combination of room temperature FPP and AIH-FPP is effective for the formation of fine grains at the surfaces of Fe–Cr alloys.

Metadynamics-based simulation sheds light on a longstanding issue on the temperature dependence of solid-liquid interfacial energy of metallic materials. The solid-liquid interfacial energy at various temperatures is directly derived from free energy profile of the solid-liquid biphasic system. Negative temperature dependence of solid-liquid interfacial energy is found both for Fe and Ni systems near the melting point. Moreover, the origin of negative temperature dependence is discussed from atomistic viewpoint. The effect of positive excess entropy becomes dominant at high temperature near the melting point due to the severe thermal vibration. This results in the negative temperature dependence of solid-liquid interfacial energy.

In the die design of high pressure die casting (HPDC), in order to prevent the casting from the warpage, it is essential to predict ejection force of each ejector pin before locating it. Recently, Shiga et al. revealed that their developed elasto-plastic-creep model is more accurate than conventional elasto-plastic models to estimate the thermal stress of casting during cooling with constraint of dies.

In this study, by using the elasto-plastic-creep model, thermal stress analysis was conducted to predict the force generated on each ejector pin in the HPDC process. As a result, the force was estimated within the error of 30% in respect to the measured value. It was also predicted that increasing in the curing time increases the force.

This Paper was Originally Published in Japanese in J. JFS 92 (2020) 583–588. The caption of Figs. 1, 4, 6, 8, 9 and Tables 3-1, 3-2 are slightly changed. Figures 4, 6, 8, 9 and Tables 3-1, 3-2 are slightly changed.

Multilayer iron-doped indium-saving indium–tin oxide (ML ITO50:Fe₂O₃) thin films with high conductivity and high transmittance in the visible spectrum have been fabricated by sputtering method. Structures consisting of very thin layer of conventional indium tin oxide (90 mass% In₂O₃–10 mass% SnO₂) and iron-doped indium-saving indium–tin oxide layer with reduced content of In₂O₃ (ITO50:Fe₂O₃) to 50 mass% are discussed. By optimizing oxygen flow rate in iron-doped indium-saving indium–tin oxide layer, the lowest volume resistivity of 3.78 × 10⁻⁴ Ω·cm, mobility of 29.8 cm²/(V·s), carrier concentration of 4.60 × 10²⁰ cm⁻³ and transmittance larger than 90% in the visible range have been achieved. ML ITO50:Fe₂O₃ thin films deposited under optimal conditions demonstrated lower volume resistivity and higher transmittance than undoped multilayer indium-saving ITO thin films and iron-doped single-layer thin films obtained under the same oxygen flow rate Q(O₂) = 0.1 sccm. ML ITO50:Fe₂O₃ thin films demonstrated optimal parameters at the lower oxygen flow rate (Q(O₂) = 0.1 sccm) than undoped ML ITO50 thin films. ML ITO50:Fe₂O₃ thin films are crystallized and show In₄Sn₃O₁₂ structure.

We investigated the relationship between the material properties and axial collapse behavior of hat-section hollow columns made of high-strength steels. Dynamic collapse tests of the columns and FEM simulations were conducted using high-strength steels with various tensile strengths and n-values. Materials that exhibited yield point elongation (YPEl) underwent enhanced accordion-type deformation in axial collapse owing to the occurrence of long wave buckling on the hat walls in the early stage of collapse. In addition, it was important to avoid fracture in the initial buckling region to induce progressive crumpling behavior. A high n-value of the material was favorable for avoiding fracture by increasing the bend radii and decreasing the strain concentration at the buckling regions on the hat walls, thereby resulting in stable progression of accordion-type folds. In conclusion, steels exhibiting YPEl and high n-values are favorable for inducing stable accordion-type deformation during axial collapse.

This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 123–129.

The tensile residual stress near pierced holes deteriorates the fatigue properties and hydrogen embrittlement resistance of automotive parts. Consequently, a simple shearing method that can reduce this type of stress is highly desired in the automotive industry. In this study, the effect of a coining method using scrap material on the stress near a 10 mm diameter pierced hole was investigated. The fracture surfaces of the pierced scrap material and pierced hole presented similar shapes because these parts were produced by the separation of a single sheet via crack propagation. Therefore, it is possible to use the pierced scrap material as a precise coining tool. The effectiveness of this method was investigated for 980 MPa grade steel sheets with a thickness of 1.6 mm. To examine the effect of the coining stroke, the test was conducted at two levels: a coining stroke in which the product and scrap were at the same level and a coining stroke in which the scrap passed through a hole in the product. The tensile residual stress on the sheared surface significantly reduced as the length of the coining stroke increased. The results of finite element method simulations indicated that the above effects were mainly caused by the change in the shape of the pierced hole and the stress generated by the contact between the pierced scrap material and the hole in the product.

This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 301–306. The Abstract is slightly modified.

The effects of grain size on the hydrogen embrittlement susceptibility of pure Ni and Ni–20Cr alloy were investigated. The hydrogen embrittlement susceptibility was evaluated through tensile testing under electrochemical hydrogen charging. Relative elongation, defined as the elongation under hydrogen charging divided by elongation in air, increased with increasing grain size in pure Ni (the grain size was in the range of 11–22 µm). In contrast, the relative elongation of the Ni–20Cr alloy increased with decreasing grain size from 13 to 1.8 µm. Correspondingly, the intergranular fracture was suppressed by grain coarsening in pure Ni and grain refinement in the Ni–20Cr alloy. In addition, the intergranular fracture surface in pure Ni exhibited curved slip lines and that in the Ni–20Cr alloy exhibited straight line marks. These fractographic features imply that the mechanisms of the hydrogen-assisted intergranular crack growth in pure Ni and Ni–20Cr alloy were different, which could be attributed to the difference in their stacking fault energies.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 49–58.

In this study, the mechanical properties of aluminum foam were classified by machine learning from their X-ray computed tomography (CT) images. It was found that aluminum foam samples with high and low compressive strengths can be classified with an accuracy rate of more than 95%. In addition, it was indicated that the accuracy rate can be further improved by increasing the amount of training data. From these results, it is expected that the quality assurance method of aluminum foam can be established by nondestructively acquiring the images of the manufactured aluminum foam product.

This study investigated the evolution of hardness homogeneity of pure niobium subjected to equal channel angular pressing (ECAP) through microhardness measurements. Niobium samples were deformed via ECAP die with channel angle of Φ = 90° and curvature angle Ψ = 0° using Bc processing route at room temperature. Deformations occurred up to 8 passes, and hardness was measured at the longitudinal sections in the samples. The ECAP process increased the hardness by 55% between the 1st and 8th passes. The results demonstrate that hardness homogeneity was reached after six passes considering the steady-state of deformation.

MnS inclusions in Type 304 stainless steel were mainly transformed into oxide/oxysulfide inclusions by heat-treatment at 1673 K. In 0.1 M MgCl₂ under both the no-stress and stress conditions, the pitting potential of the steel heat-treated at 1673 K was higher than the steel heat-treated at 1373 K. The pitting potential of the stainless steel heat-treated at 1673 K did not decrease due to applied stress. For the steel heat-treated at 1673 K, the dissolution current of oxide/oxysulfide inclusions was lower than that at MnS inclusions. The results show no significant decrease in the pitting potential of the steel heat-treated at 1673 K even under applied stress.