The effects of post-annealing treatment temperature and ambient gas on the properties of ZnO thin films were investigated. The results showed that the properties of the ZnO thin film depended on the crystal quality, the type and density of the intrinsic defects. Treating the ZnO thin films at 300°C, regardless of the ambient gas, affected only on the crystal quality of the thin films. When the samples were treated at 500°C and 600°C, the structural properties were significantly improved, accompanied by changes in the optical and electrical properties. The extent of improvements varied with the treatment ambient. These variations in the optical and electrical properties of the thin films were attributed to the different reaction mechanisms when ZnO was treated under different ambient conditions. The thin films, treated at 600°C under N₂O ambient, exhibited the best structural, optical, and electrical properties for metal-semiconductor-metal photodetector (MSM-PD) applications.

Although severe plastic deformation causes grain refinement of polycrystalline materials, saturation of grain refinement is known to be caused by simultaneous grain coarsening. In the present study, for ultra-fine-grained (UFG) pure Al prepared by equal-channel angular pressing, we studied the grain refinement and coarsening processes during large simple-shear deformation using scanning electron microscopy/electron backscatter diffraction. The changes of the crystal orientations in the grains as a function of position were analyzed using log angles as rotation angles around reference axes. This analysis enabled the evaluation of order of magnitude of the in-plane components of geometrically necessary dislocation (GND) density tensors. Detailed changes in the crystal orientations during shear deformation were measured for identical regions in UFG Al, and the changes in the components of the GND density tensors were discussed. Our findings indicate that the grain refinement and coarsening can be explained by the appearance and disappearance of grain boundaries composed of dislocation walls, respectively.

In the present study, the diffusion coefficient of Cu in Fe was experimentally estimated from the precipitation kinetics down to 390°C. At this temperature, diffusion couples, which is a typical method to obtain diffusion coefficients, cannot be applied. The matrix Cu concentration and the number density of Cu precipitates in Fe–Cu alloy, which were the main parameters used to estimate the diffusion coefficient, were directly obtained using atom probe tomography. The temperature dependency of the diffusion coefficient of Cu in Fe estimated in the present study was more reliable than that obtained in a previous study, which also reported the diffusion coefficient of Cu in Fe from precipitation kinetics. This indicated that our estimation of the diffusion coefficient of Cu in Fe with atom probe tomography measurements yielded greater accuracy. In addition, the estimated diffusion coefficient of Cu in Fe tended to deviate to higher values from the extrapolated diffusion coefficient of Cu in Fe, which was obtained by diffusion couples, with decreasing temperature. This deviation is discussed by employing a kinetic Monte Carlo simulation.

TiB-reinforced Ti–3Al–2.5V, in which the TiB whiskers are oriented parallel to the direction of heat extrusion, was fabricated via powder metallurgy. To investigate the effects of TiB whisker volume fraction on fatigue properties in Ti–3Al–2.5V, four-point bending fatigue tests were conducted on plate-type specimens having TiB whiskers of varying volume fraction and orientation. The fatigue limit and fatigue life of Ti–3Al–2.5V having TiB whiskers that are oriented parallel to the loading direction tended to be increase with increasing TiB volume fraction, whereas the fatigue limit and fatigue life of specimens having TiB whiskers oriented along the short transverse direction did not depend on the TiB volume fraction. The fatigue limit of specimens with TiB whiskers oriented along the long transverse direction tended to decrease with increasing TiB volume fraction, because fatigue cracks were initiated and propagated along the TiB whiskers. Thus, the resistances of fatigue crack initiation and propagation behavior in Ti–3Al–2.5V changed with both the TiB volume fraction and orientation.

Automobile manufacturers are accelerating adoption of spot welding of Advanced High-Strength-Steels (AHSS) sheets to reduce weight of automobile bodies. Rapid evaluation of the hydrogen embrittlement (HE) resistance for the spot-welds of AHSS sheets is required, since it is worried that the HE resistance of the nugget will deteriorate compared to the base metal due to the difference in microstructure caused by rapid cooling and solidification during spot welding. However, evaluation of the HE resistance for the spot-welds has not been established. In this study, we prepared spot-welded specimens using AHSS sheets and performed tensile shear tests with varying tensile rates under hydrogen charging to evaluate the relationship between diffusible hydrogen content and tensile shear strength. As a result, the tensile shear strength of spot welds decreased as the amount of diffusible hydrogen increased. The quasi-cleavage fracture surface and intergranular fracture surface were observed at the nugget and inside the crack generated at the nugget-heat affected zone interface. Furthermore, as the results of crack growth behavior and hydrogen thermal desorption spectroscopy analysis, hydrogen embrittlement in spot welds can be attributed to the stress-induced diffusion of hydrogen and the hydrogen trapped in dislocation and vacancy clusters at crack tips.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 75–83.

The effects of the thickness (t), grain size (d), ratio t/d, and tensile direction on tensile properties were carefully examined using ASTM grade 1 pure titanium having a strong B texture with the controlled thickness (0.2–0.4 mm), grain size (20–175 µm), and t/d (2.3–19.4). 0.2%-proof stress followed a Hall-Petch relationship in each tensile direction regardless of thickness, and this indicates that 0.2%-proof stress does not depend on t/d. On the other hand, tensile strength and uniform elongation were confirmed to depend on t/d and significantly decreased in some thin sheets, compared to thick sheets with the same grain size, except for the case tested in the transverse direction. Namely, both tensile strength and uniform elongation exhibited a drastic decrease when t/d becomes smaller than the critical values. The critical value of t/d depended on tensile direction and decreased with an increasing tensile angle in the rolling direction. Local elongation increased with decreasing grain size except for the case tested in the transverse direction. In addition, local elongation was not related to t/d. Tensile properties that depend on t/d are inferred to be affected by work-hardening behavior, enhanced by twinning deformation in hcp titanium. However, the formation of deformation twinning in the surface regions was found to be suppressed, presumably due to the relaxed, constrained conditions. Consequently, the work-hardening rate decreased with decreasing t/d because of the attribution from the surface region where twinning deformation is retarded.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 227–236.

In order to reveal the variation trend of stress in T9A complex helical structure during long period of service, the stress relaxation of T9A helical compression spring was investigated via experiment and finite element analysis. The stress relaxation experiments of the T9A helical compression spring were carried out for up to 30 h at 90°C, 110°C, 130°C and 150°C, and a modified constitutive relation was established on the basis of the experimental data. As a result, it proves that the model is effective to predict the whole relaxation process. Meanwhile, the finite element analysis of the helical structure spring was carried out. It is found that the steel wire in the spring is relaxed from outside to inside in the process of relaxation.

The temperature dependence of the fatigue crack propagation rate in stage IIb in bimodal Ti–6Al–4V was investigated at different stress ratios R. Fatigue tests were conducted between room temperature and 550 K at R of 0.1, 0.7, 0.8, and 0.9, and two phenomena were elucidated consequently. First, the fatigue crack growth rates were nearly temperature independent for R = 0.1, 0.7, and 0.8 while it is temperature dependent at R = 0.9. This difference in the temperature dependence can be explained by the assumptions that the fatigue crack growth is controlled by the dislocation activities associated with work-hardening for R ≤ 0.8 while it is controlled by dislocation glide at R = 0.9. Second, the fatigue crack growth rates at R = 0.9 was higher than those at R = 0.1, 0.7, and 0.8. This increase in the fatigue crack growth rate at R = 0.9 can be explained by the change in the stress intensity factor of crack opening. Both the controlling mechanisms emanated from the change in the dislocation structure in front of the crack tip.

Silicon single crystals were deformed in tensile tests along the [134] direction between 1173 K and 1373 K. The yield point phenomenon was observed in the specimens deformed below 1273 K, while a continuous yield was observed in the specimens deformed above 1323 K. The values of work-hardening rate in stage II were the same as those reported in other single crystals. Orientation maps of the specimen obtained by using electron backscattered electron diffraction method indicated that stage II starts before the Schmid factor of the secondary slip system became larger than that of the primary slip system. Because of the constraint due to the gripping of the test piece, kink bands are formed during stage I before the onset of stage II, and then the stress state becomes non-uniaxial. This suggests that the formation of kink bands triggers the activation of the secondary, i.e., conjugate slip system to increase the resolved shear stress on the conjugate slip systems.

The equilibrium crystal structure of LnMnO₃ (Ln: lanthanide) has been reported to be orthorhombic when La³⁺ to Dy³⁺ are used as Ln³⁺, and hexagonal when Ho³⁺ to Lu³⁺ are used. Whereas Kumar et al. reported a two-phase structure of orthorhombic and hexagonal phases is formed in DyMnO₃ when it was solidified from the undercooled melt under containerless state. The reason for the formation of the two-phase structure was not thoroughly addressed and discussed. We investigated the formation mechanism for the two-phase structure from the undercooled melt of DyMnO₃ in detail. As a result, the surface morphology, microstructure, and crystal structure of the samples, in which the nucleation was forced at a predetermined temperature with a Mo needle, indicated that the hexagonal and orthorhombic phases are dominant at high and low temperatures, respectively. When the sample was quenched from below 1670 K in a water bath, as-solidified sample consisted of h-DyMnO₃ and o-DyMnO₃. Whereas a single phase of h-DyMnO₃ was obtained in the sample quenched from above 1670 K. This phenomenon can be quantified in terms of nucleation-rate determined phase selection. That is, the activation energy for forming a critical nucleus calculated based on the model of the crystal-melt interface proposed by Turnbull and Speapen suggests that the o-DyMnO₃ phase can be heterogeneously nucleated on the interface of the initially formed h-DyMnO₃ phase.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 155–161. The abstract and captions of Figs. 1–10 are modified.

In this study, sintered Nd–Fe–B magnets with high residual flux densities and magnetic coercivities were manufactured by filling a die cavity with granulated Nd–Fe–B magnetic powder during pressing. In the binder-bound granules prepared by a spray drying method, the bonding force between the primary particles was relatively strong. However, the magnets obtained from these granules could not achieve high residual magnetic flux densities because it was difficult to align each particle in these granules along the magnetization easy axis through magnetic field pressing. Therefore, a weak liquid bridge force was utilized for particle binding. After preparing the liquid-bound granules containing 0.25 mass% terpineol, which does not evaporate easily at room temperature (∼293 K) but undergoes debinding on baking, the angle of repose of the magnetic powder (representing its flowability index) decreased from 63° to 44.2°, while the deterioration of its magnetic properties was considerably suppressed. As a result, the residual magnetic flux density of the sintered magnet was equal to 99.3% of the value obtained for the non-granulated powder. In addition, the liquid-bound granules exhibited high filling rates and low weight variations, demonstrating their applicability for mass production.

Twin-electrode GTAW is a novel welding technology in recent years and attracts lots of attention to researchers. However, stainless steel cladding with twin-electrode GTAW has been scarcely reported. This paper investigates the microstructure and mechanical performances of austenite stainless steel cladding by twin-electrode GTAW, and particularly the heat input is concerned. Experimental results of hardness tests, bending tests, and corrosion resistance tests show that both single GTAW and twin-electrode GTAW produce defect-free weld beads which meet engineering standards. Compared to single GTAW, twin-electrode GTAW improves the welding productivity at a lower heat input because of its higher welding speed and melting rate. Oscillation twin-electrode GTAW cladding also produces fine weld bead formation, but causes excessive heat input due to its very low welding speed and relative large welding currents. During oscillation twin-electrode GTAW, Fe–Cr(–Mo) intermetallic compound (σ phase) tends to precipitate in the weld bead, which leads to undesirable ferrite content results.

A four-year atmospheric exposure test was performed on steel with a Zn–30 mass%Al thermal-sprayed coating using an accelerated atmospheric exposure test, and then the atmospheric corrosion properties were evaluated. X-ray diffraction results showed that the corrosion products formed on the coating owing to the accelerated atmospheric exposure test were the same as those formed in a typical atmospheric exposure test. The corrosion weight loss of the thermally sprayed coatings in the accelerated atmospheric exposure test was promoted by approximately 1.5 to 2.0 times compared to the atmospheric exposure test.

This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 68 (2019) 187–193.

The alignment and angular dependences of the coercivity of (Sm, Ce)₂(Co, Fe, Cu, Zr)₁₇ magnets were investigated. The coercivity of an aligned magnet is slightly larger than that of the isotropically aligned magnet. This result differs from those of Nd–Fe–B sintered magnets and ferrite magnets. In regard to the angular dependence, the coercivity of (Sm, Ce)₂(Co, Fe, Cu, Zr)₁₇ magnets decreases from 0° to 40° and increases thereafter. The trend is similar to that observed in Ga-doped Nd–Fe–B sintered magnets and ferrite magnets and reproduces those expected from a Stoner–Wohlfarth model or a coherent rotation of magnetization, even though the magnetization reversals for these magnets proceed through the motion of magnetic domain walls. The angular dependence of the coercivity of isotropically aligned magnets agrees well with the calculation results for angles up to 50° obtained under the assumption that the magnetization of every grain reverses independently through the motion of magnetic domain walls. These results support strongly the conclusion that the coercivity of Ga-doped Nd–Fe–B sintered magnets and ferrite magnets, both which have an angular dependence similar to (Sm, Ce)₂(Co, Fe, Cu, Zr)₁₇ magnets, is determined by the motion of magnetic domain walls.

In a creep damage analysis for a weld joint (WJ) using a damage model, it is important to select a damage model that properly reflects complex conditions of stress generated in the interface between the base metal and the heat-affected zone (HAZ). For 2 1/4Cr–1Mo steel, we examined what type of scalar stress is appropriate to properly reflects the conditions of stress generated around the HAZ for accurately predicting creep rupture time, in the framework of the time exhaustion rule, which evaluates damage as a function of stress. For a specimen of a round rod WJ through which an HAZ penetrates obliquely, we compared actual tests and finite element method (FEM) analysis, in temperature-stress ranges from 120 to 160 MPa at 823 K, and from 80 to 100 MPa at 873 K. Using FEM calculations, we evaluated the rupture time based on the time exhaustion rule, considering three types of scalar stress: the maximum principal stress, the equivalent stress, and the Huddleston stress. Comparing with the tests, we found that the scalar stress that gives an appropriate rupture time is the Huddleston stress. In this stress, the damage accumulation due to the creep deformation in the base metal (BM)-HAZ interface and the cavity formation due to the hydrostatic pressure are taken into account. Analyses of the stress distribution inside and outside of the HAZ and in the BM-HAZ interface indicated that, for the 2 1/4Cr–1Mo steel, the cavity formation due to the tensile hydrostatic pressure is also important when we evaluate damage. We optimized a parameter b, which regulates the balance in the Huddleston stress, so that the observed results are reproduced with the determination coefficient of R² = 0.948, and we obtained the value of b as b = 0.34.

Al–10Si–Mg alloy have been used for die casting and additive manufacturing. In this study, solution-treated Al–10Si–Mg and Al–10Si alloys (mass%) were heat-treated at several temperatures and the linear dimensional changes arising from the heat-treatments were investigated. A theoretical model for calculating the linear dimensional change in Al–Si–Mg ternary alloys was proposed, and the theoretically and the experimentally determined linear dimensional changes were compared. It was found that the linear dimensional changes of Al–10Si–Mg and Al–10Si alloys were almost the same. In both alloys, the linear dimensional changes increased with the decrease of the heat-treatment temperature. The linear dimensional changes of Al–10Si–Mg and Al–10Si alloys agreed well with the values estimated by the proposed theoretical model.

For the distillation separation of organic solvents from lithium-ion battery (LIB) cells and their recycling, the volatilization behavior characteristics of lithium hexafluorophosphate (LiPF₆) with several types of alkyl carbonates in open cells were studied using mass spectrometry from 296 to 773 K. Similar volatilization behaviors were observed for three different dialkyl carbonates, i.e., dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), and diethyl carbonate (DEC). When the electrolytic solution from single dialkyl carbonate with LiPF₆ was heated, solution mass loss occurred in three steps. In the first step near the boiling points of the dialkyl carbonates, it is expected that dialkyl carbonates (with trace POF₃ for DMC and EMC) can be recovered. The residues after heating to 773 K contained LiF and Li₃PO₄. For electrolytic solution from mixed alkyl carbonates with LiPF₆, dialkyl carbonates with higher volatilities could be recovered near their boiling points (with trace POF₃ for EMC + ethylene carbonate (EC)). EC with lower volatilities than the dialkyl carbonates could be recovered at higher temperatures near the EC boiling point with trace PF₅ and POF₃. The residues after heating to 773 K contained LiF and Li₃PO₄. With moisture, POF₃ generation, which must accompany hydrofluoric-acid generation, was observed from a lower temperature than that without moisture. Moisture also affected the chemical form of the residues. Lithium salts, such as Li₄P₂O₇, LiPO₃, LiF, or Li₃PO₄ were observed. The presence of water should be avoided when recovered electrolytic solution from LIBs is separated and recycled by distillation. Oxygen in air does not affect the volatilization behavior of LIB electrolytic solutions. These insights will be of practical importance to consider electrolytic solution recovery from LIB cells and its recycling by distillation without incineration to avoid damage by hydrofluoric-acid generation.

The effect of addition of boron on the long afterglow property of SrAl₂O₄: Eu²⁺, Dy³⁺ by preparing the phosphors at various amount and types of boron compounds as flux was investigated. The addition of boron compounds as flux improved the long afterglow property and the sample with addition of H₃BO₃ showed higher afterglow intensity than that with addition of B₂O₃. The investigation of long afterglow property of the samples with various amounts of addition of H₃BO₃ revealed that there is a distinct correlation between afterglow intensity and boron content in the samples. Molar ratio of Sr₄Al₁₄O₂₅ phase increased and unit cell volume of SrAl₂O₄ crystal decreased with an increase in the amount of addition of H₃BO₃. The result indicated that the substitution of BO₄ for AlO₄ in SrAl₂O₄ crystal phase promoted both incorporation of Dy³⁺ ions, which could act as electron traps, into Sr²⁺ sites and formation of strontium vacancies, which could act as hole traps, and then afterglow property of the phosphors was enhanced.

This paper presents a current research trend based on the best papers awarded in 2019 and 2020 by Materials Transactions. The summary includes the 8 papers carefully selected from the articles in the research areas of materials physics, microstructures of materials, mechanics of materials, materials chemistry, materials processing, and engineering materials and their applications. Four out of the 8 best papers are those specially selected for young scientists who are the age of 35 or younger. Here, brief summary is given for the introduction of high quality papers published in Materials Transactions.

Cu-doped amorphous calcium phosphate (ACP) coatings were fabricated on the surface of pure titanium (Ti) by electrochemical deposition at initial electrolyte temperatures of 35, 45, and 55°C. The antibacterial activities of the coatings were then evaluated by the plate counting method using Escherichia coli as the indicator. The Cu concentrations on the surfaces of samples are increased from 6.90 to 15.05 mass% as initial electrolyte temperature is increased from 35 to 55°C. The Cu-doped ACP coatings show that they can fully inhibit the growth of E. coli.