It is desirable to develop high temperature shape memory alloys (HTSMAs) with a martensitic transformation start temperature (M_s) above 100°C and a recoverable strain of about 4–6% in the shape memory effect. The latter property is achieved with low variant reorientation stress due to easy detwinning of martensite and high plastic deformation stress due to precipitation strengthening. We previously demonstrated facilitation of detwinning for Ti–Zr–Ni–Pd quaternary alloy systems through controlling the crystal structure of martensite, and proposed that Ti–(15–20)Zr–49.7Pd (at%) and surrounding Ni-containing compositions are candidates of HTSMAs having low variant reorientation stress. On the other hand, the aging condition for precipitation strengthening was not optimized, since the candidate alloys show a complex precipitation behavior of two types of precipitates, Ti₂Pd-type and H-phase. Therefore, in this study, the effects of aging on precipitation behavior, martensitic transformation temperatures, and shape memory properties were investigated for one of the candidate alloys, Ti–20Zr–49.7Pd, and an excellent shape memory effect with M_s above 130°C, a low reorientation stress around 200 MPa, a high plastic deformation stress around 1800 MPa, and a large recovery strain of 4.5% was achieved after an optimum aging treatment. On the other hand, short-range ordering of solute atoms occurs just above the reverse transformation temperature and decreases M_s, which would limit the number of shape recovery operations when the alloy is used as a device.

The relationship between the microstructural changes and hydrogen permeability of rolled and annealed Nb₃₀Ti₃₅Co₃₅ alloys was investigated. The as-cast alloy had a lamellar structure that consisted of bcc-(Nb, Ti) and B2-TiCo phases. Hydrogen permeability decreased with increasing rolling reduction. The granule (Nb, Ti) phase formed in the TiCo matrix after annealing at temperatures of 1173 to 1373 K. The hydrogen permeability of the non-rolled alloys after annealing, which caused microstructural changes, was almost independent of annealing temperature. In contrast, the hydrogen permeability of the rolled alloys increased with increasing annealing temperature. It recovered to 90% of that for the as-cast alloy after annealing at 1373 K, which is higher than the value predicted by the mixing rule for the two phases. Therefore, rolling and annealing effectively increase the hydrogen flux for the industrial preparation of Nb–TiCo alloy foil for hydrogen production.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 338–344. The captions of Fig. 1, 2, 3, 5, 6 are slightly modified.

In the previous studies, the surface modification effect of formic acid on the bonding strength of tin and copper has been investigated. As a result of previous investigations, it was found that the lowering of the bonding temperature is achieved by removing the oxide film on the bonding surfaces with formic acid, forming a metal salt film, and thermally decomposing the film. However, it has been pointed out that formic acid is toxic and irritating and difficult to handle. Therefore, in this study, we decided to investigate the effects on the removal of the oxide film on the bonding surface and the bonding strength using citric acid, which is relatively harmless to the human body. In addition to observing the fractured surfaces and the bonded interfaces of the solid-state bonded tin and copper by SEM, thermal analysis of the compound produced by surface treatment with citric acid was carried out. The metal salt coating treatment using citric acid was carried out by boiling the bonding surfaces of tin and copper in citric acid for 300 s. Solid-state bonding was performed in a vacuum chamber under the following conditions; 383–473 K for bonding temperature, 7 MPa for bonding pressure and 1800 s for bonding time. Regardless of the metal salt coating treatment, the bond strength of the joint increased with the increase of the bonding temperature. When the metal salt coating treatment using citric acid was applied, the bonding temperature was reduced by 70 K to fabricate a joint with the base metal strength of tin. However, when the metal salt-coated surface was exposed to the atmosphere, the bonded surface was again covered with an oxide film and the bond strength decreased.

Sand erosion is a phenomenon in which the collision of solid particles erodes a material surface. The rate of sand erosion is higher in carbon fiber reinforced plastics (CFRP) than in metallic materials. Therefore, CFRP requires a light and protective coating material. Herein, to improve the erosion resistance of CFRP, five polyurethane coated CFRPs with different glass transition temperatures were investigated at elevated temperatures, and a prediction formula of the erosion rate at the elevated temperatures was established. Furthermore, computational fluid dynamics was used to predict the surface temperature and erosion rate of fan exit guide vane (FEGV) when polyurethane coating was applied, and the coating thickness for FEGV in the erosion environment was estimated based on these predictions.

This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Jpn. 70 (2021) 896–903. The caption of Fig. 10 is slightly modified.

The effect of W and B addition on the superplastic deformation and grain growth of electrodeposited nanocrystalline Ni alloys with a crystal grain size of approximately 20 nm was investigated with the aim of improving the mechanical properties by maintaining fine grain sizes during superplastic deformation. A maximum elongation of 12% was recorded for electrodeposited Ni–1.8 at% W at a temperature of 350°C and a strain rate of 1.0 × 10⁻⁴ s⁻¹. The electrodeposited Ni–W failed to exhibit superplasticity because the segregated W at the grain boundaries increased the energy required for grain boundary sliding. In contrast, the electrodeposited Ni–0.06 at% B exhibited superplasticity with a recorded elongation of 362% at a temperature of 450°C and a strain rate of 1.0 × 10⁻⁴ s⁻¹. With the addition of B, the optimal superplastic strain rates of the electrodeposited Ni–B shifted to lower values than that of the electrodeposited Ni. The grain size and hardness of the electrodeposited Ni–B after superplastic deformation were smaller and higher, respectively, than those of the electrodeposited Ni. The addition of B successfully suppressed grain growth and improved the mechanical properties after superplastic deformation.

Lightweight porous metals have been focused on as structural and energy absorbing materials. Mechanical properties of classical porous metals fall under the specific specimen dimension. Such cell size effects are mainly caused by the disordered cells. The present study experimentally evaluates the cell size effects of additively manufactured porous metals consisting of ordered cells. Different sized porous titanium specimens consisting of ordered rhombic dodecahedron cells are manufactured through electron beam melting process. Mechanical properties are determined by quasi-static compression tests. The plateau stress and the energy absorption are almost independent of the specimen dimension divided by the cell diameter. Experimental results conclude that the cell size effects are suppressed in additively manufactured porous metals consisting of well-defined ordered cells.

The mechanical properties of aluminum alloys fabricated by selective laser melting (SLM) were examined with particular focus on their fatigue properties. The SLM aluminum alloys with various amounts of internal porosities were fabricated to investigate the effect of porosity on the fatigue properties. There were no remarkable differences in the microstructures of the SLM alloys. In terms of tensile properties, the tensile strength was significantly higher in the specimen with low porosity and decreased with increasing porosity. In terms of fatigue properties, crack initiations were observed at the internal pores in all specimens with the number of crack initiations increasing as a function of porosity. The fatigue strength decreased with increasing porosity, and porosity had a more significant effect on fatigue strength than tensile strength. The fatigue limit, which corresponds to the size and number of internal pores, changed remarkably in the low porosity regime because the size and number of internal pores changed drastically in this range. Considering the residual stress, these results demonstrate that by considerably suppressing the internal pores in the SLM aluminum alloys, a good fatigue performance can be expected, regardless of the occurrence of fatigue cracks at the internal pores.

This Paper was Originally Published in Japanese in J. JILM 70 (2020) 128–135.

In this study, the microstructure and the thermal fatigue life of Sn–3.0 mass%Ag–0.5 mass%Cu–6.0 mass%In–1.0 mass%Sb (SAC305–6In–1Sb) were investigated and they were compared with those of Sn–3.0 mass%Ag–0.5 mass%Cu (SAC305). As the test pieces, chip resistors were joined on a printed wiring board by reflow soldering with each solder. Although precipitates of Ag and Cu were present in both solder joints, the precipitation state and size of Ag precipitates were different for each solder joint. In the solder joint with SAC305–6In–1Sb, the precipitate of In and Sb was also confirmed. In addition, In and Sb were solid-soluted in Sn. Solder joints with SAC305–6In–1Sb were polycrystal with approximately 20 crystal grains per cross section of the joint. On the other hand, the joint of SAC305 was consisted of a single crystal. Average of thermal fracture life of SAC305–6In–1Sb was approximately 4.3 times longer than that of SAC305. In addition, the variation in the life of SAC305–6In–1Sb was smaller.

To evaluate the effect of acetic acid on material removal from stainless steel, sliding tests were performed. Martensitic stainless steel pins were unidirectionally slid against smooth and rough alumina (Al₂O₃) disks in water, in ethanol, in acetic acid aqueous solutions and in acetic acid ethanolic solutions. Morphological and chemical analyses of the worn pin and disk surfaces were then performed. The addition of acetic acid to water and ethanol resulted in reduction of adhesion of stainless steel to Al₂O₃, inhibition of the formation of metal oxides, and increased wear of the stainless steel. The surface roughness of the mating Al₂O₃ disk affected the wear mechanism of the stainless steel pin. The solvents and total concentration of acetic acid in the solution affected electrochemical actions such as anodic dissolution and galvanic corrosion.

The structures of a hydraulic excavator with one-sided welded joints are investigated in this study. We investigated the fatigue lifetime evaluation method using S-version FEM. Especially we considered combination of cracks from multiple crack origins. Moreover, to achieve a more precise evaluation, we obtained the crack growth rate of the excavator weld using the unloading elastic compliance method instead of some engineering standard values. Results show that, the fatigue lifetime obtained from the analysis of all width crack; type1 - all cracks named in this study is extremely short compared with test results. Considering the combination of cracks from multiple crack origins, the S-version FEM provides an evaluation similar fatigue lifetime of a test piece imitating the excavator.

The purpose of this study is to develop commercially pure (CP) titanium having a higher fatigue strength than titanium alloys developed via heterogeneous nitrogen diffusion. The microstructure of CP titanium having a heterogeneous nitrogen diffusion phase, which was fabricated by consolidating gas-nitrided powders, was characterized, and its fatigue properties were examined. The nitrogen content and hardness of CP titanium compacts having a heterogeneous nitrogen diffusion phase increased with increasing powder gas nitriding temperature and sintering temperature. The fatigue limit and fatigue life of CP titanium compacts increased with increasing sintering temperature and with decreasing powder gas-nitriding temperature. In particular, CP titanium having a heterogeneous nitrogen diffusion phase that is fabricated by high-temperature sintering of powders treated with low-temperature nitriding has a higher fatigue limit than un-nitrided bulk Ti–6Al–4V alloy. The fatigue limit of CP titanium can be controlled by optimizing the powder gas nitriding and sintering temperatures.

Kink formation and kink strengthening mechanisms were examined in the polycrystalline Ti₃SiC₂ MAX phase prepared by a reaction sintering process using a spark-plasma-sintering machine. The creep behavior tested by compression at 1200°C showed two deformation regions depending on the applied stresses; at stresses lower than 120 MPa, the stress exponent n exhibited ≈1.8, whereas at higher stresses, it exhibited n ≥ 6. The creep behavior can be ascribed to grain boundary sliding mechanisms for the lower stresses with n ≈ 1.8 and to dislocation-related creep mechanisms for the higher stresses with n ≥ 6. The kink bands were frequently observed to form in the grains deformed only at the higher stresses when its basal plane was inclined by about 10–20° against the compressive axis. This suggests that the kink bands might be formed only when two factors of the high stresses acting on the basal plane and the resultant dislocation activities were satisfied. Nanoindentation tests conducted around the formed kink bands showed that the nanohardness linearly changed with the distance from the kink bands and showed higher values around the kink bands. Since the kink bands blocked the slip line caused by the nanoindentation, those become obstacles against the dislocation motion caused by the indentation deformation. This suggests that the kink bands would contribute to improving the mechanical properties of the Ti₃SiC₂ MAX phase.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 439–448.

The effect of formic acid (HCOOH) in simulated boiler water containing chloride ions (Cl⁻) on the pitting corrosion behavior of steam turbine blade material (13Cr steel) was investigated using electrochemical corrosion tests in addition to pit morphology analyses. Pit initiation was inhibited by the addition of 25–75 ppm HCOOH into the boiler water containing 100 ppm of Cl⁻ ions. Additions of 50 ppm and 75 ppm HCOOH into the boiler water containing 100 ppm of Cl⁻ both resulted in the formation of corrosion products as re-passivation films on the inner surfaces of the pits, which suppressed pit growth.

To explore the effect of copper content in a copper-doped hydroxyapatite coating (CuHAp) on its antibacterial activity and biocompatibility, a layer of CuHAp was deposited on the surface of pure Ti using an electrochemical deposition method. An orthogonal experiment was used to determine the Cu content in the coating by varying the concentration of Cu²⁺ in the electrolyte. The antibacterial properties and biocompatibility of the CuHAp coatings were also evaluated. The antibacterial effect increased with increasing Cu content in the sample. When the content of Cu in the coating was 1.57%, the deposited CuHAp coating exhibited excellent biocompatibility, and the number of living cells after culturing was significantly higher than that for the HAp coating. When the content of Cu increased to 6.85%, the coating exhibited cytotoxicity. Thus, the CuHAp sample with a Cu content of 1.57% exhibited good antibacterial and biocompatibility, and thus could be a suitable material for use in biomedical applications.

Copper (Cu) alloy thin films deposited on barrierless substrates via sputtering and annealing processes have been essential for numerous microelectronic products and continue to be so in the new nanometer-range manufacture era. The search for better new films thus is crucial for further technical and manufacture advancement. The requirements on the new films lie in their stability in existence under high-temperature manufacture environments, low electric resistivity, less leakage current under various electric fields, and sufficient adhesion strength. For the search and advancement, I have developed a new type of films by co-sputtering impurities of niobium, Nb, and zirconium, Zr, with Cu within a vacuum chamber without any gas or with nitrogen (N) under low pressure, resulting in new Cu(NbZr) or Cu(NbZrN_x) films whose fabrication processes and test results are detailed herein. The new type of films displays good physical features and seems desirable for microelectronic manufacture and to material science, too.

Ti–27.5Al–13Nb is an intermetallic alloy that incorporates the orthorhombic Ti₂AlNb phase (O phase) and the α₂ phase, and was previously developed by the authors for high-temperature applications. The aim of the present study was to develop less expensive orthorhombic alloys. For this purpose, a portion of the cost-prohibitive Nb in this baseline Ti–27.5Al–13Nb alloy was replaced with an equivalent amount of Fe and/or Mo, yielding three new derivative alloys: Mo-replaced Ti–27.5Al–8.7Nb–1Mo, Fe-replaced Ti–27.5Al–5.5Nb–1Fe and (Mo and Fe)-replaced Ti–27.5Al–4.9Nb–1Mo–0.5Fe. Further, a minor amount of B (boron), i.e., 0.1 pct B, was added to these derivative alloys and baseline alloy. The microstructures and corresponding tensile and creep properties were examined in four cases: B-free and B-modified alloys with fully lamellar microstructures, and B-free and B-modified alloys with duplex microstructures. With the addition of 0.1 pct B, the prior B2 grain size of each ingot was drastically reduced by about one order of magnitude, from 600∼1000 µm for the B-free alloy to 100∼250 µm, and thereby a refined fully lamellar microstructure was obtained. However, this high degree of grain refinement did not markedly improve ductility at room temperature. A duplex microstructure consisting of a globular α₂-phase and a lamellar microstructure led to the improved ductility and tensile strength. The addition of B exerts either a positive or negative effect on the creep properties depending on the compositions and microstructures, creep test temperature and applied stress. The Mo-replaced alloys had better creep properties than the baseline alloy, whereas creep properties of the Fe-replaced and (Mo and Fe)-replaced alloys were considerably inferior to those of the baseline alloy. Among the four alloys, the 0.1 pct B-modified Mo-replaced alloy with a duplex microstructure exhibited the highest ductility of 4.7 pct at room temperature and higher tensile strength up to 1073 K and better creep properties than the other two derivative alloys and the baseline alloy.

CeFe₁₁Ti melt-spun ribbon with the ThMn₁₂ phase was produced by annealing of the rapidly quenched specimen, but the resultant specimen contained some α-Fe phase and showed a coercivity of less than 0.1 MAm⁻¹, below the expected coercivity value for permanent magnets. The coercivity of the CeFe₁₁Ti melt-spun ribbon was improved by Sm substitution. The optimally annealed (Ce,Sm)Fe₁₁Ti melt-spun ribbons exhibited a relatively high coercivity, although these specimens contained α-Fe phase together with the ThMn₁₂ phase. The coercivity and Curie temperature of the annealed (Ce,Sm)Fe₁₁Ti melt-spun ribbons increased with increasing the Sm content. The optimally annealed SmFe₁₁Ti melt-spun ribbon showed a high coercivity of 0.40 MAm⁻¹ when annealed at 1073 K.