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 alloy.

Mater. Trans. 64 (2023) 131-137に掲載

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.

Mater. Trans. 64 (2023) 155-164に掲載

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.

Mater. Trans. 64 (2023) 78-85に掲載

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 O. 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).

Mater. Trans. 64 (2023) 61-70に掲載．Fig. 2，Table 2を修正．

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℃ for 3.6 ks and then hot-rolled at 850℃ into 1.5 mm thick sheets. The disk specimen of 3 mm diameter was heated up to β phase field at 1000℃ in the differential thermal analysis apparatus. And then specimen was cooled to room temperature at a cooling rate of 20℃/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-20at％Nb alloy.

Mater. Trans. 64 (2023) 71-77に掲載．文献[19]を追加．

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℃ 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.

Mater. Trans. 64 (2023) 95-103に掲載．Table 4：鍛造材の熱処理条件および文献リストを修正.

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 1000 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.

Mater. Trans. 64 (2023) 121-130に掲載

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.

Mater. Trans. 64 (2023) 147-154に掲載

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.

Mater. Trans. 64 (2023) 111-120に掲載．3.1.4項にてFig. 7（a）と表記．試料の呼称においてDuplex(α+α′)組織，Bimodal(α+β)組織など，構成相を付記．

Machine learning is a powerful tool that can predict the iron loss of electromagnetic steels with high accuracy. However, there is no theoretical explanation as to why machine learning can make such accurate predictions; it is a black box. Therefore, it is unclear how to select an algorithm for predicting iron loss behaviors among many machine learning algorithms available. In this study, we used approximately 10000 iron loss data for electromagnetic steels under stress and 19 machine learning algorithms to evaluate and compare the learning algorithms in terms of prediction accuracy, robustness, the number of data required, avoidance of overfitting, and agreement with experimental results. As a result, we conclude that LightGBM is the best algorithm for predicting the iron loss properties of electromagnetic steels. Although the discussion in this study has not yet led to a theoretical breakthrough in the field of machine learning, we hope that it will provide an effective guideline for selecting the learning algorithm in materials science.