Faculty Profiles - TAKEMOTO Yoshito

Papers - TAKEMOTO Yoshito

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Strange phase transformation of Ti-10Mo-7Al alloy Invited Reviewed

Titanium Japan 67 ( 2 ) 112 - 115 2019.2

Control of Heterogeneous Microstructure for Improving Delayed Fracture Resistance of Ultrahigh Strength Hot Stamping Steel Sheets Reviewed

Takehide Senuma, Yoshito Takemoto, Tomohiko Hojo

TETSU TO HAGANE-JOURNAL OF THE IRON AND STEEL INSTITUTE OF JAPAN 105 ( 2 ) 51 - 59 2019.2

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：IRON STEEL INST JAPAN KEIDANREN KAIKAN

Toward a high demand for ultrahigh strength automotive components for reducing body weight and increasing crashworthiness, hot stamped components have been employed worldwidely, and the further increase in strength of hot stamped components is being requested. A large obstacle of the strength increase is the concern about the occurrence of delayed fracture.In this study, we investigated the effects of microstructure on the resistance to delayed fracture of hot stamped steel sheets from a viewpoint of the control of heterogeneous microstructure, and the following results were obtained:(1) The grain refinement of martensite increases the resistance to delayed fracture.(2) Samples containing some amount of ferrite in the martensite matrix have a lower or higher resistance to delay fracture than samples with full martensitic microstructure, depending on the morphology of ferrite.(3) The presence of residual austenite of several % increases the resistance to delayed fracture remarkably but if it is strained and transformed to martensite, the resistance to delayed fracture drastically decreases.(4) Concerning 2000 MPa class hot stamping steel sheets, a higher carbon steel quenched and tempered at low temperature has higher resistance to delayed fracture than a quenched lower carbon steel with the same strength.In the paper, we discussed these results including their mechanisms in detail.

DOI： 10.2355/tetsutohagane.TETSU-2018-079

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Martensitic Transformation Induced in Ti-15V-7Al Alloy at Low and High Temperatures Reviewed

Y. Takemoto, M. Tsunekawa, Y. Manabe, S. Itano and Y. Muraoka

Materials Transactions 59 ( 10 ) 1560 - 1566 2018.10

Multilayer formation via spinodal decomposition in TiO2-VO2 epitaxial films on sapphire substrates Reviewed

Y. Matsuura, F. Yoshii, T. Otsuka, K. Kadowaki, M. Ijiri, Y. Takemoto, K. Terashima, T. Wakita, T. Yokoya and Y. Muraoka

Journal of the European Ceramic Society 38 5043 - 5050 2018.8

Change of Electrical Resistivity with Martensite Transformation of Ti-42mass%Nb Alloy Reviewed

J. Japan Inst. Met. Mater. 82 ( 7 ) 256 - 261 2018.7

Martensitic Transformation Induced at Low and High Temperatures on Ti-15V-7Al Alloy Reviewed

J. Japan Inst. Met. Mater. 82 ( 8 ) 307 - 313 2018

Effect of aliovalent dopants on the kinetics of spinodal decomposition in rutile-type TiO2-VO2 Reviewed

Makoto Ogata,Kenji Kadowaki,Masataka Ijiri,Yoshito Takemoto*,Kensei Terashima*,Takanori Wakita*,Takayoshi Yokoya*,Yuji Muraoka*

Journal of the European Ceramic Society, International Journal of High Technology Ceramics 37 ( 9 ) 3177 - 3183 2017.8

Stress-Strain Behavior of Ti-Nb Alloys under Compressions along Linear Strain Paths and Bilinear Plane Strain Path Reviewed

SHIMIZU Ichiro, TAKEMOTO Yoshito, ISHIKAWA Shinichi, KUMURA Tomohiro

Advanced Experimental Mechanics 2 99 - 104 2017.7

Language：English Publishing type：Research paper (scientific journal) Publisher：The Japanese Society for Experimental Mechanics

<p>Titanium alloys containing beta stabilizing elements exhibit several superior properties compared to other conventional metals. However, there are unknowns about the mechanical behavior of titanium alloys under biaxial compressive stress conditions. In this study, uniaxial compression and biaxial compressions along linear strain paths and bilinear strain path were performed on titanium-niobium alloys (Ti-Nb alloys) in order to investigate the influence of Nb contents on the compressive deformation behaviors. The results of compressions along linear strain paths revealed that the flow stress decreased while the ductility increased with the increase of the Nb content. It was found that the phenomenon was mainly due to changes of the microstructure and the primary plastic deformation mechanism. Those changes also induce anisotropic hardening. Based on those results, quantitative expression of the stress-strain relation with taking into account the Nb contents was discussed. The investigation on the effect of strain path change on the flow stress revealed the niobium dependency of the stress-strain response before and after the abrupt strain path change.</p><p></p>

DOI： 10.11395/aem.2.0_99

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Influence of Nb Content of Ti-xNb-7Al Alloys on beta ->alpha '' Transformation with Tempering Reviewed

Masataka Ijiri, Yuki Tomita, Takafumi Ishikawa, Kenji Kadowaki, Yoshito Takemoto

JOURNAL OF THE JAPAN INSTITUTE OF METALS 81 ( 7 ) 345 - 351 2017

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

The purpose of this study is to investigate the influence of Nb content of Ti-xNb-7Al alloys (x= 20-35 mass%, abbreviated as xNbA) on beta (bcc) -> alpha ''(orthorhombic) transformation with tempering through microstructure, isothermal age-hardening, and shape change of a U-shaped specimen with heating. Full alpha ''-martensite structure was obtained on the quenched 20 NbA. With increasing of Nb content, residual beta-phase increased, and single beta-phase was obtained in the alloys of more than 25 NbA. From the result, the M-S temperature commonly decreases with increasing of Nb content. The hardness of 20 NbA and 23 NbA rapidly increased in a few seconds by isothermal aging at 450 degrees C, and the hardness of the alloys of more than 25 NbA abruptly increased after an incubation period for 30-60 s. No compositional distribution between alpha '' and matrix in the aged 23 NbA was found by STEM-EDS analysis, but the obvious distribution of Nb was detected in 35 NbA. U-shaped specimen of 20 NbA exhibited no shape change by heating. On the other hand, the specimen of 23 NbA composed of beta+alpha '' phases showed the shape recovery (SR) first due to work induced alpha ''->beta inverse transformation, then deformed toward the bending direction (SA: shape advance) due to beta ->alpha '' transformation. The specimens of more than 28 NbA composed of single beta-phase exhibited only SA without SR. The starting temperature of SA (M-SA: beta ->alpha '') with heating increases with increasing of Nb content. Merging M-S(upper) and M-SA(lower), it is proposed that the whole M-S of xNbA alloy forms a bow shape curve. It is suggested that the low Nb alloys inside the M-S curve transform immediately without atom diffusion by tempering at 450 degrees C, but the higher Nb alloys outside the curve need an incubation period to distribute Nb concentration with atom diffusion, and martensitic alpha '' transformation will abruptly occur in the local domains less than 25 NbA which is the inside of the curve.

DOI： 10.2320/jinstmet.J2017011

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Influence of Nb and Mn content on resistance to delayed fracture of ultrahigh strength hot stamped steel sheets Reviewed

Takuya Kishimoto, Yoshito Takemoto, Takehide Senuma

6th International Conference Hot Steel Metal Forming of High-Performance Steel, CHS2 2017- Proceedings 187 - 194 2017

Language：English Publishing type：Research paper (international conference proceedings) Publisher：Association for Iron and Steel Technology, AISTECH

One of the important objectives of hot stamping technology is an increase in tensile strength up to 2000 MPa. The largest obstacle to the realization of the strength increase to this height is a concern over the occurrence of delayed fracture. However, the principle idea for designing chemical compositions and operational conditions for increasing the resistance to delayed fracture has not been established yet. In this paper, the effect of Mn and Nb on the resistance to delayed fracture has been investigated to obtain the principle idea for designing chemical compositions for increasing the resistance to delayed fracture of ultrahigh strength hot stamping steel sheets. The following results were obtained. Nb addition increased the resistance to delayed fracture. A decrease in added Mn also increases the resistance to delayed fracture. Detailed investigations on the effect of Nb and Mn on the resistance to delayed fracture indicated that these alloys in solution, especially segregated in the grain boundary, affected the resistance to delayed fracture. A key requirement to improve the delayed fracture property is to control the segregation behavior of Nb and Mn.

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Stress-Strain Behavior of Ti-Nb Alloys under Compressions along Linear Strain Paths and Bilinear Plane Strain Path Reviewed

SHIMIZU Ichiro, TAKEMOTO Yoshito, ISHIKAWA Shinichi, KUMURA Tomohiro

Advanced Experimental Mechanics 2 1 - 6 2016.11

Language：English Publishing type：Research paper (international conference proceedings) Publisher：The Japanese Society for Experimental Mechanics

DOI： 10.11395/aem.2.0_99

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Effect of Nb in Ti-xNb-7Al Alloys on Microstructure and Tempering Behavior Reviewed

Masataka Ijiri, Yuki Tomita, Takafumi Ishikawa, Yousuke Taguchi and Yoshito Takemoto

The Ninth Pacific Rim International Conference on Advanced Materials and Processing (PRICM9) 648 - 650 2016.8

Effect of Stress Loading on Elastic Behavior of Quenched Ti-Nb Alloys Reviewed

Yoshikazu Mantani and Yoshito Takemoto

The Ninth Pacific Rim International Conference on Advanced Materials and Processing (PRICM9) 644 - 647 2016.8

Shape Change Behavior and Microstructure with Tempering of Ti-10Mo-7Al Alloy Reviewed

Yoshito Takemoto, Masataka Ijiri and Tatsuya Tanaka

The Ninth Pacific Rim International Conference on Advanced Materials and Processing (PRICM9) 651 - 653 2016.8

Oil-in-water emulsion as fabrication platform for uniform plasmon-controlled two-dimensional metallic nanoparticle array Reviewed

Ryusuke Kagawa, Nobuyuki Takeyasu, Takashi Kaneta, Yoshito Takemoto

APPLIED PHYSICS EXPRESS 9 ( 7 ) 075003 2016.7

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Gold/silver nanoparticles were trapped at the oil/water interface of oil droplets dispersed in water. The metallic nanoparticles were self-assembled into a uniform two-dimensional large array structure through the aggregation and coalescence of the nanoparticle-covered oil droplets. The plasmon resonance of the array structure was tunable and a surface-enhanced Raman scattering measurement was performed with the silver nanoparticle array. The enhancement factor was similar to 10(5) and enhanced Raman signals were observed over the whole array (greater than or similar to cm(2)) with high reproducibility, which is an advantage of a self-assembly method using a liquid/liquid interface. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/APEX.9.075003

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Phase Transformation Behavior of Ti-35Nb-7Al Alloy with Tempering Reviewed

Masataka Ijiri, Yuki Tomita, Takafumi Ishikawa, Yoshito Takemoto

JOURNAL OF THE JAPAN INSTITUTE OF METALS 80 ( 9 ) 547 - 552 2016

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

Certain beta-Ti alloys, Ti-4Fe-7Al and Ti-10Mo-7Al, the compositions of which are characterized by a 7% aluminum addition to the beta lower. limit compositions, exhibit an interesting phenomenon upon tempering. For example, these alloys drastically harden upon tempering at 450 degrees C for several seconds, and the sample surface becomes severely uneven. When a strip specimen is bent plastically or elastically into a U-shape and heated to 450 degrees C, the shape spontaneously deforms towards the inside, which is different from the behavior of shape-memory alloys. In this study, Ti-35Nb-7Al alloy was selected as a candidate novel alloy, and the microstructure, the characteristics of age hardening and the shape evolution of a U-shaped specimen upon heating were investigated. The quenched alloy was a single beta-phase, and its hardness was considerably higher than that of binary Ti-35Nb alloy. The smooth surface of the quenched specimen became uneven at around 300 degrees C upon heating. Either case of U-shaped specimen, which was plastically or elastically bent, exhibited spontaneous bending towards the inside by heating up. It was found that, with tempering, Ti-35Nb-7Al also exhibits the novel phenomenon, similarly to Ti-4Fe-7Al and Ti-10Mo-7Al. Fine needle-alpha '' and omega-particles were formed in the beta matrix after isothermal aging at 450 degrees C for 3 min and gave rise to a remarkable hardening. STEM. EDS analysis revealed that the Nb content decreased in the alpha '' products formed by tempering. Consequently, the beta ->alpha '' transformation upon tempering on this alloy is accompanied with atomic diffusion. Extra spots at < 100 >*, such as those of the B2 structure, were observed in the selected. area diffraction patterns of the TEM foils prepared by common electropolishing techniques. However, a TEM foil prepared with an FIB ( focused ion beam) did not exhibit the extra spots. These results suggested that the extra spots were caused by hydrogen absorption in the specimen during electropolishing.

DOI： 10.2320/jinstmet.JAW201601

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Microstructure of Ti-4Fe-7Al Alloy Quenched in a Salt Bath after Solution Treatment Reviewed

Masataka Ijiri, Akira Okumura, Takafumi Ishikawa, Kenji Kadowaki, Yoshito Takemoto

JOURNAL OF THE JAPAN INSTITUTE OF METALS 80 ( 11 ) 691 - 696 2016

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

The Ti-4Fe-7Al alloy exhibits some interesting phenomena upon tempering. The noteworthy features are rapid hardening and shape change in a U-shaped specimen with heating, which are caused by the beta(bcc) -> alpha '' (orthorhombic) transformation at elevated temperature. The transformation behavior on the TTT ( time-temperature-transformation) diagram estimated from the age hardening at various temperatures indicated a C-curve with a nose at 450 degrees C, which means that the transformation occurs through a thermal activation process. However, STEM-EDS analysis of alpha '' revealed that the transformation had occurred without atom diffusion. In this study, every solution treatment was carried out at 1050 degrees C for 30 min under a vacuum, then the specimen was quenched into a salt bath of 200-550 degrees C and held for a certain period prior to water quenching. These treated specimens were compared with the quenched and tempered specimens with respect to the microstructure and the hardening behavior. The hardness of the specimens held at 200-500 degrees C (30 s) increased with increasing holding temperature. The maximum hardness, 610Hv, which was almost the same as that of a tempered specimen, was obtained at around 450 degrees C. The microstructure of the specimen held at 450 degrees C (30 s) showed a common morphology composed of acicular martensite variants, except for the size. Because the several variants would cancel the transformation strain of each other, no severe unevenness of the surface was generated, unlike the tempered specimen. On the other hand, the hardness of the specimen held at 550 degrees C (30 s) decreased drastically owing to the phase separation to alpha + beta, and the TiFe precipitates were generated from beta upon the extension of the holding time.
By the TEM observation of the electropolished thin foil of the specimen held at 450 degrees C (30 s), unreasonable microstructure that was different from the XRD result was obtained. Just after the electropolishing, the SAD patterns showed that the alpha '' structure has a commensurate relation with the beta-lattice. However, alpha '' turned spontaneously into an incommensurate relation after one year. On the other hand, TEM observations of the foil prepared by FIB (focused ion beam) were consistent with the XRD result. It was suggested that the hydrogen absorption during electropolishing caused a structural transition of alpha '' martensite.

DOI： 10.2320/jinstmet.J2016032

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Change in structure and mechanical properties of Ti-10Mo-7Al by heat treatments Reviewed

M. Ijiri, Y.Takemoto

Proc. and Fabrication of Advanced Materials -XXIV 318 - 324 2015.12

Phase Transformation Behavior of Ti-10Mo-7Al Alloy with Heat Treatment Reviewed

Masataka Ijiri, Yoshito Takemoto

JOURNAL OF THE JAPAN INSTITUTE OF METALS 79 ( 9 ) 468 - 473 2015.9

Language：Russian Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

A novel phenomenon recently observed in the tempering of Ti-4Fe-7Al alloy, has attracted considerable attention. This phenomenon is induced by the beta ->alpha '' transformation during tempering, and its noteworthy features are rapid hardening and a shape change in a U-shaped specimen. It is of interest whether this phenomenon occurs in other alloy systems. In this study, Ti-10Mo-7Al alloy, which contains a quantity of molybdenum corresponding to the lower beta limit, was prepared and its microstructure, aging behavior, and shape change during tempering were investigated. An acicular alpha '' martensites were observed preferentially at the beta grain boundaries in the quenched specimen, but it became invisible at the optical microscope level upon tempering. However, TEM (transmission electron microscopy) observation of the tempered specimen revealed nano size martensites instead of micron-size martensites. The novel phenomenon observed in the tempering of Ti-4Fe-7Al, was also confirmed to occur in Ti-10Mo-7Al alloy. In the heating of a U-shaped specimen, shape recovery occured first, then a shape return into a U-shape followed, and a curvature progressed more with increasing of temperature. As a result, the phenomenon was clarified to be a universal one. The transformation mechanism through tempering is explained as follows: Though aluminum addition to beta-alloy raises the M-S chemically, on the other hand, it causes a remarkable solution strengthening, which increases shear stress required for martensite transformation, and decreases the apparent M-S. Consequently, the beta phase is maintained by a rapid cooling. However, temperature rise decreases the shear stress, and the apparent M-S increases to a chemically intrinsic one. If the M-S increases beyond the tempering temperature, martensite transformation occurs abruptly.

DOI： 10.2320/jinstmet.J2015027

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Change in Crystal Structure and Material Properties with Deformation of Quenched Martensite in Ti-Nb Alloys Reviewed

Yoshikazu Mantani, Yoshito Takemoto

JOURNAL OF THE JAPAN INSTITUTE OF METALS 79 ( 9 ) 461 - 467 2015.9

Language：Russian Publishing type：Research paper (scientific journal) Publisher：JAPAN INST METALS

The purpose of this study was to investigate and discuss the relationship between the crystal structure of martensites and the material properties, such as the Young's modulus and internal friction, in quenched Ti-Nb alloys with plasto-elastic deformation. In the alloys, minimal Young's modulus was indicated by the alpha' martensite of Ti15Nb, and maximal internal friction was indicated by the alpha '' martensite of Ti18Nb. After a weak plastic deformation of 5% rolling reduction, the Young's modulus of Ti-(15, 18, 20)Nb decreased, and the internal friction of Ti20Nb increased though that of Ti-(15, 18)Nb hardly changed. From the XRD profiles, the extreme values of the material properties were predicted to be associated with the unit volume and phases (hexagonal close-packed or orthorhombic) of the materials. It was also predicted to be associated with the change of phase from alpha' to alpha '' and from alpha '' to alpha' + alpha '' or alpha '' + alpha '' with different lattice parameters. Cyclic tensile test results after weak plastic deformation revealed characteristic changes in elastic incline, that is, an increase from similar to 40 GPa in Ti15Nb, and a decrease until similar to 40 GPa from the initial value followed by an increase in Ti18Nb and Ti20Nb. The consequent changing mechanism of material properties was suggested from the viewpoint of the changing behavior of the phase transformation and transition on martensites with plastic deformation or loaded elastic deformation.

DOI： 10.2320/jinstmet.JAW201506

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Papers - TAKEMOTO Yoshito