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Journal Articles

Oxygen interstitials make metastable $$beta$$ titanium alloys strong and ductile

Chong, Y.*; Gholizadeh, R.*; Guo, B.*; Tsuru, Tomohito; Zhao, G.*; Yoshida, Shuhei*; Mitsuhara, Masatoshi*; Godfrey, A.*; Tsuji, Nobuhiro*

Acta Materialia, 257, p.119165_1 - 119165_14, 2023/09

 Times Cited Count:0 Percentile:77.63

Metastable $$beta$$ titanium alloys possess excellent strain-hardening capability, but suffer from a low yield strength. As a result, numerous attempts have been made to strengthen this important structural material in the last decade. Here, we explore the contributions of grain refinement and interstitial additions in raising the yield strength of a Ti-12Mo (wt.%) metastable $$beta$$ titanium alloy. Surprisingly, rather than strengthening the material, grain refinement actually lowers the ultimate tensile strength in this alloy. This unexpected and anomalous behavior is attributed to a significant enhancement in strain-induced $$alpha^{primeprime}$$ martensite phase transformation, where in-situ synchrotron X-ray diffraction analysis reveals, for the first time, that this phase is much softer than the parent $$beta$$ phase. Instead, a combination of both oxygen addition and grain refinement is found to realize an unprecedented strength-ductility synergy in a Ti-12Mo-0.3O (wt.%) alloy. The advantageous effect of oxygen solutes in this ternary alloy is twofold. Firstly, solute oxygen largely suppresses strain-induced transformation to the $$alpha^{primeprime}$$ martensite phase, even in a fine-grained microstructure, thus avoiding the softening effect of excessive amounts of $$alpha^{primeprime}$$ martensite. Secondly, oxygen solutes readily segregate to twin boundaries, as revealed by atom probe tomography. This restricts the growth of $${332}langle113rangle$$ deformation twins, thereby promoting more extensive twin nucleation, leading to enhanced microstructural refinement. The insights from our work provide a cost-effective rationale for the design of strong yet tough metastable $$beta$$ titanium alloys, with significant implications for more widespread use of this high strength-to-weight structural material.

Journal Articles

High-temperature creep properties of 9Cr-ODS tempered martensitic steel and quantitative correlation with its nanometer-scale structure

Otsuka, Satoshi; Shizukawa, Yuta; Tanno, Takashi; Imagawa, Yuya; Hashidate, Ryuta; Yano, Yasuhide; Onizawa, Takashi; Kaito, Takeji; Onuma, Masato*; Mitsuhara, Masatoshi*; et al.

Journal of Nuclear Science and Technology, 60(3), p.288 - 298, 2023/03

 Times Cited Count:2 Percentile:80.29(Nuclear Science & Technology)

JAEA has been developing 9Cr-oxide dispersion strengthened (ODS) tempered martensitic steel(TMS) as a candidate material for the fuel cladding tubes of sodium-cooled fast reactors(SFRs). The reliable prediction of in-reactor creep-rupture strength is critical for implementing the 9Cr-ODS TMS cladding tube in the SFR. This study investigated the quantitative correlation between the creep properties of 9Cr-ODS TMS at 700 $$^{circ}$$C and the dispersions of nanosized oxides by analyzing the creep data and the material's nanostructure. The possibility of deriving a formula for estimating the in-reactor creep properties of 9Cr-ODS TMSs based on an analysis of the nanostructure of neutron-irradiated 9Cr-ODS TMSs was also discussed. The creep properties of 9Cr-ODS TMS at 700 $$^{circ}$$C closely correlated with the dispersion of nanosized oxide particles. The correlation between creep-rupture lives and nanosized oxide particle dispersion was determined using existing creep models. The elucidation of correlation between the stress exponent of secondary creep rate and the nanostructure is essential to enhance future modeling reliability and formulation.

Journal Articles

High temperature mechanical properties and microstructure in 9Cr or 12Cr oxide dispersion strengthened steels

Mitsuhara, Masatoshi*; Kurino, Koichi*; Yano, Yasuhide; Otsuka, Satoshi; Toyama, Takeshi*; Onuma, Masato*; Nakashima, Hideharu*

Tetsu To Hagane, 109(3), p.189 - 200, 2023/03

 Times Cited Count:0 Percentile:0(Metallurgy & Metallurgical Engineering)

Oxide Dispersion Strengthened (ODS) ferritic steel, a candidate material for fast reactor fuel cladding, has low thermal expansion, good thermal conductivity, and excellent resistance to irradiation damage and high temperature strength. The origin of the excellent high-temperature strength lies in the dispersion of fine oxides. In this study, creep tests at 700 or 750$$^{circ}$$C, which are close to the operating temperatures of fast reactors, and high-temperature tensile tests at 900 to 1350 $$^{circ}$$C, which simulate accident conditions, were conducted on 9Cr ODS ferritic steels, M11 and MP23, and 12Cr ODS ferritic steel, F14, to confirm the growth behavior of oxides. In the M11 and F14 creep test samples, there was little oxide growth or decrease in number density from the initial state, indicating that dispersion strengthening by oxides was effective during deformation. After creep deformation of F14, the development of dislocation substructures such as dislocation walls and subgrain boundaries was hardly observed, and mobile dislocations were homogeneously distributed in the grains. The dislocation density increased with increasing stress during the creep test. In the high-temperature ring tensile tests of MP23 and F14, the strength of both steels decreased at higher temperatures. In MP23, elongation decreased with increasing test temperature from 900 to 1100 $$^{circ}$$C, but increased at 1200 $$^{circ}$$C, decreased drastically at 1250 $$^{circ}$$C, and increased again at 1300 $$^{circ}$$C. In F14, elongation decreased with increasing temperature. It was inferred that the formation of the $$delta$$-ferrite phase was responsible for this complex change in mechanical properties of MP23 from 1200 to 1300 $$^{circ}$$C.

Journal Articles

Grain refinement in titanium prevents low temperature oxygen embrittlement

Chong, Y.*; Gholizadeh, R.*; Tsuru, Tomohito; Zhang, R.*; Inoue, Koji*; Gao, W.*; Godfrey, A.*; Mitsuhara, Masatoshi*; Morris, J. W. Jr.*; Minor, A. M.*; et al.

Nature Communications (Internet), 14, p.404_1 - 404_11, 2023/02

 Times Cited Count:1 Percentile:93.17

Interstitial oxygen embrittles titanium, particularly at cryogenic temperatures, which necessitates a stringent control of oxygen content in fabricating titanium and its alloys. Here, we propose a structural strategy, via grain refinement, to alleviate this problem. Compared to a coarse-grained counterpart that is extremely brittle at 77K, the uniform elongation of an ultrafine-grained (UFG) microstructure (grain size $$sim$$2.0 $$mu$$m) in Ti-0.3wt.%O was successfully increased by an order of magnitude, maintaining an ultrahigh yield strength inherent to the UFG microstructure. This unique strength-ductility synergy in UFG Ti-0.3wt.%O was achieved via the combined effects of diluted grain boundary segregation of oxygen that helps to improve the grain boundary cohesive energy and enhanced $$<c+a>$$ dislocation activities that contribute to the excellent strain hardening ability. The present strategy could not only boost the potential applications of high strength Ti-O alloys at low temperatures, but could also be applied to other alloy systems, where interstitial solution hardening results into an undesirable loss of ductility.

Journal Articles

Effect of grain boundary on the friction coefficient of pure Fe under the oil lubrication

Adachi, Nozomu*; Matsuo, Yasutaka*; Todaka, Yoshikazu*; Fujimoto, Mikiya*; Hino, Masahiro*; Mitsuhara, Masatoshi*; Oba, Yojiro; Shiihara, Yoshinori*; Umeno, Yoshitaka*; Nishida, Minoru*

Tribology International, 155, p.106781_1 - 106781_9, 2021/03

 Times Cited Count:5 Percentile:60.31(Engineering, Mechanical)

Journal Articles

Development of high-grade VPS-tungsten coatings on F82H reduced activation steel

Tokunaga, Tomonori*; Watanabe, Hideo*; Yoshida, Naoaki*; Nagasaka, Takuya*; Kasada, Ryuta*; Lee, Y.-J.*; Kimura, Akihiko*; Tokitani, Masayuki*; Mitsuhara, Masatoshi*; Hinoki, Tatsuya*; et al.

Journal of Nuclear Materials, 442(1-3), p.S287 - S291, 2013/11

 Times Cited Count:10 Percentile:64.36(Materials Science, Multidisciplinary)

Journal Articles

Three-dimensional microstructure and high temperature strength in ODS 9Cr ferritic heat resistant steels

Mitsuhara, Masatoshi*; Harada, Erika*; Yamasaki, Shigeto*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Otsuka, Satoshi; Kaito, Takeji

Kashika Joho Gakkai-Shi, 31(122), p.98 - 103, 2011/07

no abstracts in English

Journal Articles

Three-dimensional observation of precipitates in 10Cr ferric heat-resistant steel with V and Nb

Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi

Kyushu Daigaku Cho Koatsu Denkenshitsu Kenkyu Hokoku, 32, 2 Pages, 2008/08

no abstracts in English

Journal Articles

EDS analyses and 3D observations of precipitates on lath boundaries in V-bearing heat resistant steels

Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi

Materia, 47(6), P. 301, 2008/06

no abstracts in English

Journal Articles

Electron tomographic observation of precipitates on lath boundaries in 10Cr ferritic steel bearing V

Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Hata, Satoshi*; Nakashima, Hideharu*; Wakai, Takashi

Materia, 46(12), P. 800, 2007/12

High Cr ferritic steels bearing V have high creep strength due to the precipitation hardening. In order to enhance the efficiency of the precipitation hardening, the elemental components, shape and distribution of the precipitates should be comprehended. In this study, we investigated that of the precipitates in the 10Cr ferritic steel bearing V using the STEM-EDS analysis and the three-dimensional electron tomography (3D-ET) observation. The two types of precipitates on the lath boundaries could be observed, which were lump-shaped precipitates and film-shaped precipitates. They had the different elemental component. From the 3D-ET observation, it revealed that the film-shaped precipitates covered the lath boundary widely. Therefore, the film-shaped precipitates are expected to be effective for the stabilization of martensitic microstructure during the creep deformation.

Journal Articles

Creep strengthening mechanisms in high Cr heat resistant steel

Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

Kyushu Daigaku Cho Koatsu Denkenshitsu Kenkyu Hokoku, 31, p.78 - 79, 2007/00

The effects of V and Nb on creep strength of high Cr ferritic steel were investigated based on the observation results using STEM. As a result, it is clarified that Nb contributes to the improvement of creep strength by encumber the movement of the lath boundary by fine dispersed precipitation as MX particles and V contributes as film precipitation on the lath boundary.

Journal Articles

Creep life evaluation of high Cr ferritic steels in terms of omega-method and effects of V and Nb on the omega-parameters

Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

Tainetsu Kinzoku Zairyo Dai-123-Iinkai Kenkyu Hokoku, 47(10), p.109 - 113, 2006/07

This paper studies the influence of V and Nb on the creep strength and behavior of high Cr ferritic steels. A series of creep tests of 11 steels having various V and Nb contents is conducted. The test results are analyzed by omega-method and the microstructures of the damaged specimens are examined by EBSP. As a result, it is shown that omega-method gives good prediction of creep lives of the steels and that the influence of V and Nb on the omega-parameters is significant. In addition, it is clarified that the martensitic microstructure of the steels depends upon Nb content.

Oral presentation

Omega method analysis of V$$cdot$$Nb added 12Cr-1Mo steels

Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Oral presentation

Effect of V/Nb content on microstructural changes in high Cr ferritic steels during creep deformation

Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Oral presentation

Microstructural change of V/Nb added high Cr ferritic steels during creep deformation

Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Oral presentation

Relationship between strengthening mechanism and omega parameter of high-Cr ferritic steels containing V and Nb

Yoshida, Yu*; Mitsuhara, Masatoshi*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Oral presentation

Effect of microstructure on omega value in ferritic heat resistant steels

Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Oral presentation

Effect of Vanadium and Niobium on creep strength in 10% Chromium steel analyzed by STEM-EDS

Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

The effect of V and Nb on creep strengthening was studied in 10% Cr steels. In a steel added 0.06% V where no particles were observed in the lath, creep strength was slightly increased, indicating that it was not strengthened by particle hardening effect in the lath. Lump-shaped precipitates and film-shaped precipitates including Cr and V, which were observed on lath boundaries in the steel added V, are expected to inhibit the lath boundaries from migrating. This inhibition is the mechanism of the improvement in the steel added V. In the case of steels added Nb individually, creep strength was found to be increased by dispersion hardening due to fine precipitates in the lath. These results revealed that V and Nb have each role to improve creep strength. In steels added both V and Nb, the effect on creep strength was expressed by the sum of the effects due to each element, which were the retardation of the lath boundary migrations and the pinning of the dislocations in the lath.

Oral presentation

Mechanism of creep strengthening in high Cr ferritic heat resistant steel added V and Nb

Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

Oral presentation

Effects of addition of V and Nb on omega in high Cr ferritic steels

Mitsuhara, Masatoshi*; Yoshida, Yu*; Ikeda, Kenichi*; Nakashima, Hideharu*; Wakai, Takashi

no journal, , 

no abstracts in English

40 (Records 1-20 displayed on this page)