Takuya Yamamoto

Profile Information
Name
Takuya Yamamoto
Institution
University of California-Santa Barbara
Publications:
"Characterization and comparative analysis of the tensile properties of five tempered martensitic steels and an oxide dispersion strengthened ferritic alloy irradiated at ˜295 °C to ˜6.5 dpa" Stuart Maloy, G. Robert Odette, Tarik Saleh, Takuya Yamamoto, Osman Anderoglu, T. J. Romero, Shuangchen Li, James Cole, Randall Fielding, Journal of Nuclear Materials Vol. 468 2016 232-239 Link
Tensile test results at 25 and 300 °C on five 9-12Cr tempered martensitic steels and one 14Cr oxide dispersion strengthened alloy, that were side-by side irradiated to 6.5 dpa at 295 °C in the Advanced Test Reactor (ATR), are reported. The engineering stress–strain curves are analyzed to provide true stress–strain constitutive σ(ε) laws for all of these alloys. In the irradiated condition, the σ(ε) fall into categories of: strain softening, nearly perfectly plastic and strain hardening. Increases in yield stress (Δσy) and reductions in uniform strain ductility (eu) are observed, where the latter can be understood in terms of the alloy's σ(ε) behavior. Increases in the average σ(ε) in the range of 0–10% strain are smaller than the corresponding Δσy, and vary more from alloy to alloy. The data are also analyzed to establish relations between Δσy and coupled changes in the ultimate stresses as well as the effects of both test temperature and the unirradiated yield stress (σyu). The latter shows that higher σyu correlates with lower Δσy. In five out of six cases the effects of irradiation are generally consistent with previous observations on these alloys. However, the particular heat of the 12Cr HT-9 tempered martensitic steel in this study has a much higher eu than observed for earlier heats. The reasons for this improved behavior are not understood and may be microstructural in origin. However, it is noted that the new heat of HT-9, which was procured under modern quality assurance standards, has lower interstitial nitrogen than previous heats. Notably lower interstitial solute contents correlate with improved ductility and homogenous deformation in broadly similar steels.
"Direct comparison of nanoindentation and tensile test results on reactor-irradiated materials" David Krumwiede, Takuya Yamamoto, Tarik Saleh, Stuart Maloy, G. Robert Odette, Peter Hosemann, Journal of Nuclear Materials Vol. 504 2018 135-143 Link
"Dose rate dependence of Cr precipitation in an ion-irradiated Fe18Cr alloy" Elaina Reese, Nathan Almirall, Takuya Yamamoto, Scott Tumey, G. Robert Odette, Emmanuelle Marquis, Scripta Materialia Vol. 146 2018 213-217 Link
"Evolution of manganese–nickel–silicon-dominated phases in highly irradiated reactor pressure vessel steels" James Cole, Brandon Miller, Tim Milot, G. Robert Odette, Peter Wells, Takuya Yamamoto, Yuan Wu, Acta Materialia Vol. 80 2014 205-219 Link
Formation of a high density of Mn–Ni–Si nanoscale precipitates in irradiated Cu-free and Cu-bearing reactor pressure vessel steels could lead to severe unexpected embrittlement. Models long ago predicted that these precipitates, which are not treated in current embrittlement prediction models, would emerge only at high fluence. However, the mechanisms and variables that control Mn–Ni–Si precipitate formation, and their detailed characteristics, have not been well understood. High flux irradiations of six steels with systematic variations in Cu and Ni contents were carried out at ~295 °C to high and very high neutron fluences of ~1.3 × 1020 and ~1.1 × 1021 n cm-2. Atom probe tomography shows that significant mole fractions of Mn–Ni–Si-dominated precipitates form in the Cu-bearing steels at ~1.3 × 1020 n cm-2, while they are only beginning to develop in Cu-free steels. However, large mole fractions of these precipitates, far in excess of those found in previous studies, are observed at 1.1 × 1021 n cm-2 at all Cu contents. At the highest fluence, the precipitate mole fractions primarily depend on the alloy Ni, rather than Cu, content. The Mn–Ni–Si precipitates lead to very large increases in measured hardness, corresponding to yield strength elevations of up to almost 700 MPa.
"Fracture Toughness Characterization of Reactor Pressure Alloys from the ATR-2 Experiment" M. Sokolov, X. Chen, R.K. Nanstad, G. Robert Odette, Takuya Yamamoto, Peter Wells, Vol. 2017 Link
"Microstructural changes and their effect on hardening in neutron irradiated Fe-Cr alloys" Dhriti Bhattacharyya, Takuya Yamamoto, Peter Wells, Emmanuelle Marquis, Mukesh Bachhav, Yuan Wu , Joel Davis, Alan Xu, G. Robert Odette, Journal of Nuclear Materials Vol. 519 2019 274-286 Link
"On a' precipitate composition in thermally annealed and neutron-irradiated Fe- 9-18Cr alloys" Mukesh Bachhav, Emmanuelle Marquis, G. Robert Odette, Peter Wells, Takuya Yamamoto, Elaina Reese, Journal of Nuclear Materials Vol. 500 2018 192-198 Link
Ferritic-martensitic steels are leading candidates for many nuclear energy applications. However, formation of nanoscale a' precipitates during thermal aging at temperatures above 450?°C, or during neutron irradiation at lower temperatures, makes these Fe-Cr steels susceptible to embrittlement. To complement the existing literature, a series of Fe-9 to 18 Cr alloys were neutron-irradiated at temperatures between 320 and 455?°C up to doses of 20 dpa. In addition, post-irradiation annealing treatments at 500 and 600?°C were performed on a neutron-irradiated Fe-18 Cr alloy to validate the a-a' phase boundary. The microstructures were characterized using atom probe tomography and the results were analyzed in light of the existing literature. Under neutron irradiation and thermal annealing, the measured a' concentrations ranged from ~81 to 96?at.% Cr, as influenced by temperature, precipitate size, technique artifacts, and, possibly, cascade ballistic mixing.
"On the use of charged particles to characterize precipitation in irradiated reactor pressure vessel steels with a wide range of compositions" Nathan Almirall, Peter Wells, Takuya Yamamoto, G. Robert Odette, Journal of Nuclear Materials Vol. 536 2020 Link
Nuclear reactor lifetimes may be limited by nano-scale Cu-Mn-Ni-Si precipitates (CRPs and MNSPs) that form under neutron irradiation (NI) of pressure vessel (RPV) steels, resulting in hardening and ductile to brittle transition temperature increases (embrittlement). Physical models of embrittlement must be based on characterization of precipitation as a function of the combination of metallurgical and irradiation variables. Here we focus on rapid and convenient charged particle irradiations (CPI) to both: a) compare to precipitates formed in NI; and, b) use CPI to efficiently explore precipitation in steels with a very wide range of compositions. Atom probe tomography (APT) comparisons show NI and CPI for similar bulk steel solute contents yield nearly the same precipitate compositions, albeit with some differences in their number density, size and volume fraction (f) dose (dpa) dependence. However, the overall precipitate evolutions are very similar. Advanced high Ni (>3 wt%) RPV steels, with superior unirradiated properties, were also investigated at high CPI dpa. For typical Mn contents, MNSPs have Ni16Mn6Si7 or Ni3Mn2Si phase type compositions, with f values that are close to the equilibrium phase separated values. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. Low Mn significantly reduces, but does not eliminate, precipitation in high Ni steels. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs
"Precipitation and hardening in irradiated low alloy steels with a wide range of Ni and Mn compositions" G. Robert Odette, Nathan Almirall, Peter Wells, Takuya Yamamoto, Acta Materialia Vol. 2019 119-128 Link
Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPVtype steels, with an unusually wide range of dissolved Mn (0.06e1.34 at.%) and Ni (0.19e3.50 at.%) contents, were irradiated at z 290 C to z 1.4 1020 n/cm2 at an accelerated test reactor flux of z3.6 1012 n/cm2 -s (E > 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with z Ni1.6Mn0.8. Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Ni-silicide phase type compositions are observed
"Precipitation in reactor pressure vessel steels under ion and neutron irradiation: On the role of segregated network dislocations" G. Robert Odette, Nathan Almirall, Takuya Yamamoto, Acta Materialia Vol. 212 2021 Link
"Structural and chemical evolution in neutron irradiated and helium-injected ferritic ODS PM2000 alloy" Danny Edwards, G. Robert Odette, Takuya Yamamoto, Hee Joon Jung, Richard Kurtz, Yuan Wu, Journal of Nuclear Materials Vol. 484 2017 68-80 Link
An investigation of the influence of helium on damage evolution under neutron irradiation of an 11 at% Al, 19 at% Cr ODS ferritic PM2000 alloy was carried out in the High Flux Isotope Reactor (HFIR) using a novel in situ helium injection (ISHI) technique. Helium was injected into adjacent TEM discs from thermal neutron 58Ni(nth,γ) 59Ni(nth,α) reactions in a thin NiAl layer. The PM2000 undergoes concurrent displacement damage from the high-energy neutrons. The ISHI technique allows direct comparisons of regions with and without high concentrations of helium since only the side coated with the NiAl experiences helium injection. The corresponding microstructural and microchemical evolutions were characterized using both conventional and scanning transmission electron microscopy techniques. The evolutions observed include formation of dislocation loops and associated helium bubbles, precipitation of a variety of phases, amorphization of the Al2YO3 oxides (which also variously contained internal voids), and several manifestations of solute segregation. Notably, high concentrations of helium had a significant effect on many of these diverse phenomena. These results on PM2000 are compared and contrasted to the evolution of so-called nanostructured ferritic alloys (NFA).
Presentations:
""Late Blooming Phases in RPV Steels: High Fluence Neutron and Ion Irradiations" Collin Knight, Brandon Miller, Tim Milot, G. Robert Odette, Peter Wells, Takuya Yamamoto, NuMat 2012 October 22-25, (2012)
"Influence of Irradiation Conditions on Precipitation Behavior in Fe-Cr and Ni Alloys" Emmanuelle Marquis, E Reese, LJ Yu, Nathan Almirall, Takuya Yamamoto, G. Robert Odette, Grace Burke, Annual TMS meeting March 10-14, (2019)
"Late Blooming Phases in RPV Steels at High Fluence and Flux" James Cole, Collin Knight, Brandon Miller, G. Robert Odette, Peter Wells, Takuya Yamamoto, International Group of Radiation Damage Mechanisms 17th Semiannual Meeting May 19-24, (2013)
"On the Evolution of Late Blooming Phases in RPV Steels: Theoretical Foundations, Experimental Observations and Recent Insights" Nicholas Cunningham, G. Robert Odette, Peter Wells, Takuya Yamamoto, The Minerals, Metals and Materials Society 2013 Annual Meeting March 3-7, (2013)
"On the Opportunities and Challanges for Using Test REactor and Charged Particle Irradiations to Help Predict Inaccessible In-service Neutron Irradiations Effects" Peter Hosemann, Peter Wells, Takuya Yamamoto, TMS 2014 February 16-20, (2014)
"TEM Characterization of Dislocation Loops and Precipitates in Irradiated RPV Steels" James Cole, Collin Knight, Brandon Miller, G. Robert Odette, Takuya Yamamoto, International Group of Radiation Damage Mechanisms 17th Semiannual Meeting May 19-24, (2013)
"The Evolution of Late Blooming Phases from High to Very High Fluence" Collin Knight, Brandon Miller, G. Robert Odette, Peter Wells, Takuya Yamamoto, The Minerals, Metals and Materials Society 2013 Annual Meeting March 3-7, (2013)
"The Evolution of Late Blooming Phases in RPV Steels: Theoretical Foundations, Experimental Observations Recent Insights and Implications to Life Extension" Nicholas Cunningham, G. Robert Odette, Peter Wells, Takuya Yamamoto, International Group of Radiation Damage Mechanisms 17th Semiannual Meeting May 19-24, (2013)
"The Status of the UCSB ATR2 RPV Irradiation Experiment" doug klingensmith, Thomas Maddock, Mitch Meyer, G. Robert Odette, Takuya Yamamoto, W. Server May 19-24, (2013)
NSUF Articles:
DOE Awards 33 Rapid Turnaround Experiment Research Proposals - Projects total approximately $1.2 million These projects will continue to advance the understanding of irradiation effects in nuclear fuels and materials in support of the mission of the DOE Office of Nuclear Energy. Monday, June 18, 2018 - Calls and Awards