Sho Kano

Effects of alloying elements (Sn, Nb, Cr, and Mo) on the microstructure and mechanical properties of zirconium alloys

The alloying effects of Sn, Nb, Cr, and Mo on zirconium alloys were elucidated and compared. Electron backscatter diffraction, transmission electron microscopy, tensile test, and fractographic observation were jointly utilized to carry out detailed microstructural characterization and mechanical property evaluation. Results show that Mo is the most effective among these elements from the viewpoints of strengthening and reducing grain size. The strengthening mechanism for each element is also discussed. The order of solid-solution strengthening of these alloying elements is Cr > Nb > Sn, and the sequence is Cr ≈ Mo > Nb when precipitation strengthening is considered. Further, as far as the ability to impede dislocation motion is concerned, the sequence is Mo > Cr > Nb > Sn. The experimental results demonstrate that minor amount of Mo addition in zirconium alloys is greatly effective in strengthening the alloy and reducing the grain size.

Development of advanced expansion due to compression (A-EDC) test method for safety evaluation of degraded nuclear fuel cladding materials

Expansion due to compression (EDC) test has been applied to evaluate the performance of nuclear fuel claddings where pellet-cladding mechanical interaction (PCMI) is introduced by swelling of fuel pellets and is triggered by the larger hoop deformation of the pellets, especially during accidental transients. The purpose of this study is to modify the EDC test to describe PCMI, specimen volume reduction and others. Ring-shaped specimens were cut from Zry-4 cladding tubes. Cylindrical metal pellets with 8 mm in diameter and 15 mm in maximum height were used as inner pellets. Expansion of the specimens due to the inner pellet compression was performed at room temperature. The experimental data were further analyzed by finite element method. Through the survey in the variation of the specimen and core, specimen size and inner pellet geometry were optimized. Excellent reproducibility with less error was confirmed. The uniaxial tension condition in the hoop direction up to the specimen failure was confirmed. Hoop stress–hoop strain curves were successfully derived.

Vacancy effects on one-dimensional migration of interstitial clusters in iron under electron irradiation at low temperatures

Abstract We performed in situ observation of one-dimensional (1D) migration of self-interstitial atom (SIA) clusters in iron under electron irradiation at 110–300 K using high-voltage electron microscopy. Most 1D migration was stepwise positional changes of SIA clusters at irregular time intervals at all temperatures. The frequency of 1D migration did not depend on the irradiation temperature. It was directly proportional to the damage rate, suggesting that 1D migration was induced by electron irradiation. In contrast, the 1D migration distance depended on the temperature: distribution of the distance ranged over 100 nm above 250 K, decreased steeply between 250 and 150 K and was less than 20 nm below 150 K. The distance was independent of the damage rate at all temperatures. Next, we examined fluctuation in the interaction energy between an SIA cluster and vacancies of random distribution at concentrations 10−4–10−2, using molecular statics simulations. The fluctuation was found to trap SIA clusters of 4 nm diameter at vacancy concentrations higher than 10−3. We proposed that 1D migration was interrupted by impurity atoms at temperatures higher than 250 K, and by vacancies accumulated at high concentration under electron irradiation at low temperatures where vacancies are not thermally mobile.

One-dimensional migration of interstitial clusters in SUS316L and its model alloys at elevated temperatures

For self-interstitial atom (SIA) clusters in various concentrated alloys, one-dimensional (1D) migration is induced by electron irradiation around 300 K. But at elevated temperatures, the 1D migration frequency decreases to less than one-tenth of that around 300 K in iron-based bcc alloys. In this study, we examined mechanisms of 1D migration at elevated temperatures using in situ observation of SUS316L and its model alloys with high-voltage electron microscopy. First, for elevated temperatures, we examined the effects of annealing and short-term electron irradiation of SIA clusters on their subsequent 1D migration. In annealed SUS316L, 1D migration was suppressed and then recovered by prolonged irradiation at 300 K. In high-purity model alloy Fe-18Cr-13Ni, annealing or irradiation had no effect. Addition of carbon or oxygen to the model alloy suppressed 1D migration after annealing. Manganese and silicon did not suppress 1D migration after annealing but after short-term electron irradiation. The suppression was attributable to the pinning of SIA clusters by segregated solute elements, and the recovery was to the dissolution of the segregation by interatomic mixing under electron irradiation. Next, we examined 1D migration of SIA clusters in SUS316L under continuous electron irradiation at elevated temperatures. The 1D migration frequency at 673 K was proportional to the irradiation intensity. It was as high as half of that at 300 K. We proposed that 1D migration is controlled by the competition of two effects: induction of 1D migration by interatomic mixing and suppression by solute segregation.

Effect of Cr content on the thermal stability of tempered lath structures and precipitates in strength enhanced ferritic steels

The effect of Cr content on the thermal stability of tempered laths (or elongated subgrains) and precipitates has been studied during long-term aging at 650 °C in three P122 grade steels with increasing Cr content from 9 to 10.5 and 12%. Addition of Cr accelerates the coarsening of subgrains during long-term aging. The number fraction of MX precipitates does change up to 104 h aging in 9% and 10.5% Cr steels, whereas it decreases significantly in 12% Cr steel due to the formation of Z phase. The coarsening rate of M23C6 precipitates, mostly located on the subgrain boundaries, increases from 9 to 12% Cr and steel containing 9% Cr has the highest number density of M23C6 after 104 h aging. The addition of Cr from 9 to 12% accelerates the coarsening rate of Laves phase particles during aging. As a result, 9% Cr steel shows the most stable tempered lath martesitic structure during long-term aging.

Athermal migration of vacancies in iron and copper induced by electron irradiation

Abstract Irradiation with high-energy particles induces athermal migration of point defects, which affects defect reactions at low temperatures where thermal migration is negligible. We conducted molecular dynamics simulations of vacancy migration in iron and copper driven by recoil energies under electron irradiation in a high-voltage electron microscope. Minimum kinetic energy required for migration was about 0.8 and 1.0 eV in iron and copper at 20 K, which was slightly higher than the activation energy for vacancy migration. Around the minimum energy, the migration succeeded only when a first nearest neighbour (1NN) atom received the kinetic energy towards the vacancy. The migration was induced by higher kinetic energies even with larger deflection angles. Above several electron-volts and a few 10s of electron-volts, vacancies migrated directly to 2NN and 3NN sites, respectively. Vacancy migration had complicated directional dependence at higher kinetic energies through multiple collisions and replacement of atoms. The probability of vacancy migration increased with the kinetic energy and remained around 0.3–0.5 jumps per recoil event for 20–100 eV. At higher temperatures, thermal energies slightly increased the probability for kinetic energies less than 1.5 eV. The cross section of vacancy migration was 3040 and 2940 barns for 1NN atoms in iron and copper under irradiation with 1.25 MV electrons at 20 K: the previous result was overestimated by about five times.

A comparative study of hydride-induced embrittlement of Zircaloy-4 fuel cladding tubes in the longitudinal and hoop directions

ABSTRACT In this work, the mechanical behavior of as-received and hydrogenated Zircaloy-4 fuel claddings was investigated by the newly developed advanced expansion due to compression (A-EDC) test and the conventional uniaxial tension (UT) test at room temperature, in order to, respectively, understand the hydride-induced embrittlement in tube longitudinal and hoop directions. The UT experimental results showed that the mechanical strength in the longitudinal direction slightly increased with hydrogen content, whereas the maximum strain decreased greatly with hydrogen increasing. In the case of A-EDC tests, the mechanical performance in the hoop direction seemed insensitive to the hydrogen content; no obvious decline in maximum strain was observed until 800 ppm H. The comparison between these two tests clearly reveals that the hydride-induced embrittlement is preferential to occur in the longitudinal direction, compared with the sluggish response in the hoop direction, which implied the enhanced ductility anisotropy due to hydrides. In the post-tests observation, the fracture morphologies became gradually distinct for the as-received and hydrided samples examined by UT and A-EDC methods, and different orientation relationships between the applied stresses and hydrides distribution would be responsible for that distinction.

Effects of molybdenum on microstructural evolution and mechanical properties in Zr–Nb alloys as nuclear fuel cladding materials

The Zr–Nb alloys were modified by doping of Mo as a minor alloying element to seek for the nuclear fuel cladding materials with better characteristics. The effects of Mo on microstructural evolution and mechanical properties in Zr–Nb alloys were systematically investigated and elucidated. Results showed that the martensitic microstructure, a mixture of lath martensites and lens martensites with internal twins, was observed in the alloys quenched from β-phase. Width of the lath martensite reduced with the increasing Mo concentration, and the volume fraction of lens martensite increased with increase in the Mo concentration. After final annealing, a new kind of precipitate, namely β-(Nb, Mo, Zr), was identified in the Mo-containing alloys. It was also found that Mo reduced the growth of the precipitates but increased their number density. Furthermore, Mo addition retarded the recrystallization process strongly and reduced the grain size significantly. In terms of the mechanical properties, Mo addition enhanced the yield strength and the ultimate tensile strength at room temperature, however decreased the ductility. The grain size strengthening was presumed as the greatest contributor in this system.

Application of chemical interaction between (Fe, Cr) oxides and Mo oxide at high temperature for self-healing intelligence on nuclear fuel cladding in LWRs

ABSTRACT A novel scheme for a bilayer coating with self-healing ability is proposed in this study. The candidate materials for the coatings and the potential self-healing reaction are assessed in high-temperature aqueous environments and high-temperature air. The pure Cr2O3 layer and the composite of Cr2O3 and MoO3 are the candidate materials for the outer layer and inner layer, respectively, due to their compatibility under normal condition and fabricability. Fe2O3–MoO3 reactions exhibit a potential ability to heal the cracks because of a high reaction rate under normal condition. The self-healing process proceeds via the following mechanism under normal condition: Fe2O3 (a corrosion product in the coolant) diffuses into the cracks on the coating and reacts with MoO3 (inner layer) to produce the insoluble Fe2(MoO4)3, which deposits and repairs the cracks. In the loss-of-coolant accident (LOCA) situation, Cr2O3–MoO3 reaction is expected to strengthen the adhesion of the coating.