Taira Okita

The primary origin of dose rate effects on microstructural evolution of austenitic alloys during neutron irradiation

The effect of dose rate on neutron-induced microstructural evolution was experimentally estimated. Solution-annealed austenitic model alloys were irradiated at ≃400 °C with fast neutrons at seven different dose rates that vary more than two orders difference in magnitude, and two different doses were achieved at each dose rate. Both cavity nucleation and growth were found to be enhanced at lower dose rate. The net vacancy flux is calculated from the growth rate of cavities that had already nucleated during the first cycle of irradiation and grown during the second cycle. The net vacancy flux was found to be proportional to (dpa/s)1/2 up to 28.8 dpa and 8.4×10−7 dpa/s. This implies that mutual recombination dominates point defect annihilation in this experiment, even though point defect sinks such as cavities and dislocations were well developed. Thus, mutual recombination is thought to be the primary origin of the effect of dose rate on microstructural evolution.

Effects of dose rate on microsturctural evolution and swelling in austenitic steels under irradiation

Abstract Effects of dose rate on microstructural evolution in a simple model austenitic ternary alloy are examined. Annealed specimens are irradiated with fast neutrons at several positions in the core and above core in FFTF/MOTA between 390°C and 435°C in a wide range of doses and dose rates. In Fe–15Cr–16Ni, swelling seems to increase linearly with dose without incubation dose. Cavities are observed even in the specimens irradiated to 0.07 dpa at 1.9×10 −9 dpa/s . Both cavity nucleation and growth are enhanced by low dose rates. These are mainly caused by accelerated formation of dislocation loops at lower dose rates. Low dose rates enhance swelling by shortening incubation dose for the onset of steady-state swelling. In the specimens irradiated at higher dose rates to higher doses, high density of dislocation increases average cavity diameter, however decreases cavity density.

Evaluation of radiation hardening in Fe alloys under heavy ion irradiation by micro-indentation technique

Abstract To correlate micro-structural evolution near the surface under ion irradiation with macroscopic mechanical properties, micro-indentation tests were applied to heavy ion-irradiated model alloys of ferritic steels. The derivative of the load–displacement ratio with respect to the displacement, d( L / D )/d D , was estimated to evaluate the depth dependent formation of hardening features in the model alloys irradiated with 12 MeV Ni ions at 300 °C between 8 and 25 dpa. The depth distribution of radiation-enhanced formation of Cu precipitate in Fe–0.3Cu was found to change with dose, because of over-aging of the precipitates near the peak damage depth. Hardening near the peak damage depth region in Fe–0.3Cu–0.7Ni alloy increases with dose, indicating that the addition of Ni suppresses the growth of Cu-rich precipitates.

Validation of Ultrasonic Velocity Measurements for Detecting Void Swelling in First-Wall Structural Materials

Abstract Time-of-flight ultrasonic measurements were conducted on a thick hexagonal block of 304 stainless steel irradiated to ∽33 dpa in EBR-II, and the results of ultrasonic-implied void swelling and carbide-induced densification were compared with those obtained by immersion density measurements and TEM observation. The three types of measurement were found to agree rather well with each other. This study confirmed that ultrasonic velocity measurement is a powerful non-destructive technique to measure the through-thickness-average volumetric changes induced by neutrons in thick structural materials.

Effects of solid transmutation and helium on microstructural evolution in neutron-irradiated vanadium

Pure V and V alloys with B and Cr irradiated in the HFIR to 10 dpa at 400, 500 and 600 °C are examined by TEM. Nine percent of V is transmuted to Cr during irradiation to 10 dpa, and more than 99% of 10B is transmuted to He and Li during the early stage of irradiation. The Cr generation suppresses cavity nucleation near grain boundaries in pure V. However in V–4.9Cr which contains 13at.%Cr after irradiation, cavities concentrate near grain boundaries, and there are few cavities in the matrix. At 400 and 500 °C, the effect of He on the cavity formation is not clearly observed. At 600 °C, B addition enhancese the cavity nucleation in pure V and V–4.9Cr in the matrix. Growth of cavities is also enhanced in pure V which is converted to V–9.4Cr.

Transmission Electron Microscopy of 304-type Stainless Steel after Exposure to Neutron Flux and Irradiation Temperature Gradients

1. Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, USA 2. Department of Materials , University of Oxford, Oxford, UK 3. Radiation Effects Consulting, Richland, WA, USA 4. Westinghouse Electric Company LLC, Pittsburgh, PA, USA 5. Westinghouse Electric Japan Ltd, Kobe, Japan 6. Nuclear Fuels Industries Ltd, Osaka, Japan 7. The University of Tokyo, Tokyo, Japan

Effects of stacking fault energy on defect formation process in face-centered cubic metals

Abstract To elucidate the effect of stacking fault energies (SFEs) on defect formation by the collision cascade process for face-centred cubic metals, we used six sets of interatomic potentials with different SFEs while keeping the other properties almost identical. Molecular dynamic simulations of the collision cascade were carried out using these potentials with primary knock-on atom energies (EPKA) of 10 and 20 keV at 100 K. Neither the number of residual defects nor the size distributions for both self-interstitial atom (SIA) type and vacancy type clusters were affected by the difference in the SFE. In the case of EPKA = 20 keV, the ratio of glissile SIA clusters increased as the SFE decreased, which was not expected by a prediction based on the classical dislocation theory. The trend did not change after annealing at 1100 K for 100 ps. For vacancy clusters, few stacking fault tetrahedrons (SFTs) formed before the annealing. However, lower SFEs tended to increase the SFT fraction after the annealing, where large vacancy clusters formed at considerable densities. The findings of this study can be used to characterise the defect formation process in low SFE metals such as austenitic stainless steels.

Effect of the Stacking Fault Energy on Interactions Between an Edge Dislocation and a Spherical Void in FCC Metals at Various Spatial Geometries

Abstract Molecular dynamics simulations were conducted using six interatomic potentials for face-centered cubic metals that differed only in the stacking fault energies (SFEs). We investigated the effects of the SFE on interactions between an edge dislocation and a void of 4.0 nm diameter at 13 intersection positions. In the high SFE, most interaction morphologies at the depinning are such that the two partial dislocations reverse into the perfect dislocation locally at the void interface. In contrast, in the low SFE, the partial dislocations are depinned individually from the void with some certain time lag. The critical resolved shear stress (CRSS) is not symmetrical about the center of the void. CRSS is higher when the center of the void is located not on the glide plane, but in the compressive side of the edge dislocation. In some cases for these conditions, climb motion is observed, which further increases CRSS. The probability of climb motion occurrence is higher with higher SFE. In lower SFE, climb motion occurs temporarily, followed by the disappearance of jog by dislocation releasing several vacancies inside of the void. CRSS is higher with higher SFE for all the intersection positions.

Atomic simulations to evaluate effects of stacking fault energy on interactions between edge dislocation and spherical void in face-centred cubic metals

Abstract In this study, molecular dynamics simulations were performed to elucidate the effects of stacking fault energy (SFE) on the physical interactions between an edge dislocation and a spherical void in the crystal structure of face-centred cubic metals at various temperatures and for different void sizes. Four different types of interaction morphologies were observed, in which (1) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the trailing partial; (2) two partial dislocations detached from the void separately, and the maximum stress corresponded to the detachment of the leading partial; (3) the partial dislocations detached from the void almost simultaneously without jog formation; and (4) the partial dislocations detached from the void almost simultaneously with jog formation. With an increase in void size or SFE, the interaction morphology changed in the above-mentioned order. It was observed that the magnitude of the critical resolved shear stress (CRSS) and its dependence on the SFE were determined by these interaction morphologies. The value of the CRSS in the case of interaction morphology (1) is almost equal to an analytical one based on the linear elasticity by employing the Burgers vector of a single partial dislocation. The maximum value of the CRSS is also obtained by the analytical model with the Burgers vector of the two partial dislocations.

Interactions between clusters of self-interstitial atoms via a conservative climb in BCC–Fe

ABSTRACT We conduct kinetic Monte Carlo simulations for the conservative climb motion of a cluster of self-interstitial atoms (SIAs) towards another SIA cluster in BCC–Fe; the conservative climb velocity is inversely proportional to the fourth power of the distance between them, as per the prediction based on Einstein’s equation. The size of the climbing cluster significantly affects its conservative climb velocity, while the size of the cluster that originates the stress field does not. The activation energy for the conservative climb is considerably greater than that derived in previous studies and strongly dependent on the climbing cluster size. The results presented in this study are the atomistic evaluation of the behaviour of SIA clusters through three-dimensional motion, which cannot be achieved using molecular dynamics techniques alone.