Yutai Katoh

Swelling and dislocation evolution in simple ferritic alloys irradiated to high fluence in FFTF/MOTA

Abstract Microstructures of a series of Fe Cr binary ferritic alloys were examined following neutron irradiation to 140 dpa at 698 K in FFTF/MOTA. The chromium concentration ranged from 3 to 18% in 3% increments and the irradiation temperature corresponded to the peak swelling condition for this alloy class. The swelling varied from 0.4 to 2.9% depending on chromium concentration, and the highest swelling was found in the Fe 9Cr alloy. The cavity microstructures corresponded to transient to early steady-state swelling regime. Dislocations were composed of networks with botha〈100〉 and (a/2)〈111〉 Burgers vector anda〈100〉 type interstitial loops. The dislocation density was negatively correlated with swelling. Explanation for the observed chromium concentration dependence of microstructural development and low swelling in the ferritic alloys will be studied in connection with the dislocation bias efficiency and the theory of sink strength ratio.

The influence of He/dpa ratio and displacement rate on microstructural evolution: a comparison of theory and experiment☆

Abstract A kinetic model was developed to investigate the influence of the displacement rate and helium generation rate on microstructural evolution in austenitic stainless steels. The model integrates the rate equations describing the evolution of point defects, small point defect clusters, helium-vacancy clusters, and the larger cavity size distribution that is responsible for observable swelling. Cavity (bubble) nucleation is accounted for by the helium-vacancy cluster evolution, while void formation occurs when bubbles grow beyond a critical size in the larger cavity distribution. A series of ion irradiation experiments were used to both calibrate the model and to provide a comparison between model predictions and experimental observations. The experiments involved single and dual-beam irradiations of solution annealed AISI-316 stainless steel at 873 K. The displacement rates were in the range of 2 × 10−3 to 1 × 10−2 dpa/s and the helium-to-dpa ratios were in the range of 0 to 50 appm He/dpa. The maximum displacement dose was 25 dpa. The experiments revealed a significant effect of helium on both the dislocation structure and the cavity distribution. The model predictions of helium effects over a broad range of He/dpa ratios and displacement rates were consistent with experimental observations.

Dual-ion irradiation effects on microstructure of austenitic alloys

Abstract An Fe-15Cr-20Ni ternary model alloy and a Type 316 stainless steel were irradiated by dual-ions at 1 to 50 appm of He dpa ratios, to investigate the helium effects on microstructural development in austenitic alloys under irradiation. Quantitative analysis on resultant microstructures revealed that the Frank loop nucleation rate and the network dislocation density positively correlate and Frank loop growth rate negatively correlate with the He dpa ratio, while the cavity growth rate has its peak at an intermediate helium injection rate. Although He dpa dependence of various microstructural features were similar for the model alloy and the 316SS, the rates of their development and the mechanism which had assisted cavity growth were significantly different in these two materials.

Microstructural response of titanium-modified austenitic stainless steels to neutron exposure of 70 dpa in FFTF/MOTA

JPCA, a titanium-modified austenitic stainless steel, in solution-annealed or cold-worked condition and a compositionally modified JPCA in solution-annealed condition were examined by transmission electron microscopy following irradiation in FFTF/MOTA to an exposure level of up to about 70 dpa at 390 to 600°C. At lower temperatures, all the materials developed qualitatively similar cavity-, dislocation- and precipitate-microstructures. The lower-temperature swelling peak, which appeared at near 410°C, was more efficiently suppressed by phosphorus addition than cold-working. Irradiation at or above 520°C produced substantially large swelling in solution-annealed JPCA. The cavities contributed to this higher-temperature swelling developed in association with M6C-type precipitates. Neither cavities other than very small helium bubbles nor massive particles of M6C-type precipitates were observed in cold-worked and phosphorus-modified materials, in which MC-type precipitates developed at very high concentration. The effect of pre-irradiation microstructure and compositional modification on the behavior of these precipitates is discussed.

Modeling the effects of damage rate and He/dpa ratio on microstructural evolution

Abstract The effects of displacement damage rate, helium generation rate and other irradiation conditions on irradiation-induced microstructural changes and swelling in an austenitic stainless steel were investigated by numerical calculations. The computer model developed in this study dynamically integrates rate equations which describe the evolution of point defects, small point defect cluster including cascade vacancy clusters, cavity nucleation and the evolution of the cavity size distribution. The model was calibrated using data obtained from a dual-beam ion irradiation experiment. In this experiment, solution annealed type 316 stainless steel was irradiated by 4 MeV nickel ions with simultaneous implantation of helium ions up to 25 dpa at 873 K. The He/dpa ratios and the displacement damage rates were in the range of 1 to 50 appm He/dpa and 2 × 10−3 to 1 × 10−2 dpa/sec, respectively. These experiments showed that helium effects on dislocation evolution significantly influence the nucleation and growth of cavities. The predicted effects of helium over a broad range of damage rates were calculated using the calibrated model.

Microstructural changes in welded joints of 316 SS by dual-ion irradiation

Abstract As a part of the activity to establish joining methods to fabricate in-vessel components and to evaluate their performance under fusion environment, microstructural evolution was studied by means of the dual-ion irradiation method. The base material used in this study was solution annealed 316 stainless steel. Welded joints were made by the tungsten inert gas (TIG) welding method and electron beam (EB) welding method. For the prospective improvement of swelling in welded joints, modified TIG or EB welding procedures utilizing titanium or nickel foil insertions were investigated. TEM disk specimens from various positions of welded joints were irradiated to 25 dpa at 673, 773 and 873 K. He/dpa ratio in Ni/He dual-ion irradiation was 15 appm He/dpa. The present results indicate some concern about the microstructures which result in the fusion zone and heat affected zones.

Microstructural changes in Fe-10CR-2Mo steel by neutron or charged particle irradiation

Abstract An Fe-10Cr-2Mo type ferritic/martensitic dual phase steel. JFMS (Japanese ferrite/martensite steel), has been extensively studied in the Japanese fusion reactor materials research programs as a reference ferritic alloy. This work is intended to characterize the effects of neutron or charged particle irradiation to high doses on microstructural evolution and swelling of JFMS. Fast neutron irradiation was performed in an open packet in MOTA (materials open test assembly) during FFTF (fast flux test facility) cycle 10, at 643, 679, 793 and 873 K up to the maximum dose of about 40 dpa. For charged particle irradiation, 4 MeV nickel ions were injected up to 200 dpa, with or without helium ion implantation, at 723 K, at the high-fluence irradiation test facility (HIT facility), University of Tokyo. No void swelling was found at any temperature in the neutron irradiated specimens. X-ray energy dispersive spectroscopy revealed that precipitate structure is quite stable below 679 K, while it changed significantly at 793 K. Cavities were formed by charged particle irradiation only after exposure of more than 100 dpa. Helium implantation resulted in very small volume changes in the martensitic phase even at 200 dpa and demonstrated the excellent swelling resistance of JFMS.

The influence of impurity trapping on formation and growth of defect clusters in irradiated materials

Abstract A number of experimental works have demonstrated that adding a small amount of impurity may significantly alter irradiation induced microstructures in simple alloys and metals. Some of the impurity effects have been attributed to solid solution effect, namely point defect trapping at impurity atoms, and others to various aspects of precipitation effect. This work is intended to clarify how the impurity trapping affects the point defect clustering rates and the defect cluster growth rates, in order to provide theoretical insight to stabilize microstructures of irradiated materials by a minor compositional modification. A comprehensive rate theory model developed by Katoh, Stoller and Kohyama was modified to include point defect trapping processes at solute atoms. The comprehensive model is consisted of a point defect model, which calculates the concentrations of point defects and simple or complex defect clusters in isolated or trapped state, and an extended defect evolution model, most part of which was considerably simplified in this work. Effect of cascade vacancy cluster formation was first investigated, assuming various configurations of the clusters. It was found that cascade vacancy clusters tend to disappear through vacancy emission or SIA collection before they relax into thermally stable configurations such as stacking fault tetrahedra. Employing the calibrated cascade cluster model, point defect evolution under broad range of solute trapping conditions was calculated. Results of calculation on the effects of vacancy trapping energy and initial concentration of solute atoms on point defect and defect cluster evolution, dislocation loop nucleation and cavity development in austenitic Fe-Cr-Ni alloys within temperature range of 623 to 923 K will be provided.

Rate theory investigation of influence of cascade cluster formation and solute trapping on point defect agglomeration and extended defect evolution

Abstract Using a composite model of point defect behavior and microstructural evolution, the influence of cascade vacancy cluster formation and vacancy trapping at solute atoms on the point defect fluxes, point defect clustering and extended defect development was investigated. The point defect model calculates the concentrations of isolated and trapped point defects, and of simple or complex clusters. The extended defect model consists of individual rate theory models describing the evolution of cavities, Frank loops and network dislocations. Cascade vacancy clusters were observed to become the dominant sink for point defects in the early stages of irradiation at low to intermediate temperature. Therefore, the sink strength of the vacancy clusters largely determines the point defect flux and agglomeration rates. The calculations also showed that solute traps affect the irradiated microstructure to a large extent for certain combinations of trap concentration and trapping energy. Both the trap concentration and trapping energy have a non-monotonic effect on vacancy clustering though they do not change the interstitial flux significantly.

Numerical analysis of stress effects on Frank loop evolution during irradiation in austenitic FezCrzNi alloy

Abstract Effects of applied stress on early stages of interstitial type Frank loop evolution were investigated by both numerical calculation and irradiation experiments. The final objective of this research is to propose a comprehensive model of complex stress effects on microstructural evolution under various conditions. In the experimental part of this work, the microstructural analysis revealed that the differences in resolved normal stress caused those in the nucleation rates of Frank loops on {111} crystallographic family planes, and that with increasing external applied stress the total nucleation rate of Frank loops was increased. A numerical calculation was carried out primarily to evaluate the validity of models of stress effects on nucleation processes of Frank loop evolution. The calculation stands on rate equuations which describe evolution of point defects, small points defect clusters and Frank loops. The rate equations of Frank loop evolution were formulated for {111} planes, considering effects of resolved normal stress to clustering processes of small point defects and growth processes of Frank loops, separately. The experimental results and the predictions from the numerical calculation qualitatively coincided well with each other.