T. Muroga

Microstructural evolution induced by low energy hydrogen ion irradiation in tungsten

Abstract The microstructural evolution in tungsten during hydrogen ion irradiation with energies ranging from 0.5 to 8 keV at several temperatures between room temperature and 1073 K has been observed for the purpose of evaluating performance of tungsten during exposure to plasma and elucidating the underlaying mechanisms. It is shown that defect accumulation in tungsten is very small because of a high threshold energy for defect production by hydrogen and slipping out of dislocation loops to surfaces. This would lead to low hydrogen retention. These properties, however, may be degraded when the specimen purity is lower.

Effects of phosphorus on defect behavior, solute segregation and void swelling in electron irradiated FeCrNi alloys

Abstract Interstitial loop formation and void swelling under electron irradiation have been observed with a HVEM in pure Fe-15Cr-(13 to 28)Ni and Fe-16Cr-17Ni-(0.024 to 0.1)P alloys at temperatures between 293 and 873 K. After irradiations, microchemical changes at the void surfaces and matrices were analyzed by EDS. At temperatures below 573 K, where the effect of long range migration of vacancies is negligibly small, the interstitial loop density in FeCrNiP alloys was considerably higher than that in pure FeCrNi ternaries. This indicates that the addition of phosphorus significantly influences the nucleation of interstitial loops at these temperatures. From the temperature dependence of the loop density, an interstitial-phosphorus binding energy of 0.5 eV was derived. At temperatures above 573 K void swelling or vacancy loop formation is prominent and shifted upward by 100 K. Moreover, solute (Ni) segregation to the void surface and matrix were suppressed by phosphorus addition. Such phenomena are thought to be the result of vacancy-phosphorus interaction.

Retention and desorption of implanted deuterium and high-Z plasma facing materials

Abstract Thermal desorption after D 2 + irradiation from high-Z materials (W and Mo) has been compared with radiation induced microstructural evolution to investigate the details of retention and desorption of the implanted deuterium and underlying microscopic mechanism of the processes. The experimental results indicate that residual impurities act as major trapping sites for implanted deuterium independently of the ion energy for relatively low dose. The trapped deuterium is desorbed in two steps between 470 K and 660 K for high purity Mo, and 400 K and 650 K for high purity W. In the case of Mo damaged heavily at room temperature (1 × 10 22 ions/m 2 ), small cavities trap deuterium and give a large desorption stage at around 640 K. Radiation induced dislocation loops also trap the implanted deuterium effectively.

Characteristics of point defects and their clustering in pure ferritic steels

Abstract HVEM 1 MeV-electron irradiation of Fe-10% Cr and Fe-10% Cr-1% Ni alloys and subsequent EDS analysis were carried out. A remarkable feature of the microstructural evolution is that nucleation and growth of interstitial loops is very slow. The analysis of the experimental results based on the rate theory of defect processes showed that the stable nuclei of interstitial loops were tri-interstitials and that absorption of free interstitials by the loop was restricted. These could be the reasons for the slow accumulation of damage. Addition of Ni in the alloy reduces the mobility of interstitials and thus suppresses the evolution of microstructure.

The effect of phosphorus on microstructures of Fe-15Cr-25Ni alloys irradiated with fast neutrons

Abstract Fe-15Cr-25Ni austenitic alloys with various phosphorus contents were irradiated with fast neutrons in the EBR-II reactor at temperatures ranging from 399 to 649°C and doses between 8.2 and 14.3 dpa. Observations of microstructure and microchemical analyses were carried out to determine the various roles of phosphorus. At lower irradiation temperatures and lower phosphorus contents, where no precipitate formation was observed, the swelling increased initially with increasing phosphorus and declined thereafter, with the void density decreasing monotonically. The non-monotonic response of swelling to phosphorus level suggests that more than one mechanism involving phosphorus-point defect interaction were operating in the solid solution and that the net effect was a result of the competition of several mechanisms. Phosphide precipitates were observed to form at higher irradiation temperatures and phosphorus levels. The formation of these precipitates then exerted a further influence on the void density and distribution.

In-situ observation of cascade damage in gold under heavy ion irradiation at high temperature

Formation, stability and annihilation of vacancy clusters produced during irradiation with energetic heavy ions at high temperature have been studied using gold as a model material. The experiments were carried out using a 400 kV heavy ion accelerator/200 kV TEM linked facility. In-situ observation during Xe+ or Ar+ irradiations at temperatures between 290 and 670 K reveals instantaneous formation of defect clusters. These include vacancy loops and stacking fault tetrahedra (SFT) with some unresolved defects with dot contrast. At lower temperatures the clusters tend to form in groups, reflecting subcascade structure, while at 570 and 670 K, ungrouped clusters are dominant. Both vacancy loops and SFT have considerably shorter lifetimes during irradiation than in the absence of irradiation. Moreover, the lifetime spectrum has two components; a long component is related to SFT, while loops usually have shorter lifetimes. By interrupting irradiation at 570 and 670 K, defects annihilate and the process has two components. Since the SFT survive longer, the SFT/loop ratio is history dependent.

The effect of recoil energy spectrum on cascade structures and defect production efficiencies

Cascade defect structures produced by various ion irradiations have been examined in gold by in-situ TEM observations and computer simulations. The subcascade distribution is found to be highly sensitive to the incident ion mass and energy. By comparing the results of TEM observations and computer simulations, the subcascade energy is found to be about 20 keV and 10 keV in the case of Xe+ and Al+ ion irradiations, respectively. The number of subcascades is compared with the data for defect production efficiencies using the weighted average recoil energy as a correlation parameter. It is shown that the saturation of the efficiency at high recoil energies has a close relation to the cascade splitting into subcascades.

Low dose fission neutron irradiation on P- and Ti-modified austenitic alloys with improved temperature control

Abstract The initial process of microstructure evolution in Fe16Cr17Ni and its P- and Ti-modified variants was studied during irradiation in JMTR to 1.1×10 24 n/m 2 (> 1.0 MeV) under improved and conventional temperature control conditions. Interstitial loop density showed strong temperature dependence; the nominal activation energy for the loop nucleation process in the ternary alloy was about 0.5 eV. With addition of 0.024%P, the interstitial loop density increased very much, but void formation was suppressed considerably. In 0.1%P-alloy, Fe 2 P was precipitated at 673 K instead of interstitial loops. In conventional temperature control irradiation, loop density was two to three times higher than that of improved control irradiation. This suggests the need for improvement of temperature control of fission reactor irradiation for understanding essential processes of damage evolution at the set temperature.

Radiation damage and deuterium trapping in deuterium ion injected beryllium

Abstract Thermal desorption of D from its ion injected Be at several temperatures was compared with microstructural evolution during irradiation and annealing to know the details of retention and desorption of the implanted D and to identify responsible traps. In the case of the irradiation at 300 K, black dot defects and very small bubbles are formed at low doses and at high doses, respectively. The bubbles grow by annealing above 573 K and act as major trapping sites for D at low doses less than 1 × 10 21 ions/m 2 . D trapped in the bubbles finally released at around 880 K. Long and thin bubbles are developed by the irradiation at 673 K. These are responsible for high retention under the irradiation at high temperatures.

Effect of cyclic temperature change on microstructural evolution in austenitic stainless steels under fission neutron irradiation

Abstract To understand mechanistically the influence of cyclic temperature change on microstructural evolution in Fe-Cr-Ni austenitic alloys, temperature-cycle neutron irradiations of 473 K/673 K and 573 K/723 K were carried out at JMTR (0.9−1.4 × 10 24 n/m 2 (> 1.0 MeV)). Suppression of interstitial loop formation was remarkable. Only a few large voids were formed in Fe-Cr-Ni and Fe-Cr-Ni-P alloys, but unexpectedly large void swelling (1.5–1.6%, 9–11%/dpa) was observed in Fe-Cr-Ni-Ti alloy. Rate theory of defect clustering for temperature-variant irradiation indicates that vacancy-rich condition appears temporarily at the beginning of the high-temperature period due to the reclustering of the vacancies formed in the low-temperature period through the reactions with radiation-induced vacancies and interstitials.