Minoru Takenaka

Positron annihilation lifetime study of irradiated and deformed Fe and Ni

Abstract In order to investigate the fundamental behaviors of radiation-induced defects, especially vacancy type defects and also deformation-induced dislocations and vacancies, positron annihilation lifetime measurements have been performed for Fe, FeCu and FeSi irradiated with electrons at low temperature (77 K) and also for Ni deformed at room temperature. From the isochronal annealing experiments it was found that vacancies become mobile above 200 K and form microvoids in Fe, but in FeCu and FeSi alloys the interaction between vacancies and solute atoms significantly suppresses the microvoid formation process. In FeCu alloy, instead of microvoid formation it was considered that vacancy-Cu complexes are formed by judging the value of positron lifetime. In the deformed Ni positron lifetime decreased gradually as the isochronal annealing temperature increased. From this result and positron lifetime calculation it was suggested that in deformed Ni positrons are trapped and annihilated at complexes of a dislocation and deformation-induced vacancies.

Positron annihilation lifetime measurements of irradiated iron and iron-chromium alloys

Abstract Positron annihilation lifetime measurements have been performed for irradiated Fe, Fe-10%Cr, Fe-10%Cr-0.07%C and JFMS (Japanese Ferritic/Martensitic Stainless Steel) specimens. Information on the radiation-induced defects and defect clusters, especially the behaviour of migration and annihilation of these defects was obtained. It was concluded that vacancies migrate in the temperature range between 150°C and 200°C for these alloys, (below room temperature in pure Fe) and probably small interstitial clusters disappear at higher temperatures, i.e., 300°C to 400°C. The pure Fe specimens irradiated by 14 MeV neutrons showed higher resistance to the recovery of the second component of the positron annihilation spectrum than that in fission-neutron irradiated specimens, which means that some defect clusters, probably small interstitial clusters or vacancy-gas complexes, remain in more stable form in fusion-neutron irradiated specimens by the aid of transmutation-produced gases.

Positron annihilation lifetime measurement of irradiated stainless steels

Abstract Two types of austenitic stainless steels (316SS and JPCA) and two types of ferritic/martensitic stainless steels (HT-9 and JFMS) were irradiated by high energy electrons at 77 K and positron annihilation lifetime measurements were carried out to obtain the isochronal annealing behaviour above room temperature. The main decrease of the intensity of the second component, namely, the migration of vacancies to sinks was observed at 250°C for austenitic stainless steels and at 150°C for ferritic/martensitic stainless steels. By assuming the number of jumps to sinks as 103, the vacancy migration energy was obtained as 1.36 and 1.10 eV for austenitic and ferritic/martensitic stainless steels, respectively. This result was used to discuss the low swelling behaviour of the ferritic/martensitic stainless steels.

Positron-lifetime study of electrically hydrogen charged Ni, austenitic stainless steel and Fe

Abstract Positron-lifetime study has been made for the electrically hydrogen charged Ni, austenitic stainless steel (316 stainless steel), and Fe in order to investigate the structural changes due to the presence of high concentration of hydrogen atoms in the bulk, such as hydrogen-induced defects, hydrogen-induced phase transformation and so on. Increase of mean lifetime was observed for Ni and 316 stainless steel, but almost no change was observed for Fe. The introduction of a long lifetime of about 150 ps to the Ni specimen was interpreted as the generation of vacancies by the hydrogen charging. But in the case of Fe, no vacancy generation was observed probably due to the low concentration of hydrogen atoms, even by the electrical hydrogen charging. In 316 stainless steel, both the phase transformation and the generation of vacancies were observed by the presence of high concentration of hydrogen atoms in the bulk. Elastic recoil detection (ERD) method showed that the hydrogen concentration reaches about 40% (H/atom=0.4) near the surface region of 316 stainless steel, electrically hydrogen charged.

Irradiation-induced vacancy and Cu aggregations in Fe-Cu model alloys of reactor pressure vessel steels: state-of-the-art positron annihilation spectroscopy

Theoretical and experimental aspects of positron annihilation studies of precipitate-relevant phenomena in Fe–Cu model alloys of reactor pressure vessel steels are discussed. Although the positrons are thought, in general, to be insensitive to the impurities in solids, we demonstrate that the positrons can be confined as a quantum-dot state within the nano Cu or Cu-rich precipitates in the Fe–Cu model alloys, which makes the positron annihilation one of the most promising techniques in this field. The positron probe is employed successfully to clarify that nano Cu precipitate has a bcc structure coherent with the Fe matrix. The results show clearly that the quantum-dot-state positrons exclusively detect the microscopic and electronic structures of the precipitates.

Hydrogen and deuterium uptake in helium implanted layer of Mo and W

The accumulation of hydrogen isotopes in the helium (He) implanted layer of Mo and W single crystals was studied, focusing on the effects of the high dose irradiation of He ions and on uptake behavior of hydrogen (H) and deuterium (D) in various H 2 and D 2 pressure. A remarkable H uptake occurs in the He saturated layer of the crystals irradiated by 10 kev 4 He ions to a dose about 2 x 10 22 He/m 2 , after the heat treatment up to 850 K. The integrated amount of retained H reaches the same values before the thermal release procedure, though H-uptake rates depend on the gas pressure. The H is preferentially uptaken in the He saturated layer, when D 2 and/or D 2 O gas is introduced in the vacuum chamber. The large difference between the H- and D-uptake rates suggest the oxide formation at the crystal surface where the microstructure is significantly changed due to the He irradiation.

The effect of electrical hydrogen charging on the strength of 316 stainless steel

The effect of electrical hydrogen charging on the strength of 316 stainless steel specimens has been investigated in tensile tests at 223 K, and the increase of yield stress and the decrease of total elongation were observed. These tendencies increase with increasing hydrogen content of the specimens. This is considered to be due to hydogen-induced phase transformation from γ (fcc) to e (bcc), α (bcc), which was confirmed by X-ray diffraction method. Hydrogen concentration was determined by elastic recoil detection (ERD) method, the maximum of which reached 40% near the surface region. Positron annihilation lifetime was also measured after electrical hydrogen charging and a longer lifetime of about 300 ps was observed, which suggests the formation of microvoids in the specimens.

Positron lifetime calculation for defects and defect clusters in graphite

Calculations of positron lifetime have been made for vacancy type defects in graphite and compared with experimental results. Defect structures were obtained in a model graphite lattice after including relaxation of whole lattice as determined by the molecular dynamics method, where the interatomic potential given by Pablo Andribet, Dominguez-Vazguez, Mari Carmen Perez-Martin, Alonso, Jimenez-Rodriguez [Nucl. Instrum. and Meth. 115 (1996) 501] was used. For the defect structures obtained via lattice relaxation positron lifetime was calculated under the so-called atomic superposition method. Positron lifetimes 204 and 222 ps were obtained for the graphite matrix and a single vacancy, respectively, which can be compared with the experimental results 208 and 233 ps. For planar vacancy clusters, e.g., vacancy loops, lifetime calculation was also made and indicated that lifetime increases with the number of vacancies in a cluster. This is consistent with the experimental result in the region of higher annealing temperature (above 1200°C), where the increase of positron lifetime is seen, probably corresponding to the clustering of mobile vacancies.

Positron Lifetime Measurement of Plastically Deformed Iron Single Crystals

Positron lifetime measurement and analysis were made for pure iron single crystals plastically deformed at room temperature and 77 K. Both second component I 2 (175 psec, dislocations and jogs) and third component I 3 (about 350 psec, vacancy clusters) in the specimen deformed at 77 K were larger than those in the specimen deformed at room temperature. It must be concluded that jogs in the low temperature deformed specimen play an important role in positron trapping and also in production of vacancies. However, the jog number density estimated from the experiment is too high, which suggests that some modification in the model of positron trapping by jogs may be required.

Irradiation-Enhanced Cu-Precipitation in Fe-Cu Alloys Studied by Positron Annihilation Spectroscopy and Electrical Resistivity Measurement

Abstract The positron annihilation measurements (CDB and positron lifetime measurement) and the electrical resistivity measurement were made for the isochronal annealing process of the Fe-Cu alloy specimens irradiated with electrons at low temperature. The peak in the high momentum region in CDB ratio curve grows prominently, showing the formation of Cu precipitates. W-parameter in the S-W plot already started to increase at 150K in Fe-0.6wt%Cu alloy specimens. The vacancy component I2 in the lifetime spectrum is kept almost constant beyond stage ID (110K) to about 300K. In the electrical resistivity measurement prominent recovery peaks appear at about 140K and 155K and these peak temperatures decrease with increasing Cu concentration. These results show that even at low temperature region below stage self-interstitial atoms (SIAs) contribute to the formation of Cu precipitates through the mixed dumbbell mechanism and beyond stage vacancies mainly contribute.