Masuyuki Hasegawa

Positron annihilation study of vacancy-type defects in high-speed deformed Ni, Cu and Fe

Abstract Vacancies and vacancy clusters in Ni, Cu, and Fe induced by high- and low-speed deformations are studied systematically by positron annihilation techniques and are compared with those induced by the conventional-rolling. To clarify the nature of the defects, the experimental results are compared with our superimposed-atomic-charge calculations of the positron lifetimes in the vacancy clusters as a function of their size. It is found that the deformation-induced defects in the fcc and bcc metals are significantly distinct. In the fcc metals of Ni and Cu, monovacancies with high number densities are induced by the high- and low-speed deformations and by heavy conventional-rolling (>10% in Ni and >40% in Cu). Vacancy clusters are observed after the high- and low-speed deformation for Ni and after the conventional-rolling for Cu. On the contrary, dislocations and vacancy clusters are introduced in bcc Fe regardless of the type or degree of deformation.

Positronium in silica-based glasses

Abstract Structural subnanovoids in silica-based glasses (synthetic silica, borosilicate glass, lead glass, alkali barium glass, soda-lime glass and model glass of radioactive waste glass) with several SiO 2 concentrations are studied by positron lifetime and one-dimensional angular correlation of annihilation radiation spectroscopy. The average sizes of the structural subnanovoids are estimated from the pick-off annihilation rates of the ortho -positronium and from the momentum distribution of the para -positronium. It is observed that the sizes of the subnanovoids and the positronium intensities decrease with decreasing SiO 2 concentrations in the glasses, which suggests that the positronium is a unique and useful tool for subnanovoids in glass materials.

Coincidence Doppler broadening study of the pre-precipitation stage of an Al-Cu-Mg-Ag alloy

The role of Ag in the pre-precipitation process of an Al-1.9 at.% Cu-0.3 at. % Mg-0.2 at.% Ag alloy has been investigated in view of the interaction between the quenched-in vacancies and Ag atoms. Positron annihilation methods, coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopies, and transmission electron microscopy have been employed. In an as-quenched state, quenched-in vacancies V in the All 9 at% Cu-0.3 at.% Mg-0.2 at.% Ag alloy are associated with Ag atoms as well as Mg atoms (V-Mg-Ag) while V-Mg complexes are formed in the Al-1.9at.% Cu-0.3 at.% Mg alloy. The recovery of the vacancies after 15s ageing at 180°C is suppressed by the existence of Ag atoms. These results indicate that the V-Mg complexes are stabilized by the associated Ag atoms, which promotes the co-clustering of Ag and Mg atoms by ageing rather than by the formation of dislocation loops.

Effects of temperature history during neutron irradiation on the microstructure of vanadium alloys

Abstract Void formation behavior under fission neutron irradiation was studied in vanadium based binary alloys using an improved temperature control mode. The latter method employs an electric heater and enables to avoid unwanted irradiation at temperatures during reactor startup. Emphasize was on the effect of the atomic size factor on void nucleation and growth. Transmission electron microscopy and positron lifetime measurements were used in this study. It has been found that a strong correlation exists between the void size and density and the atomic size factor; larger solute atoms yields a higher void density. Microvoid formation at 200°C detected by positron lifetime measurement agrees very well with void formation at 350°C observed by TEM. It has also been demonstrated that the temperature control mode has a striking effect on void formation.

EFFECTS OF VARYING TEMPERATURE IRRADIATION ON THE NEUTRON IRRADIATION HARDENING OF REDUCED-ACTIVATION 9CR-2W MARTENSITIC STEELS

Abstract In order to clarify the effects of varying temperature during irradiation on the irradiation hardening of 9Cr–2W steels, tensile tests and positron annihilation lifetime measurements were carried out following the varying temperature irradiation (220/420°C and 340/530°C) utilizing a so called multi-section and multi-division controlled irradiation capsule in JMTR. After the irradiation at 220°C to 0.053 dpa, the steels show irradiation hardening as much as 110 MPa. The hardening was almost completely diminished immediately after the elevation of the irradiation temperature to 420°C. Subsequent irradiation at 420°C up to 0.14 dpa did not cause any hardening. The results of positron annihilation lifetime measurements indicate that microvoids are formed by the irradiation at 220°C but disappear upon elevating the temperature to 420°C and are then formed again by the subsequent irradiation at 420°C up to a total dose of 0.14 dpa. This behavior may be interpreted in terms of decomposition of interstitial loops or migration of small interstitial loops during temperature elevation. There is no good correlation between irradiation hardening and formation of microvoids in neutron-irradiated reduced-activation martensitic steels.

Effects of corner position and operating condition on electromigration failure in angled bamboo lines without passivation layer

Abstract The atomic flux divergence due to electromigration, AFD gen , has been formulated considering two-dimensional distributions of current density and temperature within polycrystalline and bamboo line structures without passivation. The divergence AFD gen has been identified as a governing parameter of electromigration damage by experimental verification. Recently, an AFD gen -based method for predicting the lifetime and failure site in an unpassivated bamboo line was proposed based on numerical simulation of the failure process of a metal line. Availing of the advantage of the universal method, the effects of corner position, corner angle and operating conditions on the lifetime and failure location in angled metal lines are discussed. The lifetime and failure location varied with the line shape and operating conditions. Understanding these effects on metal line failure gives important knowledge for enhancing the electromigration endurance of metal lines.

Elemental analysis of positron affinitive site in materials by coincidence Doppler broadening spectroscopy

Abstract Elemental analysis study of the positron affinitive nano-structures in materials by coincidence Doppler broadening method are presented: (1) defect free Cu nano-clusters embedded in Fe, (2) nano-voids in Fe covered with Cu atoms, and (3) solute clusters (GP zones) in Al alloys (Al–Ag and Al–Zn). By utilizing positron trapping not only in vacancy-type defects but also positron affinitive, defect-free nano-clusters embedded in materials, unique and exclusive information, such as defect structure, coherency, chemical composition, and so on, on the trapping sites can be obtained.

Irradiation-induced voids in alumina single crystal studied by positron annihilation

Abstract Angular correlation of annihilation radiation (ACAR) has been measured on α-alumina single crystals irradiated with fast neutrons to a dose of 3 × 10 24 n/m 2 at about 470 K. After post-irradiation annealing above 900 K, remarkable narrowing in ACAR curves has been observed. These ACAR curves can be decomposed into three Gaussian components: a narrow (N), an intermediate (M) and a broad (B) component, with FWHM of 2–5, about 6 and 11 mrad, respectively. The N and M components are attributed to positrons trapped in the voids. A magnetic quenching effect is observed for the N component but not for the M component. This shows that the N component is due to two photon self-annihilation of positronium (Ps) formed in voids. The FWHM of the N component indicates, through the momentum of zero-point motion of Ps, that the average void diameter is 0.6 nm after annealing at 1000 K and 1.7 nm after annealing at 1525 K Striking similarity of the M component to a void-surface trapping component in metallic Al, together with nearly the same lifetime as that of voids in Al, suggests the existence of metallic surface-conduction-electron states in the voids in α-alumina.

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.

Atomic flux divergence in bamboo line for predicting initial formation of voids and hillocks

Abstract A calculation method of the atomic flux divergence due to electromigration, AFDgen, has been proposed considering two-dimensional distributions of current density and temperature and also simply considering the line structure of not only polycrystalline line but also bamboo line. The validity of AFDgen for bamboo lines has been verified in comparison of void formation calculated by using AFDgen with experimental results, and it has been shown that void formation in bamboo line is able to be predicted by using AFDgen. In this study, angled bamboo lines are treated for prediction of hillock formation, and hillock formation predicted by using AFDgen is compared with that measured in experiment. In addition to the verification of void prediction, the usefulness of the prediction method for electromigration damage using AFDgen is discussed in more detail.