M. Kiritani

Observation and analysis of defect structure evolution from radiation damage by D-T fusion neutrons☆

Abstract A previous report on the defect structure evolution in metals, alloys and other materials by D-T fusion neutron irradiation (J. Nucl. Mater. 133&134 (1985) 85) was not accompanied with figures and illustrations, and they are all presented in this paper. More than half of the figures consists of electron micrographs, including the following: disordered zones to show the flight distances of interstitial atoms, amorphous zones in a semiconductor, grouped defect clusters developed from sub-cascade damage, stereo-micrographs of three dimensional configurations of defect clusters in sub-cascade groups, variation of defect structures with irradiation temperature, comparison of defect structures developed in thin foil and bulk specimens to demonstrate the role of free interstitials, homogeneous and localized formation of interstitial clustered defects, detection of invisible defects by the aid of electron illumination, and dislocation structures introduced by the deformation of irradiated materials. The other figures contain numerical results of micrograph analysis, which can be used for the estimation of neutron collision cross-section and primary knock-on energy. Point defect processes occurring during the damage structure evolution, including the dynamical effect of collisions, are discussed on the basis of experimental observations.

Formation of secondary defects in copper by 14 MeV neutron irradiation and their effects on microstructure evolution

Pure copper specimens were irradiated at 25, 200 and 400°C by 14 MeV neutrons using RTNS-II to the dose of 3.6 × 1022 n/m2 and their damage structure was examined by means of transmission electron microscopy. n nAt 25 and 200°C, stacking fault tetrahedra (SFT), partially dissociated Frank loops, aggregates of vacancies, and interstitial loops are nucleated by cascade collapse. They have a tendency to be formed as a group up to about 10. Because SFT are very stable under irradiation, excess interstitials corresponding to the vacancies retained in SFT are accumulated in the matrix and form their clusters. Interstitial loops nucleated near a dislocation grow preferentially by absorbing the interstitials migrating towards the dislocation. n nVoids were observed at 400°C. They play a very important role in void swelling at high dose.

Confirmation of vacancy-type stacking fault tetrahedra in quenched, deformed and irradiated face-centred cubic metals

Abstract An efficient and reliable method for characterizing the nature of stacking fault tetrahedra in f.c.c. metals using electron microscope diffraction image contrasts is presented. By using the 220 reflection, and thereby eliminating the contrast from overlapping stacking faults, one can differentiate between intrinsic-type stacking fault tetrahedra due to lattice vacancies and extrinsic type due to interstitial atoms. The validity of the method is examined by the observation of vacancy-type stacking fault tetrahedra in quenched metals, and by the observation of interstitial-type faulted dislocation loops. Stacking fault tetrahedra introduced by plastic deformation, electron irradiation, neutron irradiation and ion irradiation are all confirmed to be vacancy type. It was found that interstitial-type stacking fault tetrahedra do not exist.

Ion-irradiation experiment for the fundamental studies of damage evolution of fusion materials

Abstract Cascade defects produced by ion irradiation have been studied by electron microscopy for fcc metals (Au, Ag, Cu, Ni, Al) and bcc metals (Fe, Mo, V) as well as two practical alloys (SUS316, ferritic steel). The thin foil specimens have been irradiated with 40 to 65 keV Ar+ and metal ions at a range of temperatures between − 160°C and 600°C; the ion flux and the irradiation fluence have also been varied. The defect yield varies considerably for the materials and the defect density increases proportionally only in the low fluence regime. The density of the observed defects shows a tendency to saturate at high fluences. Repeated irradiations show that saturation of defect density is due to the disappearance of defects during subsequent ion irradiation while the production rate of defect clusters by the ion remains the same. A typical example of instability of defects is seen in Al, where the observed defects appear only after high flux ion irradiation and at the relatively low temperatures. Small defects can be observed in Fe irradiated with 40 keV Fe+ ions only when the irradiation fluence is higher than 4 × 10 13 ion/cm 2 .

Parametric analysis of the disc bend test

A study was undertaken to determine the response of disc bend test data to variations in the relative size of the spherical penetrator, receiver hole and specimen thickness. Four materials - annealed and cold-worked copper and austenitic stainless steel — were lapped to three thicknesses, 0.1–0.25 mm. Specimens from each material were tested in a disc bend apparatus in which the penetrator diameter could be varied from 1 to 1.6 mm and the receiver hole diameter from 1.3 to 1.9 mm. Uniaxial tensile tests on each material were also conducted. It was found that load displacement data were most sensitive to thickness; “yield” load, maximum load and penetrator displacement to failure all increased to the greatest extent with increasing specimen thickness. Good correlations between these values and uniaxial tensile properties were found for all combinations of ball, hole and specimen size; but the data scatter was slightly reduced for the thickest specimens and the largest ball size.

Factors controlling the nature and amount of residual defects in neutron irradiated materials

Abstract The requirements for progress towards a unified understanding of the defect structure evolution by high energy neutron irradiation are described. Experimental data to be considered are categorized from the observations made on defect structures in various materials irradiated with the fusion neutron source RTNS-II. Comparison of defect structures developed in thin foils with those in bulk specimens was found to be extremely efficient in increasing our understanding of defect processes. The variation of microstructure which comes from the difference in the point defect configuration in freshly made cascade damage is discussed. The consideration of the stability of point defect clusters includes absolute instability in some materials, stacking fault tetrahedron nucleation, and cooperation among sub-cascades. Roles of interstitial atoms are evaluated, such as the annihilation within a unit cascade, free interstitial migration to eliminate vacancy clusters, and the formation of interstitial clustered defects. Dynamical effect of collisions on point defect processes is discussed. Formation of voids is explained in relation with helium production.

Correlation among a variety of miniaturized mechanical tests and their application to D-T neutron-irradiated metals☆

Abstract Tensile, disk bulge and microhardness tests with miniaturized specimens have been performed on a variety of metals and alloys. Both the bulge fracture load and the microhardness test strongly correlate with the tensile strength. Specimens as thin as 0.1 mm generally fulfil the requirements for obtaining bulk properties from miniaturized tensile tests, for materials where the grain size can be properly controlled. The tensile and bulge tests were carried out on Ni, Au. Cu and austenitic stainless steel irradiated with D-T fusion neutrons from RTNS-II. Au and Cu showed a large increase in yield strength up to factors of 7 and a reduction of ductility down to 50% by the irradiation of 1.7 × 10 22 n/m 2 . The increase of the yield strength for Ni and stainless steel is about half that of Cu and Au, and the decrease of ductility is small for stainless steel. Variation of deformation mode with increasing neutron fluence has been detected by optical and electron microscopy.

The Japanese experimental program on RTNS-II of DT-neutron irradiation of materials☆

Research project of D-T neutron radiation damage of materials with the rotating target neutron source RTNS-II of LLNL is explained with a brief explanation of the results obtained in the first year of the project. Wide variety of materials were irradiated at three temperatures; 25°C, 200°C and 400°C up to 1 × 1018n/cm2. From the result of the observations of defect structures, discussions are made on the conditions for the defect cluster formation from cascade damage, type of defects, defects made from sub-cascade damage, correlation between defects and disordered zones, and the roles of free interstitials released from cascades. Future research plans are briefly presented.

Low temperature D-T neutron irradiation and cryotransfer observation of cascade defects of metals

Electron microscope specimens of Au, Ag, Cu, Al and Fe were irradiated below 20 K by D-T neutrons with RTNS-II at a fluence of 1 × 1016 to 1.5 × 1017 n/cm2. After irradiation the foils were dismounted from the cryostat and mounted on a TEM cold holder in liquid nitrogen, and transferred to an electron microscope without warming them. The results presented here describe preliminary observations on only a few foils. Cascade defects composed of small defects in a group were observed in Au, Ag and Cu. The relaxation of cascade defects occurring during the isochronal annealing was examined. No irradiation-induced defect was detected in cryotransferred aluminum at the instant of starting the observation, while observable cascade defects appeared during a subsequent illumination. This process is due to athermal collapsing of cascades induced by the electron illumination. In iron, no defects could be observed at low temperature or after annealing. Generally cascades collapse at low temperature to non-compact metastable structures and change to stable ones during the stage III annealing.

Defect structure evolution from radiation damage with D-T fusion neutrons

Abstract Irradiation of a variety of metals in a rotating target D-T fusion neutron source RTNS-II and observation of defect structures have been performed to obtain a unified understanding of the defect processes involved in the damage structure evolution from irradiations that generate large cascades. The maximum separation of interstitial atoms from the vacancy rich zone is measured. Vacancy type defect clusters form groups reflecting the damage with subcacades, and the three-dimensional configuration of subcascades is disclosed by high resolution stereo electron microscopy. The effective collision cross-section to produce defect clusters is estimated, and the damage efficiency is obtained. The variation of point defect processes is discussed based on the variation of the stability of small clusters in different materials irradiated at different temperatures. The roles of free interstitials released from cascade zones, the elimination of vacancy clustered defects and the formation of their own clusters, are clearly understood from a comparison of the results of irradiation of a bulk specimen with those of thin foils. The necessity of a dynamical effect from collisions on the defect cluster formation is concluded and its mechanism is discussed. Only the descriptions are included in this paper; all the micrographs and figures will appear in another paper in the same journal.