Small Stacking Fault Tetrahedra Research Articles

Defect clusters formed from large collision cascades in fcc metals irradiated with spallation neutrons

Fcc pure metals were irradiated with spallation neutrons (energies up to 500MeV) at room temperature to a neutron fluence of 1×1018n m−2 at KENS, High Energy Accelerator Research Organization (KEK). Defect clusters induced by large collision cascades were examined using transmission electron microscopy (TEM). In Au, large groups of defects included more than 10 clusters, and the damage zone extended over 50nm, which was larger than that induced by fusion neutron irradiation (<20nm). Although small stacking fault tetrahedra (SFT) are formed in subcascades by fission and fusion neutron irradiation, dislocation loops were also observed in the present experiments. Large dislocation loops (>10nm) were identified as vacancy type by the conventional inside–outside contrast method. Because of the low neutron fluence, spatial overlapping of collision cascades was ignored. Large vacancy loops are formed through cooperative reactions among subcascades in a single collision cascade with large recoil energy.

陽電子消滅分光法を用いた中性子照射した Ni-Sn 合金中の Sn 原子と原子空孔の相互作用

Solute atoms greatly influence irradiation damage structures. For example, voids are formed in pure Ni at 573 K, but they are not formed in Ni-Sn alloys irradiated by fission neutrons at 573 K to 0.2 dpa. In this study, the interaction between solute atoms and vacancies and correlation between void growth and solute atoms in neutron-irradiated Ni-Sn alloys were investigated by positron annihilation spectroscopy. Positron annihilation lifetime measurements showed that void density was lower in Ni-0.05 at%Sn than pure Ni. In Ni-0.3 at%Sn, single vacancies, di-vacancies and small stacking fault tetrahedra (SFTs) were formed and the size did not change below 0.1 dpa. In Ni-2 at%Sn, single vacancies and SFTs were formed and the size also did not change below 0.1 dpa. In Ni-Sn alloys, the total intensities of lifetime related to vacancy type defects were more than 60%. Vacancy concentration was higher than in pure Ni and a large number of positrons annihilated at them. From positron annihilation coincidence Doppler broadening (CDB) measurements, the ratio of annihilation of positrons and low-momentum electrons increased in Ni-Sn alloys compared with pure Ni. The ratio increased with increasing the solute atom concentration. As positron affinity of Sn is lower than that of Ni, positrons are attracted to Sn atoms more strongly than Ni. Complexes of vacancy clusters and Sn atoms are formed and the number of Sn atoms adjacent to vacancy clusters increases with the solute atom concentration. It is expected that Sn atoms change the electron state of vacancy clusters, which leads to the increase of annihilation ratio between positrons and low-momentum electrons.

Mechanisms of stacking fault tetrahedra destruction by gliding dislocations in quenched gold

The destruction processes of stacking fault tetrahedra (SFTs) induced by gliding dislocations were examined by transmission electron microscopy (TEM) in situ straining experiments for SFTs with edge lengths ranging from 10 to 50 nm. At least four distinct SFT destruction processes were identified: (1) consistent with a Kimura–Maddin model for both screw and 60° dislocations, (2) stress-induced SFT collapse into a triangular Frank loop, (3) partial annihilation leaving an apex portion and (4) complete annihilation. Process (4) was observed at room temperature only for small SFTs (∼10 nm); however, this process was also frequently observed for larger SFTs (∼30 nm) at higher temperature (∼853 K). When this process was induced, the dislocation always cross-slipped, indicating only screw dislocations can induce this process.

Atomic-scale details of dislocation–stacking fault tetrahedra interaction

Stacking fault tetrahedra (SFTs) are formed during irradiation of f.c.c. metals and alloys with low stacking fault energy. The high number density of SFTs observed suggests that they should contribute to radiation-induced hardening and, therefore, be taken into account when estimating mechanical property changes of irradiated materials. The key issue is to describe the interaction between a moving dislocation and an individual SFT, which is characterized by a small physical scale of about 100 nm. In this paper we present results of an atomistic simulation of edge and screw dislocations interacting with small SFTs at different temperatures and strain rates and present mechanisms which can explain the formation of defect-free channels observed experimentally.

Ultra-high-speed deformation by impact of a projectile flying at speeds on the order of km s −1

An apparatus was developed to facilitate application of an electro-thermo-chemical accelerator to high-speed deformation experiments. The apparatus is designed on the principle of sequential collision of elastic bodies. Speeds ranging from 600 to 780 m s −1 were achieved, and estimated strain rate of deformation is 10 7 s −1. The newly developed apparatus can be applied to various types of accelerators for attaining deformation speeds as high as several km s −1. Transmission electron microscopy of aluminum deformed at high speed by use of the apparatus revealed the formation of very small stacking fault tetrahedra (SFTs). This observation is quite new for aluminum; previously, SFTs had not been observed in aluminum, although deformation had been carried out at strain rates lower than 10 6 s −1. Use of the apparatus promises to provide new insight into high-speed deformation.

Dynamical Behavior of Voids in Neutron-irradiated Copper at Elevated Temperature

ABSTRACTWe reported previously an experimental result which shows that voids can move in neutron-irradiated copper at elevated temperature. To study the detailed behavior of voids, we carried out annealing experiments and in-situ observations in neutron-irradiated copper. Neutron irradiation was carried out in the temperature controlled capsule at KUR (Kyoto University Reactor). Neutron irradiation was performed at 300°C to damage levels between 10-4 to 10-3 dpa. The annealing temperature was 250°C for 10, 20, 30 min sequentially for neutron- irradiated copper. After annealing TEM observation was carried out at room temperature. Experimental results show that voids moved along the [110] direction. Voids moved during 250°C annealing but small stacking fault tetrahedra (SFT) were not changed. We observed 37 voids and 8 voids moved, the others vanished during annealing. The images of in-situ observation, void contrast images were recorded on VTR tape and analyzed frame by frame. At room temperature observation voids were static and were seen as white circles; however at 300°C the contrast changed to an oval shape and sometimes disappeared. After 7 sec contrast was a white circle and moved slightly and similar phenomena were observed quite frequently at 300°C. It is concluded that void moved with dynamical structural relaxation at elevated temperature.

Observation of vacancy clustering in FZ-Si crystals during in situ electron irradiation in a high voltage electron microscope

Vacancy accumulation in pure FZ-Si crystals covered with thin Si 3N 4 films leads upon irradiation with a high intensity electron beam in a high voltage electron microscope (HVEM) at temperatures between 20 and 250°C, to the suppression of the formation of 113 -defects of interstitial type in the central part of the irradiated area. Very small secondary clusters of interstitial type with a density of about 10 11 cm −2 appear, however, in this area after prolonged irradiation. Small stacking fault tetrahedra and Frank loops of vacancy type connected with interstitial clusters were analysed on an atomic scale in the thinnest part of the irradiated area in a 400 keV high resolution electron microscope. Vacancy clusters might act as nucleartion centres for the observed secondary defect formation.

Microstructure of copper and nickel irradiated with fission neutrons near 230°C

The microstructures of pure copper and nickel specimens were examined by transmission electron microscopy (TEM) following irradiation at ∼ 230°C in a mixed spectrum fission reactor to damage levels between 0.01 and 0.25 displacements per atom (dpa). A high density of small stacking fault tetrahedra (SFT) and dislocation loops was observed in both materials, and a moderate density of small voids was observed in the irradiated copper specimens. From a comparison with published studies, the proportion of SFTs and dislocation loops in copper was observed to be nearly constant over a wide range of damage levels at temperatures between 200 and 250°C. This suggests that nucleation and growth of interstitial loops to visible sizes is rather difficult in copper in this temperature regime. On the other hand, the loop size and density in nickel increased steadily with increasing dose. The defect clusters in nickel formed a {001} planar wall pattern which became visible after damage levels of ∼ 0.1 dpa. Defect cluster patterning was not observed in the irradiated copper specimens.

Diffraction contrast images of small stacking fault tetrahedra in f.c.c. metals

Abstract Transmission electron microscopy images of stacking fault tetrahedra (SFTs) are studied in detail both experimentally and by image simulation. Experimental dark-field diffraction contrast images of SFTs are examined under various conditions. The image contrast of stacking faults and stair-rod dislocations are simulated based on standard multi-beam dynamical theory, and the image contrast of very small SFTs (< 2.6 nm) is calculated by the multi-slice method. The correspondence between experimental and calculated results under systematically varied conditions is quite satisfactory. Stacking faults make a larger contribution to the total SFT image than stair-rod dislocations. The images are generally smaller than the SFT. The difference in size depends on many parameters such as deviation from the Bragg condition, location in the foil, and specimen thickness. However, the difference is always smaller than the fringe spacing in the stacking fault image, and is generally smaller at a larger deviation ...

Mechanical property changes and defect structures of Ni and Fe irradiated with fission and fusion neutrons

Abstract The correlation between the fission and 14-MeV D-T fusion neutron irradiation effects on the mechanical property changes is discussed for Ni and Fe with reference to the defect structures induced by the irradiation. In Ni irradiated with fission and fusion neutrons at 470 K, no significant difference in size distribution was found in the defect structures of small SFTs (stacking fault tetrahedra) and dislocation loops except for the absolute value of their number density and the dependence of the change in the yield stress on the defect number density is the same. In irradiations at higher temperatures, 720 K in RTNS-II and 670 and 770 K in Joyo, the dominant defect clusters are SFTs in RTNS-II and voids in Joyo irradiation. The difference in the amount of the yield stress increase for the same defect number density is understood from the difference in the defect cluster species. The dependence of the yield stress on the fluence in Fe was discussed on the basis of the results of Ni.

Small Stacking Fault Tetrahedra Observed in High Purity Gold Shortly after Quenching

Small stacking fault tetrahedra (SFT) which just nucleated by clustering of vacancies inquenched high purity gold were observed at -160°C after specimen thinning by electron microscopy. It was found that even in a thin foil SFT grew appreciably and new SFT nucleated duringa room temperature annealing, while only SFT which were observed on the first cooling stage observation could grow during the subsequent 200°C thin foil annealing. The minimum edge size of tetrahedron image observed shortly after quenching was about 20 Å. For the nucleus of SFT which is the smallest vacancy cluster stable below 400°C as previously found expermentally, the present results suggest that they are the smallest SFT whose size is less than 20 Å. Growth of SFT in thin foil was explained with an almost perfect sink efficiency of SFT for the vacancy absorption below room temperature.

Nature of Black Spot Defects in Quenched Gold

The formation and annihilation of the small so called black spot defects in quenched gold are investigated in connection with those of the stacking fault tetrahedra. No discontinuity is found between the above two defects, and the black spot defects are concluded to be small stacking fault tetrahedra.

DISLOCATION DISTRIBUTION IN DEFORMED SILVER SINGLE CRYSTALS

The results of an electron transmission microscopy investigation of silver single crystals deformed in the three stages of the work-hardening curve are described. In stage I edge-dislocation bundles of the primary system form. With continued deformation in stage I more bundles form and the number of dislocations in each bundle increases. The end of stage I is associated with activity on secondary glide systems adjacent to the bundles. During stage II deformation, a cell structure forms and continually decreases in size by the cooperative activity of the primary and secondary systems. This cooperative activity occasionally results in the formation of dense networks of Lomer–Cottrell dislocations in local areas. Often associated with these networks are small stacking-fault tetrahedra believed to result from the stress-induced cross slip of extended dislocations at jogs. The importance of the stacking-fault energy in the deformation of f.c.c. crystals is discussed.

Electron Diffraction Contrast of Small Stacking Fault Tetrahedra with Applications to Quenched Gold

AbstractIt is shown that the characteristic diffraction contrast of small stacking fault tetrahedra obtained by imaging with a {220} reflection can be used to distinguish small unresolved stacking fault tetrahedra from dislocation loops. Application of these results to quenched gold shows that vacancy clusters exist already in their early stage as stacking fault tetrahedra.