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The formation of helium bubble precursors, i.e., helium-vacancy complexes, was investigated for Fe9Cr alloy, which was uniformly irradiated by using 100 keV helium ions with fluences up to 5 × 10 16 ions/cm 2 at RT, 523, 623, 723, and 873 K. Helium-irradiation-induced microstructures in the alloy were probed by positron annihilation technique. The results show that the ratio of helium atom to vacancy (m/n) in the irradiation induced He m V n clusters is affected by the irradiation temperature. Irradiated at room temperature, there is a coexistence of large amounts of He m V 1 and mono-vacancies in the sample. However, the overpressured He m V n (m > n) clusters or helium bubbles are easily formed by the helium-filled vacancy clusters (He m V 1 and He m V n (m ≈ n)) absorbing helium atoms when irradiated at 523 K and 823 K. The results also show that void swelling of the alloy is the largest under 723 K irradiation. [ABSTRACT FROM AUTHOR]

Here, we investigated the irradiation defect in reduced activation ferritic/martensitic steels by slow positron beam. Three ion-irradiation experiments were carried out: (i) He2+irradiation, (ii) H+irradiation and (iii) He2+irradiation followed by H+irradiation, at temperature 450 °C. The presences of vacancy defects, represented by ∆SHe+Hparameter, induced by sequential irradiations was larger than the sum of defects, ∆SHeparameter + ∆SHparameter, caused by single He ions and single H ions. The synergistic effect of He and H was confirmed clearly from the perspective of positron annihilation spectroscopy. [ABSTRACT FROM AUTHOR]

• Helium diffusion and desorption behavior was researched by DBS and TDS measurements. • Helium atoms combining with dislocation change the distribution of electron density. • Helium atoms trapped by dislocations form relatively stable complexes. • Dislocations inhibit the helium radiation damage. Suppression of transmutation helium can be achieved by introducing trapping sites such as dislocation and grain boundary in nuclear materials. In order to understand role of dislocations in helium irradiation effect in metal, positron annihilation spectroscopy and thermal desorption measurements were performed to study the interaction of helium atoms with dislocation in deformed Fe-9 wt.Cr alloys irradiated by 10 keV helium ions. The formation of irradiation defects and the thermal stability of helium-defect complexes are discussed. Analyzing the parameters of positron spectrum, the introduced dislocations will change the positron parameters of irradiated samples, which demonstrate that the dislocation has changed normal irradiation effects. Judging from thermal desorption spectrum, we can see the helium desorption peak of the deformed sample is ahead of well-annealed sample. [ABSTRACT FROM AUTHOR]

The evolution of defects with isochronal annealing between 373 and 1273 K in 316L stainless steels, irradiated by helium ions at room temperature (RT), has been studied by the slow positron Doppler broadening spectroscopy (DBS). The results showed that with annealing temperature between RT and 673 K, the S parameters of sample were maintained. It indicates that below 673 K, on the one hand, some vacancies introduced by irradiation were gathered to form vacancy clusters and on the other hand, some vacancies migrated and recovered. From 773 K to 1073 K, there was continuous growth of He n V m cluster, resulting in rapid increasing of S parameter. After 1073 K, unstable He n V m clusters dissociated into vacancies and He atoms. The vacancies were vanished rapidly, which decreases the concentration of vacancy defects, leading to decline of S parameter. [ABSTRACT FROM AUTHOR]

1 Irradiation defects in w and W-5wt% Re alloys were characterized by DBS,TEM and SEM. 2 Re restricted the behavior of vacancy-type defects in tungsten. 3 The surface irradiation damage such as crystal gap enlargement, grain shedding and pores were observed at different irradiation temperatures. 4 The formation mechanism of the coral-like structure and the Hilly-Gully structure was analyzed. W and W-5wt% Re alloy were irradiated by 50 keV helium ions at RT, 200 °C, 350 °C and 500 °C to investigate the influence of temperature on the microdefects and surface morphology in the sample. The microdefects in the subsurface region and the surface damage caused by irradiation were characterized by Doppler broadening spectroscopy (DBS) of slow positron beam, transmission electron microscope (TEM) and scanning electron microscopy (SEM), respectively. The results show that a large number of vacancy-type defects were produced after helium ions irradiation, especially at 500 °C, the vacancy-type defects of W alloy are higher than that of W-5wt% Re alloy, which indicated that Re restricted the behavior of vacancy-type defects in tungsten to some extent. The surface irradiation damage such as crystal gap enlargement, grain fell off and pores were observed at different irradiation temperatures. Compared with the coral-like structure observed in W, the circular objects and Hilly-gully structure were observed in W-5wt% Re alloy, and both of them increased and grew with the increase of irradiation temperature. It was speculated that the presence of Re led to the difference by analyzing the formation mechanism of the coral-like structure and the Hilly-gully structure. [ABSTRACT FROM AUTHOR]

The nucleation and growth of the helium bubble in the structure material usually were subjected to temperature in a thermodynamic environment. In this study, Y-bearing oxide dispersed strengthened (ODS) ferritic/martensitic steels were used as the research material. The Y-bearing ODS steel samples with a thickness of 30 microns were irradiated successively by helium ions with three energies of 300 keV, 400 keV and 550 keV at room temperature and 450°C respectively. Firstly, the evolution of defects caused by H e + irradiation is analyzed by using slow positron beam Doppler broadening spectroscopy (DBS) in positron annihilation spectroscopy technology (PAS). The formation and growth of helium bubbles are deduced from the changes in the concentration of vacancy-type defects. The increase in the size of the helium bubbles at high temperatures leads to a certain distortion of the crystal lattice, which is also verified by analyzing the grazing incidence X-ray diffraction (GIXRD) patterns. The characterization of radius and concentration of helium bubbles in samples irradiated at room temperature and 450°C have been studied by Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM). The results showed that the intensity (I) ~ q-vector (q) curves of samples vary with irradiation temperature over ranges of 0.13 ≤ q ≤ 4.6 nm−1, which is attributed to the growth of helium bubbles at different irradiation temperature. It is found that the average size of helium bubbles under room temperature irradiation is about 1.78 nm, and the size distribution is narrow, while at the irradiation temperature of 450°C, the size of helium bubbles increases to 2.9 nm, and the size distribution is wider. This result is also verified from the TEM analysis. Besides, the shape of helium bubbles was assumed roughly and tends to spherical with higher temperatures. [ABSTRACT FROM AUTHOR]

The spatial relationship between helium bubbles and dislocation loops was observed in detail by TEM in RAFM steel irradiated with 100 keV helium ion to a fluence of 1 × 1021 He+ • m−2 at 723 K. Two families of dislocation loops were observed, with b = ± <010> and b = ± <100>, and habit planes of (010) and (100), respectively. The striking observation is that all of the loops were decorated with helium bubbles. Meanwhile, bubbles are determined to be located inside the loop plane through the investigation using different g -vectors under zone axes of [111], [001] and [011]. [ABSTRACT FROM AUTHOR]

In order to study the microstructural evolution of the He + irradiated FeCrNi model alloy, 140 keV He + ions were implanted into the specimen with the fluence of 1 × 10 15 ions/cm 2 at room temperature. Positron annihilation spectroscopy was used to characterize the evolution of micro defects during isochronal annealing between 423 and 873 K. The decrease of the S parameter between 573 and 623 K has been associated to the annihilation of vacancy clusters, which were not trapped by helium atom. While the decline of the S parameter between 773 and 823 K in the damage region might be caused by the dissociation of He n V m clusters. The dissociated vacancy clusters were unstable and annihilated rapidly, which decreases the concentration of vacancy defects. [ABSTRACT FROM AUTHOR]