Effect of irradiation at low doses and incubation of voids within the rate theory approach

The evolution of defects in a material under irradiation is studied at low doses (∼5 dpa or less) using rate equations. It is shown that as a function of temperature at a critical valueT _c a transition occurs in the behaviour of the solutions of the rate equations. BelowT _c the voids show incubation effects. An expression is derived for the critical dislocation density at which the void growth starts. This is related to the trapped vacancy fraction ε in vacancy dislocation loops. AboveT _c the incubation effects are shown to be related to the gas production rate which becomes the rate controlling parameter in determining the evolution of the defects. A gas-bubble to void transition occurs at a critical void radius and expressions are derived for the critical void size and dose at which the transition appears. It is shown that closely related to this is the incubation dose for interstitial loops. Finally, these features are corroborated by actual numerical integration of the rate equations.     

The effects of irradiation temperature and preinjected gases on voids in aluminum

Voids in high purity aluminum irradiated to a fast (E>1 MeV) fluence of 4 × 10²⁰ n/cm² at 125 (0.43T _m) and 150°C (0.45T _m) are fewer in number but very much larger in size than those in material irradiated at 55°C (0.35T _m). Additionally, at 125 and 150°C, the voids adopt a variety of shapes including plates, ribbons, cylinders and more equiaxed polyhedra, and are frequently associated with particles of transmutation-produced silicon. At the higher temperatures voids are larger near grain boundaries than in grain interiors. Injection of hydrogen or helium prior to irradiation causes an increase in the number of voids and a corresponding decrease in size in specimens irradiated at 150°C; 3 at. ppm He is more effective than either 3 or 9 at. ppm H. The gases do not appear to influence swelling.

A commercial purity (99 per cent) aluminum subjected to the same irradiation treatments did not develop voids whether preinjected with gases or not; the visible radiation damage consisted solely of small loops on or near grown-in dislocations.     

Mechanism of recovery of the mechanical properties of neutron-irradiated metals in the course of thermocycling

The mechanism of recovery of the strength and strain characteristics of neutron-irradiated metals and alloys (up to their initial values observed prior to irradiation) during periodic quenching in the temperature range below the irradiation temperature is considered. It is assumed that the removal of radiation-induced defects (interstitial and vacancy loops) from radiation-hardened metals is associated with the formation of defect-free channels along the slip planes (the phenomenon of dislocation channeling) under the thermal stresses arising in each cycle of quenching. The relationships describing both the kinetics of the decrease in the yield strength σ_Y and in the ultimate strength σ_U and the kinetics of the increase in the uniform strain ε_U of a preliminarily irradiated material with increasing number of quenching cycles are derived using the equations of dislocation kinetics. The theoretical results obtained are compared with experimental data on the kinetics of recovery of the mechanical properties of neutron-irradiated samples (the austenitic FeNiCr and ferritic FeCrMo structural steels and the titanium alloy TiAlZr) in the course of periodic quenching.     

Neutron radiation damage in zirconium and its alloys

There has been much research and speculation recently on the nature of radiation induced defects in zirconium and its alloys, and in particular on the absence of voids at high fluences and temperatures in the range 0.3 to 0.5 T _m (T _m is the absolute melting temperature). Wolfenden and Farrell¹ have reviewed the evidence and suggest that α-Zr has so far resisted void formation during neutron irradiation because of: (a) the absence of a dislocation (loop or tangle) structure and/or (b) a low insoluble gas (e.g. helium) content.     

Positron-annihilation investigation of high-temperature neutron-irradiated molybdenum

Molybdenum samples have been irradiated in EBR II at temperatures ranging from 600 to 1500°C. Both lifetime and Doppler-broadening measurements of the positron annihilation characteristics have been performed and the effects of voids on these measurements determined. Good correlation of both measurements with the suspected swelling behavior has been obtained consistent with a peak in the swelling between 0.3 and 0.4 T_m, where T_m is the melting-point temperature. These positron techniques appear to be a much more sensitive probe of void swelling than other investigative methods. At temperatures greater than 600°C, a change in the positron lifetime is attributed to the possible arrival of impurities at the void surface. A third, as yet unresolved lifetime component, appears to be present that causes the values of τ₁, the shorter lifetime associated with the free-lattice annihilations, to be too large relative to an annealed sample. A possible explanation is the presence of radiation-induced dislocation loops that can act as additional positron trapping sites.     

Void formation in Nickel during high temperature proton irradiation

Pure Ni foils, doped with He from 0 to 28 appm, were irradiated with protons at temperatures in the range 0.3–0.6 T_m (T_m = melting point in °K) and void formation was studied. The influence of He doping, irradiation temperature and alloying were investigated. For constant He content and proton fluence, void number density and swelling are maximum at about 400°C, while the void size increases with temperature. Most voids are octahedral in shape with no sign of truncation. Helium is required to nucleate voids, and lowering the stacking fault energy by alloying suppresses void formation completely. Present results suggest that void nucleation is inhomogeneous. Some implications of these findings are discussed.     

空位位错环的形成

本文研究了淬火面心立方金属内位错环和空洞的形核和长大问题。两种核都能形成,但是位错环的长大速度比空洞的长大速度大很多倍,故电子显微镜能看清的大缺陷都是位错环,而不是空洞。     

Characterization of dislocation interactions in olivine using electron tomography

We have investigated by electron tomography, in a transmission electronic microscope, the interactions between dislocations in olivine single crystals and polycrystals deformed in axial compression at T < 1000 °C (T < 0.5T_m). Dislocations are mostly of the [0?0?1] type, except in the polycrystal where [1?0?0] and [0?0?1] dislocations have been activated. A few 〈1?0?1〉 junctions have been found and characterized. Many collinear interactions have been identified either involving direct interactions between crossing dislocations of opposite Burgers vectors or indirect interactions between dislocations gliding in parallel planes and sessile dislocation loops. We suggest that collinear interaction, already identified as the primary source of strain hardening in FCC metals, is the main dislocation interaction mechanism in olivine deformed at temperatures below 1000 °C.     

Defect aggregates in neutron-irradiated β-Si3N4

Abstract

Microstructural analysis of the defect aggregates formed in bulk samples of polycrystalline β-Si₃N₄ neutron-irradiated to a dose of ～2.0 × 10²⁶n/m² at temperatures of 1100 K and 925 K has been carried out. This study has shown that the defect aggregates formed are faulted dislocation loops lying on the {1010} planes with a Burgers vector of b ? 1 /10<1125>. The vector is non-rational but corresponds to the insertion of an extra layer of [SiN4] tetrahedra on the {10l0} planes plus an additional shear in the loop plane. The formation of these loops is dependent upon the temperature of irradiation. In the sample irradiated at 1100 K their formation is additionally dependent upon whether or not a particular grain contains pre-existing c-axis dislocations. If no c-axis dislocations are present then independent nucleation of the loops is apparent; if there are pre-existing c-axis dislocations then the loops form from an apparent dissociation between the arcs of the irradiation-induced helical c-axis dislocation. In the sample irradiated at 925 K only independent nucleation of the loops occurs, regardless of whether or not there are any pre-existing c-axis dislocations in the grains.     

强流脉冲电子束辐照诱发金属纯镍中的空位簇缺陷

利用强流脉冲电子束(high-current pulsed electron beam,HCPEB)技术对多晶纯Ni进行了辐照处理,采用透射电子显微镜详细分析了辐照诱发的缺陷结构.HCPEB辐照后,纯镍表层积聚了幅值极大的残余应力,沿{111}晶面形成了稠密的位错墙及孪晶结构,另外还形成了大量的包括位错圈、堆垛层错四面体(SFT)及孔洞在内的空位簇缺陷.SFT缺陷的数量远高于其他空位簇缺陷,其周围区域位错密度很低.孔洞缺陷主要出现在SFT密集区域.HCPEB瞬间的加热和冷却诱发的幅值极大的应力和极高的应变 关键词： 强流脉冲电子束 多晶纯Ni 空位簇缺陷 堆垛层错四面体     

Evolution of dislocation loops in austenitic stainless steels implanted with high concentration of hydrogen

Abstract

It has been found that under certain conditions, hydrogen retention would be strongly enhanced in irradiated austenitic stainless steels. To investigate the effect of the retained hydrogen on the defect microstructure, AL-6XN stainless steel specimens were irradiated with low energy (100 keV) H₂⁺ so that high concentration of hydrogen was injected into the specimens while considerable displacement damage dose (up to 7 dpa) was also achieved. Irradiation induced dislocation loops and voids were characterised by transmission electron microscopy. For specimens irradiated to 7 dpa at 290 °C, dislocation loops with high number density were found and the void swelling was observed. At 380 °C, most of dislocation loops were unfaulted and tangled at 7 dpa, and the void swellings were observed at 5 dpa and above. Combining the data from low dose in previous work to high dose, four stages of dislocation loops evolution with hydrogen retention were suggested. Finally, molecular dynamics simulation was made to elucidate the division of large dislocation loops under irradiation.     

Temperature dependence of positron annihilation parameters in neutron irradiated molybdenum

Positron lifetime measurements have been performed for molybdenum samples containing different densities of voids and dislocation loops. The samples consisted of single crystal molybdenum exposed to 2.7×10¹⁸ fast neutrons/cm₂ at 60°C, and subsequently annealed at 650°, 725°, 800°, and 875°C in vacuum (p<10^–7 Torr). After each annealing, where the densities of voids and loops were changed, positron lifetime measurements were performed in the temperature interval [–194°, 285°C]. In two-term fits of the measured spectra the longer lifetime, _e2-460 ps corresponds to an intensityI _e2 increasing with sample temperature. The shorter lifetime _e1 decreases with increasing temperature. A three-state trapping model with and without detrapping is discussed, and appears to be incapable of explaining the observed temperature dependences. A four-state positron trapping model including detrapping is necessary and satisfactory. It describes positron trapping to voids and trapping to dislocation loops, which is followed by a competition between detrapping and positron transition to jogs or other dislocation-bound defects. Mathematical expressions of the four-state trapping model including detrapping are worked out and calculations of the intensityI _e2 are compared with the experimental values ofI _e2. By use of special models for the temperature dependence of trapping rates, numerical values can be determined for the positron-dislocation-binding energy and for specific positron trapping rates.     

The effect of phason defects on the radiation-induced swelling of quasicrystalline materials

The radiation-induced vacancy swelling of quasicrystals is considered. In quasicrystals, the dislocation kinetics involves the formation of phasons: vacancy- and interstitial-like localized topological defects. Phasons interact with radiation-induced point defects (PD; vacancies and interstitials) and influence the swelling behavior of quasicrystals. After absorption of vacancies, phasons of the interstitial type transform into phasons of the vacancy type and vice versa. In other words, phasons are the recombination centers of alternating polarity for PD. Assuming that (i) the production rate of phasons is proportional to the dislocation climbing rate and (ii) voids are the sinks for mobile phasons, the set of rate equations for PD and phasons is formulated and analyzed both analytically and numerically. It is shown that the swelling rate is lower in quasicrystals compared with crystals. Growth rates of loops and voids, as well as the swelling rate, strongly depend on the phason concentration which is controlled by phason diffusion to sinks.     

Analysis of the temperature dependence of the heat capacity of solid tin in the vicinity of its melting temperature

Summary The experimental temperature dependence of the heat capacityC _p(T) of solid tin (Sn) in its premelting region 402.07–502.22 K was investigated and two regions with different temperature dependences ofC _p(T) were found. In the far (from the melting temperatureT _m) region 402.07–485.88 K the experimentalC _p(T) of Sn is described by the standard vacancy model. In the close region 485.88–502.22 K it is described by the formation of complicated volumetric defects in the crystalline lattice of solid Sn near itsT _m.     

Theory of melting,vacancy model

A theory of melting based on vacancy model is formulated. The polymer solution theory is used for derivation of the melting equation for a two-species model of melting solid. Under simplifying assumptions the analysis leads to a simple correlation betweenT _m and 〈v〉, the average energy of interaction between the vibrating atoms. Pseudopotential method is used for calculating 〈v〉 for the alkali metals lithium, sodium, potassium and rubidium at temperatureT _m. The calculated values ofT _m〈v〉 are in accord with those expected from our model. Application to the high pressure melting curves of solids is also discussed.     

Effects of stacking fault energy on defect formation process in face-centered cubic metals

To elucidate the effect of stacking fault energies (SFEs) on defect formation by the collision cascade process for face-centred cubic metals, we used six sets of interatomic potentials with different SFEs while keeping the other properties almost identical. Molecular dynamic simulations of the collision cascade were carried out using these potentials with primary knock-on atom energies (E_PKA) of 10 and 20 keV at 100 K. Neither the number of residual defects nor the size distributions for both self-interstitial atom (SIA) type and vacancy type clusters were affected by the difference in the SFE. In the case of E_PKA = 20 keV, the ratio of glissile SIA clusters increased as the SFE decreased, which was not expected by a prediction based on the classical dislocation theory. The trend did not change after annealing at 1100 K for 100 ps. For vacancy clusters, few stacking fault tetrahedrons (SFTs) formed before the annealing. However, lower SFEs tended to increase the SFT fraction after the annealing, where large vacancy clusters formed at considerable densities. The findings of this study can be used to characterise the defect formation process in low SFE metals such as austenitic stainless steels.     

超临界水冷堆燃料包壳材料的辐照损伤研究进展

超临界水冷堆（SCWR）是第四代核电站的主力堆型之一,高温、高压、超临界水环境下的辐照损伤问题是其燃料包壳材料面临的最大挑战。SCWR燃料包壳候选材料主要包括锆合金、奥氏体不锈钢、铁素体/马氏体不锈钢、镍基合金、ODS合金五大类,奥氏体不锈钢是最有希望的候选材料。介绍了近年来在这个领域国际上的主要研究进展。作者所在团队也对多种SCWR的候选材料进行了辐照损伤研究,包括:镍基合金C-276和718、铁素体/马氏体钢P92、奥氏体不锈钢AL-6XN和HR3C。对AL-6XN的氢离子辐照实验发现,辐照产生的缺陷主要是间隙型位错环,伯格斯矢量为1/3<111>,在较高剂量（5～7 dpa）辐照下,出现空洞肿胀。在氢滞留的影响下,位错环有着独特的演化规律,总结提出了位错环的四阶段演化过程。The Supercritical Water-cooled Reactor (SCWR) is one of the prior Generation IV advanced reactors. Irradiation damage is one of the key issues of fuel cladding materials which will suffer serious environment, such as high temperature, high pressure, high irradiation and supercritical water. The candidate materials contain zirconium alloys, austenitic stainless steels, ferritic/martensitic stainless steels, Ni-base alloys and ODS alloys. Austenitic stainless steels are the most promising materials. This paper summarized the international researches on irradiation effects in fuel cladding materials for SCWR. The group of authors also has done many researches in this field, including nickel-base alloy C-276 and 718, ferritic/martensitic steel P92 and austenitic stainless steel AL-6XN and HR3C. In AL-6XN austenitic stainless steels irradiated by hydrogen ions, dislocation loops were the dominant irradiation defects. At higher irradiation dose (5～7 dpa), the voids were found. All the dislocation loops were confirmed to be 1/3<111> interstitial type dislocation loops, and four evolution stages of dislocation loops with hydrogen retention were suggested.     

Positron studies of hydrogen-defect interactions in proton irradiated molybdenum

Molybdenum single crystals are irradiated at 20 K with 6 MeV protons. The radiation damage and lattice defect annealing is studied by positron lifetime spectroscopy in the temperature range from 15 to 720 K. Loss of vacancies due to recombination with mobile interstitials is observed at 40 K (Stage I) in agreement with resistivity measurements. This is the first time Stage I is observed by positrons below 77 K. The implanted hydrogen decorates the vacancies around 100 K, which is consistent with a hydrogen migration energy in molybdenum:E _M ^H = 0.3–0.4 eV. Clustering of spatially correlated vacancies takes place in a wide temperature region below the usual vacancy clustering stage (Stage III). Stage III is observed at rather low temperatures (400–480 K) due to the very high vacancy concentration. Hydrogen bound to vacancies and vacancy clusters is released above 540 K, which puts an upper limit to the hydrogen binding energy:E _B ^H 1.4 eV. The present work emphasizes the advantage of employing a vacancy sensitive technique to study hydrogen in metals, where its intrinsic solubility is low. In such metals (as molybdenum) both the effective solubility and the effective mobility of hydrogen are strongly influenced by the presence of vacancies.     

On stage V recovery in VIa b.c.c. metals

Abstract

A resistometric study at 4.2 °K of the high-temperature recovery spectrum of neutron-irradiated molybdenum as a function of fast fluence and of cold-worked molybdenum as a function of the heating program, yields evidence against vacancy migration in the 0.31 T_m stage for VIa b.c.c. metals.     

高温电子辐照硅中缺陷的研究

本文对高温电子辐照硅中产生的缺陷进行了研究,发现缺陷的引进率随电子辐照温度的增加而增加,在达到极值温度T_m后,缺陷的引进率将随之而下降,T_m值与缺陷的退火激活能有关。E₃缺陷(E_c—0.36eV)浓度在高温电子辐照中显著增加,在330℃高温电子辐照时,E₃缺陷浓度为室温电子辐照的6倍。研究结果表明,E₃缺陷的可能结构为与多空位和氧有关的复合体。 关键词：