Radiation defects induced by helium implantation in gold-based alloys investigated by positron annihilation spectroscopy

The formation of gas bubbles in metallic materials may result in drastic degradation of in-service properties. In order to investigate this effect in high density and medium-low melting temperature (T _M) alloys, positron annihilation spectroscopy measurements were performed on helium-implanted gold–silver solid solutions after isochronal annealing treatments. Three recovery stages are observed, attributed to the migration and elimination of defects not stabilized by helium atoms, helium bubble nucleation and bubble growth. Similarities with other metals are found for the recovery stages involving bubble nucleation and growth processes. Lifetime measurements indicate that He implantation leads to the formation of small and over-pressurized bubbles that generate internal stresses in the material. A comprehensive picture is drawn for possible mechanisms of helium bubble evolution. Two values of activation energy (0.26 and 0.53 eV) are determined below and above 0.7T _M, respectively, from the variation of the helium bubble radius during the bubble growth stage. The migration and coalescence mechanism, which accounts for these very low activation energies, controls the helium bubble growth.     

Dependence of the ultrasound velocity on the acting stress during plastic flow of polycrystals

The dependence of the propagation velocity of ultrasound in polycrystalline materials on the magnitude of the stress acting during deformation is measured. It is established that the dependence has three stages, which are apparently due to the formation of dislocation ensembles of various characteristic sizes in the deformed material. Zh. Tekh. Fiz. 69, 100–101 (December 1999)     

Evolution of helium bubbles in nickel-based alloy by post-implantation annealing

Nickel-based alloys have been considered as candidate structural materials used in generation IV nuclear reactors serving at high temperatures. In the present study, alloy 617 was irradiated with 180-keV helium ions to a fluence of 3.6×10¹⁷ ions/cm² at room temperature. Throughout the cross-section transmission electron microscopy (TEM) image, numerous over-pressurized helium bubbles in spherical shape are observed with the actual concentration profile a little deeper than the SRIM predicted result. Post-implantation annealing was conducted at 700 ℃ for 2 h to investigate the bubble evolution. The long-range migration of helium bubbles occurred during the annealing process, which makes the bubbles of the peak region transform into a faceted shape as well. Then the coarsening mechanism of helium bubbles at different depths is discussed and related to the migration and coalescence (MC) mechanism. With the diffusion of nickel atoms slowed down by the alloy elements, the migration and coalescence of bubbles are suppressed in alloy 617, leading to a better helium irradiation resistance.     

Effect of the saturation conditions and structure on the retention of helium in structural materials

A comparison between the kinetics of helium desorption upon linear heating of samples saturated using various regimes is performed, and the effect of dislocations on the retention of helium in materials is estimated. In order to investigate the effect of the conditions of saturation of materials with helium on its retention, samples of austenitic stainless steel 0Kh16N15M3B saturated using various methods were studied, namely, helium irradiation in a cyclotron, in a magnetic mass-separation setup, inside IRT-2000 and BOR-60 reactors, and using the so-called “tritium trick” technique. The investigations show that when saturation of the samples with helium is accompanied by the introduction of radiation defects (in wide limits of helium concentrations and radiation damage), the kinetics of helium evolution from samples of this type is adequate to the kinetics of its evolution from samples irradiated in a reactor. The investigation of the kinetics of helium evolution from the samples of 0Kh16N15M3B steel both after a preliminary deformation and in the process of deformation showed that, in the process of heating, the helium atoms can migrate along dislocation pipes, resulting in a significant effect on the release of helium and its redistribution in the volume of the material. The activation energy for helium pipe diffusion in austenitic steel 0Kh16N15M3B is about 0.7 eV. Mobile dislocations favor the ejection of helium onto the surface of the material, to grain boundaries, interphase interfaces, etc.     

α-Fe中氦泡极限压强的分子动力学模拟

为了深入认识α-Fe中氦泡冲出位错环的微观机制,有必要研究α-Fe中氦泡冲出位错环时的极限压强特性.本文建立金属-氦泡的立方体型代表性体积单元模型,针对8种不同初始半径的球形氦泡,以初始氦空位比为变量,开展分子动力学模拟,得到了各模型中位错环开始形成时的氦泡极限压强和临界氦空位比.研究结果表明:对于无量纲半径介于2—10的氦泡,冲出位错环时的氦泡极限压强和临界氦空位比均随着氦泡初始半径的增大而非线性减小;基体中氦泡冲出位错环时的临界氦空位比具有明显的尺寸效应;初始时刻(0 ps),在经过立方体型模型中心的横截面上,氦泡周围Fe原子阵列的剪应力集中和最大剪应力出现在对角线与氦泡边界交点(即45°)处,并且关于横截面上平行于边的两条对折线对称分布,剪应力集中区的范围和最大剪应力均随着初始氦空位比的增大而增大;位错环冲出方向对应最大剪应力方向.本文的研究加深了对金属中氦泡物理特性的认识,为后续分析氦泡对材料宏观物理和力学性质的影响奠定了有益的基础.     

Plasticization effect and the excitation of the electronic subsystem in an LiF single crystal by an ultraweak thermalized-neutron flux in the stress-relaxation regime

Plasticization is detected during stress relaxation in uniaxially loaded LiF single crystals irradiated with an ultraweak flux of thermalized neutrons (UFTN) with intensity I _n∼100 neutrons/cm²s. It is shown that when loaded LiF samples are irradiated with an UFTN, excitation of the electronic subsystem of the crystal is observed and is manifested in a stimulation of deformation exoemission of electrons and the generation of F centers. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 118–123 (25 July 1999)     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

Nanoscratch behavior of multi-layered films using molecular dynamics

Molecular dynamics simulations are performed to study the plastic deformation, stress and chip formation of scratched multi-layered films. The results showed that stick–slip and work-hardening behaviors were observed during the scratching process. There was a pile-up of amorphous disordered debris atoms and shear rupture ahead of the probe and a clear side-flow on the lateral sides of the probe when the probe moved forward. Both the plastic energy and the adhesion increased with an increase in the scratching depth. The glide band of the interface was on the {111}〈110〉 slip system with a maximum width of the glide band of about 1 nm. The strain energy stored in the deformed structure caused a higher stress region in the material in front of the tool edge, with a maximum stress of about 10 GPa. In addition, the mechanical response and thermal softness phenomenon are discussed. PACS 02.70.Ns; 46.55.+d; 47.11.Mn; 91.55.Ax; 62.40.+i     

Negative ions Ar<Superscript>−</Superscript>, Kr<Superscript>−</Superscript>, and Xe<Superscript>−</Superscript> in superfluid helium

The catalogue of negative ions in superfluid helium has been extended using the example of Ar⁻, Kr⁻, and Xe⁻. Such objects cannot exist in vacuum, since the polarization attraction of an electron to the inert A atom is insufficient for the formation of the bound state A⁻. However, these objects exist in helium as stable or metastable with a very long lifetime. The effect is due to the electron localization in liquid helium. If a mixture of excited A* atoms and electrons is prepared in the gas phase above liquid helium, the reaction A* + e = A*⁻ becomes possible for all atoms of the periodic table. Such charges can be immersed into liquid helium by the electric field. In this case, the radiative decay A*⁻ = A + e allowed in vacuum can be forbidden in liquid. This leads to the formation of the new unique objects A⁻, which can exist in liquid helium but are absent in nature. The size of such charged formations has been determined and is close the radius of a usual electron bubble in helium.     

金属表面下氦泡融合与释放研究

采用分子动力学方法研究了钛金属表面下不同深度处氦泡的行为,分析了氦泡融合与释放的竞争,对比了不同深度处氦泡的释放对金属的影响。结果表明：在接近金属表面处,氦泡很难通过融合无限长大,当达到临界尺寸后,氦泡将会释放而不再与邻近的氦泡发生融合;植入深度对氦泡的融合有一定的影响,深度越大,越有利于形成具有较高氦密度的大氦泡;较深处氦泡的释放会在金属表面形成较大的突起和表面针孔。实验中观察到的不同尺寸的表面孔,其部分原因来自于金属表面下不同深度处氦泡的释放。     

金属表面下氦泡融合与释放研究

采用分子动力学方法研究了钛金属表面下不同深度处氦泡的行为,分析了氦泡融合与释放的竞争,对比了不同深度处氦泡的释放对金属的影响.结果表明:在接近金属表面处,氦泡很难通过融合无限长大,当达到临界尺寸后,氦泡将会释放而不再与邻近的氦泡发生融合;植入深度对氦泡的融合有一定的影响,深度越大,越有利于形成具有较高氦密度的大氦泡;较深处氦泡的释放会在金属表面形成较大的突起和表面针孔.实验中观察到的不同尺寸的表面孔,其部分原因来自于金属表面下不同深度处氦泡的释放.     

Negative Ca− and Ba− ions of large radius on the surface and in the volume of liquid helium

Negative Ca⁻ and Ba⁻ ions of large radii on the surface of and in bulk liquid helium have been studied. Our results indicate that these ions are adsorbed on the helium surface. Ions on free liquid helium surfaces have not been studied previously because it was thought impossible to confine them on the surface. Ca⁻ and Ba⁻ ions have very low binding energies, therefore, like electrons, they form a bubble of large radius in bulk helium, whose energy is higher than on the surface. The behavior of ions on the surface exhibits a number of previously unknown features owing to their large masses and strong localization in the horizontal plane. Even in the absence of confining electric field, a hole is formed under an ion due to the polarization attraction between the liquid helium and the charged ion. This hole formation reduces the ion mobility by several orders of magnitude and increases its effective mass severalfold. The critical density of electrons and ions is approximately the same on the surfaces of thin and thick helium films. Zh. éksp. Teor. Fiz. 115, 593–604 (February 1999)     

聚变堆候选金属材料的惰性气体离子辐照损伤的研究

综述了有关核聚变反应堆材料的辐照损伤问题的研究,主要包括国产316L奥氏体不锈钢中氦的扩散与氦泡形核生长的研究、316L及低活化FeCrMn合金的高能Ar离子辐照缺陷与空洞肿胀的研究、近期开展的低活化马氏体钢和氧化物颗粒弥散强化合金的高能Ne离子辐照损伤和效应的研究成果。This paper gives a review of our recent studies on the irradiation damage induced by energetic inert-gasions in metallic materials candidate to fusion reactors. The work includes the study of helium diffusion and helium bubble formation in 316L stainless steels, the study of void formation and swelling in the low-activation Fe-Cr-Mn alloy irradiated with high-energy Ar ions, the study of irradiation damage in some low-activation Fe-based steels and ODS alloys by high-energy Ne ions.     

Evolution of dislocation structure during deformation of γ-irradiated LiF crystals

The effect of γ irradiation on the mechanical characteristics and dislocation structure of slip bands in LiF crystals is studied at doses D⩽7.3×10⁸ R. Irradiation causes a substantial increase (up to a factor of 30) in the yield stress τ _y of the crystals, with τ _y ∝ D ^0.4 in the first approximation. The deformation shear increases in the slip bands of irradiated crystals, as do the densities of the screw and edge dislocation components, while the dislocation mean free paths decrease. Irradiation also raises the probability of twinning cross slip for screw dislocations. The observed effects are assumed to be related to the formation of a different kind of defects in the irradiated crystals, primarily clusters of implanted atoms. Fiz. Tverd. Tela (St. Petersburg) 39, 1072–1075 (June 1997)     

Spall in Aluminium with Helium Bubbles under Laser Shock Loading

The spallation behaviors of AI+0.2 wt% ~(10)B targets and neutron irradiated AI+0.2 wt% ~(10)B targets with 5 nm radius helium bubble subjected to direct laser ablation are presented. It is found that the spall strength increases significantly with the tensile strain rate, and the helium bubble or boron inclusions in aluminum reduces the spall strength of materials by 34%. However, slight difference is observed in the spall strength of unirradiated samples compared with the irradiated sample with helium bubbles.     

Kinetics of dislocation ensembles in deformable irradiated materials

The development of plastic instability in the initial deformation stages of irradiated materials is studied. The dependence of the fraction of dislocations which overcome obstacles in the dynamic regime (dislocation “channeling”) on the degree of radiation hardening (irradiation dose) and the dislocation velocity is analyzed. It is shown that this effect plays a role in radiation embrittlement of reactor materials. Fiz. Tverd. Tela (St. Petersburg) 40, 1631–1634 (September 1998)     

Resistance to dynamic deformation and fracture of tantalum with different grain and defect structures

This paper presents the results of measurements of the strength properties of technically pure tantalum under shock wave loading. It has been found that a decrease in the grain size under severe plastic deformation leads to an increase in the hardness of the material by approximately 25%, but the experimentally measured values of the dynamic yield stress for the fine-grained material prove to be less than those of the initial coarse-grained specimens. This effect has been explained by a higher rate of stress relaxation in the fine-grained material. The hardening of tantalum under shock wave loading at a pressure in the range 40–100 GPa leads to a further increase in the rate of stress relaxation, a decrease in the dynamic yield stress, and the disappearance of the difference between its values for the coarse-grained and fine-grained materials. The spall strength of tantalum increases by approximately 5% with a decrease in the grain size and remains unchanged after the shock wave loading. The maximum fracture stresses are observed in tantalum single crystals.     

Experimental investigation of bubble occurrence and locality distribution of bubble detectors bombarded with high-energy helium ions

Large-sized bubble detectors with microscopic droplets of superheated liquids of dichlorodifluoromethane (Freon-12), dichlorotetrafluoroethane (Freon-114), tetrafluoroethane (Freon-134a), and mixture of Freon-12 and Freon-114, respectively, were irradiated with 150 MeV/amu helium ions at the HIMAC accelerator in NIRS, Chiba, Japan. Distributions of bubbles produced by the helium ions have been studied in each type of the detectors. The origin of the bubbles has been investigated. The detection efficiency of each type of the bubble detectors for helium ions with respect to the energy of the ions has been obtained. The phenomenon of bubble occurrence and its possible applications to the determination of He intensity from accelerators, research of track formation mechanism, energy loss straggling and neutron detection in the space and at higher altitude are discussed.     

Emission of Dislocation Loops from Nanovoids in an FCC Crystal Subjected to Shear Deformation under Post-Cascade Shock Waves

The method of molecular dynamics is applied to the study of the effect of post-cascade shock waves generated in a solid irradiated by high-energy particles on the heterogeneous formation of dislocation loops in a simulated gold crystal containing a spherical nanovoid, which is subjected to shear deformation. The interaction between atoms is described with the use of a potential calculated by the embedded atom method. Shock waves are created by assigning a velocity exceeding the speed of sound in the simulated material to the boundary atoms of the computational cell. It is shown that two regions of increased mechanical stress are formed under shear deformation near the surface of a nanovoid, which are the sources of emerging partial dislocations. The main mechanism for the formation of dislocations is the displacement of a group of atoms towards the inner surface of the void, which does not contradict modern ideas about the heterogeneous formation of dislocations. It is shown that, when the values of shear stress are insufficient for the formation of dislocations, loop emission can be initiated by a post-cascade shock wave generated in the computational cell. As temperature increases, the number of nucleated dislocation loops increases, and, in addition, the formation of Lomer–Cottrell dislocations is observed, which is attributed to the additional tangential stresses created by the unloading wave. In this case, the formation of a stable dislocation loop in which the linear tension is balanced by the Peach–Koehler force due to external stress requires that the shock wave front affect the regions of increased stress near the void surface while propagating through the simulated crystal.