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.     

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.     

冲击加载下孔洞贯通的微观机理研究

利用分子动力学方法计算模拟了沿〈100〉晶向冲击加载下单晶铜中双孔洞的贯通过程.发现孔洞周围发射剪切型位错环是孔洞塌缩和增长的原因.在拉伸阶段,孔洞首先分别独立增长,随后其周围塑性变形区开始交叠和相互作用,最后两个孔洞开始直接贯通.这种贯通模式和实验对延性材料中孔洞贯通过程的显微观察结果一致.对四种不同θ值（θ为两个孔洞中心连线与冲击加载方向之间的夹角）的模型分别进行了计算模拟,发现在相同的冲击加载强度下,θ＝0°和θ＝30°的孔洞之间没有相互贯通; 关键词： 纳米孔洞 分子动力学 冲击加载 贯通     

Simulation of PD patterns due to a narrow void in different E-field distribution

For simulation of Partial Discharge (PD) patterns, a Finite Difference Method (FDM) based model is put forward, in which three significant PD parameters are incorporated, viz. the critical field intensity for partial discharge occurrence E_c, residual field intensity E_r and locations of voids inside the dielectric. The void taken in this simulation is a narrow rectangular parallelepiped, i.e. the length and breadth of the void (in the direction normal to the field) is less compared to the height (in the direction of the field) of the void. In this paper, three different electrode systems are considered, viz. Plane–Plane, Point–Plane and Point–Point. The supply voltage is considered to be sinusoidal in nature. PD simulations for different locations of voids inside the dielectric for these three configurations are carried out considering occurrence of PD and residual field to be stochastic in nature for a constant E_c. The inception and extinction of PD, effect of applied voltage on PD, shape of PD patterns in relation to instantaneous released charge amplitude, and the amount of charge which is released due to PDs during a course of time, are studied and reported in this paper.     

Fluoroethylenepropylene ferroelectrets with patterned microstructure and high, thermally stable piezoelectricity

Layered fluoroethylenepropylene (FEP) ferroelectret films were prepared from sheets of FEP films by template-patterning followed by a fusion-bonding process and contact charging. The layered ferroelectret films show consistency and regularity in their void structures and good bonding of the layers. For films composed of two 12.5???m thick FEP layers and a typical void of 60???m height, the critical voltage necessary for the built-up of the ??macro-dipoles?? in the inner voids is approximately 800 V. At room temperature, Young??s modulus in the thickness direction, determined from dielectric resonance spectra of the fabricated films with a typical thickness of 85???m, is about 0.21 MPa. Initial quasistatic piezoelectric d ₃₃ coefficients of samples contact charged at a peak voltage of 1500 V are in the range of 1000?C3000 pC/N. From these, ferroelectrets with high quasistatic and dynamic (up to 20 kHz) d ₃₃ coefficients of up to 1000 pC/N and 400 pC/N, respectively, which are thermally stable at 120°C, can be obtained by proper annealing treatment. This constitutes a significant improvement compared to previous results.     

Void content and interfacial properties of composite laminates under different autoclave cure pressure

With an identical temperature profile, various cure pressures were applied to determine the effect of cure pressure on void distribution and interlaminar shear strength (ILSS) of [0]₁₀ T800/X850 composite laminate. Void shape, distribution, and void content within the composite laminates were characterized using optical microscope. The ILSS was evaluated using short-beam three-point bending tests and their interface debonding failure and fracture surfaces were determined using scanning electron microscope. The experimental results indicated that long strips voids are generated in the low-pressure curing stage. The voids mainly exist in the two forms of rod-like shape and spherical shape, and their number and size decrease as the cure pressure increases. The influence of cure pressure on the void content and ILSS shows a different trend in two stages: when the cure pressure is lower than 0.4 MPa, the ILSS decrease by 5.21% with 1% increase in void content, and their relationship is inversely linear. However, when the cure pressure is higher than 0.4 MPa, the void content is less than 1% and ILSS increase slightly by 1.69% when the cure pressure goes from 0.4 to 0.6 MPa, the influence of cure pressure on void content and ILSS is clearly presented.     

A boundary-value problem for dielectric spherical void in a homogeneous insulator by considering surface conductivity

In this paper, the boundary-value problem satisfying the Dirichlet condition has been solved for the case of a dielectric, spherical void inside a homogeneous insulator by considering surface conductivity. Mathematical equations have been derived to calculate the respective electric fields E_i(t)and E_e(t) inside and outside spherical gas voids that exist within an insulator by considering the surface conductivity γ_s of gas voids having an electric permittivity of ε_i and conductivity γ_i under DC (ωτ⪡1) and AC (ωτ⪢1) conditions. The expression for the dipole moment of a polarized spherical dielectric particle has been obtained in accordance with the determined electric field E_i(t). The derived expressions are then applied to calculate reciprocal interaction force between the spherical particle and metallic electrodes, which is very considerable for the dielectric separation processes in the DC and AC cases.     

Piezoelectric performance of fluor polymer sandwiches with different void structures

Film sandwiches, consisting of two outer layers of fluoroethylenepropylene and one middle layer of patterned porous polytetrafluoroethylene, were prepared by patterning and fusion bonding. Contact charging was conducted to render the films piezoelectric. The critical voltage to trigger air breakdown in the inner voids in the fabricated films was investigated. The piezoelectric d ₃₃ coefficients were measured employing the quasistatic method and dielectric resonance spectrum. The results show that the critical voltage for air breakdown in the inner voids is associated with the void microstructure of the films. For the films with patterning factors of?0%, 25% and?44%, the critical values are?300, 230?and 230?kV/cm, respectively. With an increase in the patterning factor, both the piezoelectric d ₃₃ coefficients determined from the dielectric resonance spectra and those determined from quasistatic measurements increase, which might be due to a decrease in Young??s modulus for the films. The nonlinearity of d ₃₃ becomes increasingly obvious as the patterning factor increases.     

Positron annihilation study of voids in a neutron irradiated aluminum single crystal

We have measured the lifetimes of positrons in an aluminum single crystal which was irradiated to a fast neutron fluence of 1.5·10²¹ n/cm² (>0.18 MeV) at 50°C. These irradiation conditions produced 4.2·10¹⁴ voids/cm³ with a mean diameter of 330 Å, as determined by both small-angle x-ray scattering and transmission electron microscopy. The positron lifetime spectra were resolved into three lifetime components of 100, 300, and 500 ps. The short lifetime component is a result of fast trapping of positrons by the voids; the long lifetime components result from annihilations within the voids. The intensity of the long lifetime components increases with temperature in the range 80 to 300 K and supports the model of a positron state at the void surface. The positron diffusion coefficient appears to have aT ^1/2-dependence. A magnetic quenching experiment shows no indication of positronium formation. Finally, an isochronal heat treating sequence shows that the voids anneal out between 200 and 300°C, and that the lifetime spectrum after annealing is described by a single component of 170 ps, the observed lifetime in unirradiated aluminum.     

In situ TEM study of stability of TaRhx diffusion barriers using a novel sample preparation method

The atomic diffusion mechanisms associated with metallurgical failure of TaRh_x diffusion barriers for Cu metallizations were studied by in situ transmission electron microscopy (TEM). The issues related to in situ heating of focused ion beam (FIB) prepared cross-sectional TEM samples that contain Cu thin films are discussed. The Cu layer in Si/(13 nm)TaRh_x/Cu stacks showed grain growth and formation of voids at temperatures exceeding 550 °C. For Si/(43 nm)TaRh_x/Cu stacks, grain growth of Cu was delayed to higher temperatures, i.e., 700 °C, and void formation was not observed. Extensive surface diffusion of Cu, however, preceded bulk diffusion. Therefore, a 10 nm film of electron beam evaporated C was deposited on both sides of the TEM lamellae to limit surface diffusion. This processing technique allowed for direct observation of atomic diffusion and reaction mechanisms across the TaRh_x interface. Failure occurred by nucleation of orthorhombic RhSi particles at the Si/TaRh_x interface. Subsequently, the barrier at areas adjacent to RhSi particles was depleted in Rh. This created lower density areas in the barrier, which facilitated diffusion of Cu to the Si substrate to form Cu₃Si. The morphology of an in situ annealed lamella was compared with an ex situ bulk annealed sample, which showed similar reaction morphology. The sample preparation method developed in this study successfully prevented surface diffusion/delamination of the Cu layer and can be employed to understand the metallurgical failure of other potential diffusion barriers.     

The effect of impurities on the lifetime of positrons in voids in molybdenum

Following the work of Thrane, Petersen and Evans which showed that positron lifetimes for voids in molybdenum were independent of void size, it seemed necessary to re-examine earlier results where larger positron lifetimes had been obtained during the annealing of specimens containing voids. This paper outlines the problem and presents the result of an experiment pointing strongly to impurity effects being responsible for the increased values of τ₂.     

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.     

Void coalescence in metals at high irradiation doses

Mechanism of void coalescence in a metal irradiated by fast particles : p to high doses in dpa is theoretically investigated. It is shown that the decrease of void concentration with the dose increase due to the coalescence of immobile but growing voids must obey the dependence N ₀ ～ exp {-α(δ V/V)(t)}, where δV/V(t) is the void swelling at a time moment t, and α is a constant, the value of which is in the interval 1 < α < 8 and seems to be close to α ≈ 6.

It is shown that the void concentration as a function of irradiation dose : hould have a maximum when the void swelling δ V/V reaches some definite value between 2% and 17% depending upon the void size distribution form.     

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.     

Diffusion anisotropy and void development under cascade irradiation

Cascade irradiation of metals gives rise to swelling as a result of the creation of voids and the evolution of the void ensemble. Under suitable circumstances, the originally disordered void distribution transforms into to a void lattice. As demonstrated previously, the understanding of the evolution and the unique features of the void ensemble requires a difference in the anisotropy of the diffusion (DAD) of vacancies and self-interstitial atoms (SIAs), which is achieved by one-dimensional diffusion of the SIAs. On the other hand, void swelling has been successfully modeled in terms of three-dimensional diffusion of both vacancies and SIAs. In the present paper it is shown that these seemingly contradicting interpretations and all related observations can be quantitatively reconciled by a small DAD created by only ～1% of SIAs diffusing one-dimensionally. It is also demonstrated that at the initial stage of void-lattice formation, ordering occurs mainly on close-packed crystal planes, which is in contrast to the naïve expectation that one-dimensional diffusion of SIAs should result in a void ordering along close-packed directions. Finally it is found that, in the case of a small DAD, voids annihilate via stochastic shrinkage much faster than by coalescence. This falsifies the argument in the literature that one-dimensional diffusion of SIAs would necessarily lead to the coalescence of voids and destabilization of the void lattice.     

Vapor Pressure Assisted Void Growth and Cracking of Polymeric Films and Interfaces

Pores and cavities form at filler particle-polymer matrix interfaces, at polymer film-silicon substrate interfaces as well as in molding compounds of IC packages. Moisture diffuses to these voids. During reflow soldering, surface mount plastic encapsulated devices are exposed to temperatures between 210 to 260°C. At these temperatures, the condensed moisture vaporizes. The rapidly expanding water vapor can create internal pressures within the voids that reach 3–6 MPa. These levels are comparable to the yield strengths of epoxy molding compounds and epoxy adhesives, whose glass transition temperatures T _g range between 150 to 300°C. Under the combined action of thermal stress and high vapor pressure (relative to the yield strength at T _g), both pre-existing and newly nucleated voids grow rapidly and coalesce. In extreme situations, vapor pressure alone could drive voids to grow and coalesce unstably causing film rupture, film-substrate interface delamination and cracking of the plastic package.Vapor pressure effects on void growth have been incorporated into Gurson's porous material model and a cohesive law. Crack growth resistance-curve calculations using these models show that high vapor pressure combined with high porosity bring about severe reduction in the fracture toughness. In some cases, high vapor pressure accelerates void growth and coalescence resulting in brittle-like interface delamination. Vapor pressure also contributes a strong tensile mode component to an otherwise shear dominated interface loading. An example of vapor pressure related IC package failure, known as popcorn cracking, is discussed.     

A molecular dynamics investigation of the micro-mechanism for vacancy formation between Ag3Sn and βSn under electromigration

In this paper, the vacancy formation at the interface between different grains (Ag₃Sn and βSn) induced by electromigration was investigated from the perspective of atom diffusion. To explain the micro-mechanism of void formation near the interface, the diffusion coefficient was specifically studied here via molecular dynamics (MD) simulation. By comparing the atom diffusion rates of atoms in βSn and Ag₃Sn, a significant difference could be observed when the temperature is up to 400 K. The Sn atoms in βSn have a higher diffusion coefficient (8E ? 9 cm²/s) than atoms in Ag₃Sn (4E ? 9 cm²/s), which indicated that the void would be prone to appear in βSn near the interface. Moreover, the effect of grain size and pressure on atom diffusivity was studied. Results show that the atom diffusivity depends heavily on the grain size of Ag₃Sn. When the thickness of Ag₃Sn is increased from 4 to 12 nm, this difference is significant when the temperature is only 375 K. On the other hand, the atom diffusion character of Ag₃Sn and βSn changes substantially under constant pressure. The difference of the atom diffusion rate would be inhibited by pressure perpendicular to the interface, which indicated voids have less possibility to appear herein.     

Positron lifetime spectroscopy in nanocrystalline iron

First positron annihilation measurements were performed on nanocrystalline iron specimens which are polycrystals with very small crystallite size (5–10 nm). The lifetimes τ₁ = 180 ± 15 ps, τ₂ = 360 ± 30 ps and long-lived components between 1 and 4 ns are attributed to vacancy-size free volumes in the crystallite interfaces, to microvoids at the intersections of three interfaces, and to positronium formation in larger voids, respectively.     

Void formation in irradiated Nickel 270

Nickel 270 (99.98 per cent nominal purity) was irradiated in EBR-II to fluences ranging from 1 × 10¹⁸ to 1.5 × 10²² neutrons/cm² at temperatures between 375 and 525°C. Voids were observed in all specimens in concentrations of 1 to 3 × 10¹⁴/cm³, independent of temperature and fluence. At low fluences the voids were non-homogeneously distributed. These observations are interpreted in terms of void nucleation on sites existing in the material prior to irradiation. The results are compared with other observations on nickel of comparable and higher purity. Large differences exist not only in the magnitude of void concentrations but also in temperature and fluence dependencies. These differences indicate that a single, void nucleation mechanism is not operative and that impurities play an important role in determining the nature and amount of damage produced by neutron irradiation at elevated temperatures.     

Researches on the ultrasonic scattering attenuation of carbon fibre reinforced plastics with 2D real morphology void model

In order to investigate the ultrasonic propagation in carbon fibre reinforced plastics with complex void morphology, the effective mathematical model needs to be established. The current models are oversimplified on void morphology, leading to the significant inconsistency of theoretical calculation with experimental results. In view of the problem, a real morphology void model (RMVM) was established with the idea of image-based modeling. The void morphology was extracted by digital image processing technology, and the material properties were assigned subsequently. As a result of the complex and random void morphology in RMVMs, a non-unique corresponding relationship was verified between porosity P and ultrasonic attenuation coefficient α. In the scatterplot of simulation, about 66 percent of points were plotted within the ±10% error band of fitting line, while almost all the data located at the ±20% error zone. The simulation results showed good consistency with experiments, and it proved the validity of RMVM. The investigation provides a novel model to explore the ultrasonic scattering mechanism for the composite materials containing random voids.