Irradiation Electron Microscopy Research Articles

Evidence of dislocation loop preferential nucleation in irradiated aluminum under stress

To explain irradiation creep, several mechanisms have been proposed. Some are based on the effect of stress on either nucleation or growth of dislocation loops. To investigate these mechanisms in aluminum we combine in-situ transmission electron microscope irradiation under stress and two simulation approaches (object kinetic Monte-Carlo, molecular dynamics Frenkel pair accumulation). We observe the selectivity of Frank loop variants under electron irradiation and applied stress. When the stress is turned on after loop formation, there is no loop variant selectivity, suggesting the absence of preferential absorption on already formed loops. Object kinetic Monte-Carlo simulations, including the effect of stress on the diffusion of point defects, show no growth rate difference between loop variants. Frenkel pair accumulation simulations exhibit variant selectivity of nucleated loops. This shows that loop selectivity is due to preferential nucleation of well oriented loops under stress and not to differential growth of loops.

In situ investigation of ion irradiation-induced amorphization of (Ge2Sb2Te5)1−xCx [0 ≤ x ≤ 0.12

Chalcogenide thin films that undergo reversible phase changes show promise for use in next-generation nanophotonics, microelectronics, and other emerging technologies. One of the many studied compounds, Ge2Sb2Te5, has demonstrated several useful properties and performance characteristics. However, the efficacy of benchmark Ge2Sb2Te5 is restricted by amorphous phase thermal stability below ∼150 °C, limiting its potential use in high-temperature applications. In response, previous studies have added a fourth species (e.g., C) to sputter-deposited Ge2Sb2Te5, demonstrating improved thermal stability. Our current research confirms reported thermal stability enhancements and assesses the effects of carbon on crystalline phase radiation response. Through in situ transmission electron microscope irradiation studies, we examine the effect of C addition on the amorphization behavior of initially cubic and trigonal polycrystalline films irradiated using 2.8 MeV Au to various doses up to 1 × 1015 cm−2. It was found that increased C content reduces radiation tolerance of both cubic and trigonal phases.

The Mechanism of {113} Defect Formation in Silicon: Clustering of Interstitial–Vacancy Pairs Studied by In Situ High-Resolution Electron Microscope Irradiation

We report the direct visualization of point defect clustering in {113} planes of silicon crystal using a transmission electron microscope, which was supported by structural modeling and high-resolution electron microscope image simulations. In the initial stage an accumulation of nonbonded interstitial-vacancy (I-V) pairs stacked at a distance of 7.68 Å along neighboring atomic chains located on the {113} plane takes place. Further broadening of the {113} defect across its plane is due to the formation of planar fourfold coordinated defects (FFCDs) perpendicular to chains accumulating I-V pairs. Closely packed FFCDs create a sequence of eightfold rings in the {113} plane, providing sites for additional interstitials. As a result, the perfect interstitial chains are built on the {113} plane to create an equilibrium structure. Self-ordering of point defects driven by their nonisotropic strain fields is assumed to be the main force for point defect clustering in the {113} plane under the existence of an energy barrier for their recombination.

Nano-scale quasi-melting of alkali-borosilicate glasses under electron irradiation

Quasi-melting of micro- and nano-samples during transmission electron microscope irradiation of glassy materials is analysed. Overheating and true melting by the electron beam is shown not to be an explanation due to the ultra-sharp boundary between transformed and intact material. We propose that the observed fluidisation (quasi-melting) of glasses can be caused by effective bond breaking processes induced by the energetic electrons in the electron beam. The bond breaking processes modify the effective viscosity of glasses to a low activation energy regime. The higher the electron flux density the lower is the viscosity. Quasi-melting of glasses at high enough electron flux densities can result in shape modification of nano-sized particles including formation of perfect beads due to surface tension. Accompanying effects, such as bubble formation and foil bending are revisited in the light of the new interpretation.

Dislocation Driven Problems in Atomistic Modelling of Materials

Understanding the mechanical properties of technologically advanced materials from quantum mechanical predictions based on electronic structure calculations remains one of the most challenging problems in modern computational materials science. In this paper, we illustrate this challenge from our current investigations on dislocation behaviour in bcc transition metals that are promising candidates for materials subject to fast neutron irradiations in future fusion power plants. Starting with the relationship between the brittleness and the negative Cauchy pressure of elastic constants in materials within the so-called Harris-Foulkes approximation to the density functional theory (DFT), we briefly discuss the importance of the generic form of interatomic potentials in order to reproduce a correct Cauchy pressure. The latter in turn plays an important role in predicting dislocation properties in fcc iridium and therefore allows us to explain experimental observation of the intrinsic brittleness of this material. We then investigate the behaviour of the (1/2)[111] screw dislocation that controls plastic deformation in bcc metals from atomistic simulation. Here we show the atomic phenomena associated with the non-planar core structure of dislocations in bcc iron from the Stoner tight-binding bond model. The crucial point comes from the accurate evaluation of forces implemented within the charge neutrality conditions in the treatment of the spin-polarized dependence in the electronic structure calculations. In agreement with DFT studies, the magnetic bond-order potentials predict a non-degenerate core structure for screw dislocations in Fe. Finally, a new analytic expression has been derived for the migration energy barrier for the one-dimensional (1D) motion of crowdions, which are the most stable self-interstitial atom (SIA) defects predicted by our DFT calculations. Importantly, the latter study is strongly supported by the recent observation of 1D diffusion of nanometer-sized dislocation loops, observed very recently under in situ electron microscope irradiation for bcc transition metals.

Transmission electron microscopy characterization and sculpting of sub-1nm Si–O–C freestanding nanowires grown by electron beam induced deposition

The authors present a transmission electron microscopy characterization and sculpting of freestanding Si–O–C nanowires, fabricated by electron beam induced deposition from a tetraethylorthosilicate precursor, in a dual beam system. Electron energy loss spectroscopy and near edge structure analyses performed on as deposited wires show the formation of amorphous silicon dioxide with extra oxygen and carbon content. Subsequent electron beam sculpting by 200keV transmission electron microscope irradiation decreases carbon and oxygen contents leaving the silicon oxidation state unchanged and narrows Si–O–C wire width to less than 1nm.

Local phase transformations in alloys during charged-particle irradiation

Among the various mechanisms and processes by which energetic irradiation can alter the phase stability of alloys, radiation-induced segregation is one of the most important phenomena. Radiation-induced segregation in alloys occurs as a consequence of preferential coupling between persistent fluxes of excess defects and solute atoms, leading to local enrichment or depletion of alloying elements. Thus, this phenomenon tends to drive alloy systems away from thermodynamic equilibrium, on a local scale. During charged-particle irradiations, the spatial nonuniformity in the defect production gives rise to a combination of persistent defect fluxes, near the irradiated surface and in the peak-damage region. This defect-flux combination can modify the alloy composition in a complex fashion, i.e. it can destabilize pre-existing phases, causing spatially- and temporally-dependent precipitation of new metastable phases. The effects of radiation-induced segregation on local phase transformations in Ni-based alloys during proton bombardment and high-voltage electron-microscope irradiation at elevated temperatures are discussed.

High voltage electron microscope irradiation and observations in metallic glasses

High voltage electron microscope (hvem) has been extensively used to produce radiation damage and to study the characteristics of defects so produced in crystalline solids. To understand the defect production in metallic glasses and to evaluate the influence of such defects on physical properties like crystallisation temperature etc., high voltage microscopy and subsequentin situ heating and observation has been extremely useful technique. This paper gives a qualitative overview of such work performed in metallic glasses. In particular results obtained on a nickel based metallic glass using ahvem and an electron accelerator are presented. The advantages and limitations ofhvem irradiation are highlighted.

Compositional redistribution in alloy films under high-voltage electron microscope irradiation

The problem of nonequilibrium segregation in alloy films under high-voltage electron microscope (HVEM) irradiation at elevated temperatures is re-examined in the present work, taking into account the damage-rate gradients caused by radial variation in the electron flux. Axial and radial compositional redistributions in model solid solutions, representative of concentrated Ni-Cu, Ni-Al and Ni-Si alloys, were calculated as a function of time, temperature, and film thickness, using a kinetic theory of segregation in binary alloys. The numerical results were achieved by means of a new software package (DISPL2) for solving convection-diffusion-kinetics problems with general orthogonal geometries. It was found that HVEM irradiation-induced segregation in thin films consists of two stages. Initially, due to the proximity of the film surfaces as sinks for point defects, the usual axial segregation (to surfaces) occurs at relatively short irradiation times, and rapidly attains quasi-steady state. Then, radial segregation becomes more and more competitive, gradually affecting the kinetics of axial segregation. At a given temperature, the buildup time to steady state is much longer in the present situation than in the simple case of one-dimensional segregation with uniform defect production. Changes in the alloy composition occur in a much larger zone than the irradiated volume. As a result, the average alloy composition within the irradiated region can differ greatly from that of the unirradiated alloy. The present calculations may be useful in the interpretation of the kinetics of certain HVEM irradiation-induced processes in alloys.

Loop and Void Formation in Copper During High Voltage Electron Microscope Irradiation

The formation of dislocation loops and voids under neutron, ion, and electron irradiation is well documented. Since irradiation produces Frenkel pairs, the presence of biased interstitial sinks is required for void formation to procede. It is generally accepted that dislocations, generated either by cold-work or irradiation, act as the preferential sinks.During irradiation in the high voltage electron microscope, both interstitials and vacancies are lost to foil surfaces and corresponding denuded zones are set up. As a result of image forces, dislocation loops formed upon irradiation tend to unfault and glide out of the foil. Thus, below a given thickness, the foil is unable to maintain a sufficiently high dislocation density to support void nucleation and growth.