Defocused Electron Beam Research Articles

Effect of defocused electron beam surface Ni/VC alloying on microstructure and properties of 40Cr steel

ABSTRACT The use of alloying technology can improve the surface mechanical properties of 40Cr steel and prolong its service life. In this study, the Ni/VC surface alloying modification treatment of 40Cr steel was carried out using electron beam defocusing. The results show that after defocusing electron beam treatment, the cross-section of the specimen is composed of alloying layer, heat-affected zone and matrix. When the VC content is 80%, the maximum hardness is 1030 HV0.1, which is 4.3 times higher than that of the 40Cr matrix (240 HV0.1), and the lowest coefficient of friction (COF) is 0.395, which is 1.9 times lower than that of the matrix (0.757). The wear mechanism of the matrix is mainly severe abrasive wear, and the wear mechanisms of the three specimens are mainly slight abrasive wear, slight fatigue wear and adhesive wear.

Three-Dimensional Measurement of Magnetic Moment Vectors Using Electron Magnetic Chiral Dichroism at Atomic Scale.

Here we have developed an approach of three-dimensional (3D) measurement of magnetic moment vectors in three Cartesian directions using electron magnetic chiral dichroism (EMCD) at atomic scale. Utilizing a subangstrom convergent electron beam in the scanning transmission electron microscopy (STEM), beam-position-dependent chiral electron energy-loss spectra (EELS), carrying the EMCD signals referring to magnetization in three Cartesian directions, can be obtained during the scanning across the atomic planes. The atomic resolution EMCD signals from all of three directions can be separately obtained simply by moving the EELS detector. Moreover, the EMCD signals can be remarkably enhanced using a defocused electron beam, relieving the issues of low signal intensity and signal-to-noise-ratio especially at atomic resolution. Our proposed method is compatible with the setup of the widely used atomic resolution STEM-EELS technique and provides a straightforward way to achieve 3D magnetic measurement at atomic scale on newly developing magnetic-field-free TEM.

Electron beam surface heating-diffusion bonding: An effective joining method for aluminum alloy metal-matrix composite with high SiC volume

As a novel method to reduce the temperature during electron beam welding, and to subsequently inhibit interfacial reaction between SiC and Al matrix, the electron beam surface heating - diffusion bonding is proposed for joining SiC particle reinforced aluminum alloy metal - matrix composite of 45 vol.% SiC/2024 Al. The defocused electron beam was used to heat the base metal surface, and the simultaneous pressure was applied to the butt surface to achieve bonding. The base metals were successfully joined by diffusion. The maximum temperature of the whole joint was effectively decreased to less than 650 ℃. The Gibbs free energy change of interfacial reaction was calculated, meaning a positive value reaching 218 kJ/mol and a consequently prominent inhibitory effect on the formation of brittle Al4C3 that was proved by microstructural observation. The tensile strength for the bonded joint was increased by 35 % compared to that for ordinary welded joint. When the TC4 layer was added, TiC strengthening particles were formed with the deficiency of Al4C3, corresponding to the significantly increased tensile strength of 63 % of base metal (154 MPa).

Influence of focus and deflection when comparing electron beam welds to laser welds at varying parameters in 304 SS

In many cases, both laser and electron beam welding may be considered for critical applications involving a wide range of structural materials. The ability to use both processes to make comparable welds in terms of both weld profile (penetration) and microstructure provides considerable process selection flexibility. In this study, autogenous, partial penetration welds on 304 and 304L SS were made using both fiber laser and electron beam processes. To simplify the analysis, many parameters were kept constant between processes, including working distance and spot size. The main variables, power and travel speed, were varied individually. Beam analysis was conducted using a PRIMES Focus Monitor to characterize the laser beam and a pro-beam diagnostic tool (PBD) for the electron beam. Electron beam welds were deflected or defocused to achieve a spot size similar to that of the laser welds, approximately 500 μm. The deflection pattern chosen for electron beam welding was made to mimic the power distribution of the laser. A similar melting efficiency at varying powers and travel speeds was maintained for both processes. Geometries and microstructures of the deflected and defocused electron beam welds and the laser beam welds are compared and related to process parameters.

Fusion-diffusion electron beam welding of aluminum‑lithium alloy with Cu nano-coating

As a new method to reduce the temperature in melt pool and subsequently alleviate the Li evaporation loss, fusion-diffusion electron beam welding is proposed for aluminum‑lithium alloy joining achieved by defocused electron beam and pressure. The Li evaporation loss, which was only 9%, was significantly suppressed compared to the ordinary electron beam welding. However, joining failed within the unfused region due to the poor atomic diffusion. As a result, Cu nano-coating, with a 300 nm thickness, was sputtered to the butt surface of base metal to accelerate atomic diffusion. After adding Cu nano-coating, the unfused region achieved an effective joining since the eutectic α-Al + Al2Cu appeared at 821 K, indicating that an equiaxed crystal zone, which joined two base metals, was formed at unfused region. Reasons for the presence of equiaxed crystal zone were examined. Dispersed strengthening phases including θ′ (Al2Cu) and T1 (Al2CuLi) were observed to be present in both the fusion zone and equiaxed crystal zone, corresponding to a higher tensile strength and increased hardness value in equiaxed crystal zone.

Surface potential and charging of polymer films submitted to defocused electron beam irradiation

The surface potentials and dynamic charging properties of polymers subjected to electron irradiation in scanning electron microscopy are investigated by using a novel method applied in parallel with simulation and experimental methods. We perform our simulation through a parallel computing method with various microscopic parameters. The results show that under the given microscopic parameters, the charging time concomitantly decreases as the film thickness, electron mobility, trap cross and recombination rate increase. The surface potential decreases as electron energy, beam current density, capture cross section, sample thickness and electron-hole recombination rate increase and as electron mobility decreases. Charging time increases with increasing beam density, sample thickness, recombination rate and change in capture cross-section, and decreases with increasing electron mobility. The primary-electron energy exhibits an inflection point at 35 keV. This study offers an intuitive analytical and measurement method for understanding microscopic charge characteristics in electron-based surface microscopy.

Analytical model of secondary electron emission yield in electron beam irradiated insulators

The study of secondary electron emission (SEE) yield as a function of the kinetic energy of the incident primary electron beam and its evolution with charge accumulation inside insulators is a source of valuable information (even though an indirect one) on charge transport and trapping phenomena. We will show that this evolution is essentially due, in plane geometry conditions (achieved using a defocused electron beam), to the electric field effect (due to the accumulation of trapped charges in the bulk) in the escape zone of secondary electrons and not to modifications of trapping cross sections, which only have side effects.We propose an analytical model including the main basic phenomena underlying the space charge dynamics. It will be observed that such a model makes it possible to reproduce both qualitatively and quantitatively the measurement of SEE evolution as well as to provide helpful indications concerning charge transport (more precisely, the ratios between the mobility and diffusion coefficient with the thermal velocity of the charge carrier).

Analysis of thermal detrapping of holes created by electron irradiation in high purity amorphous SiO2 using the induced and secondary current measurements

Isothermal detrapping of holes after electron irradiation (using a SEM) in high purity amorphous SiO2 is evaluated at different temperatures (in the range 300–663K) by means of the induced and secondary current measurements. In order to single out the hole detrapping, the specific charging conditions (1keV defocused electron beam of low density) leading to positive charging are adopted. The thermal detrapping, which stems from a single trap, begins at 523K and is completed at 663K. After annealing in air at 973K during 48h, two detrapping stages are revealed: the former is connected with an additional shallow trap, while the latter requires temperatures above 663K for a complete detrapping. The first order kinetics describes reasonably well the detrapping process. The frequency factors (near 1010s−1) and the activation energies (about 1.6eV) deduced from this analysis could be assigned, respectively, to the relaxation connected to detrapping and to the trap energy level of the charged oxygen vacancy.

Mineralogy, petrography, geochemistry, and classification of the Košice meteorite

AbstractThe Košice meteorite was observed to fall on 28 February 2010 at 23:25 UT near the city of Košice in eastern Slovakia and its mineralogy, petrology, and geochemistry are described. The characteristic features of the meteorite fragments are fan‐like, mosaic, lamellar, and granular chondrules, which were up to 1.2 mm in diameter. The fusion crust has a black‐gray color with a thickness up to 0.6 mm. The matrix of the meteorite is formed mainly by forsterite (Fo80.6); diopside; enstatite (Fs16.7); albite; troilite; Fe‐Ni metals such as iron and taenite; and some augite, chlorapatite, merrillite, chromite, and tetrataenite. Plagioclase‐like glass was also identified. Relative uniform chemical composition of basic silicates, partially brecciated textures, as well as skeletal taenite crystals into troilite veinlets suggest monomict breccia formed at conditions of rapid cooling. The Košice meteorite is classified as ordinary chondrite of the H5 type which has been slightly weathered, and only short veinlets of Fe hydroxides are present. The textural relationships indicate an S3 degree of shock metamorphism and W0 weathering grade. Some fragments of the meteorite Košice are formed by monomict breccia of the petrological type H5. On the basis of REE content, we suggest the Košice chondrite is probably from the same parent body as H5 chondrite Morávka from Czech Republic. Electron‐microprobe analysis (EMPA) with focused and defocused electron beam, whole‐rock analysis (WRA), inductively coupled plasma mass and optical emission spectroscopy (ICP MS, ICP OES), and calibration‐free laser induced breakdown spectroscopy (CF‐LIBS) were used to characterize the Košice fragments. The results provide further evidence that whole‐rock analysis gives the most accurate analyses, but this method is completely destructive. Two other proposed methods are partially destructive (EMPA) or nondestructive (CF‐LIBS), but only major and minor elements can be evaluated due to the significantly lower sample consumption.

Effect of the sample annealing temperature and sample crystallographic orientation on the charge kinetics of MgO single crystals subjected to keV electron irradiation.

This paper focuses on the effect of sample annealing temperature and crystallographic orientation on the secondary electron yield of MgO during charging by a defocused electron beam irradiation. The experimental results show that there are two regimes during the charging process that are better identified by plotting the logarithm of the secondary electron emission yield, lnσ, as function of the total trapped charge in the material QT. The impact of the annealing temperature and crystallographic orientation on the evolution of lnσ is presented here. The slope of the asymptotic regime of the curve lnσ as function of QT, expressed in cm(2) per trapped charge, is probably linked to the elementary cross section of electron-hole recombination, σhole, which controls the trapping evolution in the reach of the stationary flow regime.

Analysis of microscopic parameters of surface charging in polymer caused by defocused electron beam irradiation

The relationship between microscopic parameters and polymer charging caused by defocused electron beam irradiation is investigated using a dynamic scattering-transport model. The dynamic charging process of an irradiated polymer using a defocused 30keV electron beam is conducted. In this study, the space charge distribution with a 30keV non-penetrating e-beam is negative and supported by some existing experimental data. The internal potential is negative, but relatively high near the surface, and it decreases to a maximum negative value at z=6μm and finally tend to 0 at the bottom of film. The leakage current and the surface potential behave similarly, and the secondary electron and leakage currents follow the charging equilibrium condition. The surface potential decreases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. The total charge density increases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. This study shows a comprehensive analysis of microscopic factors of surface charging characteristics in an electron-based surface microscopy and analysis.

Charging effects of SiO2 thin films under defocused electron beam irradiation

Based on a three-dimensional self-consistent numerical model with consideration of electron scattering, trapping and transport, the charging effects due to low-energy defocused electron beam irradiation are simulated for a SiO2 thin film with a grounded conductive substrate. The results show that because of electron drift and diffusion, electrons can transit the electron scattering region, forming negative space charges. The space charge is, therefore, positive and negative within and outside the scattering region, respectively. Some electrons can flow to the conductive substrate, forming the leakage current, and the transient negative charging process tends to equilibrium as the leakage current increases. In comparison, the transient positive charging process approaches equilibrium with the number of returned electrons increasing due to the positive surface potential. In the equilibrium state, the surface potential of the film negatively charged decreases with film thickness and trap density increasing; it increases with electron mobility and dielectric constant. However, the equilibrium surface potential of the film positively charged varies slightly with film parameter.

Self-consistent charging in dielectric films under defocused electron beam irradiation

We clarify the transient and equilibrium charging characteristics of grounded dielectrics due to low-energy defocused electron beam irradiation by a three-dimensional self-consistent simulation model. The model incorporates the electron scattering, transport and trapping. Results show that some electrons can arrive at the grounded substrate due to the internal field and density gradient, forming the leakage current. The transient charging process tends to equilibrium as the surface potential decreases and the leakage electron current increases. The positive and negative space charges are distributed alternately along the beam incident direction. In the equilibrium state, the surface potential and leakage electron current decrease to stable values with increasing film thickness and the trap density, but with decreasing electron mobility. Moreover, the surface potential of the dielectric thin film exhibits a maximum negative value with variation of the beam energy; for example, under the condition of the film thicknesses 0.5μm and 2μm, the maximum negative values of surface potentials are −13V and −98V in beam energies 2keV and 3.5keV, respectively. However, for the thick film, the surface potential decreases with the increase in beam energy.

The fabrication of three-dimensional nano-structures by defocused electron beam energy

We demonstrate a novel method to fabricate three-dimensional structure with various shapes using defocused electron beam lithography (DEBL) for optical elements. In this study, we experimentally analyzed the relationship between the shape profiles of exposed patterns and defocusing length by varying parameters for e-beam lithography. Through these experiments, we found out that the shape profiles of exposed patterns depended on the defocusing length at the specific conditions in the e-beam lithography. This is due to the fact that the distribution and intensity of electron beam energy are changed by the defocusing length degree in the DEBL. These results allow the fabrication of three-dimensional (3D) structures with various shapes for optical elements by varying the defocusing length under the proper conditions of e-beam lithography. Based on these results, we can fabricate the 3D patterns with hemispherical shape for nano lens array and nano cone shape for SERS (surface enhanced Raman spectroscopy) and optical diffraction grating patterns with nano scale depth and width.

Study of discharge after electron irradiation in sapphires and polycrystalline alumina

The fraction R of charges undergoing discharge during the time separating two electron pulses is derived from the induced current method developed in a scanning electron microscope. Irradiation is performed via a 10 keV defocused electron beam and low current density. The evolution of R with temperature (in the range 300–663 K) obeys to an Arrhenius type relation. Activation energies connected with the processes involved are deduced. In sapphire, no discernible discharge is observed due to the dominance of deep traps. In silver doped sapphire, R increases sharply from 10% to 70% as the temperature rises from 360 to 420 K, with a corresponding activation energy of 0.51 eV. In contrast, in polycrystalline alumina processed by solid state sintering (grain diameters of 1.7, 2.7, and 4.5 μm) the degree of discharge increases continuously with temperature and grain size. The enhancement with grain size indicates that the sintering conditions influence strongly the efficiency of a gettering effect. The activation energy below 573 K is about 0.12 eV independently of grain size. Above 573 K, a second activation energy of 0.26 eV appears for the smallest grain size sample. The results suggest that discharge may stem from a density of trapping states, associated to grain boundaries in sintered samples, rather than from a single trapping level linked to the doping element as Ag in sapphire.

Effects of zonal heat treatment on residual stresses and mechanical properties of electron beam welded TC4 alloy plates

Zonal heat treatment(ZHT) was conducted in situ to 14.5 mm-thick TC4 alloy plates by means of defocused electron beam after welding. The effects of ZHT on residual stresses, microstructures and mechanical properties of electron beam welded joints were investigated. Experimental results show residual stresses after welding are mostly relieved through ZHT, and the maximum values of longitudinal tensile stress and transverse compressive stress reduce by 76% and 65%, respectively. The tensile strength and ductility of welded joint after ZHT at slow scanning velocity are improved because of the reduction of residual stress and the microstructural changes of the base and weld metal. ZHT at fast scanning velocity is detrimental to the ductility of welded joint, which is resulted from insufficiently coarsened alpha phase in the fusion zone and the appearance of martensite in the base metal.

Volume and composition surface changes in alkali silicate glass irradiated with electrons

Alkali silicate glasses were irradiated with 50 keV defocused electron beam of 60 µm diameter. The compositional as well as volume changes were measured by means of electron microprobe and scanning electron microscopy. The compositional changes were found to significantly extend beyond the directly irradiated volume. The expansion of the volume reached up to nearly 100%.

Changes in surface morphology of silicate glass induced by fast electron irradiation

Binary potassium-silicate glass was irradiated with a defocused electron beam. During the irradiation the alkali ions migrate from the surface into the depth and alkali ions depleted layer is created near the surface. Such changes in the chemical composition are also accompanied with changes of the glass structure and finally result in the volume changes of the irradiated glass. This was directly studied using atomic force microscope (AFM). A series of exposures with energy of electrons of 7–50 keV and with different doses were performed. For low doses the irradiated glass is continuously depressed with the increasing dose, indicating this way the structural changes leading to the volume compaction. It is suggested the compaction is caused by the relaxation processes of the silica subnetwork. A further increase of the electron dose causes a formation of the small bump inside the center of the depression. The bump arises with the dose and finally exceeds the surrounding surface. It is suggested that the expansion is connected with the migration of alkali ions and the formation of Si–O–O–Si bonds which result in the formation of new rings with new space requirements.

Alternative method of using an electron beam for charge compensation during ultralow energy secondary-ion-mass spectroscopy experiments

Secondary-ion-mass spectroscopy (SIMS) analysis of insulators using positive primary beams is routinely performed by compensating the induced charge with a coincident electron beam. In the case of SIMS depth profiling, the established method consists of focusing an electron beam into the SIMS crater with a current well in excess of that of the primary ion beam. In this article we used both caesium and oxygen beams to bombard float glass, and intrinsic and doped diamond samples while varying the electron beam current and the area bombarded by electrons. We have studied how the electron beam to primary ion current density ratio modifies the charging conditions. We demonstrate that, for certain insulating and highly resistive materials, defocusing of the electron beam so as to cover the whole of the sample surface and part of the sample holder is extremely effective. It is also observed that the defocused electron beam works efficiently for an electron to primary ion current density ratio less than 1. We attribute this to the enhancement of surface conductivity through the creation of carriers in the conduction band, and observe similar effects when irradiating the surface with a laser diode. The ability to use a defocused electron beam will significantly aid profiling of insulating and highly resistive materials where alignment of the coincident electron and ion beams is problematic. Defocusing of the electron beam also offers the possible advantage of reducing or eliminating localized electron beam damage of certain material surfaces prior to and during profiling.

Microstructure and microchemistry variation during thermal exposure of low alloy steels

The microstructure of an ex-service (136,000 h) Fe–1Cr–1Mo–0.25V (wt%) low alloy steel turbine rotor was investigated using energy dispersive X-ray spectroscopy (EDXS) techniques in TEM/STEM on extraction replicas. Since different stages of the rotor experience different thermal histories, an examination of microstructural variations can serve as a method to indicate metallurgical factors affecting service-lifetime of the material. The coldest, hottest and most embrittled stages were taken for investigation. The overall fraction of carbide precipitation was measured using image analysis techniques and results indicated that the hottest stage possessed the highest precipitate volume fraction. Large area EDXS analysis using a defocused electron beam was also performed on the replica samples and this was supported by spot analyses using scanning TEM to identify individual carbides so as to allow quantification of the enrichment of solute into carbides as a function of service temperature. Three-dimensional atom probe field ion microscopy was also used to assess the chemistry at the interface between matrix and precipitate.