New mechanism for field-induced anisotropy of cobalt-rich amorphous alloys

It is generally assumed that amorphous magnetic alloys respond to field annealing by a process of local directional ordering which leaves the amorphous structure intact. We have made a comparative microstructural study of field-annealed Co_95-xFe₅(BSi)x amorphous alloys using transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) with thin sections parallel to the ribbon surface. Field annealing response was measured from anisotropy in low-field hysteresis loops. These alloys show appreciable surface crystallization for annealing as much as 80 K below the bulk crystallization temperature. The surface crystallization proceeds by a known mechanism (selective oxidation) to which we have added a more detailed understanding. Three steps are involved: (1) formation of an amorphous borosilicate surface oxide layer during annealing; (2) depletion of glass stabilizing elements (boron and silicon) from the underlying amorphous metal substrate; (3) primary crystallization of the destabilized, metal-enriched subsurface layer to an fee or hep cobalt-rich phase. Striking differences in the microstructural morphology were revealed for different glass former ratios B/Si. For high B/Si ratios, the surface crystallites are predominantly fee Co and show a high density of oxygen faults. For low B/Si ratios, the surface crystallites are predominantly hep Co and almost free of faults. Response to field annealing is proportional to the B/Si ratio and correlates with the presence of oxygen faults in surface crystallites. Electron diffraction and microprobe analysis indicate that the surface oxide in silicon-rich alloys is a dense silica glass which appears to be an effective diffusion barrier to oxygen. The surface oxide in boron-rich alloys is a more porous oxide richer in B₂O₃. These observations appear to be related to those from perminvar alloys where oxygen was found to be necessary for field annealing to be effective.     

氢化物对锆合金薄板焊缝断裂行为的影响

锆合金被广泛应用于反应堆的燃料管包壳材料,当服役过程中锆合金的力学性能改变时,对其力学性能完整性的评估至关重要。基于小尺寸三点弯曲试样,建立锆合金薄板焊缝的断裂韧度测试方法,完成腐蚀渗氢后锆合金焊缝在室温和360℃下准静态断裂韧度试验,分析氢腐蚀和温度对锆合金焊缝断裂性能的影响。研究结果表明,氢腐蚀和温度均能对锆合金焊缝断裂性能产生显著影响,由于高温条件下聚集在裂尖附近的氢化物溶解,使得360℃下锆合金渗氢焊缝断裂韧度较室温下的断裂韧度有显著提升。     

Bloch-wave-based STEM image simulation with layer-by-layer representation

In a dynamical STEM image simulation by the Bloch-wave method, Allen et al. formulated a framework for calculating the cross-section for any incoherent scattering process from the inelastic scattering coefficients: thermal diffuse scattering (TDS) for high-angle annular dark-field (HAADF) and back-scattered electron (BSE) STEM, and ionization for electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (EDX) STEM. Furthermore, their method employed a skilful approach for deriving the excitation amplitude and block diagonalization in the eigenvalue equation. In the present work, we extend their scheme to a layer-by-layer representation for application to inhomogeneous crystals that include precipitates, defects and atomic displacement. Calculations for a multi-layer sample of Si–Sb–Si were performed by multiplying Allen et al.'s block-diagonalized matrices. Electron intensities within the sample and EDX STEM images, as an example of the inelastic scattering, were calculated at various conditions. From the calculations, 3-dimensional STEM analysis was considered.     

The influence of reciprocating sliding wear on the oxidation behaviour of Fe-12Cr steel

Medium-chromium ferritic alloys are used extensively in the boiler and core sections of advanced gas cooled reactors (AGRs). It was discovered in the early 1970s, that under certain conditions these alloys could undergo the phenomenon known as breakaway oxidation. In this type of oxidation the rate-limiting step is located at the oxide/metal interface rather than the more usual gas/oxide interface and results in linear oxidation kinetics. It has been shown that repeated removal of oxide layers can expose chromium-depleted metal to the oxidizing gas and promote nucleation of breakaway oxidation. The question has been addressed as to whether high temperature sliding wear processes can also disrupt the surface so as to make the material potentially susceptible to breakaway oxidation.High temperature reciprocating wear tests of Fe-12Cr material in both low and high pressure reactor gas have been caried out. As expected, compact adhesive load-bearing oxide and mixed oxide/metal beds form in wear regions. These contacting features wear at very low rates of less than 10⁻¹⁶m³ (Nm)⁻¹. It has also been demonstrated that preformed oxides wear at sufficiently low rates at high temperature as to preclude the possibility of exposure of the underlying metal to the reactor gas. It is thus unlikely that sliding wear processes will accelerate the tendency for initiation of breakaway oxidation.     

Contribution of energy-filtering TEM to the detection of calcium: Application to mast cells

The ultrastructural distribution and quantification of calcium in mast cells prepared by anhydrous processing was investigated by energy-filtering transmission electron microscopy (EFTEM) using a Zeiss 902 electron microscope. Optimal conditions for calcium detection were determined using inorganic (calcium phosphate) and organic (calcium-loaded chelex beads) standards with known amounts of calcium. Electron energy-loss spectroscopy (EELS) revealed calcium at the L_2,3 edge and also at the M_2,3 edge for all specimens examined. Comparison with X-ray microanalysis confirmed the results obtained with EELS. Electron spectroscopic imaging (ESI) was applied for mapping calcium both in standards and in cells and we showed that mast cell granules were the main site of calcium localization. Although, results have shown that a combination of analytical techniques is required to obtain reliable results.     

A Study of Oxidation Phenomena in Corrosive Wear

A mathematical model based on the existing theories of corrosion and wear is proposed for analyzing the controlling phenomena in corrosive wear caused by atmospheric oxygen. The model is applied to experimental wear data, making the assumption that all of the wear proceeded by a corrosion mechanism, i.e., the growth of an oxide layer and its subsequent removal by rubbing. The analytical results show that the rate of metal oxidation is the more important factor in determining the wear rate. The proposed model, assuming oxidation to follow a parabolic rate law, predicts a much higher rate constant and a lower activation energy for oxidation in corrosive wear than in static corrosion at the same “hot spot” temperature. It is postulated that this is due to a mechanical activation phenomena caused by rubbing action. The increase of wear at higher loads can be accounted for by a simple increase in oxidation rate at a higher surface temperature.     

Zirconium oxidation on the atomic scale

Zirconium alloys are used in the nuclear industry as fuel rod cladding. They are chosen for this role because of their good mechanical properties and low thermal neutron absorption. Oxidation of these alloys by coolant is one of the chief limiting factors of the fuel burn-up efficiency. The aim of the present study is to understand these oxidation mechanisms. As a first step, a fundamental study of the oxidation of commercially pure zirconium has been conducted using the 3D atom probe (3DAP). The current generation of 3DAPs allows both voltage and laser pulsing, providing data sets of many millions of ions.     

Analytical electron microscopy study of high Tc superconductor YBa2Cu3O7

The high T_c superconducting material YBa₂Cu₃O₇ shows a complex relationship between microstructure and oxygen content, which are controlled by length of heat treatment, atmosphere, and quench rate. An AEM investigation studying changes in the oxygen near edge features was undertaken. Electron energy loss spectroscopy (EELS) measurements have the important advantage that they can be made on single crystal grains, allowing orentation-dependent studies. Both ion-milled and crushed samples with varying O₂ content were analyzed. The structure of YBaCu₃O₇ was determined by neutron diffraction to be orthorhombic with distinct Cu-O chains along the b-axis as well as Cu-O planes in the a—b plane. Therefore, by looking for a crystallographic dependence of the oxygen K-edge one might be able to distinguish inequivalent oxygen atoms by their core level binding energy and correlate site occupancy with varying O₂ content. The EELS results on the oxygen K-edge are strongly dependent on oxygen content, most noticeably when the c-axis is parallel to the electron beam.     

High‐temperature oxidation of nickel and chromium studied with an in‐situ environmental scanning electron microscope

Investigations of the morphology of metal oxide scales formed at high temperatures in oxidative environments are usually undertaken after exposure of the samples is completed. In this study, an environmental scanning electron microscope (ESEM) was used as a tool for the in‐situ observation of oxide scale formation. Pure nickel and chromium samples were oxidized at a temperature of 973 K in either pure oxygen or water vapour at a pressure of 667 Pa. The evolution of an oxide scale was followed in‐situ for up to 3 h. The morphology of the developing oxide scales was found to be a function of the metal substrate and the gaseous species. The growth mechanisms of the different metal oxide scales are reviewed and related to the analysed in‐situ images. Emphasis is placed on the relationship between oxidation mechanism and scale morphology. Nickel is seen to oxidise by outward diffusion of nickel probably on oxide grain boundaries when exposed to oxygen. Water vapour changes the scale morphology and a duplex‐type scale arises due to preferential overgrowth. The scale which develops due to chromium oxidation in oxygen is a fine‐grained, thin, and dense layer. In contrast, water vapour leads to whisker growth on chromium and an open, felt‐like structure forms. The applicability of the ESEM to the study of such systems is demonstrated, and its limitations are outlined. The results are encouraging examples of the possibilities which the in‐situ ESEM technique possesses.     

Dynamics of frictional interaction between the shell of a fuel element and a fuel assembly spacing grid cell

Studies of the frictional interaction between samples of standard elements simulating contact conditions in a friction couple consisting of a fuel cell shell and a spacing grid cell were conducted. The studies were carried out with a special test bench. The effects of the load parameters, the contact geometry of the specimens, and the state of the working surfaces on friction processes were studied. The values of the friction factors for elements made of zirconium alloys were obtained, which are needed for calculation and experimental testing of the strength of a fuel assembly. The presence of protective oxide films on the friction surfaces of specimens is shown to significantly reduce the friction factor and diminish the probability of seizure.     

Quantitative and qualitative characterization of aquatic iron oxyhydroxide particles by EF-TEM

Electron energy-loss spectroscopy (EELS) and elemental imaging under the energy-filtered transmission electron microscope are powerful tools for the characterization of iron-rich particles present in natural waters. Features present in EEL spectra (Fe-M_2,3 Fe-L_2,3 and O-K ionization edges) of goethite (α-FeOOH) have been studied with an energy filter operated at 80 keV to determine optimal quantification and elemental imaging of Fe-rich natural aquatic particles in the 30–200 nm range of thickness. For quantitative aims, the Fe-M_2,3 ionization edge cannot be used easily, but the Fe-L_2,3 edge provides more accurate results owing to a better background extrapolation. The partial cross-section of the Fe(III) M shell has been determined for iron oxide. The use of two-windows (jump-ratio) and three-windows (background stripping) imaging methods is discussed in relation to the specimen thickness.     

A new analytical method for characterising the bonding environment at rough interfaces in high-k gate stacks using electron energy loss spectroscopy

Determining the bonding environment at a rough interface, using for example the near-edge fine structure in electron energy loss spectroscopy (EELS), is problematic since the measurement contains information from the interface and surrounding matrix phase. Here we present a novel analytical method for determining the interfacial EELS difference spectrum (with respect to the matrix phase) from a rough interface of unknown geometry, which, unlike multiple linear least squares (MLLS) fitting, does not require the use of reference spectra from suitable standards. The method is based on analysing a series of EELS spectra with variable interface to matrix volume fraction and, as an example, is applied to a TiN/poly-Si interface containing oxygen in a HfO₂-based, high-k dielectric gate stack. A silicon oxynitride layer was detected at the interface consistent with previous results based on MLLS fitting.     

Dynamics of Growth of the Native Oxide of Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te

The growth of the native oxide of the Cd_xHg_1?xTe (MCT) compound is studied by methods of laser and spectral ellipsometry. It is found that a non-absorbing oxide film is formed from the very beginning in the case of MCT oxidation with hydrogen peroxide vapors, whereas oxidation with atmospheric oxygen leads to the formation of absorbing layers on the surface at the first stages of the process. When the oxide film thickness reaches 1–2 nm, the oxidation rate drastically decreases. If MCT samples that were stored for a long time (for years) in air at room temperature are heated at T = 200 °C, the optical thickness of the oxide film decreases.     

Spatially resolved electron energy-loss studies of metal–ceramic interfaces in transition metal/alumina cermets

Composites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy-loss spectroscopy (EELS), and, to a lesser extent, by high-resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal-ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near-edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al-terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O₃M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion-bonded Nb/Al₂O₃ interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.     

The improvement of wettability,biotribological behavior and corrosion resistance of titanium alloy pretreated by thermal oxidation

The thermal oxidation process was performed on biomedical titanium alloys to improve surface properties for the application in artificial cervical disc. The pretreated Ti6Al4V was characterized with XPS, XRD and SEM. The wettability, biotribological behavior and corrosion resistance were evaluated under distilled water and 25 wt% bovine serum lubricant. Rutile TiO₂ as the main compound was formed with the increase in hardness. The wettability was improved significantly after oxidation. Compared with the untreated, the friction coefficients and wear volumes of treated samples all decreased with about 50% reduction in both dry sliding and lubrication conditions. Corrosion resistance for oxidized samples was also enhanced with a big reduction of corrosion current density and a shift in corrosion potential towards the positive direction.     

Thermal oxidation of Zr-2.5Nb alloy and its biotribological properties

Zirconium alloys are potential materials for permanent implants due to their suitable mechanical strength, good biocompatibility, and superior corrosion resistance. However, Zr-2.5Nb alloys are greatly limited to the applications of artificial joints due to their poor tribological properties. Thermal oxidation is considered as a good way to improve the tribological properties of Zr-2.5Nb alloys. The effect of temperature on the properties of thermal oxidized Zr-2.5Nb alloys was investigated to understand the growth mechanism of ZrO₂ coating. In addition, the surface hardness of the thermal oxidized Zr-2.5Nb alloys was examined. Finally, the tribological properties of the thermal oxidized Zr-2.5Nb alloys were investigated to evaluate the potential application in artificial hip joints. The results showed that ZrO₂ coating was successfully synthesized on the surface of Zr-2.5Nb alloys, and the temperature has significantly effects on the thickness of the ZrO₂ coatings. It also found that thermal oxidation could significantly improve the surface hardness and wear resistance of Zr-2.5Nb alloys.     

Optimisation of mass ranging for atom probe microanalysis and application to the corrosion processes in Zr alloys

Atom probe tomography uses time-of-flight mass spectrometry to identify the chemical nature of atoms from their mass-to-charge-state ratios. Within a mass spectrum, ranges are defined so as to attribute a chemical identity to each peak. The accuracy of atom probe microanalysis relies on the definition of these ranges. Here we propose and compare several automated ranging techniques, tested against simulated mass spectra. The performance of these metrics compare favourably with a trial of users asked to manually range a simplified simulated dataset. The optimised automated ranging procedure was then used to precisely evaluate the very low iron concentration (0.003-0.018 at%) in a zirconium alloy to reveal its behaviour in the matrix during corrosion; oxygen is injected into solution and has the effect of increasing the local iron concentration near the oxide-metal interface, which in turn affects the corrosion properties of the metal substrate.     

EELS and optical response of a noble metal nanoparticle in the frame of a discrete dipole approximation

Surface plasmons of noble metal nanoparticles have recently been studied by Electron Energy-Loss Spectroscopy (EELS) performed in an electron microscope. We present here the basic formalism for EELS simulations in a Discrete Dipole Approximation (DDA) framework for such surface excitations. We compare EELS data and optical properties of a silver triangular nanoprism and show that the spatial variation of surface plasmon excitation probabilities allows to discriminate modes of similar energies. We also emphasize the importance of the substrate polarization effect to reliably describe experimental data.     

Structures of adsorbed zinc dithiophosphates and their relationship to engine wear

Zinc dithiophosphates (ZnDTPs) have been used extensively as anti-wear agents for over 30 years. The mechanism of action of these remarkable materials is not fully understood. In particular, the molecular composition of the antiwear film produced by the ZnDTPs remains an area requiring investigation. In this paper, the molecular composition of adsorbed ZnDTPs on aluminium oxide (Al₂O₃) surfaces has been studied using Inelastic Electron Tunnelling Spectroscopy (IETS), a highly sensitive molecular spectroscopy specifically applicable to surface species at low coverages. As such, it complements other surface science techniques for determining surface compositions. A vibrational spectroscopy, IETS utilises the literature on group characteristic frequencies available from the very extensive correlations developed for infrared and Raman spectra. Thus, functional groups may be identified that are formed by the reaction of a lubricating oil additive with a metal surface covered by a native oxide. Using IETS, we show that ZnDTP aryl esters are dissociatively chemisorbed, and do not appear to form P=S groups bonded to the surface. By contrast, primary and secondary ZnDTPs retain the phosphate ester groups and bond to the surface by the P=S bond to different degrees. These spectral differences are qualitatively correlated with the observed antiwear performance of these materials.     

Sliding Characteristics of Silver Against Iron as Influenced by Oxygen Concentration

Friction, wear and metal transfer of the silver-iron friction couple have been studied as a function of oxygen percentage of the surrounding atmosphere at different levels of temperature. The results show that the replacement of high purity argon by a mixture of argon and oxygen causes an instantaneous increase in friction, followed by local transfer of silver to the iron surface and a high rate of wear. The observed effect is reversible. It is found that in the severe wear region the diffusion of oxygen towards the friction interface determines the wear rate, although visible oxidation does not occur. Experimental evidence strongly suggests that the formation of thin films of iron oxide is responsible for increased adhesion and metal transfer.