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Electron and proton microprobes, along with electron backscatter diffraction (EBSD) analysis were used to study the microstructure of the contemporary Al–Cu–Li alloy AA2099-T8. In electron probe microanalysis, wavelength and energy dispersive X-ray spectrometry were used in parallel with soft X-ray emission spectroscopy (SXES) to characterize the microstructure of AA2099-T8. The electron microprobe was able to identify five unique compositions for constituent intermetallic (IM) particles containing combinations of Al, Cu, Fe, Mn, and Zn. A sixth IM type was found to be rich in Ti and B (suggesting TiB2), and a seventh IM type contained Si. EBSD patterns for the five constituent IM particles containing Al, Cu, Fe, Mn, and Zn indicated that they were isomorphous with four phases in the 2xxx series aluminium alloys including Al6(Fe, Mn), Al13(Fe, Mn)4 (two slightly different compositions), Al37Cu2Fe12 and Al7Cu2Fe. SXES revealed that Li was present in some constituent IM particles. Al SXES mapping revealed an Al-enriched (i.e., Cu, Li-depleted) zone in the grain boundary network. From the EBSD analysis, the kernel average misorientation map showed higher levels of localized misorientation in this region, suggesting greater deformation or stored energy. Proton-induced X-ray emission revealed banding of the TiB2 IM particles and Cu inter-band enrichment.
Understanding oxide–metal interfaces is crucial to the advancement of materials and components for many industries, most notably for semiconductor devices and power generation. Atom probe tomography provides three-dimensional, atomic scale information about chemical composition, making it an excellent technique for interface analysis. However, difficulties arise when analyzing interfacial regions due to trajectory aberrations, such as local magnification, and reconstruction artifacts. Correlative microscopy and field simulation techniques have revealed that nonuniform evolution of the tip geometry, caused by heterogeneous field evaporation, is partly responsible for these artifacts. Here we attempt to understand these trajectory artifacts through a study of the local evaporation field conditions. With a better understanding of the local evaporation field, it may be possible to account for some of the local magnification effects during the reconstruction process, eliminating these artifacts before data analysis.
When analyzing an unknown by electron-excited energy dispersive X-ray spectrometry, with the entire periodic table possibly in play, how does the analyst discover minor and trace constituents when their peaks are overwhelmed by the intensity of an interfering peak(s) from a major constituent? In this paper, we advocate for and demonstrate an iterative analytical approach, alternating qualitative analysis (peak identification) and standards-based quantitative analysis with peak fitting. This method employs two “tools”: (1) monitoring of the “raw analytical total,” which is the sum of all measured constituents as well as any such as oxygen calculated by the method of assumed stoichiometry, and (2) careful inspection of the “peak fitting residual spectrum” that is constructed as part of the quantitative analysis procedure in the software engine DTSA-II (a pseudo-acronym) from the National Institute of Standards and Technology. Elements newly recognized after each round are incorporated into the next round of quantitative analysis until the limits of detection are reached, as defined by the total spectrum counts.
5xxx series aluminum alloys, as Al-4.5Mg-1.0Mn (AA5083), are strengthened by Mg solid solution and work hardening. A drawback of this alloy is the fact that β phase, Al3Mg2, can precipitate on grain boundaries causing sensitization and intergranular corrosion, which is detrimental to the integrity of the structure. Metallography is an important technique to study the grain structure and highly sought for intergranular corrosion evaluation; however, revealing the grains of completely un-sensitized AA5083 is challenging. This paper introduces a new procedure to etch AA5083 samples that were solutionized at 450°C for 1.5 h. The new procedure is a two-step etching method, including a phosphoric acid pre-etching step and a Weck’s reagent coloring step. Solutionized, lightly sensitized, and as-received AA5083 were evaluated, and the grains were observed using optical microscopy. The microetching mechanism was further studied by optical profilometry, atomic force microscopy, scanning electron microscopy, and energy dispersive spectrometry. The phosphoric acid created a surface profile determined by the grain orientations and its reactivity, and the Weck’s reagent was then able to color grains by preferential MnO2 formation over some pre-etched grains. Moreover, the final polishing with colloidal silica was essential to reach a high contrast image.
Layered transition metal dichalcogenides (TMDs) have attracted interest due to their promise for future electronic and optoelectronic technologies. As one approaches the two-dimensional (2D) limit, thickness and local topology can greatly influence the macroscopic properties of a material. To understand the unique behavior of TMDs it is therefore important to identify the number of atomic layers and their stacking in a sample. The goal of this work is to extract the thickness and stacking sequence of TMDs directly by matching experimentally recorded high-angle annular dark-field scanning transmission electron microscope images and convergent-beam electron diffraction (CBED) patterns to quantum mechanical, multislice scattering simulations. Advantageously, CBED approaches do not require a resolved lattice in real space and are capable of neglecting the thickness contribution of amorphous surface layers. Here we demonstrate the crystal thickness can be determined from CBED in exfoliated 1T-TaS2 and 2H-MoS2 to within a single layer for ultrathin ≲9 layers and ±1 atomic layer (or better) in thicker specimens while also revealing information about stacking order—even when the crystal structure is unresolved in real space.
A method is presented to determine the spatial distribution of electrons in the focused beam of a scanning electron microscope (SEM). Knowledge of the electron distribution is valuable for characterizing and monitoring SEM performance, as well as for modeling and simulation in computational scanning electron microscopy. Specifically, it can be used to characterize astigmatism as well as study the relationship between beam energy, beam current, working distance, and beam shape and size. In addition, knowledge of the distribution of electrons in the beam can be utilized with deconvolution methods to improve the resolution and quality of backscattered, secondary, and transmitted electron images obtained with thermionic, FEG, or Schottky source instruments. The proposed method represents an improvement over previous methods for determining the spatial distribution of electrons in an SEM beam. Several practical applications are presented.
Cryo-electron microscopy (cryo-EM) is a powerful tool for macromolecular to near-atomic resolution structure determination in the biological sciences. The specimen is maintained in a near-native environment within a thin film of vitreous ice and imaged in a transmission electron microscope. The images can then be processed by a number of computational methods to produce three-dimensional information. Recent advances in sample preparation, imaging, and data processing have led to tremendous growth in the field of cryo-EM by providing higher resolution structures and the ability to investigate macromolecules within the context of the cell. Here, we review developments in sample preparation methods and substrates, detectors, phase plates, and cryo-correlative light and electron microscopy that have contributed to this expansion. We also have included specific biological applications.
Electron backscattered diffraction (EBSD) is a technique regularly used to obtain crystallographic information from inorganic samples. When EBSD is acquired simultaneously with emitting diodes data, a sample can be thoroughly characterized both structurally and compositionally. For organic materials, coherent Kikuchi patterns do form when the electron beam interacts with crystalline material. However, such patterns tend to be weak due to the low average atomic number of organic materials. This is compounded by the fact that the patterns fade quickly and disappear completely once a critical electron dose is exceeded, inhibiting successful collection of EBSD maps from them. In this study, a new approach is presented that allows successful collection of EBSD maps from organic materials, here the extreme example of a hydrocarbon organic molecular thin film, and opens new avenues of characterization for crystalline organic materials.
The work is to investigate the relationships between the microstructures and mechanical behaviors of lobster cuticles and reveal the inner mechanisms of the anisotropic mechanical properties of the cuticles and give the helpful guidance for the design of high-performance man-made composites. First, the tensile mechanical properties of the longitudinal and transverse specimens of the cuticles of American lobsters were tested with a mechanical-testing instrument. It is was found that the fracture strength and elastic modulus of the longitudinal specimens are distinctly larger than those of the transverse specimens. Then, the microstructural characteristics of the fracture surfaces of the specimens were observed with scanning electron microscope. It was observed that the pore canals in the cuticles are elliptic and their orientations are along the longitudinal orientation of the cuticles. Furthermore, the stresses and micro-damage of the longitudinal and transverse specimens were calculated with the rule of progressive damage by finite element method. It was revealed that the shape and orientation of the pore canals in the cuticles give rise to the anisotropic mechanical property of the cuticles and ensure that the cuticles possess the largest fracture strength and elastic modulus along their largest main-stress orientation.
The antennal sensilla of female Quadrastichus mendeli Kim & La Salle (Hymenoptera: Eulophidae: Tetrastichinae) were observed with scanning and transmission electron microscopy in this study. The antenna of Q. mendeli was geniculate, and the flagellum was composed of seven subsegments. Six distinct types of sensory receptors were observed, including sensilla basiconic capitate peg, sensilla böhm, sensilla chaetica, sensilla campaniformia, sensilla placodea and sensilla trichodea. Sensilla basiconic capitate pegs were found on the flagellomeres, and Böhm sensilla were found on the basal part of scape and the pedicel. Two morphological subtypes of sensilla chaetica were found on the antennae, and sensilla campaniformia were only found on the pedicel. Sensilla placodea were divided into two morphological subtypes that were found on the flagellomeres. Sensilla trichodea were found on the 2nd–6th flagellomere. By comparison to existing antennal sensilla, it was found that sensilla basiconic capitate peg, sensilla chaetica, sensilla placodea and sensilla trichodea were the most common sensilla of the parasitoids of Eulophidae. The external and internal morphology, types, number, distribution, length, and width of these sensilla were described, and their possible functions are discussed in conjunction with the host-detection behavior. Future studies on the host location mechanisms in Q. mendeli will be facilitated by these observations.
The distribution of lignin, 8-5′ and 8-8′ linked lignin substructure, and noncellulosic polysaccharides in hemp (Cannabis sativa L.) phloem fibers were explored based on histochemical and immunological methods. Ultraviolet absorption and potassium permanganate staining were observed mainly in the compound middle lamella (CML) and S1 layers, and rarely in the G-layer of phloem fibers, suggesting that lignin concentration is high at the CML and S1 layers, and very low at the G-layer of hemp fibers. Acriflavine staining, uniform KM1 labeling (8-5′ linked lignin substructure), and no KM2 labeling (8-8′ linked structure) were observed in the G-layer, suggesting that there is a small amount of lignin-like compound with 8-5′ linked structure in the G-layer. In addition, some fiber cells showed a multilayered structure. Uniform arabinogalactan protein (AGP) labeling was observed on the S1 layers and G-layers using JIM14, but little appeared in the CML of hemp fibers, indicating that these layers of the phloem fibers contain AGP. Immunogold labeling of xylan (LM11) and glucomannan (LM21) showed that xylan and glucomannan were mainly present in the S1 layers and the G-layers, respectively. In some phloem fibers, LM21 immunofluorescence labeling showed multilayered structure, suggesting the heterogeneous distribution of glucomannan.