Asher C. Leff

Direct Observation of Sink-Dependent Defect Evolution in Nanocrystalline Iron under Irradiation

Crystal defects generated during irradiation can result in severe changes in morphology and an overall degradation of mechanical properties in a given material. Nanomaterials have been proposed as radiation damage tolerant materials, due to the hypothesis that defect density decreases with grain size refinement due to the increase in grain boundary surface area. The lower defect density should arise from grain boundary-point defect absorption and enhancement of interstitial-vacancy annihilation. In this study, low energy helium ion irradiation on free-standing iron thin films were performed at 573 K. Interstitial loops of a0/2 [111] Burgers vector were directly observed as a result of the displacement damage. Loop density trends with grain size demonstrated an increase in the nanocrystalline (<100 nm) regime, but scattered behavior in the transition from the nanocrystalline to the ultra-fine regime (100–500 nm). To examine the validity of such trends, loop density and area for different grains at various irradiation doses were compared and revealed efficient defect absorption in the nanocrystalline grain size regime, but loop coalescence in the ultra-fine grain size regime. A relationship between the denuded zone formation, a measure of grain boundary absorption efficiency, grain size, grain boundary type and misorientation angle is determined.

Direct Detection Electron Energy-Loss Spectroscopy: A Method to Push the Limits of Resolution and Sensitivity

In many cases, electron counting with direct detection sensors offers improved resolution, lower noise, and higher pixel density compared to conventional, indirect detection sensors for electron microscopy applications. Direct detection technology has previously been utilized, with great success, for imaging and diffraction, but potential advantages for spectroscopy remain unexplored. Here we compare the performance of a direct detection sensor operated in counting mode and an indirect detection sensor (scintillator/fiber-optic/CCD) for electron energy-loss spectroscopy. Clear improvements in measured detective quantum efficiency and combined energy resolution/energy field-of-view are offered by counting mode direct detection, showing promise for efficient spectrum imaging, low-dose mapping of beam-sensitive specimens, trace element analysis, and time-resolved spectroscopy. Despite the limited counting rate imposed by the readout electronics, we show that both core-loss and low-loss spectral acquisition are practical. These developments will benefit biologists, chemists, physicists, and materials scientists alike.

Evidence of a temperature transition for denuded zone formation in nanocrystalline Fe under He irradiation

ABSTRACT Nanocrystalline materials are radiation-tolerant materials’ candidates due to their high defect sink density. Here, nanocrystalline iron films were irradiated with 10 keV helium ions in situ in a transmission electron microscope at elevated temperatures. Grain-size-dependent bubble density changes and denuded zone occurrence were observed at 700 K, but not at 573 K. This transition, attributed to increased helium–vacancy migration at elevated temperatures, suggests that nanocrystalline microstructures are more resistant to swelling at 700 K due to decreased bubble density. Finally, denuded zone formation had no correlation with grain size and misorientation angle under the conditions studied. GRAPHICAL ABSTRACT IMPACT STATEMENT Denuded zone formation and bubble density/swelling vs. grain size trends were shown to occur over a threshold temperature in helium-irradiated nanocrystalline iron.

Coupling Quantitative Dislocation Analysis with In Situ Loading Techniques: New Insight into Deformation Mechanisms

The vast majority of our understanding about the deformation mechanisms in nanocrystalline materials is limited to information gained from experimental and theoretical characterization of FCC materials. Related behavior in nanocrystalline BCC materials is not as frequently studied, and thus outstanding questions remain regarding deformation regimes and Hall-Petch trends. Through the use of coupled insitu TEM tensile testing and quantitative dislocation density analysis via precession electron diffraction, a study of deformation in nanocrystalline iron films was performed.

Application of Electron Counting to Electron Energy-loss Spectroscopy and Implications for Low-Dose Characterization

Electron counting with direct detection (DD) sensors offers large improvements in resolution and signal to noise ratio (SNR) compared to conventional indirect detection (ID) sensors. The benefits offered by DD sensors have yielded remarkable results for low-dose imaging [1]; however, it is unclear how electron counting would affect electron energy-loss spectroscopy (EELS). Here we quantify the performance of electron counting for EELS by comparing the Gatan K2 summit (DD sensor operated in counting mode) and the Gatan US1000FTXP (ID sensor with scintillator/fiber-optic/CCD design). The results indicate DD EELS will offer major advantages for low-dose spectroscopy.

Applications of Forward Modeling to Refinement of Grain Orientations

Determination of the crystal orientation from a diffraction pattern falls in the general class of inverse problems, in which one uses the results of actual observations to infer parameter values that best characterize the system under investigation, in this case the crystal orientation. Inverse problems are inherently ill-posed and often difficult to solve. The inverse of the inverse problem is the forward problem which describes the explicit relationship between given model parameters and the resulting measurements. While the inverse problem may or may not use prior information of the physical processes involved in the experiment, the forward model is a physics-based model, compatible with known laws and principles of physics. Contrary to the inverse problem, the forward problem will always have a unique solution. The accuracy and precision of the prediction will depend on the validity of the model, but a unique solution is always guaranteed. The solutions of inverse problems can be greatly improved if we have accurate forward models for these processes.

A Statistical Dictionary Approach to Automated Orientation Determination from Precession Electron Diffraction Patterns

The precession electron diffraction (PED) technique has been widely used for structure determination since Vincent and Midgley proposed it in 1994 [1]. By acquiring electron diffraction patterns using electron beam precession, the usually strong dynamical scattering effects can be averaged out. Moreover, PED patterns can be indexed with a higher angular resolution than conventional diffraction patterns since more reflections can be collected. In order to determine the crystal orientation from a PED pattern, one would need to measure distances and angles between diffraction spots, a process that rapidly becomes tedious for multigrain samples. A faster approach would be to use a physics-based model to produce simulated diffraction patterns which are pre-calculated according to the microscope settings and the crystal parameters; this is known as a dictionary-based approach. The crystal orientation for a given PED pattern is then obtained by finding the best match against the dictionary. Rauch et al. [2] have developed an automated orientation and phase mapping method in TEM by using PED, which has been implemented in NanoMEGAS’ commercial software ASTAR. This method makes use of template-matching algorithms, where the templates are pre-calculated based on the crystal symmetry using kinematical scattering within the fundamental zone in Euler angle space. The locations and intensities of the diffraction spots in an experimental PED pattern are then cross-correlated with the template patterns to find the best match. However, the extensive use of image filtering in ASTAR can make the indexing process somewhat less reproducible because the filtering parameters used prior to cross-correlation may yield variable indexing results. Additionally, the method described in [2] does not always reliably identify orientations from patterns taken on or near grain boundaries because diffraction spots from both orientations are present in the patterns.

Nye Tensor Dislocation Density Mapping From Precession Electron Diffraction: Effects of Filtering and Angular Resolution

The Nye Tensor describes the geometrically necessary dislocation (GND) density in terms of the number of dislocations required to accommodate a given contortion in the crystal lattice. It has been used extensively in recent years to quantify GND densities from data acquired using electron backscatter diffraction (EBSD) in a scanning electron microscope [1,2]. Thanks to the development of precession electron diffraction automated crystallographic orientation mapping (PED-ACOM) for transmission electron microscopes (TEM) [3], the same type of spatially resolved orientation data produced using EBSD can now be acquired at a much smaller length-scale. By utilizing nano-scale orientation data to calculate GND densities, the detailed dislocation structure of deformed metals can be observed and quantified.