Assessing electron beam sensitivity for SrTiO3 and La0.7Sr0.3MnO3 using electron energy loss spectroscopy

Thresholds for beam damage have been assessed for La0.7Sr0.3MnO3 and SrTiO3 as a function of electron probe current and exposure time at 80 and 200 kV acceleration voltage. The materials were exposed to an intense electron probe by aberration corrected scanning transmission electron microscopy (STEM) with simultaneous acquisition of electron energy loss spectroscopy (EELS) data. Electron beam damage was identified by changes of the core loss fine structure after quantification by a refined and improved model based approach. At 200 kV acceleration voltage, damage in SrTiO3 was identified by changes both in the EEL fine structure and by contrast changes in the STEM images. However, the changes in the STEM image contrast as introduced by minor damage can be difficult to detect under several common experimental conditions. No damage was observed in SrTiO3 at 80 kV acceleration voltage, independent of probe current and exposure time. In La0.7Sr0.3MnO3, beam damage was observed at both 80 and 200 kV acceleration voltages. This damage was observed by large changes in the EEL fine structure, but not by any detectable changes in the STEM images. The typical method to validate if damage has been introduced during acquisitions is to compare STEM images prior to and after spectroscopy. Quantifications in this work show that this method possibly can result in misinterpretation of beam damage as changes of material properties

On the Atomic Structure of the β′′ Precipitate by Density Functional Theory

The substitution of elements into the precipitate in the 6xxx-series of aluminium alloys is an interface sensitive problem. The strain caused by the misfit between the precipitate and the matrix interacts with the size misfit of solute substitutions. The full cross-section of the precipitate and interface was simultaneously studied without the influence of periodic images by applying rigid boundary conditions in a cluster-based model that contained two regions, one fixed and one relaxed. An optimised geometry of the fixed region allows partially occupied atomic columns which enable a more precise description of the various precipitate configurations. A subtle shift in the atomic columns occurred during relaxation from the initial atomic positions in the precipitate, which breaks the notion of a 4-fold symmetry of the eyes. The underlying C2/m symmetry was intact through this shift. The methodology was applied to calculate the relative formation enthalpy of substituting lithium and copper at different atomic sites and benchmarked against previously published density functional theory and transmission electron microscopy studies. The results correspond well with expectations based on the experimental studies available.publishedVersio

Strategy for reliable strain measurement in InAs/GaAs materials from high-resolution Z-contrast STEM images

Geometric phase analysis (GPA), a fast and simple Fourier space method for strain analysis, can give useful information on accumulated strain and defect propagation in multiple layers of semiconductors, including quantum dot materials. In this work, GPA has been applied to high resolution Z-contrast scanning transmission electron microscopy (STEM) images. Strain maps determined from different g vectors of these images are compared to each other, in order to analyze and assess the GPA technique in terms of accuracy. The SmartAlign tool has been used to improve the STEM image quality getting more reliable results. Strain maps from template matching as a real space approach are compared with strain maps from GPA, and it is discussed that a real space analysis is a better approach than GPA for aberration corrected STEM images

The evolution of precipitate crystal structures in an Al-Mg-Si(-Cu) alloy studied by a combined HAADF-STEM and SPED approach

This work presents a detailed investigation into the effect of a low Cu addition (0.01 at.%) on precipitation in an Al-0.80Mg-0.85Si alloy during ageing. The precipitate crystal structures were assessed by scanning transmission electron microscopy combined with a novel scanning precession electron diffraction approach, which includes machine learning. The combination of techniques enabled evaluation of the atomic arrangement within individual precipitates, as well as an improved estimate of precipitate phase fractions at each ageing condition, through analysis of a statistically significant number of precipitates. Based on the obtained results, the total amount of solute atoms locked inside precipitates could be approximated. It was shown that even with a Cu content close to impurity levels, the Al-Mg-Si system precipitation was significantly affected with overageing. The principal change was due to a gradually increasing phase fraction of the Cu-containing Q'-phase, which eventually was seen to dominate the precipitate structures. The structural overtake could be explained based on a continuous formation of the thermally stable Q'-phase, with Cu atomic columns incorporating less Cu than what could potentially be accommodated.Comment: 13 pages, 10 figures, 2 table

Bandgap measurement of high refractive index materials by off-axis EELS

In the present work, Cs aberration corrected and monochromated scanning transmission electron microscopy electron energy loss spectroscopy STEM-EELS has been used to explore experimental set-ups that allows bandgaps of high refractive index materials to be determined. Semi-convergence and -collection angles in the micro-radian range were combined with off-axis or dark field EELS to avoid relativistic losses and guided light modes in the low loss range to contribute to the acquired EEL spectra. Off-axis EELS further suppressed the zero loss peak and the tail of the zero loss peak. The bandgap of several GaAs-based materials were successfully determined by direct inspection and without any background subtraction of the EEL spectra. The presented set-up does not require that the acceleration voltage is set to below the Cerenkov limit and can be applied over the entire acceleration voltage range of modern TEMs and for a wide range of specimen thicknesses.Comment: 16 pages, 8 figure

Precipitate/matrix incompatibilities related to the {111}Al Ω plates in an Al-Cu-Mg-Ag alloy

The atomic structure of Ω plates forming on {111}Al planes in an Al-Cu-Mg-Ag alloy has been investigated by Z-contrast atomic-resolution scanning transmission electron microscopy imaging and ab initio density functional theory calculations. Ω plates with different thicknesses have been studied in two peak-aged conditions: 150 °C for 24 h and 190 °C for 1.5 h. Volumetric and structural incompatibilities as unrelaxed misfit strains and shear components, respectively, between the Ω plates involving orthorhombic θ-phase fragments and Al matrix were found to be in the plates with thicknesses from 0 to 2.5 cθ (a normal direction to {111}Al). Two types of shear components: [−101]Al // [0−10]θ (τI) and [1−21]Al // [100]θ (τII) related to precipitate/matrix structural incompatibilities have been predicted by calculations. The shear components τI and τII have been found to be energetically favorable in the plates with different thicknesses. Comparing τI and τII absolute values in supercells involving the plates with different thicknesses, 2 cθ thick plates have a shear component close to zero. All the plates analyzed have precipitate/matrix volumetric incompatibilities with Al matrix as misfit strains along [111]Al // [001]θ, which distribute non-uniformly across the plate thickness. Large misfit strains concentrate at the broad plate interfaces, i.e. in Ag2Cu and Cui layers, and cause a prohibitively high barrier to thickening of the Ω precipitates.acceptedVersio

AutomAl 6000: Semi-automatic structural labelling of HAADF-STEM images of precipitates in Al–Mg–Si(–Cu) alloys

When the Al–Mg–Si(–Cu) alloy system is subjected to age hardening, different types of precipitates nucleate depending on the composition and thermomechanical treatment. The main hardening precipitates extend as needles, laths or rods along the directions in the aluminium matrix. It has been found that the structures of all metastable precipitates may be generalized as stacks of columns, where most of these columns are replaced by solute elements. In the precipitates, a column relates to neighbour columns by a set of simple structural principles, which allows identification of species and relative longitudinal displacement over the (100) cross-section. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is an important tool for studying such precipitates. With the goal of analysing atomic resolution HAADF-STEM images of precipitate cross-sections in the Al–Mg–Si(–Cu) system, we have developed the stand-alone software AutomAl 6000, which features a column characterization algorithm based on the symbiosis of a statistical model and the structural principles formulated in a digraph-like framework. The software can semi-autonomously determine the 3D column positions in the image, as well as column species. In turn, AutomAl 6000 can then display, analyse and/or export the structure data. This paper describes the methodology of AutomAl 6000 and applies it on three different HAADF-STEM images, which demonstrate the methodology. The software, as well as other resources, are available at http://automal.org. The source code is also directly available from https://github.com/Haawk666/AutomAl-6000.publishedVersio

Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

Stress corrosion cracking (SCC) in Cu-free Al-Zn-Mg (7xxx) aluminium alloys limits its use in many applications. In this work, we study in detail the microstructure of a peak and slightly overaged condition in an AA7003 alloy using transmission- and scanning electron microscopy in order to provide a comprehensive understanding of the microstructural features related to SCC. The SCC properties have been assessed using the double cantilever beam method and slow strain rate tensile tests. Grain boundary particles, precipitate free zones, and matrix precipitates have been studied. A difference in the SCC properties is established between the two ageing conditions. The dominating difference is the size and orientation of the hardening phases. Possible explanations correlating the microstructure and SCC properties are discussed.</jats:p

Local mechanical properties and precipitation inhomogeneity in large-grained Al–Mg–Si alloy

Al–Mg–Si (6xxx series) alloys show excellent mechanical properties due to the precipitates formed during heat treatment. However, heat treatment of these alloys results in a soft precipitation free zone (PFZ) close to grain boundaries that weakens them and promotes fracture, and thereby reduces the ductility of the material. This study provides quantitative insights into the mechanical properties and underlying plasticity behavior of Al–Mg–Si (6xxx series) alloys through combined nanoindentation hardness measurements and in-depth characterization of the microstructure adjacent to the PFZ region and in the grain interior. Experimental nanoindentation, transmission microscopy (TEM) and electron channeling contrast imaging results confirm the weakening effect from PFZ by means of a reduced hardness close to grain boundaries. The nanoindentation hardness mapping also revealed an increase in hardness a few micrometers from the grain boundary with respect to the grain interior. Precipitate quantification from TEM images confirms that the hardness increase is caused by a locally higher density of precipitates. To the authors’ best knowledge, this harder zone has not been recognized nor discussed in previously reported findings. The phenomenon has important implications for the mechanical properties of large-grained ( µm) aluminium alloys.publishedVersio