Mechanical properties of microalloyed steels are enhanced by fine precipitates, that ensure grain growth control during subsequent heat treatment. This study aims at predicting austenite grain growth kinetics coupling a precipitation model and a grain growth model. These models were applied to a titanium and niobium microalloyed steel. The precipitate size distributions were first characterized by TEM and SEM and prior austenite grain boundaries were revealed by thermal etching after various isothermal treatments. From CALPHAD database, a solubility product was determined for (Ti,Nb)C precipitates. A numerical model based on the classical nucleation and growth theories was used to predict the time evolution of (Ti,Nb)C size distributions during various isothermal heat treatments. The precipitation model was validated from TEM/SEM analysis. The resulting precipitate size distributions served as entry parameters to a simple grain growth model based on Zener pinning. The pinning pressure was calculated using the whole size distribution. The resulting austenite grain growth kinetics were in good agreement with the experimental data obtained for all investigated heat treatments.

Electronic states are responsible for most material properties, including chemical bonds, electrical and thermal conductivity, as well as optical and magnetic properties. Experimentally, however, they remain mostly elusive. Here, we report the real-space mapping of selected transitions between p and d states on theÅngström scale in bulk rutile (TiO 2 ) using electron energy-loss spectrometry (EELS), revealing information on individual bonds between atoms. On the one hand, this enables the experimental verification of theoretical predictions about electronic states. On the other hand, it paves the way for directly investigating electronic states under conditions that are at the limit of the current capabilities of numerical simulations such as, e.g., the electronic states at defects, interfaces, and quantum dots.Electronic states shape the world around us as their characteristics give rise to nearly all macroscopical properties of materials. Be it optical properties such as colour and refractive index, chemical properties such as bonding and valency, mechanical properties such as adhesion, strength and ductility, electromagnetic properties such as conductance and magnetisation, or the properties of trap states: ultimately, all these properties can be traced back to the electronic states in the material under investigation. Therefore, it is not surprising that electronic states are of paramount importance across many fields, including physics, materials science, chemistry and the life sciences. It does come as a surprise, however, that while some of their aspects can be inferred indirectly from macroscopical material properties or some diffraction techniques, the direct observation of individual electronic states in real space so far has succeeded only under very special circumstances (e.g. on an insulating surface using a scanning tunnelling microscope (STM) with a specially functionalised tip [1]) due to both experimental and theoretical challenges. In this work, we endeavour to remedy this situation by using a combination of transmission electron microscopy (TEM), electron energy-loss spectrometry (EELS), and state-of-the-art simulations.TEM is a well-known technique for studying materials on the nanoscale while EELS adds element-specific information. Both are widely-used on a regular basis in many fields and are readily commercially available. Owing to these two techniques, tremendous progress has been made over the last decade in mapping atom positions with ≈ 10 pm accuracy [2-4], determining charge densities [5][6][7], and performing atom-by-atom chemical mapping [8][9][10][11][12]. Furthermore, the fine-structures of the spectra allow the determination of the local chemical and structural environment as well as the hybridisation state of the scattering atoms [11][12][13][14][15][16][17][18] in the bulk, which can be substantially different from the surface states probed by STM. This suggests to use the EELS signal to probe the local environment in real-space and map, e.g., crystal fields, conduction states,...

Biodegradation of magneto‐plasmonic nanohybrids (magnetic, gold‐decorated nanoflowers) are investigated within a multi‐faceted cellular environment by Ali Abou‐Hassan, Claire Wilhelm, and co‐workers in article number 1605997. Two extreme transformations are evidenced: the gold seeds are either released from magnetic cores which almost entirely dissolve and load ferritin with free iron (left), or the gold shell almost completely protects the magnetic core from dissolution (right).

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BaTiO 3 (BTO) is a widely studied material with several potential applications as a result of its intrinsic ferroelectricity. It undergoes multiple structural phase transitions across a range of accessible temperatures, which have an effect on its ferroelectric properties. While BTO is ferroelectric in its low‐temperature phases—rhombohedral below ∼183 K, orthorhombic in the range ∼183–273 K, and tetragonal in the range ∼273–393 K;  it becomes paraelectric above ∼393 K [1]. The ferroelectricity of BTO is directly related to the deviation of the TiO 6 octahedra from perfectly undistorted units, which is linked to the off‐centering of the Ti 4+ cation within a octahedron constituted of six O atoms. Nevertheless, the phase transition mechanisms are still widely discussed, and the exact structure of the paraelectric phase remains unclear. Probing the structural distortion within TiO 6 octahedra of the different BTO phases is therefore of particular interest, especially at the nanoscale for BTO thin films and nanostructures. Recently, it was shown that the O‐K energy‐loss near‐edge structures (ELNES) permitted the probing of this subtle structural distortion [2]. The broadening of the ELNES at lower energy is directly related to the Ti 4+ off‐centering. The O‐site symmetry affects the core‐hole potential created during excitation, which then induces the broadening in the ELNES. In this contribution, the structural distortion of BaTiO 3 (BTO) is studied in its ferroelectric (rhombohedral and tetragonal), and paraelectric phases from the O‐K and Ti‐L 23 near‐edge structures in electron energy‐loss spectroscopy [3]. The high energy‐resolution O‐K and Ti L 23 ELNES of ferroelectric and paraelectric BTO are recorded in a monochromated scanning transmission electron microscope (STEM), using cooling and heating stages to reach the phase transitions in a single crystal thin foil. Modifications of the electronic structure are detected in the lowest energy fine structure of the O‐K edge in the ferroelectric phases (Fig. 1a), and are interpreted by core‐hole valence‐electron screening geometry (Fig. 1c). The broader and more asymmetric lowest energy fine structure at low temperature, suggest that the magnitude of the Ti 4+ off‐centering along ⟨111⟩ increases in lower‐temperature phases. Interestingly, the lowest energy fine structure of the paraelectric phase is comparable to the one obtained at room temperature, hence supporting reports in the literature that the paraelectric phase is actually not cubic [4]. First principles calculations support these experimental evidences: they confirm that the lowest energy fine structure of the O‐K edge is broader for a lower O‐site symmetry, but do not reproduce the asymmetry and the overall shape of this fine structure (Fig. 1b). These discrepancies are ascribed to the approximations inherent to the static core‐hole used within the DFT framework. Furthermore, while the Ti‐L 23 ELNES is commonly used to probe and interpret the structural distortions in titanates, we show that they are only as sensitive as the O‐K ELNES to the structural distortions in BTO (Fig. 2). This finding indicates that the O‐K edge can be used instead of, or complementary to, the Ti‐L 23 edge to probe the structural distortion, and therefore the ferroelectricity, in BTO. The sensitivity of the O‐K edge to subtle structural distortions in BTO shows a new way to probe and better understand the ferroelectricity at the nanoscale, on defective or strained BTO thin films for example [5].

The front cover artwork for Issue 22/2015 is provided by the Microscopy of Nanoscale Materials Research Group at McMaster University, Canada. The image depicts the marriage between in situ heating and advanced electron microscopy techniques that enabled this research team to track structural, compositional, and configurational evolution of the same alloy nanoparticle over the course of heat treatment. See the Full Paper itself at http://dx.doi.org/10.1002/cctc.201500380.

Ternary InGaN and AlGaN alloys have been sought after for the application of various optoelectronic devices spanning a large spectral range between the deep ultraviolet (DUV) and infrared (IR), including light‐emitting diodes, and laser diodes. Conventional planar devices suffer from a high density of dislocations due to the large lattice mismatch, which together with the non‐ideal alloy mixing, are established as the cause for various phase separation, surface segregation, and chemical ordering processes commonly observed in nitride alloys. Growth in a nanowire (NW) geometry can overcome these processes by providing enhanced strain relaxation at the free surfaces. In both InGaN and AlGaN, their superior operational characteristics can be attributed to enhanced charge carrier localization at alloy inhomogeneities down to the atomic‐scale. Atomic‐level chemical ordering in wurtzite InGaN and AlGaN epilayers, describing preferential site occupancy of the cation sublattice by the group III atoms, has been reported mostly with a 1:1 periodicity along the [0001] growth direction [1]. Reports of atomic ordering in cubic ternary III‐V alloys (including III‐As and III‐P) have remained limited to planar thin films; its prevalence within NWs had not been explored. InGaN/GaN dot‐in‐a‐wire nanostructures grown on Si(111) by molecular beam epitaxy (MBE) were recently developed to achieve more controlled light emission across the entire visible spectrum [2], and characterized using aberration‐corrected scanning transmission electron microscopy (STEM) [3]. High‐angle annular dark‐field (HAADF) Z‐contrast imaging shows the InGaN quantum dots (QDs) with atypical oscillating HAADF image intensity at the atomic‐level along the c ‐axis growth direction, exhibiting alternating bright/dark atomic‐planes within the QDs [3]. Electron diffraction patterns obtained from the QDs show the presence of otherwise forbidden superlattice reflections, unambiguously confirming the presence of 1:1 bilayer atomic ordering [1]. In addition, atomic‐resolution elemental mapping using electron energy‐loss spectroscopy (EELS) shows significant In‐enrichment in alternating c ‐planes matching the maxima in the ADF signal collected concurrently, with a deviation from the local mean composition by >25%. Corresponding annular bright field imaging (ABF) enables the visualization of light elements like N, and was used to directly deduce the NWs as N‐face polarity. It also indicates that the In‐atoms have a preferential occupation at the lower‐coordination site along a pyramidal surface facet, which is the first experimental evidence [3] validating the existing theoretical structure model for ordered InGaN layers [4]. Compositional inhomogeneities were also investigated in MBE‐grown self‐catalyzed AlGaN NWs, which exhibit high luminescence efficiency in the DUV range [5]. With increasing Al concentration, atomic‐scale compositional modulations can be induced due to differences in Ga‐ and Al‐adatom migration and incorporation at the growth front. The modulating HAADF intensities were confirmed as Ga‐rich/Al‐rich regions using EELS elemental mapping at atomic‐resolution. Furthermore, their QD/quantum dash‐like nature was determined based on multi‐orientation views of the same atomic‐scale Ga‐rich regions. Such atomic‐scale compositional modulations in AlGaN can provide energy band fluctuations leading to strong three‐dimensional confinement of charge carriers [6].

This graph shows the perspective view of two connecting atomic models, for two different interface reconstructions, reported in the article number https://doi.org/10.1002/admi.201701664 by Fang Liu, Guo‐zhen Zhu and co‐workers. The warm background (near red) stands for the high heat‐treatment temperature, while, the cold background (near blue) is for the low heat‐treatment temperature. This graph perfectly demonstrates the idea of temperature‐induced interface reconstructions, the structure and their transition.