Atomic motifs govern the decoration of grain boundaries by interstitial solutes.

Grain boundaries, the two-dimensional defects between differently oriented crystals, tend to preferentially attract solutes for segregation. Solute segregation has a significant effect on the mechanical and transport properties of materials. At the atomic level, however, the interplay of structure and composition of grain boundaries remains elusive, especially with respect to light interstitial solutes like B and C. Here, we use Fe alloyed with B and C to exploit the strong interdependence of interface structure and chemistry via charge-density imaging and atom probe tomography methods. Direct imaging and quantifying of light interstitial solutes at grain boundaries provide insight into decoration tendencies governed by atomic motifs. We find that even a change in the inclination of the grain boundary plane with identical misorientation impacts grain boundary composition and atomic arrangement. Thus, it is the smallest structural hierarchical level, the atomic motifs, that controls the most important chemical properties of the grain boundaries. This insight not only closes a missing link between the structure and chemical composition of such defects but also enables the targeted design and passivation of the chemical state of grain boundaries to free them from their role as entry gates for corrosion, hydrogen embrittlement, or mechanical failure.

Effect of Pore Formation on Redox-Driven Phase Transformation.

When solid-state redox-driven phase transformations are associated with mass loss, vacancies are produced that develop into pores. These pores can influence the kinetics of certain redox and phase transformation steps. We investigated the structural and chemical mechanisms in and at pores in a combined experimental-theoretical study, using the reduction of iron oxide by hydrogen as a model system. The redox product (water) accumulates inside the pores and shifts the local equilibrium at the already reduced material back toward reoxidation into cubic Fe_{1-x}O (where x refers to Fe deficiency, space group Fm3[over ¯]m). This effect helps us to understand the sluggish reduction of cubic Fe_{1-x}O by hydrogen, a key process for future sustainable steelmaking.

Interstitial Segregation has the Potential to Mitigate Liquid Metal Embrittlement in Iron.

The embrittlement of metallic alloys by liquid metals leads to catastrophic material failure and severely impacts their structural integrity. The weakening of grain boundaries (GBs) by the ingress of liquid metal and preceding segregation in the solid are thought to promote early fracture. However, the potential of balancing between the segregation of cohesion-enhancing interstitial solutes and embrittling elements inducing GB de-cohesion is not understood. Here, the mechanisms of how boron segregation mitigates the detrimental effects of the prime embrittler, zinc, in a Σ5 [001] tilt GB in α-Fe (4 at.% Al) is unveiled. Zinc forms nanoscale segregation patterns inducing structurally and compositionally complex GB states. Ab initio simulations reveal that boron hinders zinc segregation and compensates for the zinc-induced loss in GB cohesion. The work sheds new light on how interstitial solutes intimately modify GBs, thereby opening pathways to use them as dopants for preventing disastrous material failure.

Dual phase patterning during a congruent grain boundary phase transition in elemental copper.

The phase behavior of grain boundaries can have a strong influence on interfacial properties. Little is known about the emergence of grain boundary phases in elemental metal systems and how they transform. Here, we observe the nanoscale patterning of a grain boundary by two alternating grain boundary phases with distinct atomic structures in elemental copper by atomic resolution imaging. The same grain boundary phases are found by computational grain boundary structure search indicating a first-order transformation. Finite temperature atomistic simulations reveal a congruent, diffusionless transition between these phases under ambient pressure. The patterning of the grain boundary at room temperature is dominated by the grain boundary phase junctions separating the phase segments. Our analysis suggests that the reduced mobility of the phase junctions at low temperatures kinetically limits the transformation, but repulsive elastic interactions between them and disconnections could additionally stabilize the pattern formation.

Reconstructing dual-phase nanometer scale grains within a pearlitic steel tip in 3D through 4D-scanning precession electron diffraction tomography and automated crystal orientation mapping.

The properties of polycrystalline materials are related to their microstructures and hence a complete description, including size, shape, and orientation of the grains, is necessary to understand the behavior of materials. Here, we use Scanning Precession Electron Diffraction (SPED) in the Transmission Electron Microscope (TEM) combined with a tilt series to reconstruct individual grains in 3D within a polycrystalline dual-phase cold wire-drawn pearlitic steel sample. Nanoscale ferrite grains and intragranular cementite particles were indexed using an Automated Crystallographic Orientation Mapping (ACOM) tool for each tilt dataset. The grain orientations were tracked through the tilt datasets and projections of the individual grains were reconstructed from the diffraction data using an orientation-specific Virtual Dark Field (VDF) approach for tomographic reconstruction. The algorithms used to process and reconstruct such datasets are presented. These algorithms represent an extension to the ACOM approach that may be straightforwardly applied to other multi-phase polycrystalline materials to enable 3D spatial and orientation reconstructions.

Free, flexible and fast: Orientation mapping using the multi-core and GPU-accelerated template matching capabilities in the Python-based open source 4D-STEM analysis toolbox Pyxem.

This work presents the new template matching capabilities implemented in Pyxem, an open source Python library for analyzing four-dimensional scanning transmission electron microscopy (4D-STEM) data. Template matching is a brute force approach for deriving local crystal orientations. It works by comparing a library of simulated diffraction patterns to experimental patterns collected with nano-beam and precession electron diffraction (NBED and PED). This is a computationally demanding task, therefore the implementation combines efficiency and scalability by utilizing multiple CPU cores or a graphical processing unit (GPU). The code is built on top of the scientific Python ecosystem, and is designed to support custom and reproducible workflows that combine the image processing, template library generation, indexation and visualization all in one environment. The tools are agnostic to file size and format, which is significant in light of the increased adoption of pixelated detectors from different manufacturers. This paper details the implementation and validation of the method. The method is illustrated by calculating orientation maps of nanocrystalline materials and precipitates embedded in a crystalline matrix. The combination of speed and flexibility opens the door for automated parameter studies and real-time on-line orientation mapping inside the TEM.

Segmentation of Static and Dynamic Atomic-Resolution Microscopy Data Sets with Unsupervised Machine Learning Using Local Symmetry Descriptors.

We present an unsupervised machine learning approach for segmentation of static and dynamic atomic-resolution microscopy data sets in the form of images and video sequences. In our approach, we first extract local features via symmetry operations. Subsequent dimension reduction and clustering analysis are performed in feature space to assign pattern labels to each pixel. Furthermore, we propose the stride and upsampling scheme as well as separability analysis to speed up the segmentation process of image sequences. We apply our approach to static atomic-resolution scanning transmission electron microscopy images and video sequences. Our code is released as a python module that can be used as a standalone program or as a plugin to other microscopy packages.

Ultrastrong lightweight compositionally complex steels via dual-nanoprecipitation.

High-performance lightweight materials are urgently needed, given the pressing quest for weight reduction and the associated energy savings and emission reduction. Here, by incorporating the multi-principal element feature of compositionally complex alloys, we develop the concept of lightweight steels further and propose a new class of compositionally complex steels (CCSs). This approach allows us to use the high solid solution strengthening and shift the alloys' compositions into previously unattainable phase regions where both nanosized shearable κ-carbides and non-shearable B2 particles are simultaneously formed. The achievement of dual-nanoprecipitation in our CCSs leads to materials with ultrahigh specific tensile strength (up to 260 MPa·cm3 g-1) and excellent tensile elongation (13 to 38%), a combination outperforming all other high-strength high-entropy alloys and advanced lightweight steels. Our concept of CCSs is thus useful for guiding the design of ultrastrong lightweight metallic materials.

Observations of grain-boundary phase transformations in an elemental metal.

The theory of grain boundary (the interface between crystallites, GB) structure has a long history1 and the concept of GBs undergoing phase transformations was proposed 50 years ago2,3. The underlying assumption was that multiple stable and metastable states exist for different GB orientations4-6. The terminology 'complexion' was recently proposed to distinguish between interfacial states that differ in any equilibrium thermodynamic property7. Different types of complexion and transitions between complexions have been characterized, mostly in binary or multicomponent systems8-19. Simulations have provided insight into the phase behaviour of interfaces and shown that GB transitions can occur in many material systems20-24. However, the direct experimental observation and transformation kinetics of GBs in an elemental metal have remained elusive. Here we demonstrate atomic-scale GB phase coexistence and transformations at symmetric and asymmetric [Formula: see text] tilt GBs in elemental copper. Atomic-resolution imaging reveals the coexistence of two different structures at Σ19b GBs (where Σ19 is the density of coincident sites and b is a GB variant), in agreement with evolutionary GB structure search and clustering analysis21,25,26. We also use finite-temperature molecular dynamics simulations to explore the coexistence and transformation kinetics of these GB phases. Our results demonstrate how GB phases can be kinetically trapped, enabling atomic-scale room-temperature observations. Our work paves the way for atomic-scale in situ studies of metallic GB phase transformations, which were previously detected only indirectly9,15,27-29, through their influence on abnormal grain growth, non-Arrhenius-type diffusion or liquid metal embrittlement.

Publisher Correction: Unveiling the Re effect in Ni-based single crystal superalloys.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Unveiling the Re effect in Ni-based single crystal superalloys.

Single crystal Ni-based superalloys have long been an essential material for gas turbines in aero engines and power plants due to their outstanding high temperature creep, fatigue and oxidation resistance. A turning point was the addition of only 3 wt.% Re in the second generation of single crystal Ni-based superalloys which almost doubled the creep lifetime. Despite the significance of this improvement, the mechanisms underlying the so-called "Re effect" have remained controversial. Here, we provide direct evidence of Re enrichment to crystalline defects formed during creep deformation, using combined transmission electron microscopy, atom probe tomography and phase field modelling. We reveal that Re enriches to partial dislocations and imposes a drag effect on dislocation movement, thus reducing the creep strain rate and thereby improving creep properties. These insights can guide design of better superalloys, a quest which is key to reducing CO2 emissions in air-traffic.

Ti and its alloys as examples of cryogenic focused ion beam milling of environmentally-sensitive materials.

Hydrogen pick-up leading to hydride formation is often observed in commercially pure Ti (CP-Ti) and Ti-based alloys prepared for microscopic observation by conventional methods, such as electro-polishing and room temperature focused ion beam (FIB) milling. Here, we demonstrate that cryogenic FIB milling can effectively prevent undesired hydrogen pick-up. Specimens of CP-Ti and a Ti dual-phase alloy (Ti-6Al-2Sn-4Zr-6Mo, Ti6246, in wt.%) were prepared using a xenon-plasma FIB microscope equipped with a cryogenic stage reaching -135 °C. Transmission electron microscopy (TEM), selected area electron diffraction, and scanning TEM indicated no hydride formation in cryo-milled CP-Ti lamellae. Atom probe tomography further demonstrated that cryo-FIB significantly reduces hydrogen levels within the Ti6246 matrix compared with conventional methods. Supported by molecular dynamics simulations, we show that significantly lowering the thermal activation for H diffusion inhibits undesired environmental hydrogen pick-up during preparation and prevents pre-charged hydrogen from diffusing out of the sample, allowing for hydrogen embrittlement mechanisms of Ti-based alloys to be investigated at the nanoscale.

Joint non-rigid image registration and reconstruction for quantitative atomic resolution scanning transmission electron microscopy.

Aberration corrected scanning transmission electron microscopes (STEM) enable to determine local strain fields, composition and bonding states at atomic resolution. The precision to locate atomic columns is often obstructed by scan artifacts limiting the quantitative interpretation of STEM datasets. Here, a novel bias-corrected non-rigid registration approach is presented that compensates for fast and slow scan artifacts in STEM image series. The bias-correction is responsible for the correction of the slow scan artifacts and based on a explicit coupling of the deformations of the individual images in a series via a minimization of the average deformation. This allows to reduce fast scan noise in an image series and slow scan distortions simultaneously. The novel approach is tested on synthetic and experimental images and its implication on atomic resolution strain and elemental mapping is discussed.

Bidirectional Transformation Enables Hierarchical Nanolaminate Dual-Phase High-Entropy Alloys.

Microstructural length-scale refinement is among the most efficient approaches to strengthen metallic materials. Conventional methods for refining microstructures generally involve grain size reduction via heavy cold working, compromising the material's ductility. Here, a fundamentally new approach that allows load-driven formation and permanent refinement of a hierarchical nanolaminate structure in a novel high-entropy alloy containing multiple principal elements is reported. This is achieved by triggering both, dynamic forward transformation from a faced-centered-cubic γ matrix into a hexagonal-close-packed ε nanolaminate structure and the dynamic reverse transformation from ε into γ. This new mechanism is referred to as the "bidirectional transformation induced plasticity" (B-TRIP) effect, which is enabled through a near-zero yet positive stacking fault energy of γ. Modulation of directionality in the transformation is triggered by local dissipative heating and local micromechanical fields. The simple thermodynamic and kinetic foundations for the B-TRIP effect render this approach generally suited for designing metastable strong and ductile bulk materials with hierarchical nanolaminate substructures.

Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries.

The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning.

Segregation-Induced Nanofaceting Transition at an Asymmetric Tilt Grain Boundary in Copper.

We show that chemistry can be used to trigger a nanofaceting transition. In particular, the segregation of Ag to an asymmetric tilt grain boundary in Cu is investigated. Aberration-corrected electron microscopy reveals that annealing the bicrystal results in the formation of nanometer-sized facets composed of preferentially Ag-segregated symmetric Σ5{210} segments and Ag-depleted {230}/{100} asymmetric segments. Our observations oppose an anticipated trend to form coarse facets. Atomistic simulations confirm the nanofacet formation observed in the experiment and demonstrate a concurrent grain boundary phase transition induced by the anisotropic segregation of Ag.

Correlating Atom Probe Tomography with Atomic-Resolved Scanning Transmission Electron Microscopy: Example of Segregation at Silicon Grain Boundaries.

In the course of a thorough investigation of the performance-structure-chemistry interdependency at silicon grain boundaries, we successfully developed a method to systematically correlate aberration-corrected scanning transmission electron microscopy and atom probe tomography. The correlative approach is conducted on individual APT and TEM specimens, with the option to perform both investigations on the same specimen in the future. In the present case of a Σ9 grain boundary, joint mapping of the atomistic details of the grain boundary topology, in conjunction with chemical decoration, enables a deeper understanding of the segregation of impurities observed at such grain boundaries.

Ferritic Alloys with Extreme Creep Resistance via Coherent Hierarchical Precipitates.

There have been numerous efforts to develop creep-resistant materials strengthened by incoherent particles at high temperatures and stresses in response to future energy needs for steam turbines in thermal-power plants. However, the microstructural instability of the incoherent-particle-strengthened ferritic steels limits their application to temperatures below 900 K. Here, we report a novel ferritic alloy with the excellent creep resistance enhanced by coherent hierarchical precipitates, using the integrated experimental (transmission-electron microscopy/scanning-transmission-electron microscopy, in-situ neutron diffraction, and atom-probe tomography) and theoretical (crystal-plasticity finite-element modeling) approaches. This alloy is strengthened by nano-scaled L21-Ni2TiAl (Heusler phase)-based precipitates, which themselves contain coherent nano-scaled B2 zones. These coherent hierarchical precipitates are uniformly distributed within the Fe matrix. Our hierarchical structure material exhibits the superior creep resistance at 973 K in terms of the minimal creep rate, which is four orders of magnitude lower than that of conventional ferritic steels. These results provide a new alloy-design strategy using the novel concept of hierarchical precipitates and the fundamental science for developing creep-resistant ferritic alloys. The present research will broaden the applications of ferritic alloys to higher temperatures.

Interaction of L-Cysteine with ZnO: Structure, Surface Chemistry, and Optical Properties.

Zinc oxide (ZnO) nanoparticles (NPs) were stabilized in water using the amino acid l-cysteine. A transparent dispersion was obtained with an agglomerate size on the level of the primary particles. The dispersion was characterized by dynamic light scattering (DLS), pH dependent zeta potential measurements, scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, photoluminescence (PL) spectroscopy, and X-ray absorption fine structure (EXAFS, XANES) spectroscopy. Cysteine acts as a source for sulfur to form a ZnS shell around the ZnO core and as a stabilizer for these core-shell NPs. A large effect on the photoluminescent properties is observed: the intensity of the defect luminescence (DL) emission decreases by more than 2 orders of magnitude, the intensity of the near band edge (NBE) emission increases by 20%, and the NBE wavelength decreases with increasing cysteine concentration corresponding to a blue shift of about 35 nm due to the Burstein-Moss effect.