Performing electron microscopy and atom probe tomography at the same location on the same specimen combines the strengths of electron microscopy, which is primarily the analysis of defects and ...crystal structures, with the strengths of atom probe tomography, which is primarily the robust, accurate and sensitive three dimensional compositional analysis. This viewpoint article provides a summary of the broad range of electron microscopy techniques that have been performed on atom probe specimens to date. It describes what technique is best suited to address a specific materials science question and finishes with an outlook on possible future developments in the field.
For 5000 years, metals have been mankind's most essential materials owing to their ductility and strength. Linear defects called dislocations carry atomic shear steps, enabling their formability. We ...report chemical and structural states confined at dislocations. In a body-centered cubic Fe–9 atomic percent Mn alloy, we found Mn segregation at dislocation cores during heating, followed by formation of face-centered cubic regions but no further growth. The regions are in equilibrium with the matrix and remain confined to the dislocation cores with coherent interfaces. The phenomenon resembles interface-stabilized structural states called complexions. A cubic meter of strained alloy contains up to a light year of dislocation length, suggesting that linear complexions could provide opportunities to nanostructure alloys via segregation and confined structural states.
In many cases, the three-dimensional reconstructions from atom probe tomography (APT) are not sufficiently accurate to resolve crystallographic features such as lattice planes, shear bands, stacking ...faults, dislocations or grain boundaries. Hence, correlative crystallographic characterization is required in addition to APT at the exact same location of the specimen. Also, for the site-specific preparation of APT tips containing regions of interest (e.g. grain boundaries) correlative electron microscopy is often inevitable. Here we present a versatile experimental setup that enables performing correlative focused ion beam milling, transmission electron microscopy (TEM), and APT under optimized characterization conditions. The setup was designed for high throughput, robustness and practicability. We demonstrate that atom probe tips can be characterized by TEM in the same way as a standard TEM sample. In particular, the use of scanning nanobeam diffraction provides valuable complementary crystallographic information when being performed on atom probe tips. This technique enables the measurement of orientation and phase maps as known from electron backscattering diffraction with a spatial resolution down to one nanometer.
•Atom probe tips can be characterized in TEM like any standard TEM sample.•In principal all TEM techniques can be performed on atom probe tips.•Scanning NBD enables the measurement of orientation maps on AP tips with a lateral spatial resolution of 2nm or better.•Grain orientations can be measured by NBD even in the topmost 10nm of an AP tip.
•Segregation engineering: grain boundary manipulation by solute decoration.•A concept to manipulate grain boundary structure, composition and properties.
Grain boundaries influence mechanical, ...functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as ‘segregation engineering’ solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing.
Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number ...of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation.
In this article, we probe the strain partitioning between the microstructural features present in a continuously cooled carbide-free bainitic steel together with damage nucleation and propagation. ...These features mainly comprise of phases (bainitic ferrite, martensite, and blocky/thin film austenite), interfaces between them, grain size and grain morphology. A micro Digital Image Correlation (μ-DIC) technique in scanning electron microscope is used to quantify the strain distribution between these microstructural features. The results show a strong strain partitioning between martensite, bainitic ferrite and retained austenite that provides weak links in the microstructure and creates conditions for the crack initiation and propagation during deformation. Blocky austenite islands accommodate maximum local strains in the global strain range of 0–2.3% and undergo strain-induced austenite to martensite transformation governing the local strain evolution in the microstructure. However, the local strains are minimum in martensite regions during entire in-situ deformation stage. Narrow bainitic ferrite channels in between martensitic islands and martensite-bainitic ferrite interfaces are recognised as primary damage sites with high strain accumulation of 30 ± 2% and 20 ± 3% respectively, at a global strain of 9%. The inclination of these interfaces with the tensile direction also affects the strain accumulation and damage.
Purpose To identify materials and processes which effect supersaturation of the GI milieu for low solubility drugs in order to increase oral bioavailability. Methods A variety of small and polymeric ...molecules were screened for their ability to inhibit drug precipitation in supersaturated solutions. The best polymeric materials were utilized to create spray-dried dispersions (SDDs) of drug and polymer, and these were tested for drug form and homogeneity. Dispersions were tested in vitro for their ability to achieve and maintain drug supersaturation, for a variety of drug structures. Results Of the 41 materials tested, HPMCAS was the most effective at maintaining drug supersaturation. Drug/HPMCAS SDDs were consistently more effective at achieving and maintaining drug supersaturation in vitro than were SDDs prepared with other polymers. Drug/HPMCAS SDDs were effective in vitro for eight low solubility drugs of widely varying structure. Drug/HPMCAS SDDs were more effective at achieving and maintaining supersaturation than were rotoevaporated Drug/HPMCAS dispersions or physical mixtures of Drug and HPMCAS. The degree of achievable drug supersaturation increased with increasing polymer content in the SDD. The drug in Drug /HPMCAS SDDs was amorphous, and the dispersions were demonstrated to have a single glass transition and were thus homogeneous. Conclusion HPMCAS has been identified as a uniquely effective polymer for use in SDDs of low solubility drugs, with broad applicability across a variety of drug structures and properties.
White etching cracks (WECs) and the associated white etching areas (WEAs) are responsible for failure of widely spread engineering applications such as bearings and railways. Although the phenomenon ...is known for more than 100 years, the underlying mechanisms are still a matter of debate. In this work, we thoroughly investigate a 100Cr6 wind turbine gearbox bearing after failure in service operation. Based on our findings from detailed microstructure characterization on multiple length scales we formulate a new consistent explanation for the formation of WEAs during rolling contact fatigue. We propose a mechanism of moving WECs - not only in terms of conventional crack propagation but also as a movement of the crack normal to its plane. During cyclic loading the crack continuously changes its position and leaves behind a severely plastically deformed area consisting of ferritic nano-grains, i.e. the WEAs. The atomic-scale delocalization of the crack plane in a single loading cycle adds up to micron-sized WEAs during repetitive loading/unloading. After the initial formation of a fatigue crack around inclusions, crack face rubbing occurs during compressive loading cycles. This leads to the formation of WEA by local severe plastic deformation. It also leads to partial cohesion of the abutting crack faces and material transport between them. As a result, the WEC opens at a slightly shifted position with respect to its former location during unloading.
Premature failure of rail and bearing steels by White-Etching-Cracks leads to severe economic losses. This failure mechanism is associated with microstructure decomposition via local severe plastic ...deformation. The decomposition of cementite plays a key role. Due to the high hardness of this phase, it is the most difficult obstacle to overcome in the decaying microstructure. Understanding the mechanisms of carbide decomposition is essential for designing damage-resistant steels for industrial applications.
We investigate cementite decomposition in the bearing steel 100Cr6 (AISI 52100) upon exposure to high-pressure torsion (maximum shear strain, Ƴmax = 50.2). Following-up on our earlier work on cementite decomposition in hardened 100Cr6 steel (Qin et al., Act. Mater. 2020 1), we now apply a modified heat treatment to generate a soft-annealed microstructure where spherical and lamellar cementite precipitates are embedded in a ferritic matrix. These two precipitate types differ in morphology (spherical vs. lamellar), size (spherical: 100–1000 nm diameter, lamellar: 40–100 nm thickness) and composition (Cr and Mn partitioning). We unravel the correlation between cementite type and its resistance to decomposition using multi-scale chemical and structural characterization techniques.
Upon high-pressure torsion, the spherical cementite precipitates did not decompose, but the larger spherical precipitates (≥ 1 μm) deformed. In contrast, the lamellar cementite precipitates underwent thinning followed by decomposition and dissolution. Moreover, the decomposition behavior of cementite precipitates is affected by the type of matrix microstructure. We conclude that the cementite size and morphology, as well as the matrix mechanical properties are the predominating factors influencing the decomposition behavior of cementite. The compositional effects of Cr and Mn on cementite stability calculated by complementary density functional theory (DFT) calculations are minor in the current scenario.
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