Design of Laves phase-reinforced compositionally complex alloy.

Topologically close-packed (TCP) phases such as Laves phases are usually considered to harm the mechanical properties of classical superalloys for high-temperature applications. However, if an optimal fraction and size are designed, this situation can completely change for some compositionally complex alloys (CCA). Based on existing studies on austenitic or ferritic steels, we propose in this paper a design strategy aimed at exploiting the role of the Laves phase in defining the mechanical properties of wrought CCAs at elevated temperatures. We demonstrate its efficiency by applying it to the design and production of a new Laves phase-reinforced CCA and present the results of their experimental and theoretical investigation. The results show that a new Laves phase-reinforced CCA can have fine-grained microstructures, lower density, and superior mechanical strength at elevated temperatures while maintaining workability. These new alloys show promising properties compared to existing CCA wrought alloys and actual Ni-based superalloys.

Hydrogen Evolution Reaction on Ultra-Smooth Sputtered Nanocrystalline Ni Thin Films in Alkaline Media-From Intrinsic Activity to the Effects of Surface Oxidation.

Highly effective yet affordable non-noble metal catalysts are a key component for advances in hydrogen generation via electrolysis. The synthesis of catalytic heterostructures containing established Ni in combination with surface NiO, Ni(OH)2, and NiOOH domains gives rise to a synergistic effect between the surface components and is highly beneficial for water splitting and the hydrogen evolution reaction (HER). Herein, the intrinsic catalytic activity of pure Ni and the effect of partial electrochemical oxidation of ultra-smooth magnetron sputter-deposited Ni surfaces are analyzed by combining electrochemical measurements with transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, and atomic force microscopy. The experimental investigations are supplemented by Density Functional Theory and Kinetic Monte Carlo simulations. Kinetic parameters for the HER are evaluated while surface roughening is carefully monitored during different Ni film treatment and operation stages. Surface oxidation results in the dominant formation of Ni(OH)2, practically negligible surface roughening, and 3-5 times increased HER exchange current densities. Higher levels of surface roughening are observed during prolonged cycling to deep negative potentials, while surface oxidation slows down the HER activity losses compared to as-deposited films. Thus, surface oxidation increases the intrinsic HER activity of nickel and is also a viable strategy to improve catalyst durability.

Deformation Induced Structure and Property Changes in a Nanostructured Multiphase CrMnFeCoNi High-Entropy Alloy.

A nanocrystalline CrMnFeCoNi high-entropy alloy produced using severe plastic deformation using high-pressure torsion was annealed at selected temperatures and times (450 °C for 1 h and 15 h and at 600 °C for 1 h), causing a phase decomposition into a multi-phase structure. The samples were subsequently deformed again by high-pressure torsion to investigate the possibility of tailoring a favorable composite architecture by re-distributing, fragmenting, or partially dissolving the additional intermetallic phases. While the second phase in the 450 °C annealing states had high stability against mechanical mixing, a partial dissolution could be achieved in the samples subjected to 600 °C for 1 h.

Can Severe Plastic Deformation Tune Nanocrystallization in Fe-Based Metallic Glasses?

The effects of severe plastic deformation (SPD) by means of high-pressure torsion (HPT) on the structural properties of the two iron-based metallic glasses Fe73.9Cu1Nb3Si15.5B6.6 and Fe81.2Co4Si0.5B9.5P4Cu0.8 have been investigated and compared. While for Fe73.9Cu1Nb3Si15.5B6.6, HPT processing allows us to extend the known consolidation and deformation ranges, HPT processing of Fe81.2Co4Si0.5B9.5P4Cu0.8 for the first time ever achieves consolidation and deformation with a minimum number of cracks. Using numerous analyses such as X-ray diffraction, dynamic mechanical analyses, and differential scanning calorimetry, as well as optical and transmission electron microscopy, clearly reveals that Fe81.2Co4Si0.5B9.5P4Cu0.8 exhibits HPT-induced crystallization phenomena, while Fe73.9Cu1Nb3Si15.5B6.6 does not crystallize even at the highest HPT-deformation degrees applied. The reasons for these findings are discussed in terms of differences in the deformation energies expended, and the number and composition of the individual crystalline phases formed. The results appear promising for obtaining improved magnetic properties of glassy alloys without additional thermal treatment.

Conditions near a crack tip: Advanced experiments for dislocation analysis and local strain measurement.

The local stress state and microstructure near the crack-tip singularity control the fracture process. In ductile materials multiple toughening mechanisms are at play that dynamically influence stress and microstructure at the crack tip. In metals, crack-tip shielding is typically associated with the emission of dislocations. Therefore, to understand crack propagation on the most fundamental level, in situ techniques are required that are capable to combine imaging and stress mapping at high resolution. Recent experimental advances in x-ray diffraction, scanning electron microscopy, and transmission electron microscopy enable quantifying deformation stress fields from the bulk level down to the individual dislocation. Furthermore, through modern detector technology the temporal resolution has sufficiently improved to enable stress mapping during in situ experiments.

Antibacterial activity, cytocompatibility, and thermomechanical stability of Ti40Zr10Cu36Pd14 bulk metallic glass.

This paper envisions Ti40Zr10Cu36Pd14 bulk metallic glass as an oral implant material and evaluates its antibacterial performance in the inhabitation of oral biofilm formation in comparison with the gold standard Ti-6Al-4V implant material. Metallic glasses are superior in terms of biocorrosion and have a reduced stress shielding effect compared with their crystalline counterparts. Dynamic mechanical and thermal expansion analyses on Ti40Zr10Cu36Pd14 show that these materials can be thermomechanically shaped into implants. Static water contact angle measurement on samples' surface shows an increased surface wettability on the Ti-6Al-4V surface after 48 â€‹h incubation in the water while the contact angle remains constant for Ti40Zr10Cu36Pd14. Further, high-resolution transmission and scanning transmission electron microscopy analysis have revealed that Ti40Zr10Cu36Pd14 interior is fully amorphous, while a 15 â€‹nm surface oxide is formed on its surface and assigned as copper oxide. Unlike titanium oxide formed on Ti-6Al-4V, copper oxide is hydrophobic, and its formation reduces surface wettability. Further surface analysis by X-ray photoelectron spectroscopy confirmed the presence of copper oxide on the surface. Metallic glasses cytocompatibility was first demonstrated towards human gingival fibroblasts, and then the antibacterial properties were verified towards the oral pathogen Aggregatibacter actinomycetemcomitans responsible for oral biofilm formation. After 24 â€‹h of direct infection, metallic glasses reported a >70% reduction of bacteria viability and the number of viable colonies was reduced by ∼8 times, as shown by the colony-forming unit count. Field emission scanning electron microscopy and fluorescent images confirmed the lower surface colonization of metallic glasses in comparison with controls. Finally, oral biofilm obtained from healthy volunteers was cultivated onto specimens' surface, and proteomics was applied to study the surface property impact on species composition within the oral plaque.

Mapping Shear Bands in Metallic Glasses: From Atomic Structure to Bulk Dynamics.

A deep understanding of the mechanisms controlling shear banding is of fundamental importance for improving the mechanical properties of metallic glasses. Atomistic simulations highlight the importance of nanoscale stresses and strains for shear banding, but corresponding experimental proofs are scarce due to limited characterization techniques. Here, by using precession nanodiffraction mapping in the transmission electron microscope, the atomic density and strain distribution of an individual shear band is quantitatively mapped at 2 nm resolution. We demonstrate that shear bands exhibit density alternation from the atomic scale to the submicron scale and complex strain fields exist, causing shear band segmentation and deflection. The atomic scale density alternation reveals the autocatalytic generation of shear transformation zones, while the density alternation at submicron scale results from the progressive propagation of shear band front and extends to the surrounding matrix, forming oval highly strained regions with density consistently higher (∼0.2%) than the encapsulated shear band segments. Through combination with molecular dynamic simulations, a complete picture for shear band formation and propagation is established.

Quantitative Strain and Topography Mapping of 2D Materials Using Nanobeam Electron Diffraction.

It is known that 2D materials can exhibit a nonflat topography, which gives rise to an inherent strain. Since local curvature and strain influence mechanical, optical, and electrical properties, but are often difficult to distinguish from each other, a robust measurement technique is needed. In this study, a novel method is introduced, which is capable of obtaining quantitative strain and topography information of 2D materials with nanometer resolution. Relying on scanning nanobeam electron diffraction (NBED), it is possible to disentangle strain from the local sample slope. Using the positions of the diffraction spots of a specimen at two different tilts to reconstruct the locations and orientations of the reciprocal lattice rods, the local strain and slope can be simultaneously retrieved. We demonstrate the differences to strain measurements from a single NBED map in theory, simulation, and experiment. MoS2 monolayers with different shapes are used as simulation test structures. The slope and height information are recovered, as well as tensile and angular strain which have an absolute difference of less than 0.2% and 0.2° from the theoretical values. An experimental proof of concept using a freely suspended WSe2 monolayer together with a discussion of the accuracy of the method is provided.

Tuning Hole Mobility of Individual p-Doped GaAs Nanowires by Uniaxial Tensile Stress.

Strain engineering provides an effective way of tailoring the electronic and optoelectronic properties of semiconductor nanomaterials and nanodevices, giving rise to novel functionalities. Here, we present direct experimental evidence of strain-induced modifications of hole mobility in individual gallium arsenide (GaAs) nanowires, using in situ transmission electron microscopy (TEM). The conductivity of the nanowires varied with applied uniaxial tensile stress, showing an initial decrease of ∼5-20% up to a stress of 1-2 GPa, subsequently increasing up to the elastic limit of the nanowires. This is attributed to a hole mobility variation due to changes in the valence band structure caused by stress and strain. The corresponding lattice strain in the nanowires was quantified by in situ four dimensional scanning TEM and showed a complex spatial distribution at all stress levels. Meanwhile, a significant red shift of the band gap induced by the stress and strain was unveiled by monochromated electron energy loss spectroscopy.

X-ray Diffraction Computed Nanotomography Applied to Solve the Structure of Hierarchically Phase-Separated Metallic Glass.

The structure of matter at the nanoscale, in particular that of amorphous metallic alloys, is of vital importance for functionalization. With the availability of synchrotron radiation, it is now possible to visualize the internal features of metallic samples without physically destroying them. Methods based on computed tomography have recently been employed to explore the local features. Tomographic reconstruction, while it is relatively uncomplicated for crystalline materials, may generate undesired artifacts when applied to featureless amorphous or nanostructured metallic alloys. In this study we show that X-ray diffraction computed nanotomography can provide accurate details of the internal structure of a metallic glass. We demonstrate the power of the method by applying it to a hierarchically phase-separated amorphous sample with a small volume fraction of crystalline inclusions, focusing the X-ray beam to 500 nm and ensuring a sub-micrometer 2D resolution via the number of scans.

Functional Materials Under Stress: In Situ TEM Observations of Structural Evolution.

The operating conditions of functional materials usually involve varying stress fields, resulting in structural changes, whether intentional or undesirable. Complex multiscale microstructures including defects, domains, and new phases, can be induced by mechanical loading in functional materials, providing fundamental insight into the deformation process of the involved materials. On the other hand, these microstructures, if induced in a controllable fashion, can be used to tune the functional properties or to enhance certain performance. In situ nanomechanical tests conducted in scanning/transmission electron microscopes (STEM/TEM) provide a critical tool for understanding the microstructural evolution in functional materials. Here, select results on a variety of functional material systems in the field are presented, with a brief introduction into some newly developed multichannel experimental capabilities to demonstrate the impact of these techniques.

High density of genuine growth twins in electrodeposited aluminum.

We demonstrate electrodeposition as a synthesis method for fabrication of Al coatings, up to 10 µm thick, containing a high density of genuine growth twins. This has not been expected since the twin boundary energy of pure Al is very high. TEM methods were used to analyze deposited Al and its nanoscaled twins. DFT methods confirmed that the influence of the substrate is limited to the layers close to the interface. Our findings are different from those achieved by sputtering of Al coatings restricted to a thickness less than 100 nm with twins dominated by epitaxial effects. We propose that in the case of electrodeposition, a high density of twins arises because of fast nucleation and is additionally promoted by a monolayer of adsorbed hydrogen originating from water impurities. Therefore, electrodeposition is a viable approach for tailoring the structure and properties of thicker, deposited Al coatings reinforced by twins.

Direct measurement of nanostructural change during in situ deformation of a bulk metallic glass.

To date, there has not yet been a direct observation of the initiation and propagation of individual defects in metallic glasses during deformation at the nanoscale. Here, we show through a combination of in situ nanobeam electron diffraction and large-scale molecular dynamics simulations that we can directly observe changes to the local short to medium range atomic ordering during the formation of a shear band. We observe experimentally a spatially resolved reduction of order prior to shear banding due to increased strain. We compare this to molecular dynamics simulations, in which a similar reduction in local order is seen, and caused by shear transformation zone activation, providing direct experimental evidence for this proposed nucleation mechanism for shear bands in amorphous solids. Our observation serves as a link between the atomistic molecular dynamics simulation and the bulk mechanical properties, providing insight into how one could increase ductility in glassy materials.

Exceptional fracture resistance of ultrathin metallic glass films due to an intrinsic size effect.

Metallic glasses typically fail in a brittle manner through shear band propagation but can exhibit significant ductility when the sample size is reduced below a few hundreds of nanometers. To date the size effect was mainly demonstrated for free-standing samples and the role of extrinsic setup parameters on the observed behavior is still under debate. Therefore, in the present work we investigated the mechanical properties of polymer-supported sputtered amorphous Pd82Si18 thin films with various thicknesses. We show that the films exhibit brittle fracture for thicknesses far below 100 nm. A pronounced size effect resulting in extended crack-free deformation up to 6% strain was observed only in films as thin as 7 nm - a thickness which is lower than the typical shear band thickness. This size effect results in exceptional cyclic reliability of ultrathin metallic glass films which can sustain cyclic strains of 3% up to at least 30,000 cycles without any indication of fatigue damage or electrical conductivity degradation. Since the enhancement of mechanical properties is observed at ambient conditions using inexpensive substrates and an industrially scalable sputter deposition technique, a new research avenue for utilization of ultrathin metallic glasses in microelectronics, flexible electronics or nanoelectromechanical devices is opened up.

Correlation between Electrical Transport and Nanoscale Strain in InAs/In0.6Ga0.4As Core-Shell Nanowires.

Free-standing semiconductor nanowires constitute an ideal material system for the direct manipulation of electrical and optical properties by strain engineering. In this study, we present a direct quantitative correlation between electrical conductivity and nanoscale lattice strain of individual InAs nanowires passivated with a thin epitaxial In0.6Ga0.4As shell. With an in situ electron microscopy electromechanical testing technique, we show that the piezoresistive response of the nanowires is greatly enhanced compared to bulk InAs, and that uniaxial elastic strain leads to increased conductivity, which can be explained by a strain-induced reduction in the band gap. In addition, we observe inhomogeneity in strain distribution, which could have a reverse effect on the conductivity by increasing the scattering of charge carriers. These results provide a direct correlation of nanoscale mechanical strain and electrical transport properties in free-standing nanostructures.

Optimizing disk registration algorithms for nanobeam electron diffraction strain mapping.

Scanning nanobeam electron diffraction strain mapping is a technique by which the positions of diffracted disks sampled at the nanoscale over a crystalline sample can be used to reconstruct a strain map over a large area. However, it is important that the disk positions are measured accurately, as their positions relative to a reference are directly used to calculate strain. In this study, we compare several correlation methods using both simulated and experimental data in order to directly probe susceptibility to measurement error due to non-uniform diffracted disk illumination structure. We found that prefiltering the diffraction patterns with a Sobel filter before performing cross correlation or performing a square-root magnitude weighted phase correlation returned the best results when inner disk structure was present. We have tested these methods both on simulated datasets, and experimental data from unstrained silicon as well as a twin grain boundary in 304 stainless steel.

Functionalizing Aluminum Oxide by Ag Dendrite Deposition at the Anode during Simultaneous Electrochemical Oxidation of Al.

A novel synthesis strategy is presented for depositing metallic Ag at the anode during simultaneous electrochemical oxidation of Al. This unexpected result is achieved based on galvanic coupling. Metallic dendritic nanostructures well-anchored in a high surface area supporting matrix are envisioned to open up a new avenue of applications.

In Situ Study of Lithiation and Delithiation of MoS2 Nanosheets Using Electrochemical Liquid Cell Transmission Electron Microscopy.

We report the observation of lithiation/delithiation of MoS2 nanosheets in a LiPF6/EC/DEC commercial electrolyte for the application of lithium-ion batteries using electrochemical liquid cell transmission electron microscopy (TEM). Upon discharge in a voltage range of 1.8-1.2 V, MoS2 on the Ti electrode underwent irreversible decomposition resulting in fragmentation of the MoS2 nanosheets into 5-10 nm MoS2 nanoparticles. Repeated experiments also show that some MoS2 nanosheets do not decompose upon lithiation. Instead, lithiation induced structural expansion and deformation has been observed. A solid-electrolyte interface (SEI) was formed on the anode side of the Ti electrode in contact with Li metal. The SEI layer is composed of LiF nanocrystals distributed within the entire layer with the constituent elements C, O, and F. However, no passivation film was observed on the cathode side of the Ti electrode with MoS2 nanosheets on it. Such an in situ electrochemical liquid cell TEM study sheds light on battery degradation mechanisms.

Diffraction contrast imaging using virtual apertures.

Two methods on how to obtain the full diffraction information from a sample region and the associated reconstruction of images or diffraction patterns using virtual apertures are demonstrated. In a STEM-based approach, diffraction patterns are recorded for each beam position using a small probe convergence angle. Similarly, a tilt series of TEM dark-field images is acquired. The resulting datasets allow the reconstruction of either electron diffraction patterns, or bright-, dark- or annular dark-field images using virtual apertures. The experimental procedures of both methods are presented in the paper and are applied to a precipitation strengthened and creep deformed ferritic alloy with a complex microstructure. The reconstructed virtual images are compared with conventional TEM images. The major advantage is that arbitrarily shaped virtual apertures generated with image processing software can be designed without facing any physical limitations. In addition, any virtual detector that is specifically designed according to the underlying crystal structure can be created to optimize image contrast.

Preparation of CoNi high surface area porous foams by substrate controlled electrodeposition.

We demonstrate that nanofabrication of 3D dendritic CoNi alloy foams with an open porous structure can be achieved by electrodeposition onto a single-crystalline Cu(111) substrate at ambient conditions. The very low wettability of this substrate caused by its low surface energy allows tailoring the CoNi deposit morphology. This is concluded from a comparison of polycrystalline Cu substrates with single-crystalline ones of different orientations. The advantages of the present CoNi alloy foams are low internal stresses and good mechanical stability on the substrate. In a second step, by comparing the catalytic properties of the achieved foam with those of CoNi layers obtained on polycrystalline Cu substrates, it is shown that the morphology of the CoNi layers has a decisive influence on the kinetics of the surface redox reaction. The higher reaction rate makes the open foam suitable as catalyst for oxygen evolution in electrolysers. The reversibility of the redox process provides great potential for the achieved porous layers to be used as positive material in alkaline batteries.