Naturally deformed biotite in contact-metamorphosed slates affected by a shear zone of the Southern Iberian Massif near Jaén (SE Spain) were studied by X-ray diffraction, optical microscopy, scanning electron microscopy, electron probe microanalysis and high-resolution transmission and analytical electron microscopy. Biotite is found in the contact metamorphism aureole produced by the intrusion of a granodioritic stock, but shear strain caused its deformation. The southern part of the shear band is strongly deformed, containing thick clay gouge zones. The northern part is less deformed and develops weaker planar-linear fabrics. X-ray diffraction data reveal the predominance of the 2M1 biotite in the undeformed samples whereas the 1M polytype is predominant in the sheared samples. Chemical data and electron images of the biotite from unsheared slates do not show the presence of intercalated phases. This biotite is almost defect-free and electron diffraction patterns have sharp reflections indicating a two-layer polytype (probably 2M1). Back-scattered electron images from the deformed biotite in the moderate deformation part of the shear zone do not reveal intergrown minerals, but the electron microprobe analyses show some Fe- and Mg-enriched compositions. Transmission electron microscopy indicates that disordered polytype packets are predominant (probably 1Md). Their electron diffraction patterns have diffuse streaking along c*. These packets have high dislocation densities, microcavities with ∼5 Å latticefringe regions (probably brucite-like sheets) and interlayering of chlorite-berthierine. Kaolinized biotite can be observed in the clay gouges from the strongly deformed south part of the shear zone. The degree of streaking, as an indication of the intensity of deformation, revealed that the disordered polytype packets are more deformed than the two-layer polytype packets. The microcavities of the disordered polytype packets could act as potential channels for transport of fluids during the shearing stage and serve as sites for chloritization of biotite, producing chlorite-berthierine domains within biotite. Berthierine is an intermediate metastable phase replaced by chlorite with along-layer transitions.

Conditioning of a metal surface in a high-voltage system is the progressive development of resistance to vacuum arcing over the operational life of the system. This is relevant for accelerator cavities, where high level of performance is only achievable after a long conditioning period. Beyond the accelerator research field, this is an important topic for any technology where breakdowns can cause device failure, either by directly disrupting device operation or by causing cumulative hardware damage.

We are developing a direct method to measure the surface resistivity of a metal surface that is being conditioned with a HV DC system by inducing a high frequency (GHz) radio-frequency current in the parallel-plate electrode system. If the system can function as a resonant cavity, the surface resistivity data would be encoded in its quality factor (Q-factor). The changes in the resistivity measured in cryogenic conditions would indicate a formation of dislocations under the surface, something that has been speculated as an important process behind the conditioning.

In this paper, we present two modified designs of the electrode system, which will act as a resonant cavity, the results of 3D EM simulations and experimental results regarding the characterization of this resonant system.

The characterization of the three-dimensional arrangement of dislocations is important for many analyses in materials science. Dislocation tomography in transmission electron microscopy is conventionally accomplished through intensity-based reconstruction algorithms. Although such methods work successfully, a disadvantage is that they require many images to be collected over a large tilt range. Here, we present an alternative, semi-automated object-based approach that reduces the data collection requirements by drawing on the prior knowledge that dislocations are line objects. Our approach consists of three steps: (1) initial extraction of dislocation line objects from the individual frames, (2) alignment and matching of these objects across the frames in the tilt series, and (3) tomographic reconstruction to determine the full three-dimensional configuration of the dislocations. Drawing on innovations in graph theory, we employ a node-line segment representation for the dislocation lines and a novel arc-length mapping scheme to relate the dislocations to each other across the images in the tilt series. We demonstrate the method for a dataset collected from a dislocation network imaged by diffraction-contrast scanning transmission electron microscopy. Based on these results and a detailed uncertainty analysis for the algorithm, we discuss opportunities for optimizing data collection and further automating the method.

The strain rate sensitivity (m) of (Ni0.92Zr0.08)100−xAlx (0 ≤ x ≤ 4 at.%) eutectic with varying average lamellae thickness (λw) in the range of 39–275 nm has been investigated in the strain rate range of 8 × 10−5 and 8 × 10−3 s−1 at room temperature. The microstructure of the nano-/ultrafine eutectic composites (NECs) is comprised of alternate lamellae of fcc γ-Ni and Ni5Zr along with 20–31 vol% γ-Ni dendritic phase. The m value of all the investigated NECs lies between 0.0080 and 0.0102, whereas the activation volume (V*) has been estimated to be between 29.7b3 and 49.8b3. High-resolution transmission electron microscopy studies confirm the dislocation-mediated plastic flow including dislocation–lamellae interaction, and their pile-up at the interface, which result in the narrow variation of m for a wide range of λw due to its interlocked lamellar microstructure. A mathematical model has been developed to correlate the m with λw for the experimented NECs with wide microstructure length scale and solute content.

With the ever-increasing importance of nanoscale deformation phenomena in contemporary technologies, basic understanding of material behavior at the nanoscale has become of critical importance. Especially, nanomechanical testing that provides the capability to study fundamental nanoscale deformation and phase change phenomena in real time and under controlled loading conditions is essential for nanomaterial research. In this study, acoustic emission (AE) was used in situ to characterize nanoindentation-induced deformation, microfracture, and phase transformation processes intrinsic of bulk single-crystal MgO and polycrystalline Al, thin films of polycrystalline SiC, and thick films of austenitic TiNi shape-memory alloy. Scale-dependent plastic deformation and microfracture affected by the indenter tip radius and the applied normal load are interpreted in terms of the type and intensity of AE events revealed by abrupt displacement excursions in the loading response of the indented materials. The amplitudes of AE waveforms are used to examine characteristic deformation, microfracture, and phase change mechanisms in the time domain. Fast Fourier transformation and short-time Fourier transformation analyses provide further insight into the material behavior and structural changes due to indentation loading in the frequency and time-frequency domain, respectively. The methodology developed in this study represents an effective approach for nanomechanical testing and in situ characterization of nanoscale deformation, microfracture, and phase transformation phenomena.

The stress field induced by an edge dislocation or a point force located near a coated triangle-like hole in an infinite plate is provided in this paper. Based on the method of analytical continuation and the technique of conformal mapping in conjunction with the alternation technique, a series solution for the displacement and stresses in the coating layer and the matrix is obtained analytically. Examples for the interaction between an edge dislocation and a coated triangle-like hole for various material constant combinations, coating thicknesses and shape factors are discussed. The analysis discovers that the so-called trapping mechanism of dislocations is more likely to exist near a coated triangle-like hole. The result shows that the dislocation will first be repelled by the coating layer and then attracted by a hole when the coating layer is slightly stiffer than the matrix. However, when the coating layer is sufficiently thin, the dislocation will always be attracted by a hole even the coating layer is stiffer than the matrix.

The effect of length scale on mechanical strength is a significant consideration for semiconductor materials. In III-V semiconductors, such as InSb, a transition from partial to perfect dislocations occurs at the brittle-to-ductile transition temperature (~150 °C for InSb). High temperature micro-compression reveals InSb to show a small size effect below the transition, similar to ceramics, while in the ductile regime it shows a size effect consistent with fcc metals. The source truncation model is found to agree with the observed trends in strength with size once the change in Burgers vector and bulk strength are taken into account.

In situ nanomechanical testing provides critical insight into the fundamental processes that lead to deformation phenomena in materials. Often, in situ tests are performed in relevant conditions such as high or low temperatures, tribological contact, gas environments, or under radiation exposure. Modern diffraction and imaging methods of materials under load provide high spatial resolution and enable extraction of quantitative mechanical data from local microstructure components or nano-sized objects. The articles in this issue cover recent advances in different types of in situ nanomechanical testing methods, spanning from dedicated nanomechanical testing platforms and microelectromechanical systems devices to deformation analyses via in situ diffraction and imaging methods. This includes scanning electron microscopy, advanced scanning transmission electron microscopy, electron diffraction, x-ray diffraction, and synchrotron techniques. Emerging areas such as in situ tribology enable novel insights into the origin of deformation mechanisms, while the evolution of microelectromechanical systems for controlled in situ testing provide opportunities for advanced control and loading strategies. Discussion on the current state of the art for in situ nanomechanical testing and future opportunities in imaging, strain sensing, and testing environments are also addressed.

The mechanical response of modern alloys results from a complex interplay between existing microstructure and its evolution with time under stress. To unravel these processes, in situ approaches intrinsically have a critical advantage to explore the basic mechanisms involving dislocations, grain boundaries (GBs), and their interactions in real time. In this article, we discuss recent findings using in situ nanomechanical testing techniques and refined crystallographic analysis tools. Advancements in in situ nanomechanics not only include multiaxial loading conditions, which bring us closer to real-world applications, but also high strain-rate testing, which is critical to compare experiments and simulations. In particular, unraveling the details of GB-based mechanisms and related microstructural changes will facilitate significant breakthroughs in our understanding of the behavior of materials on macroscopic length scales.

In situ nanomechanical testing in (scanning) transmission electron microscopy provides unique opportunities for studying fundamental deformation processes in materials. New insights have been gained by combining advanced imaging techniques with novel preparation methods and controlled loading scenarios. For instance, by applying in situ high-resolution imaging during tensile deformation of metallic nanostructures, the interplay of dislocation slip and surface diffusion has been identified as the key enabler of superplasticity. Evidence for dislocation pinning by hydrogen defect complexes has been provided by in situ imaging under cyclic pillar compression in a tunable gas environment. And, for the very first time, individual dislocations have been moved around in situ in two-dimensional materials by combining micromanipulation and imaging in a scanning electron microscope.

Post-irradiation plastic strain spreading in ferritic grains is investigated by means of three-dimensional dislocation dynamics simulations, whereby dislocation-mediated plasticity mechanisms are analyzed in the presence of various disperse defect populations, for different grain size and orientation cases. Each simulated irradiation condition is then characterized by a specific “defect-induced apparent straining temperature shift” (ΔDIAT) magnitude, reflecting the statistical evolutions of dislocation mobility. It is found that the calculated ΔDIAT level closely matches the ductile-to-brittle transition temperature shift (ΔDBTT) associated with a given defect dispersion, characterized by the (average) defect size D and defect number density N. The noted ΔDIAT/ΔDBTT correlation can be explained based on plastic strain spreading arguments and applicable to many different ferritic alloy compositions, at least within the range of simulation conditions examined herein. This systematic study represents one essential step toward the development of a fully predictive, dose-dependent fracture model, adapted to polycrystalline ferritic materials.

In this work, we investigate misfit dislocations in PbTe/PbSe heteroepitaxial systems using the concurrent atomistic–continuum (CAC) method. A potential model containing the long-range Coulombic interaction and short-range Buckingham potential is developed for the system. By considering the minimum potential energy of relaxed interface structures for various initial conditions and PbTe layer thicknesses, the equilibrium structure of misfit dislocations and the dislocation spacings in PbTe/PbSe(001) heteroepitaxial thin films are obtained as a function of the PbTe layer thicknesses grown on a PbSe substrate. The critical layer thickness above which misfit dislocations inevitably form, the structure of the misfit dislocations at the interfaces, and the dependence of average dislocation spacing on PbTe layer thickness are obtained and discussed. The simulation results provide an explanation for the narrowing of the spread of the distribution of misfit dislocation spacing as layer thickness increases in PbTe/PbSe(001) heteroepitaxy.

Predicting the structural response of advanced multiphase alloys and understanding the underlying microscopic mechanisms that are responsible for it are two critically important roles that modeling plays in alloy development. The demonstration of superior properties of an alloy, such as high strength, creep resistance, high ductility, and fracture toughness, is not sufficient to secure its use in widespread applications. Still, a good model is needed to take measurable alloy properties, such as microstructure and chemical composition, and forecast how the alloy will perform in specified mechanical deformation conditions, including temperature, time, and rate. Here, we highlight recent achievements using multiscale modeling in elucidating the coupled effects of alloying, microstructure, and mechanism dynamics on the mechanical properties of polycrystalline alloys. Much of the understanding gained by these efforts relies on the integration of computational tools that vary over many length scales and time scales, from first-principles density functional theory, atomistic simulation methods, dislocation and defect theory, micromechanics, phase-field modeling, single crystal plasticity, and polycrystalline plasticity.

Dislocation-mediated plasticity in stable nanocrystalline metals, where grain boundary motion is suppressed, is revisited in the context of dislocation elastodynamics. The effect of transient waves that emanate from the generation and motion of dislocations is quantified for an idealized Cu–10 at.% Ta system with grain sizes on the order of 50 nanometers. Simulations indicate that for this material, as dislocation velocities approach 0.6–0.8 times the shear wave speed, grains several grain diameters away from the initial glide event experience a large transient shear stress for a finite duration. These transient shear stresses increase with increasing glide velocity and can activate nucleation sites far from the original nucleation event. These considerations are used to explain recent experimental observations of a lack of increase in flow stress with increasing loading rate, as well as localization resistance, in this class of stable nanocrystalline metals.

Recent molecular dynamics simulations revealed that 〈c + a〉 dislocations in Mg were prone to dissociation on the basal plane, thus becoming sessile. Basal dissociation of 〈c + a〉 dislocations is significant because it is a major factor in the limited ductility and high work-hardening in Mg. We report an in situ transmission electron microscopy study of the deformation process using an H-bar-shaped thin foil of Mg single crystal designed to facilitate 〈c + a〉 slip, observe 〈c + a〉 dislocation activity, and establish the validity of the largely immobile 〈c + a〉 dislocations caused by the predicted basal dissociation. In addition, through detailed observations on the fine movement of some 〈c + a〉 dislocations, it was revealed that limited bowing out movement for some non-basal portions of 〈c + a〉 dislocations was possible; under certain circumstances, i.e., through attraction and reaction between two 〈c + a〉 dislocations on the same pyramidal plane, at least portions of the sessile configuration were observed to be reversed into a glissile one.

Discs of CoCuFeMnNi face centered cubic high entropy alloy were subjected to monotonic and cyclic high pressure torsion (HPT) in a single step and multiple steps of 5° forward and reverse cycle for 100° and 360° twist, respectively, under 5 GPa pressure at room temperature. It was observed that the 100° cyclic HPT sample shows the highest hardness at the periphery comparable to 360° monotonic HPT sample, while the cyclic 360° HPT sample shows the lowest hardness throughout the sample. High hardness of 100° cyclic HPT sample can be attributed to finer grain size and unstable dislocation substructure by continuous change in strain path from initial compression to forward–reverse torsion, while stable dislocation structure corresponding to shear contributes to increase in hardness from 100° to 360° for monotonic HPT sample. The unstable dislocation substructure promotes grain boundary migration–enabled grain growth leading to low hardness throughout the 360° cyclic HPT sample.

The effects of specimen size, Hall–Petch (H-P) grain or subgrain size, particle size plus spacing, and crack size on the yield strength, plastic deformation, and fracturing properties of crystalline materials are described on a dislocation mechanics basis. The size effects are assessed at relevant macro- and/or micro-and/or nano-scale dimensions; in the latter case, at the upper-limiting strength levels. The description is applied mostly to face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal close-packed (HCP) metals but also involves grain size/particle size–dependent (composite) steel material behaviors. Competition is described for the role of dislocation pile-ups versus hole-joining mechanisms for ductile failure. Grain size–dependent microhardness and strain rate sensitivity measurements are presented for nano-grain size strengthening and grain size weakening, respectively. An intrinsic size effect is demonstrated for silicon crystal nano-indentation hardness testing, which, on microscale loading, leads to evaluation of crack size dependence and, for polycrystalline alumina, to associated H-P behavior for the fracture mechanics stress intensity.

Atomic scale defects critically limit performance of semiconductor materials. To improve materials, defect effects and defect formation mechanisms must be understood. In this paper, we demonstrate multiple examples where molecular dynamics simulations have effectively addressed these issues that were not well addressed in prior experiments. In the first case, we report our recent progress on modelling graphene growth, where we found that defects in graphene are created around periphery of islands throughout graphene growth, not just in regions where graphene islands impinge as believed previously. In the second case, we report our recent progress on modelling TlBr, where we discovered that under an electric field, edge dislocations in TlBr migrate in both slip and climb directions. The climb motion ejects extensive vacancies that can cause the rapid aging of the material seen in experiments. In the third case, we discovered that the growth of InGaN films on (0001) surfaces suffers from a serious polymorphism problem that creates enormous amounts of defects. Growth on ($11\bar{2}0$) surfaces, on the other hand, results in single crystalline wurtzite films without any of these defects. In the fourth case, we first used simulations to derive dislocation energies that do not possess any noticeable statistical errors, and then used these error-free methods to discover possible misuse of misfit dislocation theory in past thin film studies. Finally, we highlight the significance of molecular dynamics simulations in reducing defects in the design space of nanostructures.