Kink Formation Research Articles

Stressed during cycling: Electrochemically induced mechanical deformation in Ni-rich cathode materials

The polycrystalline architecture of Ni-rich cathode materials faces complex stresses during electrochemical cycling. Recently in Matter, Wang et al. discussed different modalities of the O3-O1 phase transformation due to electrochemically induced mechanical deformation using aberration-corrected scanning transmission electron microscopy and applying an artificial-intelligence-assisted super-resolution algorithm. The polycrystalline architecture of Ni-rich cathode materials faces complex stresses during electrochemical cycling. Recently in Matter, Wang et al. discussed different modalities of the O3-O1 phase transformation due to electrochemically induced mechanical deformation using aberration-corrected scanning transmission electron microscopy and applying an artificial-intelligence-assisted super-resolution algorithm. Direct observation of chemomechanical stress-induced phase transformation in high-Ni layered cathodes for lithium-ion batteriesWang et al.MatterFebruary 27, 2023In BriefMechanical degradation and phase transformation are two main causes of the failure of battery cathodes. Aided by deep-learning-aided super-resolution imaging, we discover that severe chemomechanical deformation during battery operation triggers phase transformation and the formation of bending bands and kinks in layered oxide cathodes. The failure mechanism uncovers the connection between mechanical degradation and phase transformation in layered oxides, and it provides useful guidance for the development of advanced layered cathodes for next-generation lithium-ion batteries. Full-Text PDF

Structural Anisotropy Governs the Kink Formation in Cellulose Nanocrystals

Understanding the defect structure is fundamental to correlating the structure and properties of materials. However, little is known about the defects of soft matter at the nanoscale beyond their external morphology. We report here on the molecular-level structural details of kink defects of cellulose nanocrystals (CNCs) based on a combination of experimental and theoretical methods. Low-dose scanning nanobeam electron diffraction analysis allowed for correlation of the local crystallographic information and nanoscale morphology and revealed that the structural anisotropy governed the kink formation of CNCs. We identified two bending modes along different crystallographic directions with distinct disordered structures at kink points. The drying strongly affected the external morphology of the kinks, resulting in underestimating the kink population in the standard dry observation conditions. These detailed defect analyses improve our understanding of the structural heterogeneity of nanocelluloses and contribute to the future exploitation of soft matter defects.

Kink Formation and Strengthening Effects in TiNi–V Eutectic Alloys with Mille-Feuille Structure

Kink formation by rolling and its strengthening effects on the mechanical properties of TiNi-V alloy were investigated. The as-cast TiNi-V alloy had a eutectic lamellar structure consisting of B2-TiNi and bcc-V phases. When the alloy was rolled, kinks were formed in the layered structure. The Vickers hardness of the alloy increased after it was subjected to 30% rolling reduction, and then decreased with increasing annealing temperature. After the rolled alloy was annealed at 973 K, its hardness recovered to the original value; however, the layer structure and kinks remained in the alloy. Thus, both dislocations and kinks were introduced into the alloy by rolling, but only dislocations disappeared upon annealing. Therefore, a layered TiNi-V alloy with only kinks but no dislocations was obtained. A comparison of the yield stresses of as-cast, 30% rolled, and annealed alloy specimens revealed that the magnitude of strengthening by dislocations and kinks was about 170 and 40 MPa, respectively. Therefore, kink formation and kink strengthening effects similar to those observed in Mg-based long-period stacking ordered (LPSO) alloys were observed.

Orientation Dependence of High Temperature Compressive Behavior of Textured Ti<sub>3</sub>SiC<sub>2</sub>

In order to clarify the orientation dependent deformation behavior of the MAX phase ceramics, compressive deformation behavior was examined in a textured Ti3SiC2 (TSC) at a high temperature of 1200°C. Depending on the relationship between the texture and loading directions, both the deformation behavior and microstructure were strongly influenced, and the resultant basal slip, kink formation and delamination affected the compression behavior of the textured TSC. When the stress was loaded parallel or perpendicular to the basal plane (0TSC and 90TSC), the stress-strain (S-S) curves showed higher peak stresses followed by the reduction in the flow stress. When the stress was loaded 45° to the basal plane (45TSC), the S-S curve showed strain hardening after yielding, but did not show peak stress. Although the strength was higher both in 0TSC and 90TSC than in 45TSC, both 0TSC and 90TSC showed the formation of cracks and delamination, resulting to the large drop in the flow stress. In contrast to 0TSC and 90TSC, although 45TSC did not exhibit the peak stress, it exhibited work hardening due to the kink boundary formation, irrespective of the formation of delamination. It is reasonable to conclude from the deformation behavior and the deformed microstructures that for the TSC, the kink boundary plays an important role for attaining both deformability and strength.

Kink Modeling and Simulations Based on Field Theory of Multiscale Plasticity (FTMP) Part II: Implicit Kink Model and Scale-Free Treatment

We discuss not only kink formations but also the attendant primary strengthening taken place during the formation processes, by proposing anew an “Implicit” kink model based on Field Theory of Multiscale Plasticity (FTMP) only by utilizing the incompatibility tensor-incorporated Rank-1 degrees of freedom in the hardening law of the slip constitutive equation. Demonstrated is either realistically inhomogeneous kink field morphologies and successfully-raised flow stress level throughout the deformation, with the help of prescribed soft/hard structure accompanied by initial misorientations. A preliminary attempt is also made for identifying the critical roles of the empirically-found “scale-free” nature in the AE characteristics, based on the proposed model combined with the interaction field formalism in FTMP, resulting in further improved kink morphologies as well as slightly increased flow stress level.

Effects of a Preannealing Process on the Morphology of Developed Kinks in Mille-Feuille Structured Cu/A5052 Alloy Fabricated by Accumulative Roll Bonding: Criteria for Kink Formation

Cu/A5052 dissimilar metal laminates (DMLs) were fabricated by an accumulative roll bonding process. The Cu/A5052 DML consisted of hard and soft layers, in other words, high and low shear modulus layers, which is called a mille-feuille structure (MFS). The Cu/A5052 DML with the MFS showed kink deformation during monotonic compression. Kink formation was analyzed by in-situ optical microscopy and digital image correlation. The development behavior and morphology of the ortho-type kink depended on the annealing conditions prior to the compression tests. Based on the observations and measurements, criteria for kink formation in the DML with the MFS were proposed.

Two-Directional Micro-Laue Diffraction Mapping to Observe Kink Deformation in Long-Period Stacking-Ordered Mg–Zn–Y Alloys under Compression

We modified a previously developed compact compression-test stage for synchrotron radiation micro-Laue diffraction mapping to investigate the deformation of the long-period stacking-ordered phase of a magnesium polycrystal. This modification made it possible to obtain grain-boundary images from two orthogonal directions, realizing to obtain depth information about the position of kink formation. Also, to compare bulk and surface images, a long-focus optical microscope, which can take photographs of the surface morphology of the sample, was introduced into the apparatus.

Using Molecular Dynamic Simulation to Understand the Deformation Mechanism in Cu, Ni, and Equimolar Cu-Ni Polycrystalline Alloys

The grain boundaries and dislocations play an important role in understanding the deformation behavior in polycrystalline materials. In this paper, the deformation mechanism of Cu, Ni, and equimolar Cu-Ni alloy was investigated using molecular dynamic simulation. The interaction between dislocations and grain boundary motion during the deformation was monitored using the dislocation extraction algorithm. Moreover, the effect of stacking fault formation and atomic band structure on the deformation behavior was discussed. Results indicate that dislocations nucleate around the grain boundary in copper, the deformation in nickel changes from planar slip bands to wavy bands, and high density of dislocation accumulation as well as numerous kink and jog formations were observed for the equimolar Cu-Ni alloy. The highest density of the Shockley dislocation and stacking faults was formed in the equimolar Cu-Ni alloy which results in the appearance of a huge gliding stage in the stress–strain curve. The grain boundaries act as a sinking source for vacancy annihilation in Ni and Cu; however, this effect was not observed in an equimolar Cu-Ni alloy. Finally, radial distribution function was used to evaluate atom segregation in grain boundaries.

A study of step defects on NiAl(110) using a curved single crystal surface

In contrast to common flat single crystal surfaces, curved surfaces provide the opportunity to study a large range of vicinals using only one single crystal. Here, we employ a curved NiAl single crystal surface to study step defects on the (110) plane using scanning tunneling microscopy under UHV conditions and at room temperature. The direction of curvature is chosen to align with the alternating Al and Ni atom rows, and we image the surface at various positions along its curvature. With very few exceptions, we find only monoatomic step heights to occur with local step densities consistent with the expected values along the macroscopically curved surface. We use our home-built Python-based analysis script to analyze STM images and obtain statistical information on step and kink distributions, a.o. it allows us to determine the kink formation energy.

Role of disclinations around kink bands on deformation behavior in Mg–Zn–Y alloys with a long-period stacking ordered phase

Mg–Zn–Y alloys that contain an Mg-based long-period stacking ordered (LPSO) phase exhibit excellent mechanical properties because of the kink bands formed by plastic deformation. Such “kink-band strengthening” has attracted significant attention. Herein, we conducted deformation tests on directionally solidified single-phase LPSO Mg85Zn6Y9 alloys to investigate the kink-band strengthening mechanism. High-angular resolution electron backscatter diffraction was performed to detect misorientation between the matrices on either side of the kink bands. The misorientations corresponded to the magnitudes of the Frank vectors of the disclinations around the kink, and closely matched the estimations from geometric analysis, which supported the existence of disclinations. Moreover, the Frank vector of the kink bands increased after shear deformation, which indicates that shear deformation introduced new disclinations around the kinks. Scanning electron microscopy demonstrated that the kinks clearly obstructed basal ⟨a⟩ slip. Furthermore, scanning transmission electron microscopy of a sheared kink band revealed dislocation pile-ups on both sides of the kink/matrix interface, as well as the formation of secondary kinks that stabilized the structure against shear deformation (i.e., self-accommodation) and bend contours caused by elastic stress fields. The results suggest that disclinations behave as long-range obstacles to dislocation motion (e.g., by reducing the increase in kink-band self-energy or by the elastic stress field of disclinations). We believe that this study will play a key role in identifying the factors responsible for kink-band strengthening in LPSO-phase Mg alloys and in understanding the phenomena underlying the strengthening mechanism.

Solitary-like and modulated wavepackets in the Couette-Taylor flow with a radial temperature gradient.

We have performed numerical and experimental studies of the flow in a large aspect ratio Couette-Taylor system with a rotating inner cylinder and a fixed radial temperature gradient. The base flow state is a superposition of an azimuthal flow induced by rotation and an axial large convective cell induced by the temperature gradient. For a relatively large temperature gradient, the rotation rate of the inner cylinder destabilizes the convective cell to give rise to travelling wave pattern through a subcritical bifurcation. This wave pattern is associated with a temperature mode and it consists of helical vortices travelling in the annulus. In a small range of the rotation rate, helical vortices have longitudinal meandering leading to the formation of kinks randomly distributed, leading to spatio-temporal disordered patterns. The flow becomes regular for a large interval of rotation rate. The friction, the momentum and the heat transfer coefficients are computed and found to be independent of the heating direction. This article is part of the theme issue 'Taylor-Couette and related flows on the centennial of Taylor's seminal Philosophical Transactions paper (part 1)'.

Mobility of carbon-decorated screw dislocations in bcc iron

We investigate the mobility of screw dislocations decorated by carbon solutes in body-centred cubic iron based on density functional theory (DFT) calculations and transmission electron microscopy (TEM) observations of in-situ straining experiments. We focus on the high-temperature domain, between 500 and 800 K, where plasticity is controlled by the slow glide of decorated screw dislocations with a mobility similar to the Peierls mechanism existing below 300 K, at variance with the athermal regime observed at intermediate temperatures. We propose that due to the strong pinning of reconstructed dislocation lines by carbon solutes, dislocation glide occurs by the formation and migration of kinks, both controlled by the jumps of carbon atoms. We calculate the associated kink-pair nucleation and formation energies as well as the migration energies of isolated kinks and use these DFT values to predict the dislocation velocity as a function of temperature, applied stress and dislocation length. The validity of the mobility law is assessed by the comparison with dislocation velocities measured in TEM observations at different temperatures and local shear stresses. The quantitative agreement between the experimental measurements and the analytical model supports the proposed glide mechanism and the DFT values obtained for kink energies.

Изучение кинетики сближения ступеней поверхности Si(100)

In this work, the convergence kinetics investigations of the SA and SB steps on Si(100) substrates with inclination 0.5o and 0.1o were carried out. Analysis of the time dependence of reflection high-energy electron diffraction (RHEED) intensity was used to establish the growth kinetics character on vicinal Si(100) surfaces. It is shown that, in a Si flow at the growth rate of 0.37 ML/s, the step convergence velocity has a decreasing exponential dependence with the temperature increase. It is determined that the single-domain surface formation velocity increases with an increase in the terrace width on the surface, which may be due to the partial participation of growth due to the formation of two-dimensional islands. Above a temperature of 650°C, the dominant growth mode is due to the step movement and the single-domain surface formation velocity decreases with an increase in the terrace width. Thus, the single-layer step convergence is determined by both the MBE growth conditions and the Si(100) substrate orientation. The convergence of SA and SB steps of the Si(100) surface is explained by the slowdown of the step SA motion, which is associated with complex permeability mechanisms and a kink formation of steps. It is assumed that the reason for the slowdown of the step convergence with increasing temperature is an increase in the kink density at the SA step, which reduces the step SA permeability coefficient.

Study of Si(100) surface step convergence kinetics

In this work, the convergence kinetics investigations of the SA- and SB-steps on Si(100) substrates with inclination 0.5o and 0.1o were carried out. Analysis of the time dependence of reflection high-energy electron diffraction (RHEED) intensity was used to establish the growth kinetics character on vicinal Si(100) surfaces. It is shown that, in a Si flow at the growth rate of 0.37 ML/s, the step convergence velocity has a decreasing exponential dependence with the temperature increase. It is determined that the single-domain surface formation velocity increases with an increase in the terrace width on the surface, which may be due to the partial participation of growth due to the formation of two-dimensional islands. Above a temperature of 650oC, the dominant growth mode is due to the step movement and the single-domain surface formation velocity decreases with an increase in the terrace width. Thus, the single-layer step convergence is determined by both the MBE growth conditions and the Si(100) substrate orientation. The convergence of SA- and SB-steps of the Si(100) surface is explained by the slowdown of the step SA-motion, which is associated with complex permeability mechanisms and a kink formation of steps. It is assumed that the reason for the slowdown of the step convergence with increasing temperature is an increase in the kink density at the SA-step, which reduces the step SA-permeability coefficient. Keywords:: Molecular-beam epitaxy, reflection high-energy electron diffraction, surface, terraces, steps, kinks.

Plastic Deformation of High Density Polyethylene with Extended-Chain Crystal Morphology.

Samples of polyethylene with extended-chain crystal morphology, obtained by crystallization under high pressure, were subjected to uniaxial compression to various strains. Accompanying structural changes were analyzed using scanning electron microscopy. At the true strain of e = 0.2−0.3 the microbuckling instability was observed in longitudinally loaded lamellae, resulting in the formation of angular kinks. This induced a rapid reorientation of the lamellae, facilitating their further deformation by crystallographic slip. Microbuckling instability was found to occur earlier than in samples with folded-chain crystal morphology (e = 0.3−0.4) due to a smaller ratio of the amorphous to crystalline layer thickness. SEM observations demonstrated that the microbuckling instability begins with small undulation in long lamellae. Sharp angular lamellar kinks develop from the initial undulation through intense plastic deformation by crystallographic slip along the chain direction. The same slip system was found to operate throughout the kink, including the tip region as well as both limbs. In contrast to thin folded-chain lamellae that often undergo fragmentation during deformation, the thick extended-chain lamellae deform stably by chain slip and retain their continuity up to high strains, e > 1.6. This stability of deformation is related to the large thickness of extended-chain lamellae.

Real-Time Study of Surface-Guided Nanowire Growth by In Situ Scanning Electron Microscopy.

Surface-guided growth has proven to be an efficient approach for the production of nanowire arrays with controlled orientations and their large-scale integration into electronic and optoelectronic devices. Much has been learned about the different mechanisms of guided nanowire growth by epitaxy, graphoepitaxy, and artificial epitaxy. A model describing the kinetics of surface-guided nanowire growth has been recently reported. Yet, many aspects of the surface-guided growth process remain unclear due to a lack of its observation in real time. Here we observe how surface-guided nanowires grow in real time by in situ scanning electron microscopy (SEM). Movies of ZnSe surface-guided nanowires growing on periodically faceted substrates of annealed M-plane sapphire clearly show how the nanowires elongate along the substrate nanogrooves while pushing the catalytic Au nanodroplet forward at the tip of the nanowire. The movies reveal the timing between competing processes, such as planar vs nonplanar growth, catalyst-selective vapor-liquid-solid elongation vs nonselective vapor-solid thickening, and the effect of topographic discontinuities of the substrate on the growth direction, leading to the formation of kinks and loops. Contrary to some observations for nonplanar nanowire growth, planar nanowires are shown to elongate at a constant rate and not by jumps. A decrease in precursor concentration as it is consumed after long reaction time causes the nanowires to shrink back instead of growing, thus indicating that the process is reversible and takes place near equilibrium. This real-time study of surface-guided growth, enabled by in situ SEM, enables a better understanding of the formation of nanostructures on surfaces.

Kink Formation through Creep Deformation and Possibility of Kink Strengthening in Ti<sub>3</sub>SiC<sub>2</sub> MAX Phase

Kink formation and kink strengthening mechanisms were examined in the polycrystalline Ti3SiC2 MAX phase prepared by a reaction sintering process using a spark-plasma-sintering machine. The creep behavior tested by compression at 1200°C showed two deformation regions depending on the applied stresses; at stresses lower than 120 MPa, the stress exponent n exhibited ≈1.8, whereas at higher stresses, it exhibited n ≥ 6. The creep behavior can be ascribed to grain boundary sliding mechanisms for the lower stresses with n ≈ 1.8 and to dislocation-related creep mechanisms for the higher stresses with n ≥ 6. The kink bands were frequently observed to form in the grains deformed only at the higher stresses when its basal plane was inclined by about 10–20° against the compressive axis. This suggests that the kink bands might be formed only when two factors of the high stresses acting on the basal plane and the resultant dislocation activities were satisfied. Nanoindentation tests conducted around the formed kink bands showed that the nanohardness linearly changed with the distance from the kink bands and showed higher values around the kink bands. Since the kink bands blocked the slip line caused by the nanoindentation, those become obstacles against the dislocation motion caused by the indentation deformation. This suggests that the kink bands would contribute to improving the mechanical properties of the Ti3SiC2 MAX phase.

The formation and growth of a cross kink in a rope under torsion: An interpretation based on structural mechanics

The application of large twistings to a thin rope is known to cause the occurrence and evolution of an intermediate cross kink. Using classical linear elastic structural mechanics, the branched equilibrium path, which characterizes the kink formation, has been obtained. This path is characterized by increasing torsional stiffness. Some energy considerations have been formulated to motivate why the rope moves along the branched path generating the cross kink.

Structural basis for the tethered peptide activation of adhesion GPCRs.

Adhesion G-protein-coupled receptors (aGPCRs) are important for organogenesis, neurodevelopment, reproduction and other processes1-6. Many aGPCRs are activated by a conserved internal (tethered) agonist sequence known as the Stachel sequence7-12. Here, we report the cryogenic electron microscopy (cryo-EM) structures of two aGPCRs in complex with Gs: GPR133 and GPR114. The structures indicate that the Stachel sequences of both receptors assume an α-helical-bulge-β-sheet structure and insert into a binding site formed by the transmembrane domain (TMD). A hydrophobic interaction motif (HIM) within the Stachel sequence mediates most of the intramolecular interactions with the TMD. Combined with the cryo-EM structures, biochemical characterization of the HIM motif provides insight into the cross-reactivity and selectivity of the Stachel sequences. Two interconnected mechanisms, the sensing of Stachel sequences by the conserved 'toggle switch' W6.53 and the constitution of a hydrogen-bond network formed by Q7.49/Y7.49 and the P6.47/V6.47φφG6.50 motif(φ indicates a hydrophobic residue), are important in Stachel sequence-mediated receptor activation and Gs coupling. Notably, this network stabilizes kink formation in TM helices 6 and 7 (TM6 and TM7, respectively). A common Gs-binding interface is observed between the two aGPCRs, and GPR114 has an extended TM7 that forms unique interactions with Gs. Our structures reveal the detailed mechanisms of aGPCR activation by Stachel sequences and their Gs coupling.

Cathodoluminescence and EBIC investigations of stacking fault expansion in 4H-SiC due to e-beam irradiation at fixed points

The effect of e-beam irradiation in the local and scan modes on the stacking fault expansion in 4H-SiC has been studied. It is shown that the distance, at which the e-beam affects the glide of partial dislocations driving the stacking fault expansion, does not exceed 10–12 μm. The dislocations were found to glide as straight lines with a velocity independent of their length, even when this length essentially exceeds the size of excitation volume. The irradiation at fixed points allows to separate the excess carrier effects on the kink formation and kink migration. The results obtained were explained under an assumption that the irradiation is necessary only to stimulate the kink pair formation and then the kinks can migrate without any excitation. That could mean that the barrier for the kink migration along Si-core 30° partial dislocations in 4H-SiC is very small.