Twist Component Research Articles

Concurrent grain boundary motion and grain rotation under an applied stress

Simultaneous shear coupling and grain rotation were observed experimentally during grain boundary migration in high-purity Al bicrystals subjected to an external mechanical stress at elevated temperatures. This behavior is interpreted in terms of the structure of the investigated planar 18.2° 〈1 0 0〉 non-tilt grain boundary with a 20° twist component. For characterization of the grain rotation after annealing under stress the bicrystal surface topography across the boundary was measured by atomic force microscopy. The temperature dependence of the boundary migration rate was measured and the migration activation energy determined.

Crack–grain boundary interactions in zinc bicrystals

In polycrystalline materials that fail by transgranular cleavage, it is known that crystallographic misorientation of preferred fracture planes across grain boundaries can provide crack growth resistance; despite this, the micromechanisms associated with crack transmission across grain boundaries and their role in determining the overall fracture resistance are not well understood. Recent studies on diverse structural materials such as steels, aluminum alloys and intermetallics have shown a correlation between fracture resistance and the twist component of grain misorientation. However, the lack of control over the degree and type of misorientation in experimental studies, combined with a dearth of analytical and computational investigations that fully account for the three-dimensional nature of the problem, have precluded a systematic analysis of this phenomenon. In this study, this phenomenon was investigated through in situ crack propagation experiments across grain boundaries of controlled twist misorientation in zinc bicrystals. Extrinsic toughening mechanisms that activate upon crack stagnation at the grain boundary deter further crack propagation. The mechanical response and crack growth behavior were observed to be dependent on the twist angle, and several accommodation mechanisms such as twinning, strain localization and slip band blocking contribute to fracture resistance by competing with crack propagation. Three-dimensional finite element analyses incorporating crystal plasticity were performed on a stagnant crack at the grain boundary that provide insight into crack-tip stress and strain fields in the second grain. These analyses qualitatively capture the overall trends in mechanical response as well as strain localization around stagnant crack-tips.

Formula omitted]-invariant decomposition of ten-dimensional Yang–Mills supersymmetry

The N=1, d=10 super Yang–Mills action is constructed in a twisted form, using SU(5) invariant decomposition of spinors in 10 dimensions. The action and its off-shell closed twisted scalar supersymmetry operator Q is derived from a Chern–Simons term. The action can be decomposed as the sum of a term in the cohomology of Q and of a term that is Q-exact. The first term is a fermionic Chern–Simons term for a twisted component of the Majorana–Weyl gluino and it is related to the second one by a twisted vector supersymmetry with 5 parameters. The cohomology of Q and some topological observables are defined from descent equations. In this SU(5)⊂SO(10) invariant decomposition, the N=1, d=10 theory is determined by only 6 supersymmetry generators, as in the twisted N=4, d=4 theory. There is a superspace with 6 twisted fermionic directions, with solvable constraints.

Twist and writhe of δ-island active regions

AbstractWe study the magnetic helicity properties of a set of peculiar active regions (ARs) including δ-islands and other high-tilt bipolar configurations. These ARs are usually identified as the most active in terms of flare and CME production. Due to their observed structure, they have been associated with the emergence of magnetic flux tubes that develop a kink instability. Our main goal is to determine the chirality of the twist and writhe components of the AR magnetic helicity in order to set constrains on the possible mechanisms producing the flux tube deformations. We determine the magnetic twist comparing observations of the AR coronal structure with force-free models of the magnetic field. We infer the flux-tube writhe from the rotation of the main magnetic bipole during the observed evolution. From the relation between the obtained twist and writhe signs we conclude that the development of the kink instability cannot be the single mechanism producing deformed flux-tubes.

Spin and twist motions in the earthquake preparation processes: Analysis of records

Three seismic events in L’Aquila region and one in western Greece have been chosen for comparative analysis of two kinds of seismic motions, spin and twist. These torque components of the seismic field were detected in horizontal plane with the rotational seismometers. Homologous parts were sought in the signals of spin and twist components — curve of one component was compared to the other directly and after transformations: phase-shifting and sign reversal. To achieve better clarity, these operations were done on the data divided into several descendant signals with the use of band-pass filtration. Conformities of spin and twist, revealed in this way, are assumed to be results of distant processes in the source, where relations between rotational and shear motions include retardations.

Multi-Section Bow-Tie Steps for Full-Band Waveguide Polarization Rotation

A new compact multi-section structure for broadband waveguide polarization rotation is presented. The twist component is based on cascading multiple steps with bow-tie shape geometry, which are progressively rotated between the input and output rectangular waveguides. The twists show an excellent performance in the full operating frequency band, which is controlled with the number of sections and their shapes and can be designed for any rotation angle. Moreover, the bow-tie sections are very short and compact and can be easily manufactured, being well-suited for satellite applications. The proposed configuration is shown with design tables and curves for different rotation angles. Two 90° twists in WR75 waveguide have been tested, showing measured return losses of 30 dB (one section) and 40 dB (two sections) from 10 to 15 GHz with a total length of 5.6 mm and 8.6 mm, respectively.

Sun to 1 AU propagation and evolution of a slow streamer‐blowout coronal mass ejection

We present a comprehensive analysis of the evolution of the classic, slow streamer‐blowout CME of 1 June 2008 observed by the STEREO twin spacecraft to infer relevant properties of the pre‐eruption source region which includes a substantial portion of the coronal helmet streamer belt. The CME was directed ∼40° East of the Sun‐Earth line and the Heliospheric Imager observations are consistent with the CME propagating essentially radially to 1 AU. The elongation‐time J‐map constructed from the STEREO‐A HI images tracks the arrival of two density peaks that bound the magnetic flux rope ICME seen at STEREO‐B on 6 June 2008. From the STEREO‐A elongation‐time plots we measure the ICME flux rope radial size Rc(t) and find it well approximated by the constant expansion value Vexp = 24.5 km/s obtained from the STEREO‐B declining velocity profile within the magnetic cloud. The flux rope spatial orientation, determined by forward modeling fits to the STEREO COR2 and HI1 data, approaches the observed 1 AU flux rope orientation and suggests large‐scale rotation during propagation, as predicted by recent numerical simulations. We compare the ICME flux content to the PFSS model coronal field for Carrington Rotation 2070 and find sufficient streamer belt flux to account for the observed ICME poloidal/twist flux if reconnection during CME initiation process is responsible for the conversion of overlying field into the flux rope twist component in the standard fashion. However, the PFSS model field cannot account for the ICME toroidal/axial flux component. We estimate the field strength of the pre‐eruption sheared/axial component in the low corona and the timescales required to accumulate this energized pre‐eruption configuration via differential rotation and flux cancelation by supergranular diffusion at the polarity inversion line. We show that both mechanisms are capable of generating the desired shear component over time periods of roughly 1–2 months. We discuss the implications for slow streamer‐blowout CMEs arising as a natural consequence of the corona's re‐adjustment to the long term evolutionary driving of the photospheric fields.

Characterization of the relationship between systolic shear strain and early diastolic shear strain rates: insights into torsional recoil

Early diastolic left ventricular (LV) untwisting has been evaluated as a manifestation of LV recoil, reflecting the release of elastic energy stored during systole. The primary goal of this study was to characterize the relationship between systolic strain (e.g., circumferential strain and the shear strains that comprise twist) with the resulting early diastolic shear strain rates, including the rate of untwisting. A further goal was to characterize these relationships regionally from apical to basal locations. Cardiac magnetic resonance imaging tissue tagging was used to measure circumferential strain, global and regional (apex, mid, basal) twist (theta), and circumferential-longitudinal (epsilon(CL)) and circumferential-radial (epsilon(CR)) shear strains along with the corresponding untwisting rates (dtheta/dt) and diastolic shear strain rates (depsilon/dt) in 32 healthy males (33 +/- 7 yr). LV untwisting rates and shear strain rates measured during early diastole varied significantly with the measurement location from apex to base (P < 0.001) but demonstrated significant linear correlation with their corresponding preceding systolic strains (P < 0.001). Untwisting rates and diastolic shear strain rates were not significantly correlated with circumferential systolic strain or end-systolic volume (P > 0.05). Normalization of the untwisting rates to the peak twist (dtheta/dt(Norm) = -13.6 +/- 2.1 s(-1)) or shear strain rates to peak systolic shear strain (depsilon(CL)/dt(Norm) = -15.0 +/- 5.4 s(-1), and depsilon(CR)/dt(Norm) = -14.2 +/- 7.7 s(-1)) yielded a uniform measure of early diastolic function that was similar for all shear strain and twist components and for all locations from apex to base. These findings support a linear model of torsional recoil in the healthy heart, where diastolic shear strain rates (e.g., untwisting rates) are linearly related to the corresponding preceding systolic shear stain component. Furthermore, these findings suggest that torsional recoil is uncoupled from end-systolic volumes or the associated strains, such as circumferential strain.

FINE MAGNETIC FEATURES AND CHIRALITY IN SOLAR ACTIVE REGION NOAA 10930

In this paper, we present fine magnetic features near the magnetic inversion line in the solar active region NOAA 10930. The high-resolution vector magnetograms obtained by Hinode allow detailed analyses around magnetic fibrils in the active region. The analyses are based on the fact that the electric current density can be divided into two components: the shear component caused by the magnetic inhomogeneity and the twist component caused by the magnetic field twist. The relationships between magnetic field, electric current density, and its two components are examined. It is found that the individual magnetic fibrils are dominated by the current density component caused by the magnetic inhomogeneity, while the large-scale magnetic region is generally dominated by the electric current component associated with the magnetic twist. The microstructure of the magnetic field in the solar atmosphere is far from the force-free field. The current mainly flows around the magnetic flux fibrils in the active regions.

On the Force Control of Kinematically Defective Manipulators Interacting With an Unknown Environment

In this paper, the problem of force feedback control of kinematically defective manipulators (KDMs) is considered. KDMs are robot manipulators that have fewer joints than the dimension of the space in which their end-effector moves. It is well known that controlling the end-effector velocity of an n-joint KDM can be easily solved by appropriately selecting n components of the output twist, thus squaring the control problem. On the contrary, we show that such a component selection approach is not appropriate in general to solve the force control problem for KDMs. In particular, for advanced force control applications, such as comanipulation, where the contact geometry is not known in advance, the selection of the wrench components leads to a lack of passivity, which in turn may induce instability. This instability does not arise from the system dynamics. Rather, it can be viewed as a new form of kinematic instability. Moreover, by formulating the problem in the joint space, we show how to properly design a stable force controller for KDMs subject to arbitrary external forces applied to their end-effector. Furthermore, we propose several implementations for pure force control and damping control. Experimental results with a kinematically defective laparoscopic comanipulator illustrate these propositions.

Implementation of exact grain-boundary geometry into a 3-D Monte-Carlo (Potts) model for microstructure evolution

A three-dimensional Monte-Carlo (Potts) model was modified to incorporate the effect of grain-boundary inclination on boundary mobility. For this purpose, a straightforward geometric construction was developed to determine the local orientation of the grain-boundary plane. The combined effects of grain-boundary plane and misorientation on the effective grain-boundary mobility were incorporated into the Monte-Carlo code using the definition of the tilt–twist component. The modified code was validated by simulating grain growth in microstructures comprising equiaxed or elongated grains as well as the static recrystallization of a microstructure of deformed (elongated) grains.

Evaluation of leftventricular rotation and twist in uremia patients by two-dimensional ultrasound speckletracking imaging

：Objective To analyze theleft ventricular(LV) rotation and twist by two-dimensional ultrasound speckle-trackingimaging(STI),and evaluate the correlation between twist and other components of LVcontraction in uremia patients. Methods Twenty-nine uremia patients and twenty-ninehealthy volunteers were included. High frame rate two-dimensional images were recordedfrom the left ventricular short-axis views both of the basal and apical planes. Segmentaland global LV rotation were recorded at both The peak systolic LV twist was significantlydecreased in uremia patients(P <0. 001). The loss of LV twist occurred mainly by theimpairment of maximal global counterclockwise apical rotation(P <0. 001),and wasaugmented by somewhat less reduction in global clockwise basal rotation although thedifferences was not significantly reduced(P<0.01), while only the posterior andinferior of the basal plane was significantly fraction(r=0.645,P=0.000), LV end-diastolicvolume (r=-0.549,P=0.002), LV end-systolic volume(r=-0.673, P = 0.000) in uremia patients.Conclusions STI enables quantification of segmental and global LV rotation in an objectivemanner in uremia patients,and provides a new method for evaluation of segmental and globalLV systolic function.

SEMILEPTONIC B(Bs, Bc) DECAYS IN THE LIGHT-CONE QCD SUM RULES

Semileptonic B(Bs, Bc) decays are investigated systematically in the light-cone QCD sum rules. Special emphasis is put on the LCSR calculation on weak form factors with an adequate chiral current correlator, which turns out to be particularly effective to control the pollution by higher twist components of spectator mesons. The result for each channel depends on the distribution amplitude of the the producing meson. The leading twist distribution amplitudes of the related heavy mesons and charmonium are worked out by a model approach in the reasonable way. A practical scenario is suggested to understand the behavior of weak form factors in the whole kinematically accessible ranges. The decay widths and branching ratios are estimated for several B(Bc) decay modes of current interest.

Dislocation Networks Formed by Silicon Wafer Direct Bonding

The paper reviews methods of hydrophobic wafer bonding. Hydrophobic surfaces are obtained by removing the oxide layer from the surfaces of crystalline silicon substrates. Bonding such surfaces causes the formation of a dislocation network in the interface. The structure of the dislocation network depends only on the misalignment (twist and tilt components). The different dislocation structures are discussed. Because wafer bonding offers a method to the reproducible formation of such networks, different applications are possible

Disconnection arrays in a rhombohedral twin in α-alumina

Interfacial defects, such as grain boundary dislocations, play an important role in the creep behavior of alumina. In the present work, interfacial defects are analyzed in detail using a Volterra approach without a reference to a near-coincidence description. We investigate disconnections (boundary steps with dislocation character) in a diffusion-bonded alumina bicrystal, with a misorientation close to the rhombohedral twin, by conventional and atomic resolution electron microscopy. The bicrystal contains two arrays of parallel disconnections with Burgers vectors that have alternating equal and opposite twist components, so there is no long-range stress field. This configuration is discussed in terms of the stability of different grain boundary disconnection arrangements. The complex core structure of the defects is revealed by high resolution electron microscopy using exit wave reconstruction. It is shown that the defects are dissociated into two partials that delimit grain boundary segments with alternating structure.

Partial dislocation configurations in a low-angle boundary in α-Al2O3

The dislocation structures of a low-angle tilt grain boundary in alumina bicrystal were investigated by transmission electron microscopy. The grain boundary was found to consist of two regions: an area with pairs of partial dislocations and an area with groups of odd numbered partial dislocations (multiple-partial-structure). Eight kinds of multiple-partial-structures were found in the fabricated grain boundary. The Burgers vectors of each partial dislocation in the grain boundary can be distinguished by dark-field imaging, and thus the arrangement of partial dislocations in the multiple-partial-structures are determined. It is concluded that a slight twist component of the boundary is the origin of the characteristic multiple-partial-structures.

Experimental moments of the nucleon structure function F2

Experimental data on the F 2 structure functions of the proton and deuteron, including recent results from CLAS at Jefferson Lab, have been used to construct their n ⩽ 12 moments. A comprehensive analysis of the moments in terms of the operator product expansion has been performed to separate the moments into leading and higher twist contributions. Particular attention was paid to the issue of nuclear corrections in the deuteron, when extracting the neutron moments from data. The difference between the proton and neutron moments was compared directly with lattice QCD simulations. Combining leading twist moments of the neutron and proton we found the d / u ratio at x → 1 approaching 0, although the precision of the data did not allow to exclude the 1/5 value. The higher twist components of the proton and neutron moments suggest that multi-parton correlations are isospin independent.

An Overview of Accomplishments and Challenges in Recrystallization and Grain Growth

The study of microstructural evolution in polycrystalline materials has been active for many decades so it is interesting to illustrate the progress that has been made and to point out some remaining challenges. Grain boundaries are important because their long-range motion controls evolution in many cases. We have some understanding of the essential features of grain boundary properties over the five macroscopic degrees of freedom. Excess free energy, for example, is dominated by the two surfaces that comprise the boundary although the twist component also has a non-negligible influence. Mobility is less well defined although there are some clear trends for certain classes of materials such as fcc metals. Computer simulation has made a critical contribution by showing, for example, that mobility exhibits an intrinsic crystallographic anisotropy even in the absence of impurities. At the mesoscopic level, we now have rigorous relationships between geometry and growth rates for individual grains in three dimensions. We are in the process of validating computer models of grain growth against 3D non-destructive measurements. Quantitative modeling of recrystallization that includes texture development has been accomplished in several groups. Other properties such as corrosion resistance are being related quantitatively to microstructure. There remain, however, numerous challenges. Despite decades of study, we still do not have complete cause-and-effect descriptions of most cases of abnormal grain growth. The response of nanostructured materials to annealing can lead to either unexpected resistance to coarsening, or, coarsening at unexpectedly low temperatures. General process models for recrystallization that can be applied to industrial alloys remain elusive although significant progress has been made for the specific case of aluminum alloy processing. Thin films often exhibit stagnation of grain growth that we do not fully understand, as well as abnormal grain growth. Grain boundaries respond to driving forces in more complicated ways than we understood. Clearly many exciting challenges remain in grain growth and recrystallization.

Influence of structural imperfection of Σ5 grain boundaries in bulk multicrystalline Si on their electrical activities

Bulk multicrystalline Si with {310} Σ5 grain boundaries (GBs) was grown by Bridgman growth method using seed crystals with artificially controlled configuration. The structure of the GBs was preserved in the epitaxial growth region without formation of more GBs. However, the GBs were revealed to contain small-angle deviation of ∼5º from the perfect Σ5 relative crystal orientation both in tilt and twist components due to the inaccuracy of the seed crystal arrangement. Such an unintentional misalignment was utilized to investigate the relationship between electrical activity and the deviation angle of {310} Σ5 GB. Electron beam-induced current measurement clarified that carrier recombination velocity at the {310} Σ5 GBs decreases with decreasing deviation angle and shows a minimum at the perfect Σ5 relationship. This tendency suggests that {310} Σ5 GB is electrically inactive, as well as Σ3 and Σ9 GBs.

Modification of Local Structure and Its Influence on Electrical Activity of Near (310) &amp;Sigma;5 Grain Boundary in Bulk Silicon

We report on the modification of the local structures of (310) Σ5 grain boundary in bulk silicon as the growth of crystal and its influence on electrical activity. The grain boundary was formed via floating-zone growth method utilizing bicrystal seed. The misalignment of the seed resulted in formation of the grain boundary with small deviations of crystal orientation from the Σ5 singular coincidence orientation. The deviations consist of tilt and twist components and they were found to monotonically decrease with respect to the distance from the seed crystal accompanied by the decrease in density of dislocations on the grain boundary. The change in the grain boundary structure allows a systematic study on the correlation between the structure and the electrical activity at the GB. The density of dislocations was found to control the electrical activity and the (310) Σ5 coincidence grain boundary without any dislocations is expected to show a very low electrical activity.