Combination Of Glide Research Articles

A nucleation rate limited model for grain boundary creep

The motion of grain boundary (GB) dislocations, via a combination of glide and climb, mediates GB plasticity during processes such as creep and GB stress relaxation. Creep within the low stress regime, where non-Newtonian GB creep occurs, remains poorly understood. Models for GB dislocation nucleation rate limited creep, however, have not been sufficiently developed previously in the literature. Herein, a model for nucleation rate limited kinetics is developed and demonstrated to describe existing creep data reasonably well, using physically realistic materials properties as inputs to the model.

Displacement field due to glide and climb of rectilinear dislocations in gradient elasticity

ABSTRACT The aim of this article is to provide the displacement field of a straight dislocation in gradient elasticity that can be implemented in dislocation dynamics simulations. The displacement and plastic strain fields of a dislocation depend on the history of the dislocation motion. The cut surface (or the branch cut) represents this history. When an edge dislocation glides, the branch cut must be parallel to the Burgers vector, and when a dislocation moves by a combination of glide and climb, extra terms need to be added to the displacement field to capture the correct discontinuity in the displacement field. The existing formulas for the dislocation displacement in gradient elasticity lack such considerations, which make them unsuitable for simulations. In addition, the previous formulas both in classical and gradient elasticity have contained some inaccuracies and miscalculations. In this paper, the displacement field of a dislocation and the corresponding plastic strain are derived in detail and a distinction between the dislocation due to glide and that due to climb has been made. Moreover, we discuss the shortcomings of Mura's [Micromechanics of Defects in Solids, 2nd ed., Martinus Nijhoff Publishers, 1987.] and Lazar and Maugin's [Dislocations in gradient elasticity revisited, Proc. R. Soc. Math. Phys. Engin. Sci. 462(2075) (2006), pp. 3465–3480.] calculations. We illustrate that Orowan's law is a direct result of the correct displacement and plastic strain fields.

High-temperature tensile and creep behavior of Cu–Nb composites: A discrete dislocation plasticity investigation

Tensile and creep responses of Cu–Nb composites are predicted at 400°C using a high-temperature discrete dislocation plasticity (DDP) framework where the multilayer composites are idealized as two-dimensional unit cells with periodic boundary conditions. Considering the fact that the interfaces in multilayer composites dominate their plastic response, bi-material interfaces are treated here so as to store lattice dislocations via formation of steps at the interfaces. While dislocations in the Nb layer are assumed to move by glide-only at 400°C, they move by a combination of glide and climb in the Cu layer at this temperature. Material parameters for the layers are obtained at 400°C by fitting the DDP predictions of the tensile response of Cu–Nb composites to the experimental data corresponding to the composites with the layer thickness in the range 65nm≤h≤2μm. Using these material parameters, the framework enables quantitative predictions of the creep responses of these composites at 400°C, which are in very good agreement with the available experimental data. Investigations of the results demonstrate that: (i) the creep response of Cu–Nb composites, in line with observations, is dependent on the laminate length scale h as well as on the stress indicating that the creep behavior is dislocation dominated; (ii) the average stresses within the layers remain almost unchanged (with some fluctuations) during the time history indicating that there is a limited load transfer between the Nb and Cu layers during the creep process; and (iii) the dislocation and step densities in the Cu layers are higher than those in the Nb layers implying that the interfaces interact with a high flux of climbing dislocations from the Cu layers. Predictions of the deformed shapes of the Cu–Nb composites including the deformed configurations of the interfaces imply that annihilation of climbing Cu dislocations at the interfaces is the dominant interaction mechanism that occurs at the interfaces. This resembles a recovery mechanism in the bi-material interfaces that facilitates dislocation generation and movement in the Cu layers and thus enhances the creep strains.

Winged Aerial Manipulation Robot with Dual Arm and Tail

This paper presents the design and development of a winged aerial robot with bimanual manipulation capabilities, motivated by the current limitations of aerial manipulators based on multirotor platforms in terms of safety and range/endurance. Since the combination of gliding and flapping wings is more energy efficient in forward flight, we propose a new morphology that exploits this feature and allows the realization of dexterous manipulation tasks once the aerial robot has landed or perched. The paper describes the design, development, and aerodynamic analysis of this winged aerial manipulation robot (WAMR), consisting of a small-scale dual arm used for manipulating and as a morphing wing. The arms, fuselage, and tail are covered by a nylon cloth that acts as a cap, similar to a kite. The three joints of the arms (shoulder yaw and pitch, elbow pitch) can be used to control the surface area and orientation and thus the aerodynamic wrenches induced over the cloth. The proposed concept design is extended to a flapping-wing aerial robot built with smart servo actuators and a similar frame structure, allowing the generation of different flapping patterns exploiting the embedded servo controller. Experimental and simulation results carried out with these two prototypes evaluate the manipulation capability and the possibility of gliding and flying.

Mechanics and microstructure of deformed natural anisotropic ice

We deformed coarse-grained (∼10 mm) natural ice in axial compression at −30 °C, to an axial strain of 0.2, at three different angles to the existing strong crystallographic preferred orientation (CPO). We used three strain rates for each sample orientation. Cryo-electron backscatter diffraction (EBSD) maps show that after deformation there is a mixture of large (∼1–2 mm) relict grains, and finer (100–200 μm) recrystallised grains. The fine grains form a connected network in all samples. The large grains define a very strong CPO with equivalent but weaker CPOs in the recrystallised grains. The final CPO changed completely from its original orientation. Lattice distortion and subgrains equivalent in size to recrystallised grains suggest a subgrain rotation recrystallisation process has generated the recrystallised fraction. We suggest that strain rates were higher in the connected network of recrystallised grains because of a significant component of grain boundary sliding (GBS) that enables large grains to rotate by a combination of glide on the basal plane and rigid rotation, to define a very strong CPO. GBS weakens the CPO in the finer grained regions. The patterns of mechanical behaviour and the resultant microstructures do not bear an obvious relationship to original CPO.

On the nature of γ′ phase cutting and its effect on high temperature and low stress creep anisotropy of Ni-base single crystal superalloys

The creep anisotropy of the single crystal superalloy LEK 94 deformed in tension along [001] and [110] directions at 1293K and 160MPa was investigated. Elementary microstructural processes which are responsible for a higher increase in creep rates with strain during [110] as compared to [001] tensile loading were identified. [110] tensile creep is associated with a higher number of γ′ phase cutting events, where two dislocations with equal Burgers vectors of type 〈110〉 jointly shear the γ′ phase. The resulting 〈220〉-type superdislocation can move by glide. In contrast, during [001] tensile loading, two dislocations with different 〈110〉-type Burgers vectors must combine for γ′ phase cutting. The resulting 〈200〉-type superdislocations can only move by a combination of glide and climb. The evolution of dislocation networks during creep determines the nature of the γ′ phase cutting events. The higher [110] creep rates at strains exceeding 2% result from a combination of a higher number of cutting events (density of mobile dislocations in γ′) and a higher superdislocation mobility (〈220〉 glide) in the γ′ phase.

Climb-enabled discrete dislocation plasticity

A small strain two-dimensional discrete dislocation plasticity framework coupled to vacancy diffusion is developed wherein the motion of edge dislocations is by a combination of glide and climb. The dislocations are modelled as line defects in a linear elastic medium and the mechanical boundary value problem is solved by the superposition of the infinite medium elastic fields of the dislocations and a complimentary non-singular solution that enforces the boundary conditions. Similarly, the climbing dislocations are modelled as line sources/sinks of vacancies and the vacancy diffusion boundary value problem is also solved by a superposition of the fields of the line sources/sinks in an infinite medium and a complementary non-singular solution that enforces the boundary conditions. The vacancy concentration field along with the stress field provides the climb rate of the dislocations. Other short-range interactions of the dislocations are incorporated via a set of constitutive rules. We first employ this formulation to investigate the climb of a single edge dislocation in an infinite medium and illustrate the existence of diffusion-limited and sink-limited climb regimes. Next, results are presented for the pure bending and uniaxial tension of single crystals oriented for single slip. These calculations show that plasticity size effects are reduced when dislocation climb is permitted. Finally, we contrast predictions of this coupled framework with an ad hoc model in which dislocation climb is modelled by a drag-type relation based on a quasi steady-state solution.

Active Weight Bearing Tibio-talar Motion and Meniscal Translation in the S.T.A.R Prosthesis-A Radiographic Comparative Study of the S.T.A.R. Toward the Opposite Normal Ankle Joint

Background: The kinematics of meniscal bearing artificial ankle joints is not well documented after the implantation of the prosthesis. Purpose of the study To investigate the active weight-bearing tibio-talar motion and the translation of the meniscus radiographically after mean 36 months in patients with unilateral traumatic ankle osteoarthritis. Methods: Straight lateral radiographic views under image intensification of the replaced ankle and the opposite normal ankle were performed during active weight-bearing. The tibio-talar motion of both ankles, as well as the translation/rotation of the meniscus in the replaced ankles was measured. Results: The total range of motion in normal ankles was mean 59° (33.7–68.7). The motion in the replaced ankles was mean 37.2° (16.7–53.3), p<0.02. Normal extension was mean 24° (3.7–37.7). Replaced ankle extension was mean 14.7° (2-26.7), p<0.02. Normal plantarflexion was mean 31° (19.7–51), in replaced ankles plantar flexion was mean 22.3° (12.3–34), p<0.02. The translation of the meniscus was 0.7 mm (-1.5 to 1.2). This could also be calculated to be a rotation of 3.9°. The translation of the meniscus is most probably a combination of gliding and rotation. Conclusions: The active weight-bearing motion of the replaced ankle is about 2/3 of the normal ankle motion. Functionally this is a satisfactory result. After 36 months the translation of the meniscus is rather small suggestion that the replaced ankle has reached a steady state of conformity.

Dynamics and Force Generation by Single Motor Complexes in M. Xanthus

The gram-negative bacterium Myxococcus xanthus is an important model system for multicellular group formation. These cells move on solid surfaces using a combination of gliding and twitching motility. Whereas pilus retraction drives twitching, the mechanism controlling gliding is not fully understood. We recently identified a new class of molecular motors that power gliding motility [1]. These motors, AglQRS proton channels, assemble within focal adhesion sites that are separated by approximately 480nm along the cell [2]. It is currently unknown how much force the motors can generated and whether they act individually or in cooperative groups.Here, we combined optical tweezers and real-time tracking of motor-driven beads to address these topics. We find that gliding motors produce saltatory movements on two major time scales, 0.4s and 3.5s. These are likely associated with motor kinetics and cargo binding respectively. During bursts of motor activity, we applied a controlled load to slow single motor movement along the cell axis. We show that mechanical force is sufficient to reduce the velocity of a gliding motor complex and that motors stall under forces stronger than 12pN. We reduced the pH gradient through exposure to mild concentrations of the drug nigericin. We find that the characteristic force is independent of the driving proton motive force.[1] Sun et al., PNAS(108)7559, 2011. [2] Mignot et al., Science(315)853, 2007.

Inhomogeneity of misfit stresses in nickel-base superalloys: Effect on propagation of matrix dislocation loops

It is shown experimentally that, during annealing and creep under low applied stresses, matrix dislocation loops frequently cross-glide. The periodic length of the zigzag dislocations deposited in the interfaces is equal to that of the γ/γ′-microstructure. Initially, the zigzag dislocations move in the (001) interface by a combination of glide and climb but then they stop near the γ′-edges and align along ⟨100⟩. Reactions of such dislocations lead to the formation of square interfacial networks consisting of ⟨100⟩ oriented edge dislocations. The complex dislocation movement is explained by the inhomogeneity of the misfit stresses between γ- and γ′-lattices. The tensile components of the stress tensor drive the dislocations through the channel, whereas the shear components near the γ′-edges cause the zigzag movement and the ⟨100⟩ alignment. The total effect is the most efficient relaxation of the misfit stresses. The results are relevant, especially for single-crystal superalloys of the newest generations, which have an increased γ/γ′-misfit due to the high level of refractory elements.

Nitrous oxide processing by a combination of gliding and microwave discharges

Non-thermal plasma of microwave discharge coupled with gliding discharge was applied to convert nitrous oxide. The experiments were carried out using air or oxygen as carrier gases for N 2O (5%). The overall rates of nitrous oxide conversion determined for the N 2O + air mixture were slightly higher than those for N 2O + oxygen. No significant effect of the carrier gas (air or oxygen) on the rate of N 2O → NO conversion was observed. The effect of the power of the microwave discharge and gas flow rate (air) on the overall rate of nitrous oxide conversion and rate of N 2O conversion to NO was studied. The increase of the gas flow rate from 200 to 400 N l/h resulted in an increase of the N 2O conversion rates both overall ( r) and to NO ( r NO). For 400 N l/h, both rates were higher by about 80–100% than those determined in the experiments performed with 200 N l/h.

High-voltage Transmission Electron Microscope in situ Study on Dislocation Motion in Decagonal Al–Ni–Co Single Quasicrystals

Decagonal single quasicrystals of the composition Al70Ni15Co15have been deformed in situ in a high-voltage transmission electron microscope at 730 °C along the 10-fold periodic axis to directly observe the dislocation motion. The deformation is carried by stress-assisted climb of dislocations with periodic Burgers vectors. These dislocations may also glide and move by a combination of glide and climb. Dislocations with Burgers vectors with components in the periodic and quasiperiodic directions probably move under the action of a chemical force. The observations are interpreted by a model established by P. Schall et al. under consideration of the activation parameters of macroscopic deformation and by analogies with the behavior of icosahedral quasicrystals.

Nonbound dislocations in hexagonal patterns: pentagon lines in surface-tension-driven Bénard convection.

We report on a novel class of defects in a hexagonal pattern which we call pentalines. They are built up of two nonbound dislocations and are orientated parallel to the roll axis of the mode free of a dislocation. A pentaline has its origin in a transformation of the penta-hepta defect (PHD), taking place at higher supercriticality. The underlying mechanism consists in a combination of glide and climb motion of the original dislocations bound to the PHD. We demonstrate that the pentalines play an important role within the transition from hexagonal towards square convection cells, observed in surface-tension-driven Bénard convection.

Simulation of dislocation microstructures in two dimensions. I. Relaxed structures

The self-organization of dislocation structures produced by rearrangement under mutual interaction forces has been simulated by a computer technique based on molecular dynamics. The model employed is a two-dimensional array of straight, mixed dislocations on orthogonal slip planes. Equations of motion are developed that account for viscous drag in glide and climb motion, inertial forces, externally applied stresses and interaction forces due to other internal defects. Relaxation of an initially random dislocation structure by glide and by a combination of glide and climb has been studied. Results are expressed in the form of a two-dimensional distribution function, which reveals the degree and nature of the self-organization of the structure. Kinetics of the reduction of dislocation density by annealing are found to agree with a similar model based on mutual annihilation of dislocations in cell walls.