Dynamic Behavior Of Dislocations Research Articles

INVESTIGATING THE IMPACT OF AN EXTERNAL INFLUENCE ON THE DYNAMIC BEHAVIOR OF DISLOCATIONS IN SILICON CRYSTALS

The results of studying of the influence of a constant magnetic field on the dynamic behavior of dislocations in the crystalline block of an interferometer subsequently subjected to uniaxial mechanical tensile stress along the crystallographic axis of silicon are presented. The periodicity of the dilatation moire image after these influences, the average means free path of a dislocation in a crystal (block), the amount of slip, the average density of dislocations along the slip line, the relative deformation, and Young's modulus are calculated. To study the effect of a stable magnetic field on the dynamic behavior of dislocations in silicon crystals, a special three-block X-ray interferometer was fabricated in one of the blocks, and dislocations were introduced using the technique proposed by us. The experiments were carried out for various values of the magnetic field induction and mechanical stress. It is shown that the sequential action of these external influences leads to an increase in the starting stress, a decrease in the velocity of dislocation movement, a delay in the onset of dislocation movement, and the appearance of a hardening phenomenon. Experiments show that the displacement mobility characteristics are especially sensitive when the modulus of the magnetic induction vector reaches a certain threshold value for a given sample. It was shown that, in a sample containing silicon dislocations, the magnetic memory that appears after storage in a magnetic field is short-lived. It is found that the speed of movement of a dislocation in a magnetic field processed by a magnetic field, "then" subjected to mechanical stress (stretching), changes, but in practice the energy of its activation does not change. A theoretical analysis of the above experimental results is carried out, their scientific substantiation is given, all the factors associated with the phenomena observed in experiments are revealed.

X-ray Interferometric Study of the Effect of a Constant Magnetic Field on the Dynamic Behavior of Dislocations in Silicon Crystals

The results of studying the influence of a constant magnetic field on the dynamic behavior of dislocations in the crystal block of an interferometer, which is in a magnetic field and subsequently subjected to uniaxial mechanical tensile stress along the crystal axis [1 $$\bar {1}$$ 0], are presented. It is experimentally proved that in silicon crystals that have undergone preliminary magnetic treatment, a hardening effect is observed. The magnetic memory that appears in silicon samples with dislocations after exposure to a magnetic field is shown to be short term.

Concurrent atomistic–continuum simulation of polycrystalline strontium titanate

This paper presents the new development of a concurrent atomistic–continuum (CAC) method in simulation of the dynamic evolution of defects in polycrystalline polyatomic materials. The CAC method is based on a theoretical formulation that extends Kirkwood’s statistical mechanical theory of transport processes to a multiscale description of crystalline materials. It solves for both the deformation of lattice cells and the internal deformation within each lattice cell, making it a suitable method for simulations of polyatomic materials. The simulation results of this work demonstrate that CAC can simulate the nucleation of dislocations and cracks from atomistically resolved grain boundary (GB) regions and the subsequent propagation into coarsely meshed grain interiors in polycrystalline strontium titanate without the need of supplemental constitutive equations or additional numerical treatments. With a significantly reduced computational cost, CAC predicts not only the GB structures, but also the dynamic behaviour of dislocations, cracks and GBs, all of which are comparable with those obtained from atomic-level molecular dynamics simulations. Simulation results also show that dislocations tend to initiate from GBs and triple junctions. The angle between the slip planes and the GB planes plays a key role in determining the GB-dislocation reactions.

Assessment of interatomic potentials for atomistic analysis of static and dynamic properties of screw dislocations in W

Screw dislocations in bcc metals display non-planar cores at zero temperature which result in high lattice friction and thermally-activated strain rate behavior. In bcc W, electronic structure molecular statics calculations reveal a compact, non-degenerate core with an associated Peierls stress between 1.7 and 2.8 GPa. However, a full picture of the dynamic behavior of dislocations can only be gained by using more efficient atomistic simulations based on semiempirical interatomic potentials. In this paper we assess the suitability of five different potentials in terms of static properties relevant to screw dislocations in pure W. Moreover, we perform molecular dynamics simulations of stress-assisted glide using all five potentials to study the dynamic behavior of screw dislocations under shear stress. Dislocations are seen to display thermally-activated motion in most of the applied stress range, with a gradual transition to a viscous damping regime at high stresses. We find that one potential predicts a core transformation from compact to dissociated at finite temperature that affects the energetics of kink-pair production and impacts the mechanism of motion. We conclude that a modified embedded-atom potential achieves the best compromise in terms of static and dynamic screw dislocation properties, although at an expense of about ten-fold compared to central potentials.

X-ray and magnetic-field-enhanced change in physical characteristics of silicon crystals

The effect of low-energy (W = 8 keV) low-dose ((0.3–7.3) × 102 Gy) radiation and a dc magnetic field (B = 0.17 T) on structural, micromechanical, and microplastic characteristics of silicon crystals has been studied. The features in the dynamic behavior of dislocations in silicon crystals, which manifest themselves upon only X-ray exposure and combined (X-ray and magnetic) exposure, have been revealed.

転位の動的振る舞いと金属間化合物の力学特性

転位の動的振る舞いと金属間化合物の力学特性

Impurity effects on the generation and velocity of dislocations in Ge

The dynamic behavior of dislocations in heavily impurity (Ga, As, O) -doped Ge crystals was investigated as a function of stress and temperature by means of the etch-pit technique. The Ga and O impurities effectively suppressed dislocation generation from a surface scratch, while the As impurity did not strongly do so. Dislocations in Ga- and O-doped Ge crystals were supposed to be immobilized by the stable complexes formed through the impurity segregation and reaction. Remarkably, the O impurity, even at a low concentration of 1016 cm−3, induced the suppression of dislocation generation. The As impurity enhanced and the Ga impurity retarded dislocation velocity in motion. The O impurity had no effect on the velocity of the dislocations.

Numerical methods to improve the computing efficiency of discrete dislocation dynamics simulations

Dislocation dynamics (DD) is a method to simulate the collective dynamic behavior of dislocations and the plasticity of metals on a mesoscopic scale. A DD simulation is computationally demanding due to the fact that the stress field of a dislocation segment is long-ranged and it needs to examine a possible intersection between dislocation segments during their motion. The computing efficiency of a serial DD code is enhanced by using the so-called ‘box method’. The box method employing 21 3 boxes achieves 30-fold speed ups in the case involving 20,000 segments. The modified serial DD code has then been parallelized by using the standard message passing interface (MPI). Both the stress computation and handling segment intersection have been parallelized by using the domain decomposition method. Performance test on IBM p690 architecture shows that the parallel scheme adds up 20-fold speed ups when using 36 processors. Thus the parallel DD code presented here is about 600 times faster than the previous code. We present a parallel algorithm for highly complex dependencies in handling segment intersections and the performance test results in detail.

Crack-tip dislocation nanostructures in dynamical fracture of fcc metals: A molecular dynamics study

Dynamic behavior of dislocations near a crack tip in an fcc lattice, studied through parallel molecular dynamics (MD) simulation with visualization facilitated by newly developed software, reveals three-dimensional features of dislocation nucleation and subsequent entanglement. Results obtained for copper and aluminum show multiple emissions of dislocation loops from the crack tip and incipient evolution of plastic deformation during crack extension. 〈100〉 dislocations are found to be emitted in aluminum at zero temperature, which however are unstable and subsequently disassociate into bundles of \(\frac{1}{2}\)\langle110\rangle dislocations.

Dynamic characteristics of dislocations in Ge-doped and (Ge+B) codoped silicon

The dynamic behavior of dislocations in heavily germanium (Ge)-doped silicon (Si) crystals with concentrations up to 2.5×1020 cm−3, and Ge and boron (B) codoped Si crystals with concentrations of 4×1019 and 9×1018 cm−3, respectively, is investigated using the etch pit technique in comparison with that in undoped and B-doped Si crystals. Strong suppression of the generation of dislocations from a surface scratch is found for Ge and B codoped Si in comparison with that observed for Ge-doped and B-doped Si. The velocity of dislocations in Ge and B codoped Si crystals is found to be lower than that of dislocations in B-doped, Ge-doped, and undoped Si. The coexistence of Ge and B impurities in Si is considered to be effective at immobilizing and retarding the velocity of dislocations in Si.

Dislocation behavior in heavily impurity doped Si

The dynamic behavior of dislocations in heavily impurity doped Si is investigated. Dislocation generation from a surface scratch is suppressed when the impurity concentration exceeds 1×10 19 cm −3, which is interpreted in terms of dislocation locking due to impurity segregation. Dislocation velocity increases by increasing the impurity concentration.

High-voltage electron microscope high-temperature in situ straining experiments to study dislocation dynamics in intermetallics and quasicrystals.

The dynamic behaviour of dislocations in several intermetallic alloys, studied by in situ straining experiments in a high-voltage electron microscope, is compared at room temperature and at high temperatures. In contrast to room temperature, the dislocations move viscously at high temperatures, which is explained by diffusion processes in the dislocation cores. In quasicrystals, the viscous dislocation motion can be interpreted by models on the cluster scale.

Dynamic characteristics of dislocations in highly boron-doped silicon

The dynamic behavior of dislocations in highly boron (B)-doped Si crystals with concentration up to 2.5×1020 cm−3 is investigated using the etch pit technique. Suppression of the generation of dislocations from a surface scratch is found for B-doped Si and the critical stress for dislocation generation increases with B concentration, which is interpreted in terms of dislocation locking due to impurity segregation. The velocity of dislocations in B-doped crystals is revealed to increase by increasing the B concentration.

Dislocation behavior in highly impurity-doped Si

The dynamic behavior of dislocations in highly impurity-doped Si crystals is investigated. Suppression of the generation of dislocations from a surface scratch is found for Si doped with B and P with a concentration higher than 1×1019cm-3 and the critical stress for dislocation generation increases with B and P concentration which is interpreted in terms of dislocation locking due to impurity segregation. The velocity of dislocations in B- and P-doped crystals increases with increasing B and P concentration, respectively.

Activities of dislocations in heavily impurity-doped Si

The dynamic behaviour of dislocations in heavily impurity-doped Sicrystals is investigated. Suppression of the generation ofdislocations from a surface scratch is found for Si doped with B andP to a concentration higher than 1×1019 cm-3 andthe critical stress for dislocation generation increases with B andP concentration which is interpreted in terms of dislocation lockingdue to impurity segregation. The velocity of dislocations in B- andP-doped crystals increases with increasing B and P concentration, respectively.

Dislocation Dynamics in Intermetallic and Oxide Dispersion Strengthened (ODS) Alloys

ABSTRACTIn situ straining experiments in a high-voltage electron microscope allow the observation of the dynamic behaviour of individual dislocations. Such experiments have been performed on the intermetallic alloys NiAl, NiAl containing 0.2 at% Ta, α-TiAl, and MoSi2, and the oxide dispersion strengthened (ODS) alloys INCOLOY MA956 and INCONEL MA754 in a wide range of temperatures. There are many similarities in the dynamic behaviour of dislocations in the different materials. In the intermetallic alloys, a transition occurs between low temperature mechanisms and a viscous motion in the temperature range of the flow stress anomaly. The viscous motion at high temperatures can be explained by diffusion processes in the dislocation cores, whichcan be described by the theory of the Cottrell effect. The diffusing species can be quite different, alloying components or intrinsic point defects like vacancies and antisite defects existing in the lattice or only in the dislocation cores. If the dislocations are straight and crystallographically oriented during their motion, they may be dissociated and move by a succession of glide and conservative climb between the partial dislocations. In the ODS alloys, the dislocations move between the oxide particles again in a viscous way. The relation between the dislocation dynamics and the strain rate sensitivity of the flow stress is discussed for thedifferent materials.

Dislocation dynamics in the oxide dispersion strengthened alloy INCOLOY MA956

In situ straining experiments in a high-voltage electron microscope have been performed on the oxide dispersion strengthened alloy INCOLOY MA 956 at room temperature and between 640 and 1010°C to study the dynamic behaviour of dislocations. Macroscopic compression experiments including stress relaxation tests have been carried out between room temperature and 900°C. The dynamic behaviour of dislocations, the flow stress and its strain rate sensitivity are discussed in the different temperature ranges in terms of long-range dislocation interactions, the Orowan mechanism, the model of the thermally activated detachment of dislocations from oxide particles, solution hardening and a diffusional point defect drag. It is concluded that the latter controls the thermally activated dislocation processes in a wide temperature range around about 700°C. The detachment model may be active only above 900°C.

Dynamic behavior of dislocations in InAs: In comparison with III–V compounds and other semiconductors

The dynamic behavior of α and β dislocations in both undoped and impurity-doped InAs crystals is investigated as a function of stress and temperature by means of the etch pit technique. Suppression of the generation of dislocations from a surface scratch is found in S doped InAs, which is interpreted in terms of dislocation locking due to impurity clusters and/or impurity-defect complexes as has been observed in other III–V compounds. α dislocations move faster than β dislocations in the undoped and impurity-doped InAs. S donors reduce the velocities of both α and β dislocations. On the other hand, Zn acceptors enhance the velocity of α dislocations and reduce the velocity of β dislocations. Such features are related to an electronic state of the dislocations in the elementary process of the motion. The measured velocities are expressed using a simple empirical equation as a function of stress and temperature in the same manner as for other semiconductors. The linear dependence of the activation energies for dislocation motion on the band gap energy shows a clear distinction dependent on the group of semiconductors, i.e., the elemental and IV–IV compound, III–V compound, and II–VI compound.

Dynamic dislocation behaviour in the intermetallic compounds NiAl, TiAl and MoSi2

Abstract The dynamic behaviour of dislocations in NiAl, TiAl and MoSi 2 on ‘easy’ slip systems is studied by in situ straining experiments in a high-voltage electron microscope. At elevated temperatures, the dislocations are smoothly bent as in NiAl and TiAl or sometimes show superkinks as in MoSi 2 , and they move in a viscous way. It is suggested that this dynamic behaviour as well as the flow stress anomaly are connected with the formation of atmospheres around the dislocations. A model is proposed assuming that the lowest energy configuration of a dislocation may require a certain number of antisite defects or other point defects in the dislocation core. This cloud of disordered structure may follow partly the moving dislocations to induce an additional friction, analogous to other diffusion controlled mechanisms. The view of atmospheres controlling the dislocation mobility in intermetallics at elevated temperatures is supported by measurements of the dependence of the strain rate sensitivity on the strain rate itself.

Anin-situtransmission electron microscopy study of pyramidal slip in Ti3Al: II. Fine structure of dislocations and dislocation loops

Abstract An in-situ transmission electron microscopy investigation has been conducted of the dynamic behaviour of dislocations in the type I pyramidal (π1) slip plane of Ti3Al single crystals which operates when specimens are strained in tension along the c-axis. Superpartial dislocations are split asymmetrically into two unlike partials. The high-energy complex stacking fault involved is thought to relax by short-range diffusion and this can account for the very strong disordering, the glide softening, and the nucleation of loops reported in part I. Three loop families have been identified, all vacancy type. Mechanisms are proposed to explain the formation of loops, their interactions with mobile dislocations, and their alignment along a c + a/2 direction which is different from the Burgers vector direction of the gliding dislocations. It is concluded that the critical resolved shear stress of pyramidal slip is determined by the nucleation process of loops.