Dislocation Motion In Crystals Research Articles

Influence of magnetic field on the mechanical behavior of friction stir processed paramagnetic alloy

The study investigated the influence of an externally applied static magnetic field (MF) on the mechanical behavior of a friction stir processed paramagnetic 1100 aluminum (Al 1100) alloy. The aim was to take advantage of the solid-state friction stir processing (FSP) technique to modify the microstructure and further improve the properties by testing in the presence of MF. Al 1100 was tested in tension under different static MFs (B = 0, 150, 1000, 1400, 1800, and 2200 Gauss) perpendicular to the stress axis. The strong effect of a comparatively weak static MF was observed on the elongation of both base and processed Al 1100 alloy, in both rolling and transverse directions, attributed to the magneto-plasticity effect (MPE) where the motion of dislocations in crystals is influenced in the presence of magnetic fields. Amongst the different mechanisms associated with MPE, the major effect in the current study was observed from the magnetization force, spin transition, and increase in strain energy of dislocations. This knowledge will open new lanes to improve the properties of a processed sample by exposing it to an MF which is a non-contact, pollution-free route; saving time and cost by avoiding in-between thermo-mechanical steps to break the cast microstructure.

Vortex motion and nonlinear response in coupled noisy phase oscillator lattices under shear stress.

Vortex motion in coupled phase oscillator lattices is analogous to the dislocation motion in crystals. A single vortex exhibits a glide motion by force at the boundaries. Thermal fluctuations induce the glide motion even below the critical point corresponding to the Peierls stress. The random drift motion is approximated as a random walk in a tilted potential. If the temperature is high, vortices are spontaneously generated. A nonlinear response where the frequency profile is relatively flat in the central region and changes sharply near the boundaries is observed when the vortex density becomes large and nonuniform.

Thermodynamic theory of crystal plasticity: Formulation and application to polycrystal fcc copper

We present a thermodynamic description of crystal plasticity. Our formulation is based on the Langer-Bouchbinder-Lookman thermodynamic dislocation theory (TDT), which asserts the fundamental importance of an effective temperature that describes the state of configurational disorder and therefore the dislocation density of the crystalline material. We extend the TDT description from isotropic plasticity to crystal plasticity with many slip systems. Finite-element simulations show favourable comparison with experiments on polycrystal fcc copper under uniaxial compression, tension, and simple shear. The thermodynamic theory of crystal plasticity thus provides a thermodynamically consistent and physically rigorous description of dislocation motion in crystals. We also discuss new insights about the interaction of dislocations belonging to different slip systems.

Mechanical anisotropy and ideal strength in a multifunctional Ti–Nb–Ta–Zr–O alloy (Gum Metal)

Abstract An ideal shear strength of Gum Metal, a newly developed set of beta-type titanium alloys with the nominal composition Ti-24(Nb + V + Ta) – (Zr, Hf) – O (at.%) believed to deform without motion of crystal dislocations, has been investigated experimentally for precise determination. Tensile tests of single crystals of Gum Metal along the <100>, <110>, and <111> directions showed that the Young' moduli were 40, 60, and 86 GPa, respectively. The elastic constants were estimated as c11 – c12 = 25 – 27 GPa and c44 = 29 – 36 GPa. In terms of these values, the anisotropy factor and the ideal strength were evaluated as 2.3 – 2.6 and 1.7 – 1.9 GPa, respectively. It was found that a ratio of the actual ultimate tensile strength to the ideal strength for Gum Metal was much higher than for other bcc metals.

A New Dislocation-Dynamics Model and Its Application in Thin Film-Substrate Systems

AbstractBased on the physical background, a new dislocation dynamics model fully incorporating the interaction among differential dislocation segments is developed to simulate 3D dislocation motion in crystals. As the numerical simulation results demonstrate, this new model completely solves the long-standing problem that simulation results are heavily dependent on dislocation-segment lengths in the classical dislocation dynamics theory. The proposed model is applied to simulate the effect of dislocations on the mechanical performance of thin films. The interactions among the dislocation loops, free surface and interfaces are rigorously computed by a decomposition method. This framework can be used to simulate how a surface loop evolves into two threading dislocations and to determine the critical thickness of thin films. Furthermore, the relationship between the film thickness and yield strength is established and compared with the conventional Hall-Petch relation.

Magnetoplastic effect: Basic properties and physical mechanisms

This paper presents a review of the main results of investigations into the magnetoplastic effect, which manifests itself in motion of dislocations in crystals exposed to magnetic fields. The dependences of the mean free path of dislocations on the induction and direction of the magnetic field, the magnetic treatment time, the temperature, and the type and concentration of impurities are studied for NaCl, LiF, CsI, Zn, Al, and InSb crystals. The threshold magnetic field B c below which the effect is absent, the saturation field B 0 above which the mean free paths of dislocations remain constant with an increase in the magnetic induction B, and the critical frequency v c of rotation of a sample in the magnetic field above which the effect disappears are examined. The quantities B c , B 0, and v c are investigated as functions of the basic physical parameters. It is found that the magnetoplastic effect is highly sensitive to X-ray radiation at low doses and to simultaneous action of an electric field or mechanical loading. The hardening of NaCl(Pb) crystals in the magnetic field is revealed. The theoretical interpretation is proposed for all the findings and dependences observed.

Twins as barriers to basal slip in hexagonal-close-packed metals

The boundary structure of {10 $$\bar 1$$ 1}, {10 $$\bar 1$$ 2}, {11 $$\bar 2$$ 1} and, {11 $$\bar 2$$ 2} twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with {11 $$\bar 2$$ 1} and {11 $$\bar 2$$ 2} twin boundaries is described here and compared with the results for {10 $$\bar 1$$ 1} and {10 $$\bar 1$$ 2} twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby creating twinning dislocations.

Dislocation Motion in Crystals With a High Peierls Relief: A Unified Model Incorporating the Lattice Friction and Localized Obstacles

The combined effect of the lattice friction and localized obstacles on the individual dislocation velocity is considered. First the two effects are considered separately. The velocity of an individual dislocation is described by the Hirth-Lothe equation for the case of lattice friction and by a power law for the case of localized obstacles. The power law is modified to introduce a static waiting time: the time a dislocation has to wait in its equilibrium configuration at an obstacle until it breaks away by virtue of thermal activation. As a next step, a combination of the two mechanisms is described. A dynamic waiting time is introduced: it corresponds to a situation when a dislocation overcomes the obstacle before it reaches the equilibrium configuration. The model provides a good description of the effects when they are independent, and also gives an interpolation of the two regimes. A simulation for a model material is proposed to illustrate the transition between the two regimes. This unified model is tested against experimental data for in-situ deformation of monocrystalline germanium in a transmission electron microscope. The purpose is to determine an equivalent power law exponent in a regime of plastic flow that does not follow a proper power law. The resolution is not complete because the strength of the localized obstacles is not known. However, the results are promising and allow a discussion relating to the strength of localized obstacles.

New interpretation about deviation of glide strains: In situ measurement by synchrotron radiation of lattice rotation in a copper single crystal

When a face-centered-cubic single crystal is elongated uniaxially, the characteristics of deformation behavior such as amount of glide strain, operative slip systems, and the moving path of tensile axis could be theoretically predicted [1, 2]. Although there have been many deformation experiments on single crystals, experimental results have shown the general invalidity of this classical theory [3–5]. Especially Price and Kelly [3] reported that the glide strain calculated from the orientation change, assuming single slip, was always less than the glide strain measured directly from the length change of the sample. It has been commonly believed that the difference of glide strains is dependent only on the activation of secondary slip systems [6, 7]. But the difference of glide strains is too large to explain by secondary slip systems. There is still controversy about the phenomena. Constitutive relations for inelastic deformation are formulated by many researchers [8–10]. It is generally agreed that the inelastic deformation of metals is accomplished through the motion of crystal dislocations. But the equation derived by Bowen and Christian [2] do not consider the crystal dislocations. The accumulations of dislocations can give serious error in the calculation of glide strains. In this study, we revise the equations derived by Bowen and Christian [2] in order to explain the difference of glide strains. When a face-centered-cubic single crystal is elongated uniaxially, the length of sample and the angle φ has the relation that a represented in Equation 1 [2].

Temperature dependence of the flow stress and the strain rate sensitivity at the transition from the Peierls mechanism to pinning by localized obstacles

Barriers of very different character such as localized obstacles and the extended Peierls- Nabarro relief may control the dislocation motion in crystals in different stress and temperature ranges. The great difference in the microscopic parameters characterizing these two mechanisms, e.g. , in the activation volumes, manifests itself even on the macroscopic scale as a strong change of the plastic properties in a rather narrow transition range of the temperature. A theory describing the temperature dependence of the flow stress and the strain rate sensitivity near the transition has been developed and compared with experimental data on the plastic deformation of cubic ZrO 2 single crystals in a soft orientation.

Critical stress of Hg1−xCdxTe solid solutions

The motion of dislocations in crystals is known to be impeded by the addition of isovalent impurities at concentrations of a few percent. By treating the Hg1−xCdxTe alloy as resulting from the addition of Cd impurities into the HgTe lattice, the critical resolved shear stress for solid-solution hardening in this material is calculated at room temperature as a function of Cd composition x. In this solid solution model, the critical stress is derived from the elastic size interaction between the Cd solute atoms and the edge dislocations in a HgTe solvent. The predicted dependence of the critical stress on x at room temperature compares favorably with experimental results for the lower yield stress. Estimates of the critical stress at temperatures up to 440 °C are also made at selected compositions by using the relation for the temperature and strain rate dependence of the lower yield stress.

Low‐Temperature Fluctuational Motion of Dislocations in Crystals II. Manifestations Of Dissipative and Quantum Effects in the Low‐Temperature Plasticity

AbstractThe temperature behaviour of the fluctuation‐dissipative effects is analysed. At sufficiently low temperatures it is controlled by variations in the viscosity coefficients of the phonon and electron gas. Some experimental data are quoted which can be regarded as evidence for the influence of fluctuation‐dissipative effects upon the low temperature plasticity of materials, i.e. change in the activation energy during NS transition, correlation between the activation energy and the phonon gas viscosity, etc. The temperature dependence of such values as the flow stress, activation energy and activation volume, work hardening coefficients, and the relaxation depth of effective stresses are analysed for the range of temperatures where fluctuation‐dissipative effects are essential. The dependences are calculated both, in the framework of the classical approximation and in the fluctuation‐dissipative model, and compared with the data measured with single crystals of lead and zinc at temperatures below the Debye point. As found, many anomalies observed at low temperatures in the temperature behaviour of the plasticity parameters can be satisfactorily explained as manifestations of the impact of dissipation on the velocity of activated dislocation motion through the crystal.

Low‐temperature fluctuational motion of dislocations in crystals I. Quantum and dissipative effects

AbstractA mechanism underlying fluctuational bending of dislocation segments is considered which is based on periodical ordering in the thermal motion of atoms near the dislocation core. Basic principles of calculating the probability of dislocation unpinning from local obstacles due to the fluctuations are formulated. As found, the interaction of dislocation segments experiencing fluctuational bending with phonons and/or conduction electrons, described in the framework of the usual dissipation model, can result in a reduction of the energy of atoms near the dislocation core during the fluctuation. Consequently, the effective activation energy of the dislocation unpinning from local obstacles is increased, i.e. the mean velocity of the activated dislocation motion drops. Activated dislocation motion at low temperatures is analysed with an account of the possible quantum mechanical fluctuations which can arise in condensed structures owing to the correlation of atomic zero point vibrations. An expression is derived for the probability of low temperature fluctuational dislocation with allowance for quantum mechanical effects.

Ultrasonic Measurements Under Extreme Conditions. Part II. –Measurements under High Pressure

The meaning and importance of ultrasonic experiments under high pressure were pointed out, and some examples of sound velocity and sound attenuation measurements in solids under hydrostatic pressure were briefly cited. Special emphasis was put on explaining details of our attenuation experiment by a pulse reflection method for a solid in pressurized fluid, and origins of sound losses other than true energy loss in the specimen solid were fully discussed. An experiment to determine the pressure dependence of damping of dislocation motion in crystals was referred as an illustration.

Statistical theory of thermally activated dislocation motion in crystals in the case of zero internal stresses

The dislocation motion induced by external stress is considered using the model of individual atom migration within the dislocation core. Equations are derived which permit calculation of dislocation velocities in perfect crystals from the value of the energy barrier which atoms must overcome. Using the known average distance between point obstacles (the regions of dislocation pinning), analogous calculations are possible for real crystals. The velocities calculated agree with the experimental results available for semiconducting and alkali halide crystals. [Russian Text Ignored].

Investigation of the motion kinetics of screw dislocations in sodium chloride and potassium chloride crystals

The kinetics of the motion and the surmounting of detents by dislocations in sodium chloride and potassium chloride crystals is investigated herein. Special attention is paid to a comparative study of the mean free paths of the screw and edge dislocations. It is established that the mean free paths of the screw dislocations is somewhat less than for edge dislocations under identical test conditions. The kinetic curves obtained were processed within the frame-work of theoretical representations about the motion of dislocations in crystals containing detents of two kinds.

Internal-Friction Study of High-Purity Single Crystals of n Eicosane (C20H42)

Large pure single crystals of n eicosane (C20H42) with [210] growth axes have been prepared by slow crystallizations from the melt. Internal-friction measurements covering temperatures from 100 to 300 K at ≃ 1 kHz reveal that in the as-grown single crystals both the α and γ loss peaks characteristic of polycrystalline n alkanes of less purity are absent, from which we conclude that, in polycrystals, these processes are a result of impurities and/or crystal defects. Two very weak loss maxima of obscure origin were observed at about 270 and 287 K for all the crystals studies. In one single crystal progressive stages of light cold work induced a complex spectrum of small loss maxima between 200 and 300 K. These maxima were found to be highly sensitive to aging. We believe that the observed behavior provides evidence for dislocation motion in crystals of this type. No detectable γ peak resulted from the cold work. Data are also included for one single crystal which was irradiated with γ rays.

Burgers Vector of Dislocations Generated for Dislocation Velocity Measurements in Semiconductors

Dislocation motion in crystals is usually measured on loops generated from a deliberately introduced nucleating center such as a scratch. In germanium crystals it has been noticed that motion of an individual loop is usually asymmetric as observed by etch-pit techniques. The asymmetry has been assumed to arise from motion of dislocations having different character. The validity of this assumption was tested directly by x-ray topographic contrast experiments on crystals of germanium and silicon. From appropriate reflections that give either a strong or vanishing contrast it is possible to determine the Burgers vector of the dislocation loop. For both bending and compression tests, dislocation loops generated from a scratch and subsequently moved have a Burgers vector in the maximum resolved shear stress direction. Thus, for the specimen orientation 〈123〉 commonly used in velocity experiments both ends of the dislocation loops intersecting the surface of observation have the same character. In some cases the asymmetry may be due to the shape of the loop. The x-ray evidence refutes the conclusions reached by previous workers that asymmetrical motion is due to dislocations of different character.