Dislocation Speed Research Articles

Elastodynamic behaviors of steady moving straight dislocation within thin nano film

The elastodynamic dislocation behaviors are of great interest for understanding the performances of structural alloys under intense dynamic loading conditions. The formation, propagations, and interactions of dislocations (such as injected dislocation, accelerating dislocation, steady moving dislocation at high constant speed) are quite different from static dislocations. For steady-moving dislocation within the isotropic infinite medium, the effects of surface and interface on steady-moving dislocations within limited space are still known. In this paper, we investigate the elastodynamic image stress simulation of steady moving dislocation within film of limited thickness at constant speed using Eigenstrain theory, Lorentz transformation, and steady dynamic equilibrium equations. We propose an efficient solution method that involves complex Fourier series, transforming partial differential equations into ordinary differential equations, and ultimately into a set of algebraic equations in spectral space. The effects of dislocation speed and position near the free surface on the image stress of steady-moving climbing and gliding dislocations within the thin film are examined. The results show that relativistic effects are significant for certain dislocation configurations and stress components, whereas other stress components are less sensitive to relativistic effects near the transonic speed region.

Dislocation flow turbulence simultaneously enhances strength and ductility

Multi-principal element alloys (MPEAs) exhibit outstanding strength attributed to the complex dislocation dynamics as compared to conventional alloys. Here, we develop an atomic-lattice-distortion-dependent discrete dislocation dynamics framework consisted of random field theory and phenomenological dislocation model to investigate the fundamental deformation mechanism underlying massive dislocation motions in body-centered cubic MPEA. Amazingly, the turbulence of dislocation speed is identified in light of strong heterogeneous lattice strain field caused by short-range ordering. Importantly, the vortex from dislocation flow turbulence not only acts as an effective source to initiate dislocation multiplication but also induces the strong local pinning trap to block dislocation movement, thus breaking the strength-ductility trade-off.

Influence of High Concentration Vacancy-Type Defects on the Mobility of Edge Dislocation in <i>α</i>-Iron: An Atomistic Investigation

Many vacancy-type defects (vacancy, vacancy clusters, and hydrogen-vacancy complexes) are generated in metals by plastic deformation in hydrogen environments. In this study, we use extensive molecular dynamics calculations based on a highly accurate interatomic potential to examine how vacancy-type defects affect the mobilities of edge dislocations in α-iron at a temperature range of 300–500 K and a dislocation speed 𝑉d range of 0.1–10 m/s. Under all conditions, the edge dislocation absorbs the vacancies along the slip plane and causes them to migrate with the edge dislocation. Although the necessary shear stress to glide edge dislocation in α-iron containing vacancy increases with dislocation speed, the effect is small compared to the hydrogen effects. The dislocation absorbs the hydrogen-vacancy complex along the slip plane and causes the hydrogen and the jog to migrate with the edge dislocation at low dislocation velocity regimes (𝑉d ≤ 0.1 m/s). Therefore, the hydrogen-vacancy complex exerts a continuous drag effect on the dislocation. At higher dislocation speeds (𝑉𝑑 ≥ 1 m/s), hydrogen does not migrate with the dislocation, resulting in the formation of isolated hydrogen detached from the dislocation and diffused into the material; only vacancy is absorbed. When multiple hydrogen-vacancy complexes are arranged along the slip plane, the dislocation absorbs them if they interact with dislocation at different points rather than at a single point to avoid the formation of a large jog at the colliding segment, and the required shear stress increases as the hydrogen atoms in the dislocation core increase.

Elastodynamics Field of Non-Uniformly Moving Dislocation: From 3D to 2D

Molecular dynamics (MD) and experiments indicate that the high-speed dislocations dominate the plasticity properties of crystal materials under high strain rate. New physical features arise accompanied with the increase in dislocation speed, such as the “Lorentz contraction” effect of moving screw dislocation, anomalous nucleation, and annihilation in dislocation interaction. The static description of the dislocation is no longer applicable. The elastodynamics fields of non-uniformly moving dislocation are significantly temporal and spatially coupled. The corresponding mathematical formulas of the stress fields of three-dimensional (3D) and two-dimensional (2D) dislocations look quite different. To clarify these differences, we disclose the physical origin of their connections, which is inherently associated with different temporal and spatial decoupling strategies through the 2D and 3D elastodynamics Green tensor. In this work, the fundamental relationship between 2D and 3D dislocation elastodynamics is established, which has enlightening significance for establishing general high-speed dislocation theory, developing a numerical calculation method based on dislocation elastodynamics, and revealing more influences of dislocation on the macroscopic properties of materials.

Abnormal interactions between high-speed edge dislocation and microvoid in BCC metals

The interaction between a high speed edge dislocation and a microvoid has been investigated by molecular dynamics (MD) simulations in BCC crystals of Ta, Fe, V and W in the present work. The focus is placed on the dynamic effect on the dislocation-microvoid interaction. Three interaction scenarios different from static cases, i.e., repeated dislocation oscillations around microvoids, dislocation depinning from microvoid like releasing an elastic slingshot, and formation of abnormal “pull forward” dislocation configuration, have been found for the first time. The “repeated oscillations” can be attributed to the dislocation line tension and dynamic effect, which is analogous to under-damped mechanical oscillator system. For the “releasing slingshot” case, the dislocation first bows out after breaking away from the microvoid, and then a stable “drag backward” dislocation configuration is formed. When further increasing the applied shear stress, the dislocation can accelerate to subsonic speed of roughly0.7CT, with CT being the transverse sound speed, leading to a stable “pull forward” configuration. These different dislocation configurations are closely related to the formation of superjog driven by the dislocation-microvoid interaction. By careful analysis, it is found that the variation of relative mobility between the original edge dislocation on the (110) plane and the superjog segments on the (112) plane with shear stresses perfectly coincides with the above three dislocation configurations. In fact, when the applied shear stress is high enough and the dislocation speed is subsonic, the friction stress on the superjog segments turns to be smaller than that of the original edge dislocation, resulting in the formation of abnormal “pull forward” configuration.

Relativistic effect inducing drag on fast-moving dislocation in discrete system

Phonon scattering, a dominant source of drag, is one of key issues to understand the dynamic behaviors of a dislocation. In this paper, it is found that a relativistic effect causes additional drag that is not ignorable when the dislocation's speed is comparable to the transverse shear wave speed. By considering the emission of lattice waves from the dislocation core, we theoretically derive an equation of dislocation motion wherein the relativistic effect is well considered in the frame of phonon scattering. Consequently, the relativistic drag force is characterized by two dimensionless constants that are newly defined in this study. Given that these constants depend on structural and oscillation properties of the dislocation core, a discrete nature of the core is well-reflected. Then, the solution of the equation, or the dislocation's speed, is compared with the result obtained by molecular dynamics simulation. Furthermore, the developed equation can explain a level-off behavior at high dislocation's speed by quantifying the relativistic drag force. Thus we can broaden our understanding of dislocation dynamics to fast-moving dislocations.

The Portevin-Le Châtelier effect of gradient nanostructured 5182 aluminum alloy by surface mechanical attrition treatment

Nanocrystalline surface layers and gradient nanostructure in 5182 aluminum alloy have been produced through surface mechanical attrition treatment (SMAT). The results indicate that the gradient nanostructure can not only improve the mechanical properties of 5182 Al alloy, but also has a certain effect on the Portevin-Le Châtelier (PLC) effect. The yield and ultimate tensile strength of 5182 Al alloy with SMAT are significantly improved combining with the decrease of fracture elongation compared with the as-received one. The PLC effect of 5182 Al alloy could be effectively postponed by the formation of gradient nanostructure after SMAT. It leads to the increase of critical strain of the PLC effect, more concentrated distribution of serrated strain, and increase of average stress amplitude in special strain range. The influence of grain size and gradient nanostructure on the PLC effect of 5182 Al alloy was also discussed in detail. Grain refinement could sharply increase the density of dislocations and hinder the movement of dislocations, which results in the decrease of moving speed of dislocations and the more concentrated distribution of solute atoms. The solute atoms would aggregate to form nano precipitates and further impede movement of dislocation. The stronger interaction between the dislocations and the nano precipitates is the main mechanism of postponed PLC effect.

Adiabatic shear banding and the micromechanics of plastic flow in metals

This article studies the conditions that dislocation generation and motion must fulfil to promote the development of adiabatic shear bands in crystalline metals. First, we derive a stability criterion for the formation of shear bands, by linearising the conservation equations of thermo-plasticity. We then apply this criterion on the micromechanics-based Orowan equation for plastic flow, introducing a number of increasingly sophisticated constitutive assumptions on the model. It is found that there are two crucial promoters of shear band formation: the unfettered generation of dislocations that may be found in stage I plasticity; and the softening of the elastic constants with increasing temperature. In turn, we show that limiting the speed of dislocations tends to inhibit the formation of dislocations, even when the temperature dependence of the dislocation’s drag is accounted for. This leads to the existence of an upper temperature limit above which shear band formation appears unlikely.

Elevated temperature mechanical behaviour of nanoquasicrystalline Al93Fe3Cr2Ti2 alloy and composites

Rapidly solidified nano-quasicrystalline Al93Fe3Cr2Ti2 at% alloy has previously shown outstanding tensile and compressive strength and microstructural stability up to elevated temperatures. Despite this, no study had previously assessed the effect of plastic deformation at elevated temperature to simulate thermal-mechanical forging processes for the production of engineering components. The present work analysed bars consisting of a nano-quasicrystalline Al93Fe3Cr2Ti2 at% alloy matrix, with the addition of 10 and 20vol% pure Al ductilising fibres, produced through gas atomisation and warm extrusion. The microstructure was made primarily of nanometre-sized icosahedral particles in an α-Al matrix. Compression tests were performed across a range of temperatures and strain rates. The measured yield strength at 350°C was over 3x that of “high strength” 7075 T6 Al alloy, showing outstanding thermal stability and mechanical performance. However, the microstructure was shown by XRD to undergo a phase transformation which resulted in the decomposition of the icosahedral phase around ~500°C into more stable intermetallic phases. Serrated flow associated with dynamic strain ageing was observed and a semi-quantitative analysis matching elemental diffusion speeds with dislocation speed at specific strain rates was performed, which tentatively identified Ti as the solute species responsible within the selected range of temperatures and strain rates.

Dislocation kinetics in nonmagnetic crystals: a look through a magnetic window

We discuss new kinematic magnetoplasticity features established experimentally and by simulations. We examine the motion of a dislocation through randomly distributed point defects under the influence of a magnetic field that reduces the impurity pinning forces. In addition to the measurable characteristics of motion, hidden motion parameters amenable only to simulation studies are investigated for the first time. It is shown that the distribution of stoppers on a dislocation is independent of the impurity concentration C, whereas the average number of stoppers and the critical force for the dislocation breakaway are proportional to . A model is proposed that for the first time explains the observed concentration dependence of the average dislocation speed in a magnetic field, . The model suggests that there is hidden room for an orders-of-magnitude increase in v, something which was already realized in NaCl crystals additionally subjected to a weak electric field.

The role of the mobility law of dislocations in the plastic response of shock loaded pure metals

This article examines the role that the choice of a dislocation mobility law has in the study of plastic relaxation at shock fronts. Five different mobility laws, two of them phenomenological fits to data, and three more based on physical models of dislocation inertia, are tested by employing dynamic discrete dislocation plasticity (D3P) simulations of a shock loaded aluminium thin foil. It is found that inertial laws invariably entail very short acceleration times for dislocations changing their kinematic state. As long as the mobility laws describe the same regime of terminal speeds, all mobility laws predict the same degree of plastic relaxation at the shock front. This is used to show that the main factor affecting plastic relaxation at the shock front is in fact the speed of dislocations.

Phonon scattering during dislocation motion inducing stress-drop in cubic metals

This work proves that scattering of elastic waves in the dislocation core induces an unusual behavior called stress-drop, which is defined for the first time in this study as a phenomenon where an externally applied stress drops inside a nano-sized cubic metal during dislocation motion. We develop a theoretical phonon scattering model based on discrete lattice dynamics and derive simple analytical equations to quantify the magnitude of the stress-drop. The proposed model is supported by atomistic simulations of perfect dislocations in bcc iron, where the stress-drop resulting from bond breaking is inversely proportional to the square of the dislocation speed. The derived equation is in excellent agreement with direct atomistic simulations for edge and screw dislocations. Next, we extend the equation to the edge dislocation in fcc aluminum where the dislocation exists as a stacking fault ribbon surrounded by two Shockley partial dislocations and thus the interaction between them is important. The extended equation accurately predicts the phonon scattering, resulting in the stress-drop by the oscillation of the two partials as well as bond breaking. In addition to the discussion on the validity of our model and equations at absolute zero, we investigate the temperature and size effects on our equations. As temperature increases, the magnitude of the stress-drop decreases and converges to zero even at very low temperature because of the extremely small Peierls barrier. Moreover, the magnitude of the stress-drop decreases, as the thickness of the nanoplate increases, which shows that the stress-drop is a mechanical behavior that is prominent on the nanoscale.

Fracture strength study of internally connected zirconia abutments reinforced with titanium inserts.

The implant-abutment connection area is known to be the weakest part of an internal-connection zirconia abutment and therefore the most likely to fracture. The aim of this study was to evaluate the complementary effect of a titanium insert on the fracture strength of a zirconia abutment. Three types of abutments with internal connection structures were selected and assembled: titanium abutment-titanium abutment screw (Ti-Ti), zirconia abutment-titanium abutment screw (Zr-Ti), and zirconia abutment-titanium insert-titanium abutment screw (Zr+Ti-Ti). Fifteen abutments and 15 implants were used and divided into three groups of five specimens each. Compressive loading was applied to the specimens at 30 degrees off-axis with dislocation speed of 1 mm/min and was increased until deformation occurred. The Ti-Ti specimens showed the highest maximum fracture load, followed by the Zr+Ti-Ti specimens; the Zr-Ti assemblies were the weakest. Significant differences in fracture strength were found between the groups. All of the investigated Zr abutments fractured. However, in the Zr+Ti-Ti specimens, 60% of the Ti abutment screws fractured and 40% bent, whereas all of the abutment screws in the Zr-Ti group were only bent. The Ti insert, as a substitute for the weakest part of a Zr abutment in an implant with an internal friction connection, can reinforce the fracture strength of a Zr abutment.

Atomistic simulation of the a0 〈1 0 0〉 binary junction formation and its unzipping in body-centered cubic iron

Molecular dynamics simulation is used to study the formation of the a0 〈100〉 binary dislocation junction in body-centered cubic Fe. Results show that under an applied strain two intersecting ½ a0 〈111〉 dislocations, one mobile edge and one immobile screw, form an a0 〈100〉 binary junction of mixed character in the glide plane of the mobile edge dislocation. It appears, however, that the binary junction does not necessarily lay in one of the three possible {110} glide planes of the screw dislocation. The binary junction starts to unzip as the impinging edge dislocation bows around and moves away, which results in the formation of a screw dipole along its Burgers vector. The dipole eventually annihilates, completing the unzipping process of the junction, which liberates the edge dislocation. The effects of temperature and strain rate on the unzipping of the junction are quantified by the critical release stress needed to detach the edge dislocation from the screw one. The critical stress decreases when the temperature increases from 10 to 300K, whereas it increases with increasing applied strain rate, or dislocation speed. The interaction mechanism and strength of the a0 〈100〉 binary junction as an obstacle to the edge dislocation are compared to that of other types of defect, namely nanosized voids, Cu and Cr precipitates, and dislocation loops in Fe. It appears that the binary junction strength is in the lowest range, comparable to that of a coherent Cr precipitate.

Severity of Spinal Cord Injury in Adult and Infant Rats after Vertebral Dislocation Depends upon Displacement but not Speed

Spinal cord injury (SCI) is less common in children than in adults, but in children it is generally more severe. Spinal loading conditions (speed and displacement) are also thought to affect SCI severity, but the relationship between these parameters is not well understood. This study aimed to investigate the effects of vertebral speed and displacement on the severity of SCI in infants and adults using a rodent model of vertebral dislocation. Thoracolumbar vertebral dislocation was induced in anaesthetized infant rats (∼30 g, 13-15 days postnatal, n=40) and adult rats (∼250 g, n=57). The 12th thoracic vertebra was secured, whereas the first lumbar vertebra was dislocated laterally. Dislocation speed and magnitude were varied independently and scaled between adults and infants (Adults: 100-250mm/s, 4-10mm; Infants: 40-100mm/s, 1.6-4mm). At 5 h post-injury, rats were euthanized and spinal cords harvested. Spinal cord sections were stained to detect hemorrhage (hematoxylin and eosin) and axonal injury (β-amyloid precursor protein). For each millimeter increase in vertebral displacement, normalized hemorrhage volume increased by 1.9×10(-3) mm(3) (p=0.028) and normalized area of axonal injury increased by 2.2×10(-1)mm(2) (p<0.001). Normalized hemorrhage volume was 3.3×10(-3) mm(3) greater for infants than for adults (p<0.001). Magnitude of dislocation was found to have a different effect on the normalized area of axonal injury in adults than in infants (p=0.003). Speed of dislocation was not found to have a significant effect on normalized hemorrhage volume (p=0.427) or normalized area of axonal injury (p=0.726) independent of displacement for the range of speeds tested. The findings of this study suggest that both age and amount of spinal motion are key factors in the severity of acute SCI.

Critical dislocation speed in helium-4 crystals

Our experiments show that in ${}^{4}$He crystals, the binding of ${}^{3}$He impurities to dislocations does not necessarily imply their pinning. Indeed, in these crystals, there are two different regimes of the motion of dislocations when impurities bind to them. At low driving strain $\ensuremath{\epsilon}$ and frequency $\ensuremath{\omega}$, where the dislocation speed is less than a critical value (45 $\ensuremath{\mu}$m/s), dislocations and impurities apparently move together. Impurities really pin the dislocations only at higher values of $\ensuremath{\epsilon}\ensuremath{\omega}$. The critical speed separating the two regimes is two orders of magnitude smaller than the average speed of free ${}^{3}$He impurities in the bulk crystal lattice. We obtained this result by studying the dissipation of dislocation motion as a function of the frequency and amplitude of a driving strain applied to a crystal at low temperature. Our results solve an apparent contradiction between some experiments, which showed a frequency-dependent transition temperature from a soft to a stiff state, and other experiments or models where this temperature was assumed to be independent of frequency. The impurity pinning mechanism for dislocations appears to be more complicated than previously assumed.

Optimization of Recording of Thermal-Wave Images in BDM Method

The beam displacement modulation (BDM) method is very difficult to implement and has a lot of features and parameters which require optimization to improve the effectiveness of detection of thermal non-uniformities. The most important parameters that have to be taken under consideration during the optimization process are the width of the excitation source, the displacement speed of the excitation source, and the distance between the excitation and measuring spots. The width of the excitation source (e.g., diameter of laser beam) and speed of beam dislocation, in relation to the surface of the object, influence the spectra of excitation’s energy. It allows examination and visualization of thermal properties of the object at varied depths. This article presents the general scheme of a thermal-wave microscope with the implemented BDM mode and describes the process of optimization for which the assignment is to find the microscope’s settings that guarantee the highest temperature contrast of non-uniformities at the surface of the examined object.

Interaction of a transonic dislocation with subsonic dislocation and point defect clusters

The moving speeds of all observed dislocations in crystals are subsonic. There has been a view in the literature that the speed of subsonic dislocations can not be accelerated above the speed of sound because the energy required would be infinitely large. Recent molecular dynamics (MD) simulation had shown that it is possible to generate dislocations with an initial moving speed higher than the velocity of sound in solids. This raises a question: what will happen when a supersonic dislocation meets other defects along its moving path? This work reports the results of MD simulation on the interaction of a transonic dislocation with other subsonic dislocations as well as with point defect clusters. The results show that a vacancy cluster such as a void has an insignificant slow-down effect on the transonic dislocation, while a subsonic dislocation slows down the transonic dislocation to subsonic one. In some cases, the subsonic dislocation (or a subsonic part of a transonic dislocation) can overcome the traditional sound barrier.

Molecular dynamics simulation of shearing deformation process of silicon nitride single crystal

Shear deformation properties of α and β silicon nitride single crystals are investigated using the classical molecular dynamics (CMD) method. Four cases of shearing directions are analyzed, which were reported to be slip system on the basis of experimental observations. The simulation results show that shear deformation does not occur in one of the experimentally predicted slips (α – {1 1 ̄ 0 1} 〈1 1 2 ̄ 0〉) . In this case the crystal is broken abruptly under shear deformation. In the case of the β crystal (β – {1 0 1 ̄ 0} 〈0 0 0 1〉) , a sharp slip with edge dislocation can be found. The dislocation core width and speed are estimated. Finally one of the CMD results is compared with the corresponding first principle density functional calculation results, and it is shown that the validity about shearing CMD simulation of the 3-body interatomic potential was proposed by Vashishta.

In-situ observation of deformation micromechanisms in a rafted γ/γ′ superalloy at 850 °C

In-situ straining experiments were performed in a transmission electron microscope at temperatures ranging from 800 to 850 °C on a MC2 γ/γ′ nickel-based superalloy single crystal with a rafted microstructure. Perfect and partial dislocations were observed in motion. It is shown that deformation starts in the γ phase, which is the weaker phase at this temperature, although interaction of dislocations with point defects is still effective. The γ/γ′ interfaces play a different role depending on the propagation direction of the dislocations. From γ to γ′, interface crossing often acts as an additional obstacle that can be overcome by pile-ups of dislocations. From γ′ to γ, an increase of the dislocation speed is almost always observed, with no noticeable waiting time at the interface. Some dislocation multiplication processes are observed when interfaces are crossed from γ′ to γ. Unusual dissociation of superdislocations involving super stacking faults is observed in γ′, along with microtwinning. This may be taken as an indication of the low stacking fault energy of the MC2 alloy.