Dislocation Slip Systems Research Articles

A method to identify dislocations in a known crystal structure by transmission electron microscopy

This paper proposes a method to identify the type and the Burgers vector of dislocations visualizedviatransmission electron microscopy (TEM). The first step is to determine experimentally the orientation, with respect to the sample holder, of a grain of known crystal structure whose dislocation slip systems have been previously reported. With this determined orientation of the grain, the method calculates the orientation of the projections of the possible dislocation line vectors in the transmission electron microscope screen coordinate system and then compares them with the observed dislocations to identify their type and the Burgers vector. The coordinate transformations underlying the method are outlined, and its validity is demonstrated using TEM measurements on a titanium sample. The method is expected to simplify the related TEM determination work.

Anisotropic size effect in strength in coherent nanowires with tilted twins

When materials are deformed plastically via dislocations, a general finding is that samples with smaller dimensions exhibit higher strengths but with very limited amount of plasticity in tension. Here we report that one-dimensional coherent nanostructures with tilted internal twins exhibit anisotropic size effect: their strengths show no apparent change if only their thicknesses reduce, but become stronger as the sample sizes are reduced proportionally. Large-scale molecular dynamics simulations show that such nanowires deform primarily through twin migration mediated by partial dislocations in one active slip system, and a large amount of plasticity could be achieved in such nanowires via twin migration. The unique structure shown here is suitable to explore strengthening mechanisms in metals when plasticity is controlled by a single dislocation slip system. This study also suggests a novel approach to modulate strength and ductility in one-dimensional coherent nanostructures.

Pressure effect on forsterite dislocation slip systems: Implications for upper-mantle LPO and low viscosity zone

In order to better constrained the effect of pressure ( P) on olivine dislocation slip-system activities, deformation experiments were carried out in a Deformation-DIA apparatus (D-DIA) on pure forsterite (Fo100) single crystals, at P ⩾ 5.7 GPa, temperature T ∼ 1675 K, differential stress σ < 350 MPa and in water-poor conditions. Constant σ and specimen strain rates ( ε ˙ ) were monitored in situ by synchrotron X-ray diffraction and radiography, respectively. Two compression directions were tested, promoting either [1 0 0] slip or [0 0 1] slip in (0 1 0) crystallographic plane. Comparison of the obtained high- P rheological data with room- P data previously reported by Darot and Gueguen (1981) shows that [1 0 0] slip is strongly inhibited by pressure while [0 0 1] slip is virtually P insensitive. This translates in creep power laws into a high activation volume V a ∗ = 15 ± 3 cm 3 / mol for [1 0 0](0 1 0) slip system, and V c ∗ = 0 ± 1.2 cm 3 / mol for [0 0 1](0 1 0) slip system. Using these laws along geotherms at natural σ condition shows that the [1 0 0] slip/[0 0 1] slip transition may occur at ∼200 km depth in the upper mantle, and be responsible for the observed lattice preferred orientation (LPO) transition. A rheological law for polycrystalline forsterite is deduced from the single-crystal rheological laws, assuming that individual grains are randomly oriented in the aggregate. Applying the aggregate law within a 2D geodynamic model of upper-mantle couette flow suggests that the pressure dependence of olivine dislocation-slip activities may partly explain the low viscosity zone (LVZ) observed underneath oceanic plates.

Kinetic Monte-Carlo simulation of self-point defect diffusion in dislocation elastic fields in bcc iron and vanadium

Simulation of self-point defects (SPDs) diffusion in elastic fields of edge and screw dislocations in slip systems 〈111〉{110}, 〈111〉{112}, 〈100〉{100}, 〈100〉{110} in the temperature range 293–1000K has been performed by the object kinetic Monte-Carlo method for bcc iron and vanadium crystals with dislocation density ∼3×1014m−2. Interaction energies of SPDs (elastic dipoles) with dislocations were calculated by means of the anisotropic theory of elasticity. These elastic interactions significantly change dislocation sink efficiencies. Dislocation sink efficiencies decrease as the temperature increases and tend to the limit value for linear sink without interaction field. The dislocations are more efficient sinks for self-interstitial atoms (SIAs) than for vacancies. The difference between the sink efficiencies for SIAs and vacancies is several times less for screw dislocations than for edge dislocations.

Quantitative Atomic Force Microscopy Characterization and Crystal Plasticity Finite Element Modeling of Heterogeneous Deformation in Commercial Purity Titanium

Using a four-point bend sample of commercial purity titanium deformed to a surface strain around 1.5 pct, the active dislocation slip and twin systems in a microstructural patch of about 15 grains were quantitatively analyzed by a technique combining atomic force microscopy (AFM), backscattered electron (BSE) imaging, and electron backscattered diffraction (EBSD). Local shear distribution maps derived from z-displacement data measured by AFM were directly compared to results of a crystal plasticity finite element (CPFE) simulation that incorporates a phenomenological model of the deformation processes to evaluate the ability of the CPFE model to match the experimental observations. The CPFE model successfully predicted most types of active dislocation slip systems within the grains at correct magnitudes, but the spatial distribution of strains within grains differed between the measurements and the simulation.

Pulling CsI(Na) single crystals from melt by continuous method without plastic deformation

It is established that bromine impurity in CsI(Na) crystals not only facilitates the homogeneous incorporation of an activator into the lattice and prevents complex activator clusters from forming, but it also significantly hinders (at certain concentrations) the action of the primary and secondary dislocation slip systems. It is shown that the automatic pulling of large CsI-based crystals can be provided by the introduction of a single Br impurity into the charge; this impurity, to a large extent, strengthens only the top part of the crystal. The absence of plastic deformation in CsI-CsBr(Na) crystals with a diameter O300 mm and height h = 600 mm (grown by the continuous method) and O500 mm and h = 200 mm (grown by the automatic Kiropulos method) has been experimentally confirmed.

Deformation of diopside single crystal at mantle pressure. 1: Mechanical data

Steady-state deformation experiments were carried out in a deformation-DIA (D-DIA) high-pressure apparatus on oriented diopside single crystals, at pressure ( P) ranging from 3.8 to 8.8 GPa, temperature ( T) from 1100 to 1400 °C, and differential stress ( σ) between 0.2 and 1.7 GPa. Three compression directions were chosen in order to test the activity of diopside dislocation slip systems, i.e., ½<1 1 0>{ 1 1 ¯ 0 } systems activated together, both [1 0 0](0 1 0) and [0 1 0](1 0 0) systems together, or [0 0 1] dislocation slip activated in (1 0 0), (0 1 0) and {1 1 0} planes. Constant applied stress and specimen strain rates ( ε ˙ ) were monitored in situ using time-resolved synchrotron X-ray diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products revealed that dislocation creep was responsible for sample deformation. Comparison of the present high- P data with those obtained at room- P by Raterron and Jaoul (1991) – on similar crystals deformed at comparable T– σ conditions – allows quantifying the effect of P on ½<1 1 0>{ 1 1 ¯ 0 } activity. This translates into the activation volume V* = 17 ± 6 cm 3/mol in the corresponding creep power law. Our data also show that both ½<1 1 0> dislocation slips and [0 0 1] have comparable slip activities at mantle P and T, while [1 0 0](0 1 0) and [0 1 0](1 0 0) slip systems remain marginal. These results show that P has a significant effect on high- T dislocation creep in diopside, the higher the pressure the harder the crystal, and that this effect is stronger on ½<1 1 0> slip than on [0 0 1] slip.

Surface nanocrystallization mechanism of a rare earth magnesium alloy induced by HVOF supersonic microparticles bombarding

A nanostructured surface layer with a thickness up to 60 μm was produced on a rare earth Mg–Gd–Y magnesium alloy using a new process named HVOF-SMB (high velocity oxygen-fuel flame supersonic microparticles bombarding). The microstructural features of the treated surface at various depth of the deformed layer were characterized by optical microscopy (OM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) with an aim to reveal the formation mechanism. Results showed that three steps during grain refinement process were found, i.e., twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets at the initial stage, stacking faults are generated and a number of dislocation slip systems are activated leading to the cross slips with increasing strain and strain rate, eventually high-density dislocation networks, dislocation cells and dislocation arrays are formed, which further subdivides the twin platelets and residual microbands into sub-microstructures. As a result, homogeneous nanostructure with a grain size of about 10–20 nm is formed through dynamic recrystallization in the topmost surface layer. Based on the experimental observations, a grain refinement mechanism induced by plastic deformation with higher strain rate during the HVOF-SMB treatment in the rare earth Mg–Gd–Y alloy was proposed.

Indentation Crystal Plasticity: Experiments and Multiscale Simulations

AbstractThis work aims at a quantitative simulation of instrumented indentation test based on physics of crystal plasticity. Indentation loading is associated with a complex 3D deformation path: it can be viewed as an ideal benchmark to test various crystal plasticity assumptions. For large scale indentation (micron size), a 3D numerical simulation using finite element crystal plasticity (FEM) is setup and quantitatively compared to experimental results using critical constraints: the load/stiffness-displacement curves and the surface displacements. Various set of parameters obtained from Dislocation Dynamics (DD) are used. A comparison with experiments shows the dominant effect of initial dislocation density and slip system interactions. For smaller depth (maximum 100 nm), Dislocation Dynamics coupled simulations to FEM are setup. Since this approach does not provide defect nucleation rules, several strategies are implemented and tested: fitting to Molecular Dynamics (MD) load-depth curve for spherical tip, or automatic generation of deformation accommodating dislocation (GND) for conical tip geometry for example. In this framework, size effects show up in the modification of the dislocation structures with depth through critical expansion of dislocation loops and junction formation.

Analytical contrast factor of dislocations along orthogonal diad axes

An analytical expression valid for any crystal system is proposed for the diffraction contrast factor of dislocations lying along a diad axis with another diad perpendicular to it. This symmetry requirement is common to several slip systems in real materials, for which the proposed solution can provide a direct relationship between diffraction line broadening, dislocation slip system and elastic properties. As an example of the utility of the contrast factor in analytical form, the main slip system of Fluorite-type structures is considered and the dependence of the line broadening anisotropy is shown as a function of the elastic anisotropy and Poisson's ratio.

Evolution of the E structural unit during uniaxial and constrained tensile deformation

Molecular dynamics simulations are used to study the mechanisms by which Shockley partial dislocations are nucleated from 〈1 1 0〉 copper grain boundaries which contain the E structural unit. Simulations in this work indicate that the natural conformation of the interface porosity with respect to the primary dislocation slip systems is responsible for the easy emission of Shockley partial dislocations during a tensile deformation. Furthermore, it is found that tensile stresses parallel to the interface plane can diminish the severity of the E structural unit on the dislocation nucleation process.

Anisotropy migration of self-point defects in dislocation stress fields in BCC Fe and FCC Cu

Spatial dependence of the interaction energies of self-point defects (vacancies and self interstitial atoms in stable, metastable and saddle point configurations) with edge dislocations in slip systems 〈1 1 1〉{1 1 0} and 〈1 0 0〉{1 0 0} in BCC Fe and 〈1 1 0〉{1 1 1} in FCC Cu was calculated using the anisotropic theory of elasticity and molecular statics (hybrid method). The migration pathways of vacancies and SIA (〈1 1 0〉 dumbbell in Fe and 〈1 0 0〉 dumbbell in Cu) along which the migration of the defects with the lowest energy barriers were defined in the presence of the dislocation stress fields. These pathways are significantly different in the stress fields of dislocations.

Plastic strain-induced grain refinement in the nanometer scale in a Mg alloy

By means of surface mechanical attrition treatment, nanometer-sized grains (with an average size of 30±5nm) were generated in the surface layer of a single-phase AZ91D alloy. Transmission electron microscopy investigations showed that the strain-induced grain refinement process in AZ91D alloy includes three steps. At the initial stage twinning dominates the plastic deformation and divides the coarse grains into finer twin platelets. With increasing strain, double twins and stacking faults form and a number of dislocation slip systems are activated, including basal plane systems, prismatic plane systems and pyramidal plane systems. As a result of the dislocation slip along these systems and of the cross slips, high-density dislocation arrays are formed which further subdivide the twin platelets into subgrains. Obvious evidence of dynamic recrystallization were identified within the high-strain-energy subgrains with a further increase of strain, leading to the formation of nano-sized grains in the surface layer.

Anomalous Spiral Motion of Steps Near Dislocations on Silicon Surfaces

We have used low-energy electron microscopy to measure step motion on Si(111) and Si(001) near dislocations during growth and sublimation. Steps on Si(111) exhibit the classic rotating Archimedean spiral motion, as predicted by Burton, Cabrera, and Frank. Steps on Si(001), however, move in a strikingly different manner. The strain-relieving anomalous behavior can be understood in detail by considering how the local step velocity is affected by the nonuniform strain field arising from the dislocation. We show how the dynamic step-flow pattern is related to the dislocation slip system.

Cover Picture: Residual Stresses, Thermomechanical Behavior and Interfaces in the Weld Joint of Ni‐based Superalloys (Adv. Eng. Mater. 3/2006)

AbstractThe cover shows in the background a typical X‐ray polychromatic microdiffraction (PXM) image obtained from the heat affected zone of a Ni‐based superalloy weld with strong plastic deformation, while the inserts show a PXM image of the nondeformed material (upper insert) and the strong oscillations of the geometrically necessary dislocation slip systems and densities across the weld (lower inserts). More about X‐ray polychromatic microdiffraction together with orientation imaging microscopy, electron scanning microscopy, and optical microscopy for studying residual stresses, thermomechanical behavior and interfaces in the weld joint of Ni‐based superalloys can be found in the paper by O. M. Barabash et al on page 202.

Computer simulation of Ti3Al intermetallic cleavage fracture

The decohesion energy and the energy of unstable stacking faults for all cracking planes and dislocation slip systems observed experimentally are calculated using the molecular dynamics method with N-particle atomic potentials. A dimensionless parameter characterizing the brittle behavior of the material is calculated for basis, prism, and pyramid faces in terms of the model elaborated by Kelly et al. and extended by Rice and Thompson. Cleavage in Ti3Al is due to low decohesion energy values, which facilitates cracking, and high energies of unstable stacking faults, which prevents the formation of a plastic zone and stress relaxation at its vertex.

TEM study of the dislocations generated by hydrogen absorption/desorption in LaNi 5 and derivatives

The combination of bright-field and weak-beam transmission electron microscopy (TEM) techniques has been used to analyse the dislocation systems activated in LaNi 5 and derivatives after hydrogen absorption/desorption cycling. The TEM results are discussed and compared with those obtained from the modelling of the anisotropic diffraction peak broadening using only two dislocation slip systems of the hexagonal structure.

Evolution of dislocation density and character in hot rolled titanium determined by X-ray diffraction

X-ray Peak Profile Analysis was employed to determine the evolution dislocation density and dislocations type in hot rolled commercially pure titanium specimens. It was found that 〈 a〉 dislocation type is dominating the deformation mechanism at all rolling reduction levels studied here. A good agreement was found between the texture evolution and changes in dislocation slip system activity during the deformation process.

Application of three-dimensional dislocation dynamics simulation to the STI semiconductor structure

As the size of semiconductor devices continues to shrink, the control of dislocation nucleation is becoming a severe problem due to high accumulated stress. In this paper, we propose a method to infer the initiation points and slip systems of nucleated dislocations through a combination of TEM observation and dislocation dynamics simulation based on FEM calculation. In order to reproduce the behaviors of dislocations on the nanometer scale, we adopted the core splitting concept first proposed by Brown and employed by Schwarz. We applied our method to a shallow trench isolation (STI) structure. The initiation points and slip systems of four kinds of nucleated dislocations can be detected. It is found that the line tension of the dislocation strongly affects the loop’s final shape, unlike the macroscopic dislocations observed in wafer slip.

Dislocation generation, slip systems, and dynamic recrystallization in experimentally deformed plagioclase single crystals

Three samples of gem quality plagioclase crystals of An60 were experimentally deformed at 900 °C, 1 GPa confining pressure and strain rates of 7.5–8.7×10 −7 s −1. The starting material is effectively dislocation-free so that all observed defects were introduced during the experiments. Two samples were shortened normal to one of the principal slip planes (010), corresponding to a “hard” orientation, and one sample was deformed with a Schmid factor of 0.45 for the principal slip system [001](010), corresponding to a “soft” orientation. Several slip systems were activated in the “soft” sample: dislocations of the [001](010) and 〈110〉(001) system are about equally abundant, whereas 〈110〉{111} and [101] in (1̄31) to (2̄42) are less common. In the “soft” sample plastic deformation is pervasive and deformation bands are abundant. In the “hard” samples the plastic deformation is concentrated in rims along the sample boundaries. Deformation bands and shear fractures are common. Twinning occurs in close association with fracturing, and the processes are clearly interrelated. Glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation. Grain boundaries of tiny, defect-free grains in healed fracture zones have migrated subsequent to fracturing. These grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization. Nucleation via small fragments can explain a non-host-controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi-brittle to plastic transition.