Lattice Dislocations Research Articles

Conditions of Strain-Induced Facet Formation during Interaction between a Lattice Dislocation Pile-Up and a Grain Boundary

A model describing the formation of a strain-induced facet at a high-angle boundary during its interaction with a dislocation pile-up is proposed in this work. The model of continuous dislocation distribution in a pile-up has been used to analyze the conditions under which a facet appears and to calculate its length as a function of the total Burgers vector of the dislocation pile-up and the boundary misorientation angle. The formation of a facet at a grain boundary is possible only if the total Burgers vector exceeds a certain critical value that depends on the external stress and boundary misorientation. The facet length as a function of the initial distance between a dislocation source and a grain boundary at various external stresses and misorientation angles has been calculated in the case of a pile-up replenished by a dislocation source. It is found that the material grain structure refinement and a shift of grain boundaries misorientation spectra into the region of large angles suppress strain induced grain boundary faceting. The facet formation has been shown to be accompanied by the formation of a shear band in the body of the adjacent grain.

Structural and optical properties of Ti and Cu co‐doped ZnO thin films for photovoltaic applications of dye sensitized solar cells

ZnO, 1% Ti doped ZnO and 1% Ti and (0.5%, 1%, and 1.5%) Cu co-doped ZnO thin films are deposited on fluorine doped tin oxide glass substrates using sol-gel technique. The impact of Ti and Cu co-doping on optical, structural, and photovoltaic properties of ZnO thin films have been analyzed. X-ray diffraction also confirms the hexagonal wurtzite crystal structure of the film. The 1% Ti and 1% Cu co-doping percentage has an astonishing impact on the structural properties of the doped film, such as large grain size (19 nm), d-spacing (2.48 Å), small dislocation line density (2.96 × 1015 m−2), lattice parameters (a = b = 3.2 Å, c = 5.2 Å), bond length (1.9 Å) and positional parameter (3.7 × 10−1) of the ZnO thin film. Optical parameters like reflectance, absorbance, transmittance, refractive index, and dielectric constants are calculated in the range of spectral from (250-750) nm by using an ultraviolet - visible spectrophotometer. In the visible region, all the films have approximately 85% transmittance, which is excellent for solar cells. The ZnO film prepared at the co-doping percentage of 1% Ti and 1% Cu has high absorbance, high refractive index (2.09) and small band gap energy (3.37 eV) as compared to other doped films. The dye-sensitized solar cells of these films as a Photoanode have been fabricated and studied the efficiency variation of films. It is found that the performance of the dye-sensitized solar cell (DSSC) using a Ti-Cu co-doped film is much better than a pure ZnO film. The highest efficiency of DSSC 2.38% is achieved in the dye-sensitized solar cell by using a 1% Ti and 1% Cu co-doped ZnO thin film. When Ti is doped in ZnO then the surface area and pore size of the ZnO increased, which increase the dye loading ability. Cu reduces the recombination rate and enhances the electron injection efficiency rate from the dye to the conduction band of ZnO. Therefore, it is suggested that the thin films prepared by co-doping of Cu and Ti into ZnO will upgrade the efficiency of cell.

Dislocation substructures in tensile deformed Fe–Mn–Al–C steel

Microstructure of a Fe–25Mn–2Al–0.1C steel studied under uniaxial tension revealed a gradually decreasing three-stage strain hardening behavior. Early deformation microstructure comprised of dislocations configurations; like Taylor lattice and stair-rod dislocations. Deformation twinning nucleated in regions lacking homogenous dislocation substructure. Dislocations cells and fine twin bundles were observed near failure strain. The strain hardening was a concomitant effect of deformation twins and dislocation substructure, while the contribution of dislocations seems to overwhelm the contribution from twinning.

Effect of gamma irradiation on structural, morphological and optical properties of thermal spray pyrolysis deposited CuO thin film

This study reports the influences of gamma irradiation (GI) in the range of 20–100 kGy on CuO thin films via thermal spray pyrolysis technique on the glass substrates. The results demonstrate significant influences of GI on the crystallographic, microstructural and optical characteristics of CuO thin films. The obtained XRD results showed that the crystallinity of the films deteriorates by gradually decreasing crystallite size (from 59.13 to 46 nm) as applied gamma doses increases. However, the basic monoclinic crystal structure remains same. The dislocation density and lattice strain increased with the rise of GI absorbed dose due to the creation of defects. The values of number of crystallites per unit surface area increased as dose increased indicating the abundance of crystallization of nano CuO thin films. A UV–Vis–NIR spectrophotometer was utilized to determine the optical properties and obtained results indicated that the optical energy band gap (OBG) energies reduced from 2.00 to 1.72 eV as the doses increased from 0 to 100 kGy. No distinctions of the monoclinic phase of virgin CuO thin film have been perceived under applied absorbed doses, notwithstanding the slight deterioration of the crystallinity and narrowing the OBG.

Enhancement of the Load Capacity of High-Energy Laser Monocrystalline Silicon Reflector Based on the Selection of Surface Lattice Defects.

Various defects during the manufacture of a high-energy laser monocrystalline silicon reflector will increase the energy absorption rate of the substrate and worsen the optical properties. Micron-scale or larger manufacturing defects have been inhibited by mechanism study and improvement in technology, but the substrate performance still fails to satisfy the application demand. We focus on the changes in the optical properties affected by nanoscale and Angstrom lattice defects on the surface of monocrystalline silicon and acquire the expected high reflectivity and low absorptivity through deterministic control of its defect state. Based on the first principles, the band structures and optical properties of two typical defect models of monocrystalline silicon—namely, atomic vacancy and lattice dislocation—were analyzed by molecular dynamics simulations. The results showed that the reflectivity of the vacancy defect was higher than that of the dislocation defect, and elevating the proportion of the vacancy defect could improve the performance of the monocrystalline silicon in infrared (IR) band. To verify the results of simulations, the combined Ion Beam Figuring (IBF) and Chemical Mechanical Polishing (CMP) technologies were applied to introduce the vacancy defect and reduce the thickness of defect layer. After the process, the reflectivity of the monocrystalline silicon element increased by 5% in the visible light band and by 12% in the IR band. Finally, in the photothermal absorption test at 1064 nm, the photothermal absorption of the element was reduced by 80.5%. Intense laser usability on the monocrystalline silicon surface was achieved, and the effectiveness and feasibility of deterministic regulation of optical properties were verified. This concept will be widely applied in future high-energy laser system and X-ray reflectors.

Graphene gets bent

Graphene gets bent

Thermal desorption spectroscopy evaluation of hydrogen-induced damage and deformation-induced defects

ABSTRACT Thermal desorption spectroscopy (TDS) was performed on ultra-low carbon (ULC) steel with various degrees of hydrogen-induced damage and deformation-induced defects. First, the extent to which hydrogen-induced damage manifests itself in TDS measurements was evaluated. Application of multiple test conditions on cold deformed ULC steel with and without the presence of hydrogen-induced damage showed that such damage did not appear as a hydrogen signal on the TDS spectra. Second, interesting features observed on the TDS spectra of cold deformed ULC steel were further investigated by assessing the TDS spectra of recrystallised and annealed ULC steel. As such, the four peaks in the TDS spectra were linked to microstructural features: interstitial lattice positions, grain boundaries, dislocations and microvoids.

Si/Ge (111) Semicoherent Interfaces: Responses to an In‐Plane Shear and Interactions with Lattice Dislocations

Concurrent atomistic–continuum simulations are employed to study Si/Ge (111) semicoherent interfaces in terms of their responses to an in‐plane shear and interactions with lattice dislocations. Three types of Si/Ge interfaces, differing in interfacial structures and energy, are considered. Type I interface coincides with the shuffle‐set slip plane and contains a hexagonal network of edge dislocations. Type II and Type III interfaces both coincide with the glide‐set slip plane, yet they contain, respectively, a triangular and a hexagonal network of Shockley partial dislocations. The simulations show that among the three types of interfaces, 1) Type I interface is the least stable subject to an in‐plane shear and 2) Type III interface impedes the gliding of lattice dislocations the most significantly.

In Situ Topotactic Transformation of an Interstitial Alloy for CO Electroreduction.

Electrochemical reduction of CO to value-added products holds promise for storage of energy from renewable sources. Copper can convert CO into multi-carbon (C2+ ) products during CO electroreduction. However, developing a Cu electrocatalyst with a high selectivity for CO reduction and desirable production rates for C2+ products remains challenging. Herein, highly lattice-disordered Cu3 N with abundant twin structures as a precursor electrocatalyst is examined for CO reduction. Through in situ activation during the CO reduction reaction (CORR) and concomitant release of nitrogen, the obtained metallic Cu° catalyst particles inherit the lattice dislocations present in the parent Cu3 N lattice. The de-nitrified catalyst delivers an unprecedented C2+ Faradaic efficiency of over 90% at a current density of 727mA cm-2 in a flow cell system. Using a membrane electrode assembly (MEA) electrolyzer with a solid-state electrolyte (SSE), a 17.4 vol% ethylene stream and liquid streams with concentration of 1.45 m and 230× 10-3 m C2+ products at the outlet of the cathode and SSE-containment layer are obtained.

Photoluminescence spectroscopy of metamorphic InAsSb on GaAs and Si

Improving the production cost and yield in next-generation infrared focal plane arrays have drawn attention towards the growth of absorbing material on large-area low-cost substrates while maintaining high material quality. In this paper, we study the structural and optical properties of InAsSb bulk layers grown on lattice-mismatched semi-insulating GaAs and Ge-on-Si (Ge/Si) substrates using molecular beam epitaxy. We used X-ray diffraction (XRD) and temperature-dependent photoluminescence (PL) spectroscopy to determine the overall quality of these samples w. r.t the grown sample on the lattice-matched GaSb substrate. Analysis of the PL spectra's line shape reveals the ground state bandgap energy along with various optical transitions in each sample in the temperature range of 12 K–280 K. Furthermore, the temperature dependence of PL peak energy was analyzed, and the Integrated PL (IPL) intensity as a function of inverse temperature was used to compare the PL efficiency in each sample. The results indicate that PL is less dependent on the lattice mismatch-induced dislocations while it is mainly affected by the cracks originated from the difference in the thermal expansion coefficient of the Si substrate and the epilayer. The combination of optical and imaging analysis presented in this paper provides a deeper insight into each sample's material quality and predicts a promising path toward the application of alternative substrates in FPA imageries.

Orientation dependence of the nano-indentation behaviour of pure Tungsten

Coupling of nano-indentation and crystal plasticity finite element (CPFE) simulations is widely used to quantitatively probe the small-scale mechanical behaviour of materials. Earlier studies showed that CPFE can successfully reproduce the load-displacement curves and surface morphology for different crystal orientations. Here, we report the orientation dependence of residual lattice strain patterns and dislocation structures in tungsten. For orientations with one or more Burgers vectors close to parallel to the sample surface, dislocation movement and residual lattice strains are confined to long, narrow channels. CPFE is unable to reproduce this behaviour, and our analysis reveals the responsible underlying mechanisms.

A molecular dynamics investigation into deformation mechanism of nanotwinned Cu/high entropy alloy FeCoCrNi nanolaminates

High-entropy alloys (HEA) present high hardness, but low tensile ductility. Here, deformation behaviour of the nanotwinned Cu/HEA FeCoCrNi nanolaminates prepared by the previous experiment is investigated using atomic simulations. The result shows the nucleation and glide of lattice dislocations in the Cu layers dominate the initial plastic deformation in Cu/HEA multilayers, and these gliding dislocations are deposited at the twinning boundary. With the increasing strain, the dislocations are activated in HEA layer, and deposited in the Cu-HEA interface, finally facilitate slip transmission from the HEA layer to the Cu layer. The flow stress decreases and keeps a constant value with increasing nanotwinned Cu/HEA FeCoCrNi layer number, which depends upon the slip transmission and interfacial dislocation to govern strain hardening.

Micro-Structural and Thermoelectric Characterization of Zinc-Doped In0.6Se0.4 Crystal Grown by Direct Vapour Transport Method

Crystal of zinc-doped In0.6Se0.4 was successfully grown by direct vapour transport (DVT) method. Grown In0.6Se0.4:Zn crystal has been characterized by energy dispersive X-ray (EDAX) and powder X-ray diffractometer (XRD) techniques for compositional and micro-structural analysis, respectively. The EDAX spectra represent the grown In0.6Se0.4:Zn crystal enriched with excess indium doped with Zn, which consecutively shows enhanced n-type conductivity. The powder XRD spectrum signified that the grown sample was crystalline and had hexagonal structure. The micro-structural parameters: average crystallite size, average lattice strain, dislocation density, and domain population were determined from powder XRD spectra. The thermoelectric properties such as Seebeck coefficient (S), electrical resistivity (σ), and thermal conductivity (κ) were measured in the temperature range of 313 to 368 K. Grown In0.6Se0.4:Zn crystal reported Seebeck coefficient (S) as high as –548 μV K–1 and figure of merit of 1.14 at 368 K.

Band gap engineering, quantum confinement, defect mediated broadband visible photoluminescence and associated quantum States of size tuned zinc oxide nanostructures

A simple wet chemical method has been used to synthesize ZnO nanostructures. The ZnO nanocrystals exhibit hexagonal unit cell structure as revealed from the XRD pattern. The intensity difference of various diffraction peaks indicates that the growth of the ZnO nanostructure is anisotropic. Various lattice parameters, dislocation density, strain, particle size, and Zn-O bond length were also calculated. The band gap of the synthesized ZnO nanostructures was determined from the UV–vis absorption spectra using Tauc equation and the band gap of the synthesized ZnO varies from 3.34 eV to 3.57 eV by varying the growth duration. Crystallite sizes also decreased with increased growth duration. This variation of size leads to the change in band gap of the nanostructures. Thus, it is possible to tune the band gap of the nanostructures by varying the growth duration. The synthesized ZnO nanostructures exhibit visible photoluminescence due to deep level transitions. A simple weak quantum confinement model has been used to designate the observed photoluminescence emission peak associated with such transitions.

Dislocation nucleation from Zr–Nb bimetal interfaces cooperating with the dynamic evolution of interfacial dislocations

The nucleation of lattice dislocations (LDs) from bimetal interfaces is critical to the interface-mediated deformation and strengthening mechanism in nanoscale multilayered metallic composites. Such nucleation not only depends on external loadings but also closely correlates with interfacial dislocation (ID) networks and their dynamic evolution. In this study, we focused on LD nucleation cooperating with IDs dynamic evolution from Zr/Nb bimetal interfaces following Pitch-Schrader and Burgers orientation relationships. To mimic the cross interactions between Zr and Nb atoms in molecular dynamic simulations, an empirical embedded-atom method potential was developed. The automatic Nye tensor analysis method was employed to directly identify the characteristics of IDs, which showed that: (1) many segments with obvious directivity, named main segments, were regularly distributed within IDs networks; and (2) main segments were connected by several atomic steps at regions where IDs intersected with each other. These characteristics have demonstrated, for the first time, the significant details of hcp/bcc interfaces. The further analyses of IDs dynamic evolution as LDs nucleation indicated: (1) the nucleation, which preferred at or by IDs intersections, is attributed to the flexible movement of atomic steps, by which the line orientations of ID segments at intersections can be adaptively altered to be parallel to the traces of LDs glide planes; (2) the weakened block effect of main segments in low strain concentrations is responsible for the unexpected nucleation across main segments in a direction which largely deviates from main segments. This investigation gives helpful understanding of the interface-mediated deformation in not only Zr/Nb but also other hcp/bcc (especially hcp crystals with low c/a ratios) nanolayered composites, and can satisfy the renewed interest in the fundamental scientific field of dislocations-interfaces interactions.

Exploring the effect of Ga3+ doping on structural, electronic and optical properties of CH3NH3PbCl3 perovskites: an experimental study

Pure and gallium (Ga)-doped CH3NH3PbCl3 perovskites are systematically prepared via a simple solution-processed technique. The as-synthesized perovskites are then analysed for their structural, optical, morphological, elemental and electronic properties using X-ray powder diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (UV–Vis-DRS), field emission gun-scanning electron microscopy (FEG-SEM), energy-dispersive spectroscopy (EDS) and photoluminescence spectroscopy (PL), respectively. The detailed structural study reveals the formation of the cubic structure with space group pm3m for pristine and Ga-doped CH3NH3PbCl3 perovskites. Microstrain and lattice dislocation density of doped perovskites increase with increasing Ga composition, compared to the undoped CH3NH3PbCl3. Optical properties show the bandgap narrowing up to 10% Ga doping, and any further increase in doping concentrations again increases the bandgap. This blue shift in absorption edge, i.e. bandgap widening behaviour, of highly doped CH3NH3PbCl3 (20% and 30%) perovskites justifies the Burstein–Moss effect in doped semiconductors. The highest Urbach tail energy is observed for 10% Ga-doped compound, which in turn results in the highest electron–phonon interaction. Morphological imaging shows that optimum Ga doping (10%) can improve the characteristic in terms of homogeneity and defect-free surface, compared to pristine perovskite. However, doping beyond the optimal value can further deteriorate the surface morphology by enhancing the surface inhomogeneity and defects. XRD and EDS both confirm the purity of the synthesized materials. Here, doping increases non-radiative recombination which suppresses the radiative recombination of pure CH3NH3PbCl3. These observations unfold the prospect for a new insight of post-transition metals, to fabricate lead-free perovskite materials with advanced optoelectronic properties.

A multi-scale MCCPFEM framework: Modeling of thermal interface grooving and deformation anisotropy of titanium alloy with lamellar colony

Micro-scale thermal interface grooving (TIG), as an important breakdown mechanism of single lamella and the key to spheroidization of lamellar colony, does significant influence on the mechanical response and microstructural evolution of titanium alloy with lamellar colony during hot working. This influence may become remarkable by coupling with meso-scale deformation anisotropy of the colony. The coupled effect across different scales is crucial to the spheroidization of lamellar colony, while rare related research is reported due to the difficulty in experiment. In this work, a multi-scale MCCPFEM framework was proposed to model on the micro-scale TIG and meso-scale deformation anisotropy through the fully coupling of lattice kinetic Monte Carlo (MC) model and crystal plasticity finite element model (CPFEM). During the modeling, the modified MC algorithm was established based on the multiple-integral method (MIM) solution with Mullin's theory, which accounts for the TIG's microstructural evolution. In addition, the deformation anisotropy at different length scales including strain localization at lamella, and strain partitioning at colony was characterized by an explicit CPFEM which includes resistance anisotropy of slip systems, evolution of dislocations and high-fidelity representative lamellar colony. At the same time, geometrically necessary dislocation (GND) was also considered in the CPFEM by developing a new algorithm with mesh-free methodology and re-construct radial basic shape functions. The fully coupling of the multi-scale MCCPFEM was realized through the microstructure-based 2D grids acting as both finite elements and lattice sites. Moreover, re-set ‘zero’ of the deformation gradient of the TIG mesh was proposed to characterize the strain-free state of new TIG nucleation. Then, the TIG-induced changes in the dislocation density, lattice rotation, strain-free state and morphological characteristics were returned to MCCPFEM to determine the heterogeneous deformation and anisotropic mechanical response of the lamellar colony. With this multi-scale model, the coupled effect of the deformation anisotropy (strain localization in beta phase, stress relief in TIG and local internal stress concentration in alpha lamella) and TIG-induced microstructural evolution during the isothermal compression of the IMI834 alloy with lamellar colony is well captured and analyzed.

The effect of grain boundary on the local incipient plastic deformation of fcc metals during nanoindentation

The effect of grain boundaries (GBs) on deformation mechanisms becomes increasingly important as the volume of deformation reaches the submicrometer and nanoscale. The current work investigates the impact of grain boundaries on the incipient plasticity of small-scale deformations of fcc metals. For this purpose, the behavior of single and bi-crystal Au thin films during nanoindentation are studied, using large-scale atomistic simulations. Various symmetric ⟨110⟩ tilt GBs with a wide range of misorientation angles are included to analyze the effect of GB geometry on the nanoscale plasticity mechanisms. Potentially, GBs can act as a source, sink, or obstacle for lattice dislocation, depending on their geometry, energy level, and distance from the deformation zone. The role of the heterogeneous nucleation and emission of dislocations from GBs on the plasticity and hardness of bicrystals is analyzed. According to our results, the intrinsic free volume involved in the GB region is associated with dislocation nucleation at the GB. The volume of the plastic zone generated beneath the tip and the way it grows is strongly dependent on the GB structure. Dislocation nucleation occurs predominantly in the early stages of indentation at GBs with a dissociated interface structural unit, before the interaction of lattice dislocation and GB. Coherent twin boundaries display the lowest effect on the hardness. Based on our results, there is a strong correlation between the interfacial boundary energy and its effect on the bicrystal hardness. GBs with lower interfacial energy offer a higher barrier against slip transmission and nucleation at the GB.

Deformation across the mantle transition zone: A theoretical mineral physics view

The dynamics of the Earth's mantle is still poorly constrained due to the lack of understanding the transfer of matter between the upper and the lower mantle and their convective vigor. The transition zone (TZ) might play a crucial role as the interface connecting the upper to the lower mantle. Here, we examine the rheology of the main TZ minerals, wadsleyite, ringwoodite and majorite garnet based on a mineral physics approach. Using the results of lattice friction modeling and dislocation glide mobilities together with the available data on self-diffusion in the TZ minerals, we quantify their plastic deformation by diffusion and dislocation creep from theoretical plasticity models. We show that pure climb creep is expected to contribute to the plasticity of the TZ without the need of significant diffusion-related hydrolytic weakening, matching well the geophysical observations. Our model results predict that crystallographic preferred orientations (CPO) might only develop along with stress concentrations as present around cold subducting slabs which can be locally weaker than the surrounding TZ despite their lower temperatures.

Environmental hydrogen embrittlement associated with decohesion and void formation at soluble coarse particles in a cold-rolled Al–Cu based alloy

The combined effect of solution treatment temperature and severely cold rolling on the hydrogen partitioning and related fracture mechanism was investigated using constant heating rate thermal desorption spectroscopy to determine hydrogen desorption energies and occupancies. Besides interstitial lattice sites, dislocations, and vacancies which have been known as trap sites in pure Al, a trap site originated from coarse Al2Cu particles and negligible trapping by fine Al2Cu particles were observed. The hydrogen desorption energy of the coarse Al2Cu particles is 56.59 kJ/mol, indicating that the particles act as a relatively strong trap site for hydrogen. Based on the obtained results and tensile testing at humid air and inert atmosphere, it is firstly reported that the high-hydrogen concentration induced by environment leads to the decohesion accompanied by void formation at Al2Cu coarse particles which promotes the premature fracture.