Dislocation Nodes Research Articles

Spatial and temporal dynamics of thermal motion of a chain of liquid lead nanoinclusions attached to a fixed dislocation segment in an aluminum matrix

Thermal motion of a chain of liquid lead nanoinclusions attached to a dislocation segment fixed at its ends in the aluminum-based alloy was studied in situ using TEM in the temperature range from 442 °C to 497 °C. The projections of points of the trajectories of the inclusions onto the dislocation line were analyzed. The evidence for the collective interaction of all the inclusions and their spatially correlated collective thermal motion was obtained. Also, moderate temporal autocorrelation of thermal motion of the inclusions and a very weak temporal cross-correlation of thermal motion of the inclusions in all pairs was shown. The FFT phase spectra of the time dependencies of the inclusions positions on the dislocation segment give the evidence for the phase synchronization of thermal oscillations of the inclusions. It is shown that the thermal oscillations of the terminal inclusion attached to the dislocation node and elevation of temperature lead to decrease in the level of synchronization of thermal oscillations of the inclusions in the chain. It is indicated that the development of the process of desynchronization of thermal oscillations of the inclusions manifests itself by an increase in the density of phase steps in the initially linear unwrapped phase spectra accompanying by an increase in the interaction of the steps.

Effect of stacking fault energy (SFE) of single crystal, equiatomic CrCoNi and Cantor alloy on creep resistance

Compared to mechanisms like solid solution strengthening, the stacking fault energy (SFE) should be considered as a further factor that influences the material properties. The effect of SFE of alloys or individual elements on strength and resistance can vary considerably. In the high-temperature regime above 700 °C, there are still significant gaps in the knowledge about the effect of the SFE on the mechanical properties of single-phase alloys. The effect of SFE on creep resistance of two face-entered cubic equiatomic medium and high entropy alloys, CrCoNi and CrMnFeCoNi, respectively, is evaluated to fill parts of these gaps. Using the Bridgman solidification process, the alloys were produced as single crystals and crept under vacuum at 700 °C up to 1100 °C. This work shows a significant impact of the lower SFE of CrCoNi on the creep behavior compared to the results of previous investigations of CrMnFeCoNi. The creep resistance of the former is higher over the complete temperature range. At very high temperatures, the strengthening effect of the stacking faults is significantly present. The formation of tetragonal stacking faults and extended dislocation nodes can be identified as the reason for this effect.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

Some important considerations in creep modelling of ferritic steels and nickel-based superalloys

ABSTRACT Trends in life with respect to stress can vary significantly based on the form of the chosen Arrhenius equation, and this in turn is fundamentally influenced by the nature of the rate controlling microstructural obstacle. Recognising this crucial aspect is important since shortcomings in creep models are in many cases attributed to myriad microstructural changes, while in fact, it is the basic creep mechanism which needs to be properly accounted for. It is proposed that nano-scale vanadium carbonitrides and dislocation nodes are the rate controlling obstacles in ferritic steels and superalloys, respectively, and this defines the basic rate equation to be used for these materials. With these definitions in place, simple constitutive equations, similar in spirit to the Larson-Miller parameter are derived. The ability of these equations to predict long-term behaviour and remaining life is demonstrated using data on P91 boiler tubes and IN738 superalloy.

Achieving excellent strength and ductility synergy by δ phase strengthening in low-density high-manganese steel

ABSTRACT In this study, we developed a novel lightweight Fe-Mn-Al dual-phase with high strength, excellent ductility, and considerable toughness. The study involved a thorough exploration of heat treatment processes and mechanical behaviour. The microstructure of the Fe-Mn-Al dual-phase steel consists of austenite and δ-ferrite. In the early stages of plastic deformation, austenite undergoes primary deformation, leading to a faster increase in dislocation density and microhardness. In later stages, strain is transferred from austenite to δ-ferrite through high-density dislocation walls, resulting in coordinated deformation of the two-phase structure. The excellent strength-ductility combination of Fe-Mn-Al dual-phase steel is attributed to multiple stages of continuous work hardening. Initially, dislocation nodes in δ-ferrite contribute to work hardening. Subsequently, high-density dislocation structures and the strengthening effect of hard δ-ferrite enhance work hardening. Finally, deformation twins in austenite, along with the TWIP effect, further increase work hardening, emphasizing the importance of these interactions in improving mechanical performance.

Molecular dynamics simulations of interface structure and deformation mechanisms in metal/ceramic composites under tension

The role of interfaces on plastic deformation of metal/ceramic composites is multifaceted, which is due to the fact that interfaces can emit, hinder, annihilate or store dislocations. In this work, molecular dynamics simulation was employed to investigate the interfacial structure and deformation mechanism of the coherent and semi-coherent Al/MgO(110) configurations under uniaxial tensile loading. In particular, rectangular mismatch dislocation networks were observed at the interface of a semi-coherent Al/MgO(110) configuration after relaxation, which were due to the difference in lattice constants between the constituent phases. Besides, the strain concentration at the dislocation nodes was found, being conducive to the emission of misfit dislocations into the Al matrix. Moreover, unlike the coherent configuration, the elastic zone in the stress-strain curve of the semi-coherent Al/MgO(110) configuration was relatively narrow and had two yield points. The first yield point was attributed to the nucleation of misfit dislocations from the interface dislocation nodes, whereas the second one was ascribed to the nucleation of lattice dislocations in the Al matrix. Finally, the failure stress of the semi-coherent configuration was less than that of the coherent system.

Molecular dynamics simulation of dislocation network formation and tensile properties of graphene/TiAl-layered composites

Alternating stacked graphene/TiAl (Gr/TiAl) composites exhibit excellent mechanical properties because of their high strength, high Young's modulus, and the two-dimensional atomic structure of graphene. Herein, a molecular dynamics approach was used to investigate the uniaxial tensile properties of Gr/TiAl composites after rapid solidification. The results of the simulation show that after rapid solidification, the composites were more crystallizable and were accompanied predominantly by a Shockley type dislocation network, with large periodic hexagonal superlattices (also known as the Moiré pattern) of ∼12.519 and 10.092 Å. Increasing the tensile load activates dislocation emission, which enhances the interaction between dislocations and numerous dislocations, forming a large number of entangled dislocation nodes. This increases resistance to the motion of the remaining dislocations and creating a strengthening effect. The spacing between graphene layers has a substantial effect on the tensile strength and Young's modulus of the Gr/TiAl composites. The composites with smaller layer spacing exhibited better performance than those with larger layer spacing. Because of the dislocation-blocking mechanism between Gr/TiAl interfaces, graphene blocks the propagation of dislocations and takes up most of the load, yielding composites with high Young's modulus, tensile strength, and breaking strain than pure TiAl.

Dynamics of thermal motion of liquid lead nanoinclusions attached to fixed dislocations coupled in a dislocation node in the aluminum-based matrix

ABSTRACT Thermal motion of four liquid lead nanoinclusions attached to fixed dislocation segments joining in a dislocation node in aluminum-based alloy at 485°C is studied in-situ using TEM. The mutual interaction of the inclusions in all pairs is detected demonstrating the collective interaction of the all inclusions. The time-correlated thermal motion of each of the inclusions, and time-correlated thermal motion of the inclusions in all the pairs are indicated. Phase spectra of the time dependencies of the positions of the inclusions on the dislocations obtained using FFT reveal an existence of a common harmonic in their thermal oscillations at low frequencies demonstrating synchronous oscillations of the inclusions. The transition between synchronous and asynchronous dynamics of the thermal oscillations of the inclusions with increasing frequency is observed. The external vibrations introduced by the inclusion attached to the dislocation node are probably the reason for this transition.

Critical resolved shear stress for twinning and twinning-induced plasticity in single crystals of the CoCrFeNiMo0.2 high-entropy alloy

The effect of alloying with Mo atoms 4 at.% on the twinning deformation, critical resolved shear stresses (CRSS) for slip τcrsl and twinning τcrtw and plasticity was studied in [1‾11]-, [1‾44]- and [001]- oriented crystals of the Co24Cr24Fe24Ni24Mo4 (at.%) high-entropy alloy (HEA) under tension at 296 and 77 K. The stacking fault energy of the Co24Cr24Fe24Ni24Mo4 HEA, measured in the present paper on the triple dislocation nodes, is equal to 0.027 J/m2. It is shown that initial yield behavior along studied orientations is governed by dislocation slip and the CRSS for slip are independent of crystal orientation. Under tensile strain, twinning develops in the [1‾11]- and [1‾44]- oriented crystals only at 77 K and is not detected in the [001]-oriented crystals. Two types of twins are found in the [1‾11]- and [1‾44]-oriented crystals (thin nanotwins and macrotwins). Nanotwins develop after a low slip deformation of 5–10% and are observed only by transmission electron microscopy. Macrotwins are detected after a significant slip deformation of 20 and 60% and are determined metallographically and X-ray by the precession of the crystal axis, respectively, in the [1‾11]- and [1‾44]-oriented crystals. In the [1‾11]- oriented crystals, there is no precession of the crystal axis due to the development of slip simultaneously in several systems. It is shown that CRSS for nano- and macrotwins are dependent on the crystal orientation. In the [1‾44]-oriented crystals, τcrtw = 212 MPa for nanotwinning and τcrtw = 400 MPa for macrotwinning. In the [1‾11]-oriented crystals, τcrtw = 250 MPa for nanotwinning and τcrtw = 335 MPa for macrotwinning. For the transition to deformation by macrotwinning, it is necessary that the effective stacking fault energy γef is approaching to zero and the condition τcrtw <τcrsl is satisfied. Deformation by macrotwinning suppresses slip multiplicity and shifts the formation of the neck formation according to the Considère condition to higher stress levels. This increases plasticity from 72% at 296 K to 108% at 77 K in [1‾44]-oriented crystals and from 48% at 296 K to 57% at 77 K in [1‾11]-oriented crystals.

All‐Organic Filler with Fractal Structure for Reinforcement and Toughening of Aromatic Polyamide Film

AbstractThe simultaneous enhancement in strength and toughness seems to be an intrinsic contradiction in multiple material designs, including the structural regulation of polymer composites. In this study, benzimidazole‐containing aromatic‐polyamide thin films are used as typical rigid polymers with high strength and modulus to study their strength–toughness collaboration via a multi‐scaled stress distribution mechanism. A fully organic deformable filler with a fractal structure is designed. Utilizing the large‐scale deformability of the filler to absorb the energy and dislocation node structure of the filler to disperse the stress, the composite thin film is significantly toughened, which is reflected in the elongation at break increased by 144.9% and energy at break increased by 154.2%. In addition, the specific fractal structure of the filler provided a larger interaction area, which enhanced the interfacial bonding force and improved the strength of the film. Moreover, owing to the larger interaction area provided by the fractal structure, the interface bonding force between the filler and matrix due to the similar chemical structures is further improved and the tensile strength of the composite thin film is increased from 389.2 ± 11.98 to 426.2 ± 4.41 MPa.

Fine core structure and spectral luminescence features of freshly introduced dislocations in Fe-doped GaN

Dislocations introduced by Vickers tip microindentation of an a-plane free-standing semi-insulating Fe-doped GaN halide vapor phase epitaxy (HVPE) crystal were investigated by means of cathodoluminescence and scanning transmission electron microscopy techniques. Detailed combined analyses of both spectral properties and the core structure of the introduced a-screw dislocations revealed that Fe-doped GaN exhibit not only dislocation-bound emission at ∼3.35 eV of perfect a-screw dislocations previously found in such kind of samples but also luminescent bands at 3.1–3.2 and 3.3 eV due to dissociated a-screw dislocations and extended dislocation nodes previously observed only in low-resistance n-GaN. For the first time, all these luminescent bands were observed together in the same sample. Structural studies revealed the coexistence of the dislocations with the dissociated and the perfect core as well as with extended dislocation nodes, thus establishing a correlation between previously observed luminescence bands and a fine dislocation core structure.

Influence of milling on structural and microstructural properties of cerium oxide: Consequence of the surface activation on the dissolution kinetics in nitric acid

Ceria (CeO2) is known as a refractory oxide for dissolution in nitric acid, since the leaching reaction is thermodynamically unfavorable, except when it is complexed by nitrates but with very slow kinetics. To enhance dissolution, surface activation was achieved using high-energy milling. With the mechanically-activated cerium oxide, leaching in nitric acid reached 36%. The mechanical activation of the solid caused structural and microstructural changes (particle size, specific surface area, crystallite size, lattice strain, defects…). After one hour, the cleavage induced by energetic milling generated two populations: nanoparticles and grains containing defects like dislocations. Beside crystallite size and micro-strain evaluation using X-ray diffraction, cerium oxidation state was measured by Electron Energy-Loss Spectroscopy (EELS) analyses while linear defects were pictured by Transmission Electron Microscopy (TEM) observations. On one hand, it was found that the nanoparticles formed during milling process greatly enhance the dissolution reaction by the creation of Ce3+ thin layers of a few nanometer depth on their surfaces. On the other hand, it is shown that dislocations represent another way to increase the kinetics by activation energy. In conclusion, dissolution rate's growth can be due to different parameters like the leaching of the smallest particles, the presence of reduced oxidation state on nanoparticles and some highly reactive sites concentrating structural defects such as dislocation nodes. Finally, as ceria is also well known to be a safe analogue of PuO2, especially for dissolution studies, a solution for improving the dissolution of ceria would probably also be useful for dissolving the oxides rich in Pu.

Characteristic emission from quantum dot-like intersection nodes of dislocations in GaN

Freshly introduced a-screw dislocations in gallium nitride are an effective source of ultraviolet radiation, characterized by intense emission of narrow luminescence doublet lines in the spectral range of 3.1-3.2 eV. Furthermore, an additional narrow spectral line with an energy of 3.3 eV has been found at the points of intersection of such dislocations, where extended dislocation nodes were formed. In this communication, we report on the spectral properties of the characteristic luminescence of such nodes, which were obtained for the (0001) gallium nitride samples with dislocations introduced by nanoindentation. The spectral position of the dislocation-related luminescence doublet experiences a redshift with increasing distance from the indentation site. It follows the spectral shift of the excitonic near-bandgap emission, associated with stress relaxation. The luminescence of the intersection points exhibits a similar tendency. At certain local positions, its doublet fine structure is observed, which has a spectral linewidth of the order of or even less than that of the exciton. In this case, the spectral splitting between components of the doublet varies irregularly depending on the position of the exciton (i.e., on the mechanical stress). We see a clear indication of quantum dot-like emission. The fine structure of the luminescence of the intersection points can be easily explained by the energy dependence of emission on their size, as well as on their density, in particular, by the formation of paired nodes, which were previously observed in experiments in a transmission electron microscope.

Effects of hydrogen clusters on interface facilitated plasticity at semi-coherent bimetal interfaces

Dislocation nucleation and interface sliding are two dominant plasticity events governing the mechanical behavior of metallic nanocomposites. Recent works have shown that both events can be closely related to atomistic interface geometries, however, how compositional factors, e.g., segregated hydrogen clusters, contribute both events are nearly unknown. Herein, we demonstrate that hydrogen clusters near misfit dislocation nodes can strongly suppress dislocation nucleation and interface sliding, while clusters at other positions will contribute somehow weaker effect on dislocation nucleation but facilitate interface sliding. These findings offer a rational atomistic mechanism for the effect of hydrogen clusters on interface facilitated plasticity.

Is Cross‐Section Shape a Distinct Feature in Plant Fibre Identification?

Correct identification of textile fibres is an important issue in archaeology because the use of different materials can yield crucial information about the society that produced the textiles. Textiles made of plant and animal fibres can normally be easily distinguished, but to distinguish between different types of plant fibres, in particular different types of bast fibres, is difficult. Some years back it was shown that the features fibre diameter, lumen diameter, dislocation (nodes), and cross markings cannot be used on their own to distinguish between the typical bast fibres used for textiles in ancient Europe: flax, hemp, and nettle. Particularly not when only a few fibres are available for an examination so that statistical analysis is not possible, as is often the case in archaeology. The last two characterization features typically used to distinguish between bast fibres are cross‐section shape and lumen shape. In this paper, we present a study of retted and unretted fibres (in the stem) of flax, nettle, and hemp, and show that also cross‐section shape and lumen shape cannot be used as distinguishing features on their own.

Loss of ductility in optimized austenitic steel at moderate temperature: A multi-scale study of deformation mechanisms

A Ti-stabilized cold-worked 15Cr-15Ni steel, called AIM1 (Austenitic Improved Material #1), has been selected as a candidate for the fuel cladding tubes of sodium-cooled fast reactors. This steel exhibits an unusual loss of ductility between 20 and 200 °C for both solution-annealed and cold-worked conditions, which is similar to that observed for Twinning Induced Plasticty steels and for the 200 and 300 series stainless steels. Therefore, a multi-scale study has been carried out to determine the deformation mechanisms that are active at different temperatures. Tensile tests have been performed to characterize the macroscopic material behavior, and Electron Backscattered Diffraction and Transmission Electron Microscopy characterization techniques have been used to investigate the meso and micro-scale phenomena, such as the deformation microstructures and the evolution of the lattice defects. The parameters governing the deformation mechanisms have been examined, with particular attention paid to the conditions for mechanical twinning activation. This work required an original study of the variation of Stacking Fault Energy with temperature, based on the measurement of the dissociation extension of dislocation nodes. An increase in the SFE was observed between 20 and 200 °C. After reviewing the existing models for predicting twinning, the present study proposes an approach based on the minimization of the total energy of the material to explain why twinning is not favorable at high temperatures. At 20 °C, both dislocation slip and twinning are active and efficient mechanisms to release the strain energy. However, at 200 °C, only dislocation slip is favorable and is often associated with dislocation cross-slip.

Modeling of atomistic scale shear failure of Ag/MgO interface with misfit dislocation network

Metal/ceramic interfaces have broad applications and misfit dislocation network (MDN) is a prominent feature of the equilibrium metal/ceramic interfaces. As one main failure mode, interface shear failure is strongly affected by the motion of MDN. In this work, we investigate the equilibrium interface structure and shear failure of Ag/MgO interface via atomistic simulation method. Periodically distributed in-plane strain field caused by MDN and severe strain concentration at dislocation node regions are found by strain analysis. During interface shearing, these dislocation nodes act as strong pinning points to the gliding motion of MDN, which leads to bending of dislocation lines. Besides, energy analysis shows the interface shear stress is largely dependent on the variation of misfit dislocation energy. To understand interface shear failure under more complex conditions, we study the effect of model thickness and shear direction further. Due to transformation of nodal structure, the shear strength of thick model eventually decreases by almost a quarter; shear failure along the direction of Burgers vector is found to be energetically favored, with the lowest interface shear strength. This work reveals the crucial role of MDN in interface shear process, and the theoretical understanding gives some hints to metal/ceramic interface design.

Commensurate-incommensurate phase transition and a network of domain walls in bilayer graphene with a biaxially stretched layer

The two-chain Frenkel-Kontorova model is applied for an analytical description of the energy and structure of the network of domain walls in bilayer graphene. Using this approach, the commensurate-incommensurate phase transition upon biaxial stretching of one of the graphene layers is considered. We demonstrate that formation of the equilateral triangular network of domain walls becomes energetically favorable above the critical relative biaxial elongation of the bottom layer of $3.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. It is shown that the optimal period of the triangular network of domain walls is inversely proportional to the difference between the biaxial elongation of the bottom layer and the critical elongation as long as it is much greater than the width of domain walls. Quantitative estimates of the contribution of a single dislocation node to the system energy and the period of the network of domain walls are obtained. Experimental measurements of the period could help to verify the energy of the fully incommensurate state (such as that obtained by relative rotation of the layers) with respect to the commensurate one.

Extended core structure and luminescence of a-screw dislocations in GaN

Straight segments of a-screw dislocations introduced by scratching of basal (0001) of intentionally undoped low-ohmic GaN radiate a doublet of narrow luminescent lines in the spectral region at about 3.1-3.2 eV while the dislocation intersection points possess luminescence band at about 3.3 eV. Transmission electron microscopy reveals that the dislocation cores are dissociated into two 300 partials separated by stacking fault (SF) ribbon with the width of 4-6 nm width and that the dislocation nodes contain extended SF of sizes of 25-30 nm. Dislocation-related luminescence (DRL) is ascribed to exciton bound by the states of partial dislocation cores and of SF quantum well. The increase of the SF lateral sizes is assumed to cause the DRL spectral shift between straight dislocations and their nodes due to the system dimensionality transition from 1D to 2D respectively.

Dislocation Interaction and V-Shaped Growth of the Distorted Structure During Nanoindentation of Cu20Ni20Al20Co20Fe20 (high-entropy alloy)-Coated Copper: A Molecular Dynamics Simulation-Based Study

In this paper, the deformation behavior of Cu20Ni20Al20Co20Fe20 high-entropy alloy-coated single-crystal Cu substrate which undergoes nanoindentation has been investigated under molecular dynamic simulation with embedded-atom method potential. The dynamic structural evolutions under nanoindentation are presented using centrosymmetry parameter analysis, common neighbor analysis and radial distribution function plots. In the initial level of nanoindentation, the interface deformation is greatly confronted by the confined V-shaped growth of the distorted structure. But the sudden discrete dislocation burst account for avalanche break-down in the interface layer, which further get influenced by the evolution of multiple dislocation nodes that is significantly governed by core spreading, extended misfit dislocation generation and relative rotation. In the meanwhile, the subsequent generation of dislocation locks, complicated multiple dislocation loops, dislocation junctions and limited cross-slip in wide stacking faults (SFs) hasten the work hardening and in turn slows down the deformation progress. On the other hand, the intermediate appearance of narrow SFs and slip bands significantly reduces the work hardening rate that increases the optimum fracture strain value of the specimen. Moreover, the overall increase in dislocation density and dislocation length leads to a significant growth in dislocation sources which leads to forest hardening in the later stage.