Stacking Dislocations Research Articles

The effect of dislocation pile-up on microcrack initiation in microcrystalline materials in the hydrogen environment

In this paper, hydrogen (H) induced microcrack initiation in the first stage of crack propagation is studied. Based on the knowledge of fracture mechanics and discrete distributed dislocation method, the interaction model between hydrogen atoms at crack tip and dislocation source, the grain boundary (GB) penetration model and microcrack initiation criterion are established, and the microcrack initiation mechanism caused by dislocation pile-up near the grain boundary 2 (GB2) is studied. Meanwhile, the mechanism of dislocation synthesis (DS) caused by dislocation stacking is studied. The results show that hydrogen atoms promote the emission of dislocations at the crack tip. The distribution of H atoms at the crack tip (CT) leads to an increase in the number of dislocations penetrating the grain boundary, and the number of dislocation pile-up in the second phase grain increases. In the hydrogen environment, the number of dislocation pile-up near GB2 increases, resulting in a large stress concentration near GB2, which makes microcracks more likely to occur. At the same time, H can decrease the surface energy of materials, making materials more prone to brittle fracture. The study of hydrogen induced crack initiation is helpful to understand and control the impact of H on the performance and reliability of metal materials, which plays a crucial role in improving the working efficiency and service life of metal materials.

Optimization of quasi-β forging parameters to control trimodal microstructure parameters and performance of TC21 forgings

This study aims to explore the influence of distinct forging parameters on microstructure and mechanical properties of TC21 forgings. The trimodal microstructure, which includes equiaxial α (αeq), lath α (αlath), and fine α (αfine) phases, was achieved via quasi-β forging and dual heat treatment. Subsequently, mechanical properties were evaluated through tensile, impact toughness (ak), and fracture toughness (KIC) tests. Results indicate that the content of αeq is higher at lower forging heating temperature, whereas the content of αlath is diminished and the size of αfine remains relatively small. This tendency reverses at higher forging heating temperature due to the increased driving force of the phase transition. The rise in αlath content implies more pronounced interface strengthening and plastic deformation, which significantly contributes to strength compared to αfine and αeq but exerts a weaker influence on plasticity. Moreover, the combined deformation and fracture of αlath and αfine exert a significant influence on the initiation and propagation of cracks in ak and KIC specimens. The limited coordination of deformations exhibited by αlath leads to preferential stacking dislocations and shear fractures at interfaces, which are further influenced by its higher aspect ratio affecting crack propagation paths. This not only increases the required energy for initiating cracks but also raises energy consumption during their propagation. Crucially, the presence of αfine significantly enhances the dislocation accumulation and the plastic deformation zone at the crack tip, thereby playing a pivotal role in determining local deformation capacity and crack deflection frequency.

The deformation and friction mechanisms of NiFeCr alloys based on molecular dynamics simulations

NiFeCr alloys are widely used in aerospace, marine and power equipment due to their good mechanical properties and wear resistance. At present, the relevant research on NiFeCr alloys mainly focuses on the design and preparation of the alloys, and there are relatively few studies on their mechanical behaviour and scratch deformation. In this paper, the effect of strain rate on the mechanical behaviour and scratch deformation of NiFeCr alloys is investigated using the molecular dynamics method. The results of tensile deformation under different strain rates show that when the strain rate increases from 2.5 × 109/s to 5.0 × 109/s, multiple slip systems are initiated simultaneously, and the entanglement of dislocations in different slip systems leads to dislocation stacking, and the strain rate enhancement effect is significant. When the strain rate is from 5 × 109/s to 7.5 × 109/s, the strain rate enhancement effect is not obvious. Further molecular dynamics scratching studies were carried out, and the results showed that as the scratching rate increased, the strain rate increased, the total number and length of dislocations increased, and the scratching force increased.

Molecular dynamics analysis of the effect of temperature on the internal structure transformation of polycrystalline 3 C-SiC nanoindentation

To investigate the structure changes of polycrystalline 3 C-SiC nanoindentation process and the influence of temperature effect on its changes, the nanoindentation experiments of polycrystalline 3 C-SiC are constructed by molecular dynamics method. The initial nanoindentation model of polycrystalline 3 C-SiC and diamond indenter is constructed based on the characteristics of Voronoi method in constructing close-packed spatial lattice. The temperature conditions of 300 K, 500 K, 800 K and 1000 K are selected to analyze the dislocation structure, number and length, internal atomic structure, grain boundary structure and deformation degree of polycrystalline 3 C-SiC during nano-indentation process. The stress and deformation of diamond indenter are analyzed. The results show that the higher the temperature, the higher the degree of dislocation stacking. Temperature has an effect on the internal variation of polycrystalline silicon carbide, particularly on the transverse size of the structure. The central force of the diamond becomes weaker and the force on either side becomes stronger. The temperature effect can change the stacking degree of dislocations, reduce the internal structure transformation rate of polycrystalline 3 C-SiC, and transform fewer discrete atoms and increase the number of cubic lattice atoms.

The nucleation and growth mechanism of solid-state amorphization and diffusion behavior at the W–Cu interface

The W–Cu materials hold vast potential for applications in electronic information, nuclear energy, and aerospace sectors. Here, we report a new occurrence of solid-state amorphization on the Cu side near W–Cu interface. A potential function with accuracy close to density functional theory (DFT) is constructed using machine learning, while the atomic mechanism of solid amorphous nucleation and growth is unveiled through a combination of in-situ transmission electron microscopy (TEM) and molecular dynamics (MD) simulations. Our findings indicate that the Cu near W–Cu interface experiences an amorphous phase transition at 400 °C. This amorphous nucleation is linked to the stress coupling between the W–Cu interface and dislocations within Cu. The lattice distortion arising from dislocations, combined with interfacial stress, results in lattice twisting, leading to the formation of dislocation pileup, HCP and disordered structures. The results of the in-situ TEM show that the dislocation stacking and HCP structures exist for a short period of time, and these structures quickly turn into disordered structures, eventually forming an amorphous band with a width of about 8 nm at the W–Cu interface. However, the amorphous structure is unstable. As temperature rises, the amorphous structure undergoes recrystallization into an ordered structure. Furthermore, we investigated the atomic diffusion behavior of W–Cu. Simulation results reveal that the defect in W significantly impacts diffusion. In summary, our study provides theoretical support for the nucleation mechanism of solid-state amorphization, the understanding of interfacial stress-strain, and the application of W–Cu materials.

Coupled effect of prestrain and intermediate annealing on mechanical properties and microstructural evolution of GH5188 superalloy

The forming of sheet metal parts with complex structures often requires multiple processing. The Co-based superalloy GH5188 sheet is taken as the research object for two-stage uniaxial tensile experiments including intermediate heat treatment, where the prestrain is 10%, 20% and 30%, respectively, in the first stage. The results indicate that the macroscopic residual stress increases with increasing prestrain, which is manifested as the increase of the initial yield strength, the deterioration of ductility and work hardening ability. The increase in dislocation density in the plastic deformation process is the decisive factor. After annealing, the number of deformed grains greatly reduces, and the grain size is refined, resulting in a corresponding reduction in dislocation density. Intermediate heat treatment can eliminate dislocation stacking and stress concentration in the alloy and reduce the influence of internal stress generated during work hardening. Furthermore, the intermediate annealing can decrease the initial yield stress and increase the elongation of the predeformed sheet, while retaining desirable work hardening ability and tensile strength, ultimately enhancing the subsequent work performance.

On the structural evolutionary behavior of the CdTe/HgCdTe interface during the annealing process

In this study, secondary ion mass spectrometry and a semiconductor parameter analyzer were used to study and analyze the component interdiffusion at the CdTe/HgCdTe interface before and after annealing, as well as the effect of the formation of a gradient bandgap layer from annealing on the electrical properties of the metal–insulator-semiconductor (MIS) structure. The site-direction relationship of the contracted crystals at the CdTe/HgCdTe interface was revealed by transmission electron microscopy, and it was found that the stacking dislocations in the CdTe layer were significantly reduced after annealing and that the Hg atoms diffused into the CdTe layer through the high-temperature annealing and occupied the lattice positions of the Cd atoms, thus forming the HgCdTe structure, which formed a contracted-crystalline structure with the CdTe substrate. Molecular dynamics was applied to simulate the specific atomic diffusion behavior at different temperatures and different times during the high temperature annealing process, and the above experimental results were verified by a research note on the variation of the diffusion phenomena during the process.

Improved Soft Magnetic Properties in FeNi@MgO Composites by Sol-Gel-Based Surface Coating and High-Temperature Heat Treatment

In this study, we utilized MgO as an insulating buffer layer to enhance the thermal stability and soft magnetic properties of Fe-Ni soft magnetic composites (SMCs) and investigated the effect of high-temperature heat treatment on those soft magnetic properties. By employing the sol-gel process, a uniform MgO insulating layer with a thickness of 600 nm was coated onto Fe-Ni magnetic powder. Subsequently, high-density SMCs were fabricated through high-pressure compaction molding. The MgO layer remained intact up to 800 °C, leading to the FeNi@MgO@MK SMCs exhibiting enhanced permeability and reduced hysteresis loss due to grain enlargement and elimination of defects, such as dislocation stacking. Notably, the dynamic loss increase after high-temperature heat treatment was significantly regulated compared to the case of the uncoated counterpart. The results underscore the potential to improve the thermal stability and soft magnetic properties of MgO-coated Fe-Ni SMCs, rendering them suitable for various electromagnetic applications.

Interaction of extended dislocations with nanovoid clusters

Voids of nanoscale dimensions in irradiated metals can act as obstacles to dislocation motion and cause strengthening. In this work, nanovoid strengthening and the influences of void size, void spacing and material properties, such as stacking fault energies, on dislocation bypass mechanisms are investigated using Phase Field Dislocation Dynamics, a three-dimensional mesoscale model that predicts the minimum energy pathway taken by discrete dislocations. A broad range of face centered cubic metals (copper, nickel, silver, rhodium, and platinum) and nanovoid sizes and spacings are treated, altogether spanning void size–to–dislocation stacking fault width ratios from less than unity to ten. Material γ-surfaces, calculated from ab initio methods, are input directly into the formulation. The analysis reveals that the critical bypass stress scales linearly with the linear void fraction, effective isotropic shear modulus, and ratio of the intrinsic to unstable stacking fault energies. With only a few exceptions, the critical stress is controlled by the stress required for the leading partial to impinge the voids (to move within range of the attractive image stress field of the void). When the void diameter is nearly an order of magnitude greater than the stacking fault width, the mechanism determining critical strength shifts to the stress for the dislocation to breakaway after partially cutting the void. This situation corresponds to that treated by line tension models and is realized here for Pt, with a sub-nanometer stacking fault width.

Influence of annealing on recrystallization and mechanical properties of Zr702 alloy obtained by accumulative roll bonding

The influence of annealing time on the recrystallization evolution and mechanical behavior of Zr702 alloy obtained by accumulative roll bonding (ARB) has been investigated. Results show that the bonding degree of the ARB interface gradually improved with the increase of annealing time from 5 to 20 min, which is because the atoms can obtain relatively more diffusion driving force with the increase of annealing time. In addition, almost complete recrystallization (average size of about 3.98 μm) and excellent comprehensive mechanical properties can be obtained as the annealing time is set to 20 min, and the evolution path of the microstructure with annealing time can be described as: (I) high density stacking dislocations; (II) formation of subgrain boundaries; (III) reduction of dislocation density and formation of recrystallization; (IV) growth of recrystallized grains. In this study, with the increase of annealing time, the release of deformation storage energy and the increase of recrystallized dislocation-free grains are very helpful to improve the ductility.

Microstructure and mechanical properties of twin wire and arc additive manufactured Ni3Al-based alloy

A Ni3Al-based alloy was prepared for the first time using a twin wire and arc additive manufacturing (T-WAAM) by adjusting the relative feeding speed of the Ni and Al wires, and the microstructure and mechanical properties of the deposited alloy were investigated. The results showed that the deposited Ni3Al-based alloy consisted of a dendritic γ + γ' dual-phase structure and an interdendritic γ' block. The interdendritic γ' block was partially transformed into a γ + γ' dual-phase structure owing to multiple thermal cycles during the deposition process. Thus, the proportion of the γ + γ' dual-phase structure gradually decreased with increasing building height. Meanwhile, the tensile strength and elongation of the deposited alloy exhibited a similar decline in the height direction. Fracture analysis revealed that the interdendritic γ' block hindered dislocation slip in the γ + γ' dual-phase structure, which caused dislocation stacking and stress concentration at the dendrite/interdendritic phase interface. The increase in the interdendritic γ' block was responsible for the decline in the tensile strength and elongation of the deposited alloy along the height direction. With the lowest proportion of interdendritic γ' block, the bottom deposited alloy had an optimal combination with a tensile strength of 817.3 MPa and elongation of 20.9%. Evaluation of the mechanical properties showed that the tensile strength and elongation of the T-WAAMed Ni3Al-based alloy were equivalent to those of commercial cast IC218 and IC221M Ni3Al-based alloys.

Microstructural evolution in the cross section of Ni-based superalloy induced by high power laser shock processing

Microstructural evolution in the cross-section of Ni-based superalloy GH202 treated by high power laser shock processing (LSP) were investigated. The microstructures on surfaces and cross-sections of alloy specimens were characterized by electron backscatter diffraction and transmission electron microscopy. The results showed that dislocation slip and twinning were generated simultaneously during plastic deformation induced by high power LSP, with many low-angle grain boundaries and twins produced in the surface. The dynamic recrystallization occurred at the spot overlap after LSP due to the crystal migration and subgrain rotation. The dislocation multiplication promoted dislocations to pile up and form dislocation stacking and tangling in the subsurface. The multisystem slippage occurred in the plastic deformation induced by LSP from 40 to 130 μm depth and the motion of dislocation lines in two or more slip systems formed dislocation arrays and subgrains. In areas over 200 μm depth from the surface, dislocation density significantly decreased and a small amount of subgrains and dislocation cells remained. Therefore, LSP had a positive effect on grain refinement, formation of low-angle grain boundaries, and mechanical property improvements.

Stacking Relations and Substrate Interaction of Graphene on Copper Foil

AbstractThe crystallinity of graphene flakes and their orientation with respect to the Cu(111) substrate are investigated by means of low‐energy electron microscopy (LEEM). The interplay between graphene and the metal substrate during chemical vapor deposition (CVD) introduces a restructuring of the metal surface into surface facets, which undergo a step bunching process during the growth of additional layers. Moreover, the surface facets introduce strain between the successively nucleated layers that follow the topography in a carpet‐like fashion. The strain leads to dislocations in between domains of relaxed Bernal stacking. After the transfer onto an epitaxial buffer layer, the imprinted rippled structure of even monolayer graphene as well as the stacking dislocations are preserved. A similar behavior might also be expected for other CVD grown 2D materials such as hexagonal boron nitride or transition metal dichalcogenides, where stacking relations after transfer on a target substrate or heterostructure could become important in future experiments.

Atomic scale simulations for the diffusion-assisted crossing of dislocation anchored by vacancy clusters

Nanosize vacancy clusters, characterized in metals after plastic deformation, irradiation or specific heat treatments are suspected to participate in materials hardening through their interactions with mobile dislocations. Our numerical simulations made from combining three different simulation techniques, i.e., molecular statics, kinetic Monte Carlo and elastic line models allow us to compute the dislocations velocity in realistic conditions of applied shear stress, temperature, concentration, and size of the vacancy clusters, in face--centered-cubic aluminium. We show that the clusters behave as sources of vacancies that follow a reaction path along the dislocation line, which is recognized as a pipe diffusion process. The accumulation of vacancies in the dislocation stacking fault ribbon yields jogs that participate in the dislocation climb. Both vacancy leaks from clusters and climb of dislocation segments contribute to the dislocation crossing, which remains thermally activated. We integrated the ensemble of the thermally activated processes: Diffusion, emission, absorption processes, as well as dislocation-cluster crossing, into the same simulation allowing us to predict the dislocation mobility in good agreement with experimental deformation tests.

Edge stacking dislocations in two-dimensional bilayers with a small lattice mismatch

Incomplete stacking dislocations are predicted to form at edges of the shorter upper layer in two-dimensional hexagonal bilayers upon stretching the longer bottom layer. A concept of the edge Burgers vector is introduced to describe such dislocations by analogy with the Burgers vector of standard bulk dislocations. Analytical solutions for the structure and energy of edge stacking dislocations in bilayer graphene are obtained depending on the magnitude of elongation and angles between the edge Burgers vector, direction of elongation and edge. The barrier for penetration of stacking dislocations inside the bilayer is estimated. The possibilities to measure the barrier to relative motion of graphene layers and strain of graphene on a substrate by observation of edge stacking dislocations are discussed.

Calculation of fast pipe diffusion along a dislocation stacking fault ribbon

The diffusion of an interstitial Si atom along an edge dislocation in Al is studied by using a quantum-mechanics/molecular-mechanics coupling method. It is found that the diffusing Si atom follows approximately a sinusoidal trajectory. The diffusion energy barrier along the partial dislocation core is in excellent agreement with the experimental value. We predict that the stacking fault ribbon could provide a fast pathway for diffusion which increases the diffusivity by 6--7 orders of magnitude comparing to the bulk value at 700 K. At the same temperature, the diffusivity along the stacking fault is found to be three to four orders of magnitude greater than that along the partial dislocation core.

Depinning transition for a screw dislocation in a model solid solution

On the basis of the classical dislocation theory, the solid solution hardening (SSH) is commonly ascribed to the pinning of the edge dislocations. At the atomic level, the theoretical study of the dislocation cores contrasts with such a prediction. Using the static molecular simulations with some interatomic effective potentials, we demonstrate numerically that the critical resolved shear stress associated with a screw dislocation in a random Ni(Al) single crystal has the same order as the edge one. Such a result is imposed by the details of the dislocation stacking fault and the core dissociation into Shockley partials. The SSH statistical theory is employed to tentatively predict analytically the data acquired through our atomistic simulations at different Al concentration.

Dislocation–stacking fault tetrahedron interaction: what can we learn from atomic-scale modelling

The high number density of stacking fault tetrahedra (SFTs) observed in irradiated fcc metals suggests that they should contribute to radiation-induced hardening and, therefore, taken into account when estimating mechanical properties changes of irradiated materials. The central issue is describing the individual interaction between a moving dislocation and an SFT, which is characterized by a very fine size scale, ∼100 nm. This scale is amenable to both in situ TEM experiments and large-scale atomic modelling. In this paper we present results of an atomistic simulation of dislocation–SFT interactions using molecular dynamics (MD). The results are compared with observations from in situ deformation experiments. It is demonstrated that in some cases the simulations and experimental observations are quite similar, suggesting a reasonable interpretation of experimental observations.

Dynamics of dislocations in InSb and GaAs crystals

Abstract An asymmetry of the mobility of 60° α and screw dislocations in nominally pure InSb and GaAs single crystals has been studied as a function of shear stress (τ ≈ 1–50MPa) and temperature (T ≈ 323–773 K). The observed synchronous decrease in the mean free path length and the mean number of mobile dislocations under periodic tensile-compressive and monotonic multiple-tension loadings are most clearly manifested for dislocations prone to a noticeable cross-slip (screws containing β partials). It has been found that variations in the dislocation stacking fault width with increasing shear stress τ under compression and extension affect the dislocation velocity in InSb and GaAs in different manners. An estimation of the Peierls stress τP shows that τP≤10 −6μ (μ is the shear modulus) in A3B5 covalent compounds, whereas τP≤10−5μ in the elemental semiconductors Si and Ge. The data on acoustoplastic, electroplastic and photoplastic effects in semiconductors may be unambiguously explained using a universal m...

The triggering mechanism of the martensitic transformation

The martensitic transformation involves, at least partly, a shear deformation of the parent lattice. Because the assumption that the martensitic transformation represents a macroscopic shear instability of the parent solid is at variance with experimental observations, the shear instabilities must be microscopic in nature. A model based on the dynamics of a dislocation stacking fault configuration (DSFC) adequately describes previous data as well as our own observations of a sharp decrease of the elastic constants of a Fe-28 wt pct Ni alloy just prior to the onset of the transformation. In terms of this model, theMs temperature represents the temperature at which the dislocation stacking fault configuration becomes thermodynamically unstable. In terms of the present harmonic approximation, the unhindered expansion of the DSFC will therefore trigger the martensitic transformation.