Dislocation Network Research Articles

Transient dislocation emission from the nanosized interface of Cu–Ag composite under the coupled thermal-mechanical shock: Molecular dynamics simulations study

In this study, we use molecular dynamics to simulate the deformation of Cu–Ag nanocomposite under the coupled thermal-mechanical shock. We compare the dislocation evolution under different loading conditions by changing the strain rate and the heating rate applied to the nanocomposite. The result indicates that for a given strain rate with increasing heating rates, the dislocation density decreases, and so does the interaction between dislocations. The resistance encountered by the dislocation movement decreases as the dislocation movement mode shifts from dislocation slip to climb. The plastic deformation mechanism transforms from dislocation movement to the amorphization of the whole composite, resulting in the disruption of the interfacial dislocation network. These factors conspire to cause a reduction in yield stress. For a given heating rate with increasing strain rates, owing to the strain rate effect and the temperature difference during the deformation process, the yield stress increase. We can conclude that due to the difference in dislocation density, dislocation movement mode, and the plastic deformation mechanism, the dislocation nucleation stress and the yield stress of composite decrease with the strain rate decreases and the heating rate increases (which involves the effect of temperature difference).

Energy storage under high-rate compression of single crystal tantalum

When a material is plastically deformed the majority of mechanical work is dissipated as heat, and the fraction of plastic work converted into heat is known as the Taylor-Quinney coefficient (TQC). Large-scale molecular dynamics simulations were performed of high strain rate compression of single-crystal tantalum, and the resulting integral and differential TQC values are reported up to true strains of 1.0. A phenomenological model is proposed for the energy stored in the material as a function of plastic strain with an asymptotic limit for this energy defined by the deformation conditions. The model reasonably describes the convergence of TQC values to 1.0 with increasing plastic strain, but does not directly address the physical nature of thermo-mechanical conversion. This is instead developed in a second more detailed model that accurately accounts for energy storage with two distinct contributions, one being the growing dislocation network and the other the point defect debris left behind by moving dislocations. The contribution of the point defect debris is found to lag behind that of the dislocation network but to be substantial for the high-rate straining conditions considered here.

Investigating the strength of Ti/TiB interfaces at multiple scales using density functional theory, molecular dynamics, and cohesive zone modeling

Titanium/titanium boride (Ti/TiB) composites are interesting technological materials with prospective applications in the aerospace, automotive, and biomedical industries. However, not much has been studied about the failure mechanisms of these composites. This article thoroughly investigates the adhesion and strength of two well-known Ti/TiB interface variants formed during the production of Ti/TiB composites below and above 910 °C, respectively. The studies were carried out using different theoretical methods at multiple scales, including density functional theory (DFT), molecular dynamics (MD), cohesive zone modeling (CZM), and the finite element method (FEM). First, we employed DFT to investigate the interfacial adhesion and strength of the selected planes. Then, MD simulations were utilized to study the misfit dislocation networks and derive interfacial CZMs for FEM modeling and simulation of composites. Our FEM simulations showed that the Ti/TiB interface has sufficient strength to transfer the shear load from Ti to TiB without debonding at room temperature. The results have confirmed the same phenomenon observed in some experimental studies and interpreted this phenomenon from a multiscale point of view. The research findings can be used in quantifying the failure stress of TiB whiskers directly from tension tests on Ti/TiB composites by ruling out the possibility of interface debonding.

Microstructure Evolution of Reactor Pressure Vessel A508-3 Steel under High-Dose Heavy Ion Irradiation

The microstructure evolution of nuclear reactor pressure vessel A508-3 steel irradiated by heavy ions up to 1.5 dpa was studied by transmission electron microscopy (TEM). According to the TEM analysis, black dots were widely distributed in the irradiated A508-3 steel, with a high density of 1.782 × 1022/m3. A large number of dislocations with Burgers vectors <100> were formed in the irradiated A508-3 steel and tangled together, leading to the formation of dislocation networks. The number density of black dots at 1.5 dpa was 3.5 times higher than that at 0.08 dpa, and the corresponding average size showed an 8% increase. The higher density of dislocation defects led to a significant increase in hardness from 3.0 GPa at 0.08 dpa to 4.2 GPa at 1.5 dpa. The elastic modulus showed a slight increase and less dependence on the irradiation dose.

The effect of hydrogen on dislocation motion and cracking in tungsten foil

By investigating the change of microstructure features in the presence and absence of hydrogen, we studied the influence of internal hydrogen on the plastic deformation ahead of a crack tip in tungsten foil. After micromechanical loading, ahead of the crack tip in the hydrogen-charged tungsten, we found an intermittent plastic zone composed of dislocation tangles and a dislocation network with an L-square shape. The L-square dislocation structure tended to move along the crack propagation direction. These observations are attributed to the hydrogen effect on the energy release rate and the stress intensity factor. The propagation of crack is considered a result of the competition between H-modified dislocation motion and H-induced decohesion.

Molecular dynamics study of hydrogen isotopes at the Be/BeO interface

Molecular dynamics simulations are used to investigate the behaviour of D atoms at two interfaces between beryllium (Be) and beryllium oxide (BeO). After relaxation of the simulation cell, there are (a) localised defects at the interface and (b) a hexagonal misfit dislocation network creating a succession of compressed and expanded area from each side of the interface. The simulations between 750 K and 1500 K for tens to hundreds of nanoseconds show that both interfaces act as trapping sites for D atoms. The simulations also show that D atoms tend to migrate in the material where the hydrogen isotope solubility is the highest as predicted by thermodynamics. However, the simulations also shows that there are additional kinetic barriers (D trapping sites, D2 formation/dissociation in BeO) that slow down the path to equilibrium. These additional kinetic barriers may influence the fuel retention and permeation in Be materials.

Enhancing mechanical properties of uniformly distributed nano TiB2/2024 Al composite rolling sheet by pre-stretch aging

The effects of pre-stretching prior to natural aging on the behavior of dislocation evolution, the precipitation behavior, microstructure and mechanical properties of an in-situ TiB2/2024 Al composite were investigated. Pre-stretching introduced a strong increment in the dislocation density, and meanwhile facilitated uniform distribution of dislocations. The dislocation structure evolved from parallel dislocation lines to dislocation cells, and finally formed uniform dislocation networks. The heterogeneous nucleation of the S’’ precipitates occurred on the more uniform-distributed dislocations. Finer and more evenly distributed S’’ precipitates were achieved. Based on these refined microstructure features, the mechanical properties of TiB2/2024 Al composite presented a significant increase in yield strength and tensile strength, from 306 ± 6 MPa and 508 ± 3 MPa without pre-stretching to 504 ± 3 MPa and 542 ± 5 MPa for the composite subjected to 8% nominal strain pre-stretching, respectively. Meanwhile, a loss of plasticity took place on the pre-stretching composites. The results in this work provided important insight into tuning the thermo-mechanical treatment which possesses improved mechanical properties.

Emulation of neutron-irradiated microstructure of austenitic 21Cr32Ni model alloy using dual-ion irradiation

In this study, the capability of heavy-ion irradiation to emulate neutron irradiation was demonstrated on an austenitic 21Cr32Ni type ternary model alloy. The model alloy used in this study is chemically analogous but compositionally simpler than of alloy 800H, which is a candidate austenitic Ni alloy which has been proposed for use in Generation IV reactors.The microstructure of the 21Cr32Ni model alloy irradiated in the BOR-60 fast reactor to 17.1 dpa and 35 dpa at ∼380 °C was characterized using transmission electron microscopy (TEM). The 17.1 dpa BOR-60 irradiated microstructure was then compared with the microstructure of the same material developed under dual-ion (DI) irradiation using various He/dpa ratios between 0.1 and 16.6 appm He/dpa in the temperature range of 430 °C-500 °C. The results showed that both neutron and DI irradiation of 21Cr32Ni model alloy produced dislocations in the form of a dislocation network as well as {111}-type faulted dislocation loops, cavities, and radiation-induced Ni enrichment at radiation-induced sinks. When the dose and the He/dpa ratio were kept similar to those in neutron irradiation, DI irradiation of the 21Cr32Ni model alloy at 460 °C resulted in over-nucleation of small cavities and in a high density of faulted dislocation loops compared to those observed in the fast-neutron irradiated alloy of the same heat irradiated at ∼380 °C. The optimal condition for reproducing the neutron-irradiated microstructure was DI irradiation at 460 °C and 0.1 appm He/dpa. In that case, the faulted loop and cavity size distributions in the BOR-60 irradiated 21Cr32Ni model alloy samples closely matched with those measured in the DI irradiated 21Cr32Ni model alloy sample. The fact that the He/dpa is an order of magnitude smaller than the helium generation rate for fast neutron irradiation, stops over nucleation and allows for the development of a similar microstructure as for neutron irradiation.

Magnetic APFC modeling and the influence of magneto-structural interactions on grain shrinkage

We derive the amplitude expansion for a phase-field-crystal (APFC) model that captures the basic physics of magneto-structural interactions. The symmetry breaking due to magnetization is demonstrated, and the characterization of the magnetic anisotropy for a bcc crystal is provided. This model enables a convenient coarse-grained description of crystalline structures, in particular when considering the features of the APFC model combined with numerical methods featuring inhomogeneous spatial resolution. This is shown by addressing the shrinkage of a spherical grain within a matrix, chosen as a prototypical system to demonstrate the influence of different magnetizations. These simulations serve as a proof of concept for the modeling of manipulation of dislocation networks and microstructures in ferromagnetic materials within the APFC model.

Effect of initial dislocation density on the plastic deformation response of 316L stainless steel manufactured by directed energy deposition

The relationship between the microstructural features (such as the solidification cells and initial dislocation densities) and the tensile properties of alloys additively manufactured (AM) using techniques such as laser powder bed fusion (L-PBF) and directed energy deposition (DED) is yet to be firmly established. In this work, a detailed investigation into the structure-property relations in DED 316L austenitic stainless steel (316L SS) was conducted. The microstructural parameters were varied systematically by changing the laser energy employed. Results show that while the sizes of grains and cells and the volume fraction of the oxide particles increase with increasing laser energy, the dislocation density decreases. Importantly, a uniform distribution of dislocations, instead of dislocation networks that are reported in many AM alloys, was observed. The connection between these microstructural features and the yield strength and work hardening capability of the DED 316L SS, which vary systematically with the laser energy, are explored. The correlation shows that a Hall-Petch type relation cannot capture the measured yield strength variation. Instead, the initial dislocation density dominates both the yield strength and the work hardening behavior. These results suggest a strategy for manipulating the mechanical performance in AM alloys through the control of dislocation densities and their distribution.

Indentation and Scratching with a Rotating Adhesive Tool: A Molecular Dynamics Simulation Study

For the specific case of a spherical diamond nanoparticle with 10 nm radius rolling over a planar Fe surface, we employ molecular dynamics simulation to study the processes of indentation and scratching. The particle is rotating (rolling). We focus on the influence of the adhesion force between the nanoparticle and the surface on the damage mechanisms on the surface; the adhesion is modeled by a pair potential with arbitrarily prescribed value of the adhesion strength. With increasing adhesion, the following effects are observed. The load needed for indentation decreases and so does the effective material hardness; this effect is considerably more pronounced than for a non-rotating particle. During scratching, the tangential force, and hence the friction coefficient, increase. The torque needed to keep the particle rolling adds to the total work for scratching; however, for a particle rolling without slip on the surface the total work is minimum. In this sense, a rolling particle induces the most efficient scratching process. For both indentation and scratching, the length of the dislocation network generated in the substrate reduces. After leaving the surface, the particle is (partially) covered with substrate atoms and the scratch groove is roughened. We demonstrate that these effects are based on substrate atom transport under the rotating particle from the front towards the rear; this transport already occurs for a repulsive particle but is severely intensified by adhesion.

Creep properties and relevant deformation mechanisms of two low-cost nickel-based single crystal superalloys at elevated temperatures

In this work, the creep properties and relevant deformation mechanisms of the two self-designed low-cost single crystal superalloys were studied under the conditions of 1038 °C/172 MPa and 982 °C/248 MPa. The experimental alloys both displayed superior creep properties than the typical second-generation single crystal superalloy René N5. During high temperature creep tests, the rafted γ′ phase became more and more twisted with the decrease of distance away from the fracture surface, which was caused by the existence of a stress gradient in the gauge length section of the fractured specimen. Under high temperature and low stress conditions, the Re-free alloy displayed denser dislocation networks under both test conditions. Based on the nodal reaction theory, the dislocation networks with different shapes formed by reactions of a/2<110> dislocations at the γ/γ′ interface. Furthermore, the morphology of dislocation networks could transform from initial rectangles to octagons and then to thick hexagons. Under the condition of 982 °C/248 MPa, some parallel dislocation arrays in γ′ particles caused by dislocations pile-up was found in the Re-low alloy, and the main obstacles to the motion of dislocations were K–W locks and a<010> superdislocations. Based on the calculated critical resolved shear stresses, the primary deformation mechanism of the two experimental alloys under elevated temperature and low stress conditions was identified as dislocations slipping in γ matrix and climbing over rafted γ′ phase.

Dislocation structure of tungsten irradiated by medium to high-mass ions

Single crystalline tungsten was irradiated by the medium-mass ion Si with 7.5 MeV and high mass-ion W with 20.3 MeV up to a calculated peak damage level of 0.04 dpa and 0.5 dpa. The obtained dislocation structure of the damage zone was investigated by transmission electron microscopy and systematically compared with each other. Bright-field kinematical images were taken under four different two-beam diffraction conditions g = −200, 020, −110, 110 close to the [100] zone axis. The observed damage depth and damage peak position is in good agreement with the SRIM calculated damage depth profiles. The dislocation structures were investigated at the region of the damage peak because there the damage levels are comparable. In both irradiations (Si and W), the dislocation structures were similar. At the low damage level of 0.04 dpa dislocation loops and dislocation-loop clusters were found. The size of the dislocation loops in the W-irradiated tungsten sample was up to 20% higher than for the Si-irradiated sample. At the high damage level of 0.5 dpa a dislocation network consisting of dislocation-loop chains and dislocation lines was found for both irradiations. The dislocation line density was about 12% higher for the W-irradiated sample. Through comparison of the damage zone to SRIM damage depth profiles it was found that the transition from dislocation loops and dislocation-loop clusters to an ordered dislocation network takes place at about 0.08–0.1 dpa. Despite the large differences in ion mass and irradiation energy the dislocation structures were very similar.

The effect of helium on cavity swelling in dual-ion irradiated Fe and Fe-10Cr ferritic alloys

Ferritic-martensitic (FM) steels for in-vessel components in proposed fusion reactors are expected to suffer from high levels of displacement damage and helium (He) generation by neutron transmutation. However, a thorough understanding of the He synergistic effects on the cavity swelling in FM steels is still not well established. To gain fundamental insights into the He effect on cavity swelling, high purity Fe and Fe-10 wt.% Cr ferritic model alloys were irradiated with 8 MeV Ni ions and co-implanted He ions at 400 to 550 °C up to 30 displacements per atom (dpa) with He implantation rates of 0.1, 10 and 50 appm He/dpa. The current study focuses on the 50 appm He/dpa, 500 °C behavior vs. 0.1 and 10 appm He/dpa. Irradiation-induced defects, including cavities, dislocation loops, and dislocation networks were characterized using transmission electron microscopy (TEM). In the grain interior, a bimodal cavity size distribution was observed in the 10 and 50 appm He/dpa samples, but not for 0.1 appm He/dpa. Cavity swelling was maximized at intermediate He implantation rates of ∼10 appm He/dpa for both ion-irradiated Fe and Fe-10Cr alloys. The cavity swelling behavior as a function of He implantation rate appears to be controlled by the He/dpa-dependent variation of cavity sink strengths. Treating small bubbles as biased sinks for interstitial absorption can significantly increase the ratio of biased to unbiased sink strengths (Q) and results in maximized cavity swelling for a Q ratio close to one.

Resonant interaction between phonons and PbTe/PbSe (001) misfit dislocation networks

This work aims at a quantitative and mechanistic understanding of the dynamic process of the phonon-dislocation interaction in PbTe/PbSe (001) heterostructures using the Concurrent Atomistic-Continuum (CAC) method as the simulation tool. The misfit dislocation network and the atomic-scale dislocation core structure obtained in the simulations are found to agree reasonably well with the experimental observations of the PbTe/PbSe (001) interface. Through visualizing the dynamic interaction between phonons and dislocations, as well as quantifying the dislocation vibration amplitude, the phonon energy transmission, and the thermal resistance of the misfit interfaces, this work has illustrated and quantified two mechanisms for phonon-dislocation interaction: (1) phonon scattering by the strain field of dislocations, and (2) phonon scattering by dislocations that vibrate via the local modes of a dislocation network; the latter, leads to resonant phonon-dislocation interaction, which is manifested as local maxima of out-of-phase vibration of the atoms on the two sides of the slip plane, leading to local minima of the energy transmission in the heterostructure that contains one interface. The local vibrational modes are found to be excited only by shear stress induced by transverse phonons. Among various resonant modes, the one with the lowest frequency has the strongest effect. This work has also demonstrated the collective motion of dislocations under ultrafast phonon pulses. In addition, the dynamic properties of the misfit dislocation network localized within one interface are found to be significantly altered by the presence of misfit dislocations at other interfaces, thus further confirming the cooperative dynamic nature of the motion of dislocations and phonons.

On dislocation networks and superdislocations in Re-containing nickel-based SX superalloy under different creep conditions

In this paper, the creep behaviors and deformation mechanisms of a 3 wt% Re-containing single crystal superalloy, are investigated under two tough conditions of 1120 °C/137 MPa and 850 °C/690 MPa. The detailed TEM observation on interfacial dislocation networks and superdislocations are analyzed to interpret the creep resistance. Results show that at 1120 °C/137 MPa, dislocation networks degrade into unstable structures and give rise to the universal shearing of superdislocations. At 850 °C/690 MPa, three mechanisms including superdislocation pairs, stacking faults, and bypassing loops jointly contribute to the plastic deformation. In addition, the comparison of superdislocations and creep performance of superalloys with different Re contents are concluded. In general, the deficiency of interfacial dislocation networks and γ′ phases in resisting dislocation climbing and shearing causes the poor creep performance, which could be improved by stabilizing interfacial dislocation networks with additional W contents. This paper guides the composition optimization of low-cost third-generation superalloys in the future.

Deformation of Copper Nanowire under Coupled Tension-Torsion Loading.

Metallic nanowires (NWs) are essential building blocks for flexible electronics, and experience different deformation modes due to external mechanical loading. Using atomistic simulations, this work investigated the deformation behavior of copper nanowire under coupled tension–torsion loading. A transition in both yielding pattern and dislocation pattern were observed with varying torsion/tension strain ratios. Specifically, increasing the torsion/tension strain ratio (with larger torsional strain) triggered the nucleation of different partial dislocations in the slip system. At low torsion/tension strain ratios, plastic deformation of the nanowire was dominated by stacking faults with trailing partial dislocations pinned at the surface, shifting to two partial dislocations with stacking faults as the strain ratio increases. More interestingly, the NW under tension-dominated loading exhibited a stacking fault structure after yielding, whereas torsion-dominated loading resulted in a three-dimensional dislocation network within the structure. This work thus suggests that the deformation behavior of the NW varies depending on the coupled mechanical loading, which could be beneficial for various engineering applications.

Detailed characterization of a PWR fuel rod at high burnup in support of LOCA testing

Experimental investigations of the fuel microstructure and volatile fission products along the pellet of a high burnup specimen (local burnup 76 GWd/tHM) have been conducted to support future transient testing. Detailed microscopy examinations have been carried out at different length scales. Transmission electron microscopy has highlighted a significant amount of damage across the entire radius with the formation of networks of dislocations. The optical and scanning electron microscopy determined the formation of three zones in the pellet with different characteristics. An intermediate region with no grain subdivision, lower porosity than the central zone porosity and high retained fission gas in nanometric bubbles and in the matrix was present between r/r0 ≈ 0.55 and 0.8. The high retention of gas in this region might suggest that the region will be prone to fine fragmentation, in addition to the HBS. An abrupt transition in the structure was observed at mid radius, with a third region developing from the mid radius to the pellet center. In this part of the pellet, metallic and grey phases with size between hundreds of nanometers and a few micrometers have formed at grain boundaries. The majority of fission gas has been released from the grain matrix and the original grains have polygonised, forming sub-grain domains separated by low-angle grain boundaries. No final explanation can be given for the polygonization occurring in the center, but on the basis of the irradiation history and the analysis of all the post irradiation examination (PIE) data, it is postulated that the polygonization within the original grains is an effect of dynamic recovery occurring at high temperature in the fuel center.

Saturation effect of creep-fatigue cyclic-life for Nickel-based superalloy DZ445 under long-term tensile dwell periods at 900 °C

This investigation determined the creep-fatigue cyclic-life (Nf) saturation effect for the DZ445 superalloy with 16–128 min dwell time at 900 °C and total strain range of 1.6%. When the dwell time exceeds 8 min, the mechanical response including maximum stress response, hysteresis loop, plastic strain range, and stress relaxation amount at characteristic cycle are also saturated with dwell time gradually. The Nf saturation effect is also reflected in the creep- and ductile-dominated fracture modes. This critical phenomenon is closely related to the dynamic equilibrium reached between the superdislocations of a/2[101] in γˊ precipitation and the formation of the dislocation networks. While the rupture time is increased continuously with dwell time, which could be better used as an evaluation index when the cyclic life is saturated. The Nf saturation effect can not only reduce the cost of tests with long-dwell time, but also provide the guidance for creep-fatigue safety design.

Volume of a dislocation network

We derive a simple analytical line integral expression for the relaxation volume tensor of an arbitrary interconnected dislocation network. This quantity determines the magnitude of dislocation contribution to the dimensional changes and volumetric swelling of a material, and highlights the fundamental dual role of dislocations as sources of internal strain as well as carriers of plastic deformation. To illustrate applications of the method, we compute the relaxation volume of a stacking fault tetrahedron, a defect commonly occurring in fcc metals; the volume of an unusual tetrahedral configuration formed by the (a/2) and a dislocations in a bcc metal; and estimate the relative contribution of extended dislocations to the volume relaxation of heavily irradiated tungsten.