Evolution Of Defect Structure Research Articles

Development of Defect Structure in Shock Loaded Two-Phase Gradient Nanograined Fe95Ni05

Development of Defect Structure in Shock Loaded Two-Phase Gradient Nanograined Fe95Ni05

Nano-cutting mechanism of ion implantation-modified SiC: reducing subsurface damage expansion and abrasive wear

This study utilized ion implantation to modify the material properties of silicon carbide (SiC) to mitigate subsurface damage during SiC machining. The paper analyzed the mechanism of hydrogen ion implantation on the machining performance of SiC at the atomic scale. A molecular dynamics model of nanoscale cutting of an ion-implanted SiC workpiece using a non-rigid regular tetrakaidecahedral diamond abrasive grain was established. The study investigated the effects of ion implantation on crystal structure phase transformation, dislocation nucleation, and defect structure evolution. Results showed ion implantation modification decreased the extension depth of amorphous structures in the subsurface layer, thereby enhancing the surface and subsurface integrity of the SiC workpiece. Additionally, dislocation extension length and volume within the lattice structure were lower in the ion-implanted workpiece compared to non-implanted ones. Phase transformation, compressive pressure, and cutting stress of the lattice in the shear region per unit volume were lower in the ion-implanted workpiece than the non-implanted one. Taking the diamond abrasive grain as the research subject, the mechanism of grain wear under ion implantation was explored. Grain expansion, compression, and atomic volumetric strain wear rate were higher in the non-implanted workpiece versus implanted ones. Under shear extrusion of the SiC workpiece, dangling bonds of atoms in the diamond grain were unstable, resulting in graphitization of the diamond structure at elevated temperatures. This study established a solid theoretical and practical foundation for realizing non-destructive machining at the atomic scale, encompassing both theoretical principles and practical applications.

Low-electric-field-induced high electrocaloric properties via defect regulation in samarium-doped BaTiO3 ceramics

A small electrocaloric effect (ECE) is usually observed in aliovalent doped ferroelectric ceramics since decreased polarization is usually present. In this work, the high ECE is achieved in a barium titanate [BaTiO3 (BT)]-based ceramic by introducing defect polarization to enhance the ferroelectric polarization and polarization change. The defect dipoles consisting of donor ion and barium vacancy, or barium and oxygen vacancies were formed in samarium-doped (Ba1−1.5xSmx)TiO3 ceramics. The phase and defect structures, and dielectric and ferroelectric properties are analyzed to confirm the evolution of charge compensation states and defect structures, and the influences of defects on polarization rotation and ECE. An enhanced ferroelectric polarization and high internal bias field are present in the ceramic with a suitable defect dipole effect, leading to a high polarization change before/after applying an electric field, causing an enhanced ECE. Finally, a high electrocaloric temperature change of ∼1.11 K and a large electrocaloric strength of ∼0.037 K·cm kV−1 were achieved under a low electric field of 30 kV/cm, indicating that forming suitable defect dipoles can optimize the ECE. This work provides a significant paradigm for tuning ECE in ferroelectric materials via defect regulation.

Structural responses of heterogeneous FeB(P)CCu amorphous alloys under nanoindentation

The nano-scaled structural heterogeneity is closely associated with the magnetic softness and mechanical properties of Fe-based amorphous alloys. However, limited by the shape and size of ribbons, the details of the atomic-scale structural evolution in ribbons under external stimulation are still missing. In this work, Maxwell-Voigt model with 2 K units and 1 Mx unit was employed to investigate the structural response of FeBCCu amorphous ribbons by substitution of P for B during creeping process under nanoindentation. The P-added alloy exhibited a more sensitive mechanical response at various loading rates, especially for the reduction of hardness and the increase of maximum indentation depth and creep displacement. With the help of creep analysis using above Maxwell-Voigt model, both types of defects were inclined to be activated at a quasi-static loading mode in P-added alloy, and the small size defects seemed to be easily activated and finally coupled into large defects in the fast loading mode, revealing the atomic diffusion and reconstruction process of activated defects. These types of defects evolved into the response of structural heterogeneity to external stimulation, manifesting a large deformation behavior during creeping. This work enlightens the comprehensive insights into the evolution of structural defects in Fe-based amorphous ribbons and has certain guiding significance for the structural design of high-performance alloys.

Investigating K+ Storage Behavior of Highly Graphitized Carbon Fibers as Anodes for a Potassium-Ion Battery.

Many problems of potassium-ion batteries (PIBs) are hidden under a low mass load of the active material. However, developing research based on areal capacity is challenging for PIBs, due to the lack of an anode capable of delivering a stable capacity of more than 1 mAh cm-2. This work investigates the K+ storage behavior of highly graphitized carbon fibers (HG-CF), which exhibit automatic structural adjustments to mitigate voltage polarization. The created defects and residual K+ in the structure favor the reversible insertion/deinsertion of K+. HG-GF after structural adjustment realizes a capacity of 2 mAh (1.13 cm-2) without K deposition and a stable cyclic stability (>500 h). In situ X-ray diffraction and in situ Raman spectra were used to detect defect formation and structural evolution during cycles. This work demonstrates the feasibility of HG-GF as an anode for PIBs and provides a suitable anode for further research of PIBs based on areal capacity.

The Evolution of Structural Defects under Irradiation in W by Molecular Dynamics Simulation.

Tungsten (W) can be used in plasma-facing components in a fusion reactor because of its excellent radiation resistance. Some studies have found that nanocrystalline metals with a high density of grain boundary show a higher ability to resist radiation damage compared to conventional coarse-grained materials. However, the interaction mechanism between grain boundary and defect is still unclear. In the present study, molecular dynamics simulations were carried out to explore the difference of defect evolution in single-crystal and bicrystal W, while the effects of temperature and the energy of the primary knocked atom (PKA) were taken into account. The irradiation process was simulated at the temperature range of 300 to 1500 K, and the PKA energy varied from 1 to 15 keV. The results show that the generation of defects is more sensitive to the energy of PKA than temperature; the number of defects increases at the thermal spike stage with the increase of the PKA energy, but the correlation with temperature is not strong. The presence of the grain boundary prevented the recombination of interstitial atoms and vacancies during the collision cascades, and the vacancies were more likely to form large clusters than interstitial atoms in the bicrystal models. This can be ascribed to the strong segregation tendency of the interstitial atoms to grain boundaries. The simulations provide useful information for understanding the role of grain boundaries in the evolution of irradiated structural defects.

Quantifying X-ray diffractometer precision for measurement of dislocation density and the evolution of defect structures during solidification of additive manufacturing powders

In this work, the Phillips PW 3020 X’Pert Diffractometer and PANalytical X’Pert Pro MPD X-ray Diffractometer were used to perform in-situ monitoring of quenched microstructures to define the defect evolution of Inconel 718 additive manufacturing powder. Dislocation density analysis based on X-ray Diffraction (XRD) measurement is sensitive to XRD peak broadening. The X-ray line profile is affected by the instrumental effects of the diffractometer, so quantifying and analyzing XRD instrument performance is important for accurate dislocation density analysis. The average measurement bias of the Phillips diffractometer was found to be -1.5490·10−3 Å while the precision was found to be 6.1000·10−5 Å. The average measurement bias of the PANalytical diffractometer was found to be -1.7633·10−4 Å while the precision was found to be 7.6917·10−5 Å. The average dislocation density calculated from the data was 3.93·1014 m−2 for the smaller particle size range and 2.58·1014 m−2 for the larger particle size range. The number of diffraction peaks utilized in dislocation density analysis was found to be a more significant factor than the instrumental differences. This work confirmed that differences in defect structure density can be observed across differently sized particles sourced from additive manufacturing powder.

EVOLUTION OF THE DEFECT STRUCTURE OF Pd–11.3 at. % W ALLOY DURING RELAXATION AT MULTIPLE HYDROGENATION

The influence of the hydrogen and vacancies introduced into an alloy matrix as a result of electrolytic hydrogenation on the structural state of Pd–11.3 at % W alloy after fourfold hydrogenation has been studied. It is established that nonmonotonic changes in the sample defect structure are determined by the transformation of single vacancies into vacancy complexes (micropores and small dislocation loops), their decay during relaxation, and migration of vacancies from the matrix to the boundaries of coherent-scattering regions (CSRs) and backwards. The vacancy transformation rate into defect complexes depends on their concentration. Long-term relaxation of the alloy leads to almost complete transfer of vacancies from the matrix to the CSR boundaries. An interpretation of the obtained experimental data within the theory of development of hierarchical defect structures is proposed.

The phase transitions behavior and defect structure evolution in magnesium hydride/single-walled carbon nanotubes composite at hydrogen sorption-desorption processes

The development of hydrogen storage methods is one of the important areas of research in the field of hydrogen energy, while metal hydrides and composites based on them are the most preferred candidates for this. In this paper, one of the composite hydrogen storage materials based on magnesium hydride and single-walled carbon nanotubes was considered. The hydrogen sorption and desorption behavior in composite have been investigated for temperatures and pressures ranges (593−653) K and (0−3) MPa, respectively. Using scanning electron microscopy, it was shown that carbon nanotubes are homogeneously distributed on the surface of MgH2 particles and some of nanotubes are partially embedded in the bulk of MgH2. According to pressure-composition-temperature curves and Van’t Hoff plots it was determined that the enthalpy value of hydrogen absorption process was 69 KJ/mol H2 and 62 KJ/mol H2 for milled MgH2 and MgH2 with 5 wt% single-walled carbon nanotubes, respectively. The main regularities of phase transition in the magnesium-hydrogen system for composite and magnesium hydride during dehydrogenation have been researched for temperatures in the range of 298–750 K. The in situ analysis of the phase transitions during dehydrogenation of MgH2 and composite based on MgH2 and 5 wt% of single-walled carbon nanotubes in argon atmosphere indicates pronounced catalytic effect of carbon nanotubes on the hydrogen desorption. The thermally stimulated desorption curves showed that hydrogen begins to release from the composite where the MgH2 decomposition is minimal. An in situ Doppler broadening spectroscopy study revealed that this is caused by a significant changes magnesium defect structure as a result of the carbon nanotubes/nanoparticles embedding, and the low-temperature hydrogen release follows from the decomposition of associated defects (dislocation-hydrogen or vacancy-hydrogen complexes) as well as its effective diffusion to the surface.

Physics-separating artificial neural networks for predicting sputtering and thin film deposition of AlN in Ar/N2 discharges on experimental timescales

Understanding and modeling plasma–surface interactions frame a multi-scale as well as multi-physics problem. Scale-bridging machine learning surface surrogate models have been demonstrated to perceive the fundamental atomic fidelity for the physical vapor deposition of pure metals. However, the immense computational cost of the data-generating simulations render a practical application with predictions on relevant timescales impracticable. This issue is resolved in this work for the sputter deposition of AlN in Ar/N2 discharges by developing a scheme that populates the parameter spaces effectively. Hybrid reactive molecular dynamics/time-stamped force-bias Monte Carlo simulations of randomized plasma-surface interactions/diffusion processes are used to setup a physics-separating artificial neural network. The application of this generic machine learning model to a specific experimental reference case study enables the systematic analysis of the particle flux emission as well as underlying system state (e.g. composition, density, point defect structure) evolution within process times of up to 45 min.

Phase field modeling microstructural evolution of Fe-Cr-Al systems at thermal treatment

A phase field model to study dynamics of microstructure transformations and the evolution of defect structure during heat treatment of Fe-Cr-Al systems is developed. Statistical and kinetic properties of evolving microstructure and defect structure in alloys with different content of alloying elements and at different temperatures were studied. Point defects rearrangement during precipitation is discussed in details. Universality of statistical distributions over precipitate size is revealed for considered class of alloys.

Grain Structure Rearrangement by Means the Advanced Statistical Model Modified for Describing Dynamic Recrystallization

The study of grain and defect structure evolution in materials subjected to thermomechanical processing is still an urgent problem because the state of a structure substantially determines the physical and mechanical macro properties of polycrystals and polycrystalline products. Significant changes in the structure of polycrystalline materials are associated with the process of dynamic recrystallization (DRX). To investigate DRX, an extended statistical model of inelastic deformation with internal variables is proposed, which takes into consideration contact interactions between neighboring grains. We constructed a geometric image of the grain structure by applying a Laguerre polyhedron in order to describe such interactions in the statistical framework. During the recrystallization simulation, this image is being reconstructed as new recrystallized grains emerge. This leads to the problem of establishing correspondence between an initial grain structure and a reconstructed structure with the required statistical consistency. To provide such consistency, an optimization problem is formulated to preserve the stress and strain parameters and the recrystallization driving force from changes in a statistical sense. This problem is posed with respect to the distributions of differences in defect-stored energy, mutual misorientation angles between grains and sizes of these grains. A genetic algorithm is applied for resolution. By the example of simulating inelastic deformation of a representative volume element (a macrosample analogue) of polycrystalline copper, the influence of the mentioned distributions on the material response upon structure reconstruction is shown. Reasonable values for the objective weights and the genetic algorithm parameters were obtained. This paper presents a detailed description of the grain structure correspondence establishment method, the formulation of the optimization problem and the algorithm to resolve it.

Investigation of subsurface defects structural evolution in nano electro machining of copper

In nano EM, a voltage is applied between workpiece and tool electrode, generating extremely high electric field (> 10 9 V/m). How the electric field influences the evolution of defect structures under the surface atoms during nano EM process has not been revealed, which is one of the most fundamental problems unsolved in nano EM. To investigate the subsurface defects structural evolution in nano EM, the monocrystalline and polycrystalline coppers were used in molecular dynamics (MD) simulation. The simulation results show that in nano EM of monocrystalline copper, a large number of subsurface defect structures were generated, in which the stacking faults occupied a dominant position. As the dislocation annihilated, the number of subsurface defect structures decreased. However, in nano EM of polycrystalline copper, the grain boundaries produced a significant influence on the evolution of defect structures and atoms removal process. The subsurface defect structures produced in polycrystalline copper were much more than these of monocrystalline copper, and the stacking faults generated in nano EM of polycrystalline copper were limited inside the grains due to the existence of grain boundaries. Furthermore, it is easier to generate a large number of removed atoms in nano EM of polycrystalline copper with a relatively low electric field strength. Sectional view of local atomic structure distribution of polycrystalline copper in nano EM

Evolution of defect structures leading to high ZT in GeTe-based thermoelectric materials

GeTe is a promising mid-temperature thermoelectric compound but inevitably contains excessive Ge vacancies hindering its performance maximization. This work reveals that significant enhancement in the dimensionless figure of merit (ZT) could be realized by defect structure engineering from point defects to line and plane defects of Ge vacancies. The evolved defects including dislocations and nanodomains enhance phonon scattering to reduce lattice thermal conductivity in GeTe. The accumulation of cationic vacancies toward the formation of dislocations and planar defects weakens the scattering against electronic carriers, securing the carrier mobility and power factor. This synergistic effect on electronic and thermal transport properties remarkably increases the quality factor. As a result, a maximum ZT > 2.3 at 648 K and a record-high average ZT (300-798 K) were obtained for Bi0.07Ge0.90Te in lead-free GeTe-based compounds. This work demonstrates an important strategy for maximizing the thermoelectric performance of GeTe-based materials by engineering the defect structures, which could also be applied to other thermoelectric materials.

Molecular dynamics study on the effect of electric current on electrically-assisted scratching for crystal copper

Investigation of the effect of electric current on the plastic deformation mechanism of metals during the electrically-assisted machining process is significant in further improving surface properties. In this paper, the molecular dynamics (MD) method is adopted to simulate the electrically-assisted scratching process of crystal copper, obtaining and analyzing the surface morphology, potential energy change, von Mises stress distribution, and crystal defect structure evolution. The MD simulation results show that the electric current effectively expands the dislocation slip range, resulting in a larger plastic deformation zone. Meanwhile, the combined action of the electron wind forces and Joule heating causes more dislocations to proliferate and increases the dislocation density limit, enhancing the plastic deformation ability of the single-crystal copper. Furthermore, the electric current strengthens the dislocation-grain boundary interactions and reduces the hindering effect of the grain boundaries on dislocations, promoting more dislocations to cross the grain boundaries. This work will be helpful for guiding the optimization of surface strengthening techniques to get better surface properties of metals.

Subgrain Coalescence Simulation by Means of an Advanced Statistical Model of Inelastic Deformation

The development of technological methods for processing and manufacturing of functional (with a priori targeted properties) polycrystalline materials and products made of these materials still remains an acute problem. A multilevel modeling approach offers researchers the opportunity to describe inelastic deformation by applying internal variables that give an effective characterization of the material structure at different structural scale levels. High temperature plastic deformation is accompanied by these processes, which leads to a significant rearrangement of the meso- and microstructure of the material. The most substantial contribution to changing the properties of polycrystals is made by the evolution of grain and defect structures at the expense of dynamic recrystallization, which significantly depends on dynamic recovery. In this paper, we consider the problem of the coalescence of subgrains undergoing rotation during inelastic hot deformation. This process is called subgrain coalescence, and it is one of the dynamic recovery mechanisms responsible for changes in the fine subgrain structure. Under applied thermomechanical loads, the coalescence process promotes the formation of recrystallization nuclei and their subsequent growth, which can greatly change the grain structure of a polycrystal. The problem was solved in terms of the advanced statistical model of inelastic deformation, modified to describe the subgrain coalescence process. The model takes into account the local interactions between contacting structural elements (subgrains). These have to be considered so that the grain coalescence caused by a decrease in subboundary energies during their progressive merging can be adequately analyzed. For this purpose, a subgrain structure quite similar to the real structure was modeled using Laguerre polyhedra. Subgrain rotations were investigated using the developed model, which relies on the consideration of the excess density edge component of the same sign dislocations on incidental subgrain boundaries. The results of modeling of a copper polycrystal are presented, and the effects of temperature and strain rate on the subgrain coalescence process is demonstrated.

Role of chemical disorder on radiation-induced defect production and damage evolution in NiFeCoCr

Understanding chemical disorder in many concentrated solid solution alloys (CSAs) at the levels of electrons and atoms has attracted increasing attention as a path forward to reveal and identify underlying mechanisms for extraordinary mechanical properties and improved radiation tolerance. Single-phase NiFeCoCr CSA is a common base for many high-entropy alloys (HEAs) that have shown improved mechanical strength and radiation tolerance. In this study, defect production and damage evolution in NiFeCoCr under ion irradiation at room temperature to dose over 20 dpa are determined using ion channeling technique along both <100> and <110> directions utilizing multiple probing beam energies. The results obtained from the multi-axial and multi-energy channeling analysis are compared with those previously obtained for Ni crystals irradiated under similar conditions. The influence of chemical complexity on defect production and clustering at early-stage under room temperature irradiation up to dose of 1 dpa is discussed based on positron annihilation spectroscopy results. Defect structure evaluation in Ni and NiFeCoCr is also discussed based on transmission electron microscopy results over a prolonged irradiation at both room and elevated temperatures. Compared with chemically complex NiFeCoCr, larger dislocation loops thus less lattice strain are expected to form in pure Ni. Moreover, the role of chemical disorder in this CSA is also investigated based on ab initio calculations using large supercells. To understand the impact of chemical complexity effect on defect structure evolution, this integrated research effort attempts to link the relatively large charge redistribution due to difference in valence electron counts resulting from alloying different 3d transition metal elements, moderate lattice distortion arising from similar adaptable atomic size, and notable suppressed or delayed damage evolution in NiFeCoCr.

Quantification of the temperature-dependent evolution of defect structures in a CoNi-base superalloy

Quantifying the individual contributions of specific deformation mechanisms to the total plastic strain is of primary importance to understand the underlying deformation behavior and assess the significance of the different mechanisms. However, this quantification is nearly impossible in Ni-base superalloys due to the inherent difficulties in determining the propagation distances of the respective shearing dislocations. In this study, the occurrence of stacking faults (SFs) at temperatures between 750 °C and 1000 °C in the single crystalline CoNi-base superalloy ERBOCo-4 provides an opportunity to systematically quantify the evolution of the defect structures as a function of temperature and strain after constant strain-rate compressive tests. The quantification approach revealed that deformation is still localized in the matrix at 750 °C. With increasing temperature, the main deformation mechanism changes to SF shearing, which contributes 74% of the total deformation during the initial yielding stage (∼ 0.3% plastic strain) at 900 °C. Deformation spreads significantly more homogeneously across the γ/γ’ microstructure with increasing plastic strain. Extremely high stacking fault densities and frequent fault interactions cause a pronounced work-hardening anomaly at 850 °C. Structural characterization on the atomic scale confirms the strengthening effect of SFs as barriers to partial dislocation propagation. A strong correlation between the possible diffusion length of segregating solutes and the operant shearing mechanisms reveals the significance of segregation on the occurring deformation mechanisms.

The Three-Level Elastoviscoplastic Model and Its Application to Describing Complex Cyclic Loading of Materials with Different Stacking Fault Energies.

The development of new technologies for thethermomechanical processing of metals and the improvement of the existing ones would be unattainable without the use of mathematical models. The physical and mechanical properties of alloys and the performance characteristics of the products made of these alloys are generally determined by the microstructure of materials. In real manufacturing processes, the deformation of metals and alloys occurs when they undergo complex (non-proportional) loading. Under these conditions, the formation of defect substructures, which do not happen at simple (proportional) loading, can take place. This is due to the occurrence of a great number of slip systems activated under loading along complex strain paths, which leads, for instance, to the more intense formation of barriers of different types, including barriers on split dislocations. In these processes, the formation and annihilation of dislocations proceed actively. In this paper, we present a three-level mathematical model that is based on an explicit description of the evolution dislocations density and the formation of dislocations barriers. The model is intended for the description of arbitrary complex loads with an emphasis on complex cyclic deformation.The model is composed of macrolevel (a representative macrovolume of the material that can be considered as an integration point in the finite-element modeling of real constructions), and mesolevel-1 (description of the mechanical response of a crystallite) and mesolevel-2 (description of the defect structure evolution in a crystallite) submodels. Using the model, we have performed a series of numerical experiments on simple and complex, monotonic and cyclic deformations of materials with different stacking fault energies, analyzed the evolution of defect densities, and analyzed the challenges of a relationship between the complexity of loading processes at a macrolevel and the activation of slip systems at low scale levels.

Structural defects of monolayer wet particles during melting under vertical vibration.

The evolution of structural defects during phase transition is of great significance to the understanding of the mechanism of solid-liquid transition. However, the current research on topological defects still uses the pair-correlation function and the orientational order correlation function, so it is difficult to quantify the detailed changes of structural defects locally. In this paper, the local volume fraction is proposed as a key parameter to accurately quantify the variation of structural defects. The experimental results indicate that the evolution of structural defects in the particle system is caused by the decrease of local volume fraction, so the critical value of phase transition could be determined by the minimum local volume fraction ϕ_{min}. Furthermore, according to the evolution law of structural defects, it can be deduced that the phase transition is continuous, which is consistent with the Kosterlitz-Thouless-Halperin-Nelson-Young theory. Therefore, the quantitative analysis of structural defects by using local volume fraction can help make the mechanism of solid-liquid phase transformation clearer.