Dislocation Strain Field Research Articles

Cooperative dislocations for pressure-dependent sequential deformation of multi-principal element alloys under shock loading

Multi-principal element alloys (MPEAs) are promising materials for structural applications under extreme conditions. Their outstanding mechanical properties are closely related to the activation of multiple deformation modes of dislocation gliding, twinning, and phase transformation that appear in sequence during deformation at low temperatures, high pressures, or high strain rates. However, the inherent correlations among these deformation modes and, thus, underlying deformation mechanisms of MPEAs remain largely unknown. We report soft-recovery plate impact experiments of face-centered-cubic (FCC) CrCoNi MPEAs, demonstrating pressure-dependent deformation modes from low-pressure stacking faults to medium-pressure twinning and high-pressure FCC to hexagonal-close-packed (HCP) phase transformation. Atomic-scale characterizations unveil that the sequential deformation is manipulated by the cooperation of 90° and 30° Shockley partial dislocations at deformation fronts, which is facilitated by low stacking fault energy and pressure-dependent phase stability of the MPEAs. Moreover, the cooperative dislocation behavior can also be observed at twin fronts of shock-loaded CrMnFeCoNi MPEA, validating the universality of the cooperative deformation mode in FCC alloys with a low stacking fault energy. Theoretical analyses suggest that the distinctive cooperative dislocation behavior results in the self-compensation of dislocation strain fields and the minimization of interfacial elastic energy at incoherent twin and FCC/HCP interfaces.

Insights into the influence of Ni4Ti3 precipitates and martensite transformation on the glide of a[100] dislocation in austenitic NiTi alloys: an atomistic simulation study

Dislocation-dominated plastic deformation in austenitic NiTi alloys is highly detrimental and it could be suppressed by precipitation of Ni4Ti3. Additionally, at a certain temperature range, not only Ni4Ti3 precipitate but also the B19’ martensite produced from stress-induced martensitic transformation may prevent dislocation glide in austenitic NiTi alloys. In order to shed further light on this issue, four atomic configurations were constructed using an edge dislocation (a [100]) and two Ni4Ti3 precipitate variants (precipitate-V1 and precipitate-V2) to investigate dislocation glide in austenite containing Ni4Ti3 precipitate under shear at temperatures of 500 K or 700 K. As a result, all configurations at 500 K undergo the shear-induced martensitic transformation, producing B19’ martensite that impedes dislocation glide. And at 700 K, dislocation was pinned by precipitate or distortion zones caused by the superposition of strain field of precipitate and dislocation, without martensitic transformation. Furthermore, it was proven that precipitate-V1 impedes dislocation glide more strongly than precipitate-V2 at both temperatures of 500K and 700 K. It was attributed to the different in spatial relative location between the precipitate and dislocation and explained by the Orowan dislocation-precipitate bypass stress. Finally, three strategies for achieving high shear fatigue resistance in NiTi shape memory alloys were proposed based on these findings.

Atomistic approach to the strain field in finite-sized heterostructures

The numerical code AS_LBFG_v2 developed to simulate the atomic elastic relaxation in finite-sized heterostructures is introduced. To minimize the elastic energy the L-BFGS algorithm is utilized. A user-defined initial atomic configuration, without restrictions on the shape of the matrix embedding quantum dot (QD), can be implemented in the code. By enlarging the pyramidal QD heterostructure towards mesoscopic size, the code generates a strain field distribution similar to that obtained by continuum model for weakly polar semiconductors. The strain field obtained for semi-torus, and spherical shape QDs embedded in finite-sized matrices is also presented. The dependence of the strain field on the cap thickness and the distribution of misfit dislocations in the relaxed configuration are simulated by the code. Program summaryProgram titles:AS_LBFG_v2, AS_Calc_v2CPC Library link to program files:https://doi.org/10.17632/53dshkskct.1Code Ocean capsule:https://codeocean.com/capsule/5342364/tree/v1 and https://codeocean.com/capsule/9050452/tree/v1Licensing provisions: MIT licenseProgramming language: FORTRANNature of problem: An initial constraint atomic configuration of a heterostructure is relaxed by imposing Keating's elastic interaction. The strain field and distribution of misfit dislocations are simulated for various quantum dot shapes and heterostructure materials.Solution method: The elastic energy of the heterostructure is minimized by an L-BFGS algorithm. The strain field is calculated by a discrete mathematical expression at each atomic location.

Analysis of relaxation peaks during continuous heating of cold rolled Al–Mg–Si alloy

ABSTRACT In this study the effect of plastic deformation on internal friction peaks in Al based alloy is discussed. For that, an Al–Mg–Si alloy (AA6060) is subjected to cold rolling with a ratio ϵ = 80%. For both samples (water quenched and water quenched then cold rolled), the results revealed the appearance of a peak P1 at medium temperature (∼463 K), which is related to the dragging of Mg or Mg-Si clouds in the strain field of dislocations firmly pinned by metastable precipitates. Moreover, cold rolled sample exhibits a second peak PR at high temperature that is interpreted as a transition from high IF background to low IF one due to a recrystallisation process. Besides to dynamical mechanical analysis, DSC, dilatometry and Vickers microhardness are used to follow the structural evolution of the alloy.

Effect of Dislocation Character on the CRSS

The yield strength of a crystalline structural material is a fundamental mechanical property predominantly governed by the critical stress for dislocation slip. This Critical Resolved Shear Stress (CRSS) is strongly influenced by the character of the dislocation (e.g., screw, edge, or mixed) as shown in previous experimental studies. Existing analytical approaches for CRSS prediction assume an atomic row description of the slip plane and do not account for Wigner-Seitz (WS) cell area at each discrete lattice site. Further, inadequate consideration of the material's elastic anisotropy and the presumed dislocation “core-width” level precludes correct CRSS determination. This study proposes a predictive model applied to Face Centered Cubic (FCC) materials addressing these shortcomings in predicting glide stress of a dissociated dislocation. The core-width is rigorously determined from the minimization of total energy comprised of continuum strain energy (ESTRAIN) and atomistic misfit energy (EMISFIT) of the dislocation's core. The ESTRAIN is obtained from dislocation strain-fields calculated using the fully-anisotropic Eshelby-Stroh formalism. The EMISFIT is determined from the Generalized Stacking Fault Energy (GSFE) landscape of the slip plane. Previous EMISFIT calculations are restricted to slipped rows in ‘simple’ cubic lattices which do not represent the slip-planes in FCC crystals. The developed model is used to predict CRSS for a wide range of metallic materials correcting the overprediction of experimental CRSS levels. The results unveiled the remarkable dependence of CRSS on the dislocation character, revealing the non-trivial dependence on GSFE parameters. Thus, this study addresses a major void in structure-property prediction for structural materials.

Competitive strengthening between dislocation slip and twinning in cast-wrought and additively manufactured CrCoNi medium entropy alloys

In situ neutron diffraction experiments have been performed under loading in cast-wrought (CW) and additively manufactured (AM) equiatomic CoCrNi medium-entropy alloys. The diffraction line profile analysis correlated the faulting-embedded crystal structure to the dislocation density, stacking/twin fault probability, and stacking fault energy as a function of strain. The results showed the initial dislocation density of 1.8 × 1013m−2 in CW and 1.3 × 1014m−2 in AM. It significantly increased up to 1.3 × 1015m−2 in CW and 1.7 × 1015m−2 in AM near fracture. The dislocation density contributed to the flow stress of 470 MPa in CW and 600 MPa in AM, respectively. Meanwhile, the twin fault probability of CW (2.7%) was about two times higher than AM (1.3%) and the stacking fault probability showed the similar tendency. The twinning provided strengthening of 360 MPa in CW and 180 MPa in AM. Such a favorable strengthening via deformation twinning in CW and dislocation slip in AM was attributed to the stacking fault energy. It was estimated as 18.6 mJ/m2 in CW and 37.5 mJ/m2 in AM by the strain field of dislocations incorporated model. Dense dislocations, deformation twinning, and atomic-scale stacking structure were examined by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM).

Planar Gliding and Vacancy Condensation: The Role of Dislocations in the Chemomechanical Degradation of Layered Transition-Metal Oxides

Stacking faults driven by dislocations have been observed in layered transition-metal oxide cathodes both in cycled and uncycled materials. The reversibility of stacking-sequence changes directly impacts the material performance. Irreversible glide due to lattice invariance or local compositional changes can initiate a catastrophic sequence of degradation mechanisms. In this study we compare the chemomechanical properties of LiCoO2 and LiNiO2 by combining density functional theory and anisotropic linear elasticity theory. We calculate stacking fault energies as a function of Li content and quantify the extent to which excess Ni hinders stacking-sequence changes. We then characterize screw dislocations, which mediate stacking-sequence changes, and find a peculiarly compliant behavior of LiNiO2 due to the interaction of Jahn–Teller distortions with the dislocation strain field. Finally, we analyze the tendency of vacancies to segregate along dislocation lines. This study represents the first instance of explicit ab initio atomistic dislocation models in layered oxides and paves the way for the understanding and optimization of the chemomechanical behavior of cathode active materials during battery operation.

Dislocation assisted phase separation: A phase field study

In this paper, we show the influence of dislocations on phase separation in alloys with a miscibility gap. Specifically, solute segregation to dislocations leads to spinodal phase separation even in systems with nominal compositions outside the spinodal limit. Further, when spinodal decomposition occurs along the dislocation cores, geometrically, the evolution is like that of a perturbed cylinder. Thus, unless the faster atomic mobility along the dislocation overcomes the bulk diffusion flux, the spinodal is transient and the fluctuations die down as time proceeds due to the bulk flux. We have summarised this interaction between the chemical instability and the cylindrical geometry of the instability in the form of mechanism maps. Our simulations also show the surprising phenomena of phase separation through both (i) nucleation and growth, and, (ii) spinodal decomposition, at the same time – for the case of intersecting dislocations. Finally, the prominent role played by dislocation and solute strain fields in influencing the morphology of precipitates is explored. We show that the range of parameter values chosen by us are of relevance to real alloys, and, some of the microstrucural features and morphologies agree qualitatively with recent Atom Probe Tomography (APT) results in iron based systems.

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.

Carrier Diffusion in GaN : A Cathodoluminescence Study. III. Nature of Nonradiative Recombination at Threading Dislocations

We investigate the impact of threading dislocations with an edge component ($a$ or $a+c$ type) on carrier recombination and diffusion in $\mathrm{Ga}\mathrm{N}$(0001) layers close to the surface as well as in the bulk. To this end, we utilize cathodoluminescence imaging of the top surface of a $\mathrm{Ga}\mathrm{N}$(0001) layer with a deeply buried (In,$\mathrm{Ga}$)N quantum well. Varying the acceleration voltage of the primary electrons and comparing the signal from the layer and the quantum well enables us to probe carrier recombination at depths ranging from the close vicinity of the surface to the position of the quantum well. Our experiments are accompanied by fully three-dimensional Monte Carlo simulations of carrier drift, diffusion, and recombination in the presence of the surface, the quantum well, and the dislocation, taking into account the dislocation strain field and the resulting piezoelectric field at the dislocation outcrop. Near the surface, this field establishes an exciton dead zone around the dislocation, the extent of which is not related to the carrier diffusion length. However, reliable values of the carrier diffusion length can be extracted from the dipolelike energy shift observed in hyperspectral cathodoluminescence maps recorded around the dislocation outcrop at low acceleration voltages. For high acceleration voltages, allowing us to probe a depth where carrier recombination is unaffected by surface effects, we observe a much stronger contrast than expected from the piezoelectric field alone. This finding provides unambiguous experimental evidence for the strong nonradiative activity of edge threading dislocations in bulk $\mathrm{Ga}\mathrm{N}$ and hence also in buried heterostructures.

Dislocation-Limited Thermal Conductivity in LiF: Revisiting Perturbative Models

We revisit models to describe phonon–dislocation interactions and resulting dislocation-limited thermal conductivity (k), particularly as applied to describe the temperature-dependent k behavior of LiF with significant dislocation densities. Coupling semiempirical models of phonon scattering from dislocation strain fields provided by Klemens and Carruthers with density functional theory description of the phonons and input parameters, we find that both models significantly overpredict the measured k of LiF, as found previously. However, more direct application of the quantum perturbation theory description, from which the models were derived, gives significantly stronger phonon–dislocation scattering and strongly underpredicts the measured k data. We revisit the derivation of phonon–dislocation–strain field scattering and provide numerical details regarding its implementation. This work demonstrates that quantum perturbative calculations, now accessible to modern computational architectures, may provide a reasonable description of dislocation-limited k in materials as governed by phonon–strain field interactions provided that questions regarding the relevant strain field range can be resolved.

Investigating the microstructure of an additively manufactured FeCo alloy: an electron microscopy study

The objective of the present investigation is to correlate mechanical properties with microstructure for FeCo alloy using electron microscopy. A short parameter adaption is conducted to determine the optimum laser powder bed fusion parameters, which lead to the highest sample density. Subsequently, FeCo alloy samples are produced to address the evolving microstructure and mechanical properties. A high build rate is also attained along with the high density. X-ray micro-tomography of the samples revealed a remaining porosity of 0.02%. Electropolishing and subsequent microstructural examinations are undertaken by electron backscatter diffraction employing scanning electron microscopy. Elongated columnar grains are observed on the side of the samples, which is characteristic of selective laser melting. Based on transmission electron microscopy, the composition of Fe and Co at the submicron scale is homogeneous. The dislocation density near the grain boundaries measured by transmission electron microscopy is ≈ 2.2 × 10 13 m −2 . Unresolved lattice fringes due to dislocation strain fields are observed in high-resolution transmission electron microscopy images. Different tensile properties are obtained with loading parallel as well as perpendicular to the build direction. This is due to different Schmid factors of the grains during loading parallel and perpendicular to the build direction. A mixture of cleavage and dimple fracture is noticed upon loading parallel to the build direction. Whereas loading perpendicular to the build direction shows only cleavage fracture.

Optoelectronic Inactivity of Dislocations in Cu(In,Ga)Se2 Thin Films

High‐efficiency Cu(In,Ga)Se2 (CIGS) thin‐film solar cells are based on polycrystalline CIGS absorber layers, which contain grain boundaries, stacking faults, and dislocations. While planar defects in CIGS layers have been investigated extensively, little is still known about the impact of dislocations on optoelectronic properties of CIGS absorbers. Herein, evidence for an optoelectronic inactivity of dislocations in these thin films is given, in contrast to the situation at grain boundaries. This unique behavior is explained by the extensive elemental redistribution detected around dislocation cores, which is connected with the dislocation strain field, probably leading to a shift of defect states toward the band edges.

Thermal resistance at a twist boundary and a semicoherent heterointerface

Traditional models of interfacial phonon scattering, including the acoustic mismatch model (AMM) and diffuse mismatch model (DMM), take into account the bulk properties of the material surrounding the interface, but not the atomic structure and properties of the interface itself. Here, we derive a theoretical formalism for the phonon scattering at a dislocation grid, or two interpenetrating orthogonal arrays of dislocations, as this is the most stable structure of both the symmetric twist boundary and semicoherent heterointerface. With this approach, we are able to separately examine the contribution to thermal resistance due to the step function change in acoustic properties and due to interfacial dislocation strain fields, which induces diffractive scattering. Both low-angle Si-Si twist boundaries and the Si-Ge heterointerfaces are considered here and compared to previous experimental and simulation results. This work indicates that scattering from misfit dislocation strain fields doubles the thermal boundary resistance of Si-Ge heterointerfaces compared to scattering due to acoustic mismatch alone. Scattering from grain boundary dislocation strain fields is predicted to dominate the thermal boundary resistance of Si-Si twist boundaries. This physical treatment can guide the thermal design of devices by quantifying the relative importance of interfacial strain fields, which can be engineered via fabrication and processing methods, versus acoustic mismatch, which is fixed for a given interface. Additionally, this approach captures experimental and simulation trends such as the dependence of thermal boundary resistance on the grain boundary angle and interfacial strain energy.

Relative relevance of mobility and driving force on edge dislocation climb by the vacancy mechanism

In this work we examine the driving force for edge dislocations to climb in α-Fe from atomistic and mesoscale viewpoints. We study the bias for the climb process depending on the dislocation orientation and the applied stress due to both the gradient of the chemical potential and the transport coefficients. Both terms are modified by the applied stress and therefore contribute to climb. Surprisingly, even though the vacancy migration barrier distribution is modified by the external stress as obtained by nudged-elastic band calculations, the mobilities resulting from a kinetic Monte Carlo model applied on the obtained energy landscape are indistinguishable, independently of the stress. Moreover, an object kinetic Monte Carlo (OKMC) model including the effect of the dislocation strain field to first order shows indeed a slight anisotropic component in the diffusion in more complex dislocation configurations. However, the OKMC results highlight the fact that the thermodynamic component is the dominant driving force. We conclude that in α-Fe under thermal conditions, the main source of bias is given by the difference in vacancy chemical potentials, which is small enough to hinder the process for dynamic atomistic simulations.

Cathodoluminescence of silicon doped aluminum nitride with scanning transmission electron microscopy

Here, we apply cathodoluminescence in scanning transmission electron microscopy to infer the influence of dislocation strain fields on the formation of point defect complexes in Si doped AlN. In addition to identifying non-radiative recombination centers, tracking Si related defect emission energies reveals a red-shift at threading dislocations. We discuss these results in the context of multiple Si-vacancy defect complexes that can form and the influence of local strain on their formation energies. By correlating the electronic and structural properties at the nanoscale, cathodoluminescence elucidates the inhomogeneity of defect complexes in Si doped AlN and offers the potential for strain engineering to control the defect energy formation landscape.

Selective variant growth of precipitates in an as-extruded Mg-Gd-Zn-Mn alloy

Selective variant growth of precipitates was found in an as-extruded Mg-13.7Gd-1.7Zn-0.5Mn (wt.%) alloy. In the coarse-deformed grains, β' phases and hexagonal clusters mainly precipitate along the [101-0]Mg orientation. While in the fine dynamic recrystallized (DRXed) grains, the β' phases and a few of the hexagonal clusters are relatively uniform distributed along the three < 101-0>Mg directions. This phenomenon is attributed to the coupling of dislocation strain field and residual stress component in the as-extruded alloy. Moreover, dislocations in the deformed grains result in a significant increase in the number density of hexagonal clusters comparing to those in the DRXed grains. This selective variant growth of precipitates is expected to change the deformation behavior and modify the mechanical properties in Mg-RE (rare earth) alloys.

Heat transport in methane-palladium nanocomposites

The thermal conductivity of crystalline methane samples with spherical palladium nanoparticles of diameter 6, 8, 10, 12, 18 and 24 nm embedded in the CH4 structure was determined in the temperature range 2–35 K. The spherical nanoparticles featured a low dispersion of the sphere diameter and the volume fraction of the nanoparticles in the nanocomposites amounted to 0.15. Time relaxation approximation analysis shows that the matrix resistive phonon scattering processes are dominated by diffuse scattering of phonons by matrix-nanoparticle boundaries. Other considered intensities of resistive scattering mechanisms of phonons, i.e., scattering by point defects, by dislocation strain fields and by phonons in U-processes remain almost the same in nanocomposites and pure methane crystal. In transfer of the heat through the investigated nanocomposites non-resistive scattering of phonons by the nanoparticles, such as phonon specular scattering and multiple scattering of phonons show very high intensity. The nanocomposites formed of 18 and 24 nm nanospheres show anomalous power increase of the thermal conductivity coefficient dependence on temperature, exceeding the values known for a typical dielectric crystal.

Application of digital phase shifting moiré method in interface and dislocation location recognition and real strain characterization from HRTEM images.

In high-resolution transmission electron microscopy (HRTEM) images of heterostructures, it is always difficult to accurately determine the interface position and identify dislocations in a large field of view at tens to hundreds of nanometers due to the small lattice differences. However, in the heterostructure, the determination of the interface position is the key to obtain the true mismatch stress/strain field of the interface. Due to the magnifying effect of the digital moiré method on small differences, digital moiré technology was applied to determine Ge/Si heterostructure interfaces and large-area identification interface dislocations in HRTEM lattice diagrams in this study. By optimizing the frequency and angle of the reference lattice, the interface and dislocation position are clearly and intuitively displayed. How to accurately determine the position of the heterostructure interface and the dislocation of the large-area recognition interface from HRTEM images are studied through simulation experiments. The results show that when the frequency of the reference lattice and the specimen lattice are close, and the angle between them is within 10°, the position of the heterostructure interface can be accurately and intuitively determined by the naked eye according to the distortion characteristics of the moiré fringe. When the frequency of the reference lattice is 0.7 to 0.9 times of the specimen lattice, and the rotation angle is within 8°, the visually clear crossover phenomenon of the moiré fringes is used for large-area identification of interface dislocations. Using the phase measurement interface position sensitivity can reach the Å level. Using the phase-shifting digital moiré method the strain field on the dislocation core at the Ge/Si heterostructure interface and the interface stress distribution were quantitatively analyzed. Compared with the Peierls-Nabarro dislocation model and the Foreman dislocation model, Foreman's variable factor α = 4 is more suitable for describing the strain field of misfit dislocations on the Ge/Si heterostructure interface.

Determination of the Carrier Diffusion Length inGaNfrom Cathodoluminescence Maps Around Threading Dislocations: Fallacies and Opportunities

We investigate, both theoretically and experimentally, the drift, diffusion, and recombination of excitons in the strain field of an edge threading dislocation intersecting the GaN{0001} surface. We calculate and measure hyperspectral cathodoluminescence maps around the dislocation outcrop for temperatures between 10 to 200 K. Contrary to common belief, the cathodoluminescence intensity contrast is only weakly affected by exciton diffusion, but is caused primarily by exciton dissociation in the piezoelectric field at the dislocation outcrop. Hence, the extension of the dark spots around dislocations in the luminescence maps cannot be used to determine the exciton diffusion length. However, the cathodoluminescence energy contrast, reflecting the local bandgap variation in the dislocation strain field, does sensitively depend on the exciton diffusion length and hence enables its experimental determination.