Helical Dislocations Research Articles

Microstructure evolution and mechanical behavior of 2024 aluminum alloy under electric pulse treatment

This study examined the effects of electric pulse treatment (EPT) on solution-treated 2024 alloy. We found that the combined effect of non-uniform stress field caused by thermal expansion and localized Joule heating promoted the formation of helical dislocations. During the subsequent aging process, these severely deformed helical dislocations formed a network configuration, which accelerated the coarsening of the S phase, and the S phase also retained a helical shape. Compared to conventional solution-treated samples, the yield strength of the EPT samples increased by 24 % with almost no loss of fracture elongation. The increase in strength is primarily due to the increase of volume fraction of rod-shaped S phase，enhancing the matrix's resistance to deformation. This study proposes a simple and rapid method to adjust dislocation configurations and aging precipitation through electric pulses, offering new insights for the industrial application of aluminum alloys to swiftly enhance mechanical properties.

Transitions between equilibrium and nonequilibrium phenomena in the description of crystal growth

The close intertwining of equilibrium and nonequilibrium thermodynamic representations and transitions between the two limiting principles of thermodynamics: the second beginning and the principle of least coercion (minimum entropy production in the stationary regime) constitute the main content of phenomenological theories of crystal growth. The difference of basic postulates of two sections of thermodynamics forces to discuss problems of reversibility and irreversibility of time, scales of observed phenomena and rules of conjugation of thermodynamic forces and flows in theories of crystal growth. A variant of the solution of some conjugation problems is shown on the example of the fluctuation model of dislocation crystal growth, which is based on the stationary isothermal process of thermodynamic free energy fluctuations. In the case of the limiting mode of adsorption of impurities on the crystal face according to the Langmuir model, the free energy fluctuations possessing the absence of the memory effect allow us to identify three chemical potentials of building particles that determine the corresponding values of solution supersaturations realized at different scale levels at the growing crystal face containing a helical dislocation. The supersaturations control quasi-equilibrium and nonequilibrium thermodynamic processes that constitute a single dislocation mechanism of crystal growth.

Enhanced creep resistance and microstructure in 7055 Al alloy by in-situ (Al2O3 + ZrB2) nanoparticles and Al3(Er, Zr)

This work put forward a novel approach to enhancing high-temperature creep resistance of 7055 Al alloy via inducing reinforcements including in-situ (Al2O3 + ZrB2) nanoparticles and Al3(Er, Zr). Creep steady-state rates of the (Al2O3 + ZrB2)/7055 Al composites enhanced by synergistic Er, Zr were lower than the (Al2O3 + ZrB2)/7055 Al composites, indicating the values of stress component (n) and apparent activation energy (Q) were 1.05–1.15 and 1.06–1.08 times of (Al2O3 + ZrB2)/7055 Al composites. The dislocation climb mechanism was considered as the dominative creep mechanism because the suitable true stress exponent was 5. The uniform distribution of Al2O3 and Al3(Er, Zr) and their influence on improving the distribution of ZrB2 aggregates gave rise to relative uniform deformation during creep process. The built interface models that induce Al3(Er, Zr) between the nanoparticle/Al interface could lead to a more thermodynamically stable and well-bonded interface. The coupling effect of attachments containing Al2O3, ZrB2 and Al3(Er, Zr) played an enhanced role in terms of vacancy-trapping, precipitate-nucleation, dislocation motion during creep process. The nanoparticles and Al3(Er, Zr) together with nuclear sites containing solute atoms, vacancies and misfits nearby, caused an expansion-inhibiting effect on the helical dislocations. The in-grain misorientation axes (IGMA) concentration of some deformed grains associated with Schmid factor (SF) analysis were related to 〈001〉, 〈111〉 and 〈101〉 direction. The calculation of creep resistance mechanisms was quantified. Additionally, typical intergranular fracture appeared within the composites due to the relatively uniform reinforcements, resulting in the complex crack path.

Towards to Theory of the X-ray Diffraction Tomography of Crystals with Nano-Sized Defects

X-ray diffraction tomography is an innovative method that is widely used to obtain 2D-phase-contrast diffraction images and their subsequent 3D-reconstruction of structural defects in crystals. The most frequent objects of research are linear and helical dislocations in a crystal, for which plane wave diffraction images are the most informative, since they do not contain additional interference artifacts unrelated to the images of the defects themselves. In this work the results of modeling and analysis of 2D plane wave diffraction images of a nano-dimensional Coulomb-type defect in a Si(111) thin crystal are presented based on the construction of numerical solutions of the dynamic Takagi-Taupin equations. An adapted physical expression for the elastic displacement field of the point defect, which excludes singularity at the defect location in the crystal, is used. A criterion for evaluating the accuracy of numerical solutions of the Takagi-Taupin equations is proposed and used in calculations. It is shown that in the case of the Coulomb-type defect elastic displacement field, out of the two difference algorithms for solving the Takagi-Taupin equations used in their numerical solution, only the algorithm for solving the Takagi-Taupin equations where the displacement field function enters in exponential form is acceptable in terms of the required accuracy-duration of the calculations.

Defect-regulated synthesis of ZrB2 powders via carbon thermal reduction and elucidation of its hot-pressing densification mechanism

Through defect modulation, ZrB2 powder was controllably synthesized using the carbon thermal reduction (CTR) method, with boron powder serving as a novel additive. Thermodynamic data for the ZrO2-B2O3-C system in the presence of boron were calculated using HSC thermodynamic software. Additionally, TG-DSC thermal analysis combined with XRD was used to investigate the reaction process. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were used to analyze the defect structure and grain surface characteristics of the reaction products. The results indicate that the reaction of B with ZrO2 and C exothermically alters the nucleation environment of ZrB2 grains. The helical dislocations generated act as a source of crystal growth steps, promoting the growth of smooth interfaces and leading to a transformation of the ZrB2 grain growth mechanism. Quantitative calculations of the dislocation density of ZrB2 powders, using the convolutional multiple whole profile fitting (CMWP-fit), suggest that the inclusion of boron as an additive can modulate the defect concentration of the products. ZrB2 powders with varied dislocation densities were achieved through meticulously controlled synthesis methods, followed by subsequent densification via hot pressing sintering. The results indicated that the relative density of ZrB2 ceramics increased from 83.9 % to 96.7 % as the dislocation density of the powder increased from 2.7 × 1013 m−2 to 7.6 × 1014 m−2 with the rise in boron content from 0 wt% to 3 wt%. In addition, the defects create extra sintering-driving forces that induce a shift in the densification mechanism from grain boundary diffusion to dislocation slip during hot-pressing sintering. This shift results in an increase in the apparent activation energy from 105 kJ/mol to 210 kJ/mol.

Characterization of defects in Tb3Ga5O12 single crystals grown by the Czochralski method

Tb3Ga5O12 (TGG) single crystals, which are used as optical isolators, are grown by the Czochralski (Cz) method. Radial stripes are sometimes observed inside the TGG-grown single crystals. Polarized light microscopy, chemical etching, and X-ray topography results suggest that helical dislocations are present in crystals with radial stripes. Therefore, the radial stripes are considered to be caused by light scattering due to the strain resulting from helical dislocation.

Defect structure of high-resistance CdTe:Cl single crystals and MoOx/CdTe:Cl/MoOx heterostructures according to the data of high-resolution X-ray diffractometry

Clorine doped CdTe single crystals (CdTe:Cl) were grown by the traveling heater method. MoO x /CdTe:Cl/MoO x films were deposited using the reactive magnetron sputtering technique. The defect structure of the obtained single crystals and heterostructures was investigated using high-resolution X-ray diffractometry. The optimized models of dislocation systems in the CdTe:Cl single crystals were constructed based on the Thompson tetrahedron. The distribution of the intensity of diffracted X-rays as a function of reciprocal space coordinates and rocking curves was analyzed using the kinematic theory of X-ray scattering in real crystals. The experimental and theoretically predicted values of the helical dislocation densities in the CdTe:Cl and MoO x /CdTe:Cl crystals with perfect and mosaic structures were compared. Two-fold increase in the dislocation concentration in the MoO x /CdTe:Cl heterostructures as a result of compression deformations of the CdTe:Cl crystal lattice was found. The ~0.1 μm thick transition deformed layer at the boundary between the MoO x film and CdTe:Cl single crystal significantly affects the electrical and spectroscopic properties of the obtained systems as the materials for γ-radiation detection.

Alumina precipitate along dislocations in rutile single crystal of diffusion couple during cooling from 1200° to 1400°C

Diffusion couples of rutile and corundum single crystals were prepared and examined for the solid solution and precipitation of corundum in rutile during cooling. Alumina dissolves in rutile according to the following equation, Al2O3→2AlTi′+3OO+VO∙∙. In the sample air-quenched from 1200 °C, the supersaturated oxide ion vacancies, VO⋅⋅, reacted with the edge component of screw dislocation parallel to c axis to form helical dislocations, along which corundum was precipitated. When air-quenched from 1300 °C and 1400 °C, corundum was precipitated in the form of stacked rectangular loops. Corundum grains with eight lobes extending in the < 101 > directions were precipitated in the samples cooled at 10 °C/min from 1300 °C and 1400 °C. The solid solution reaction increased the volume of rutile single crystals and caused voids outside the reaction region at the interface. Both the corundum and rutile components of the voids also contributed to the solid solution reaction.

Three-dimensional distribution and propagation of dislocations in β-Ga2O3 revealed by Borrmann effect x-ray topography

Synchrotron radiation x-ray topography (XRT) in a transmission configuration based on the Borrmann effect (BE) was carried out to observe characteristic dislocation structures and three-dimensional distribution and propagation of dislocations in β-Ga2O3 grown via the edge-defined film-fed growth (EFG) method. Substrates with a range of surface orientations of (001), (010), and (2¯01), cut perpendicular or parallel to the ⟨010⟩ growth direction of the EFG, were observed to understand the whole picture of dislocations distributed in the bulk crystals. Using the (001)-oriented substrate, we found characteristic dislocation structures such as dislocation helices, damage-related (001)-plane dislocation networks, and tangled dislocation complexes, which exist universally in EFG crystals but have rarely been reported before. A careful measurement of the dislocation length in BE-XRT images taken with different g-vectors allows us to determine the crystal plane on which a dislocation lies. The BE-XRTs taken from the (010)-oriented and (2¯01)-oriented substrates suggested that the dislocations propagating along the [010] growth direction were dominant. Most of these b-axis threading dislocations had a Burgers vector of [010] or [001], and they tended to align in the (100) plane. The BE-XRT observations in this study provide valuable knowledge for understanding the structure and character of dislocations in β-Ga2O3.

Tuning of Single Mixed (Helical) Dislocations in Core-Shell van der Waals Nanowires.

Linear defects (dislocations) not only govern the mechanical properties of crystalline solids but they can also produce distinct electronic, thermal, and topological effects. Accessing this functionality requires control over the placement and geometry of single dislocations embedded in a small host volume to maximize emerging effects. Here we identify a synthetic route for rational dislocation placement and tuning in van der Waals nanowires, where the layered crystal limits the possible defect configurations and the nanowire architecture puts single dislocations in close proximity to the entire host volume. While homogeneous layered nanowires host single screw dislocations, the synthesis of radial nanowire heterostructures (here exemplified by GeS-Ge1-xSnxS monochalcogenide core-shell nanowires) transforms the defect into a mixed (helical) dislocation whose edge/screw ratio is tunable via the core-shell lattice mismatch. The ability to design nanomaterials with control over individual mixed dislocations paves the way for identifying the functional properties of dislocations and harnessing them in technology.

Helical dislocation in twisted bilayer graphene

We observe helical dislocation network in twisted bilayer graphene (tBLG), which is accompanied by large out-of-plane deformation. By atomistic calculations, we demonstrate two distinct out-of-plane deformation modes, breathing mode with small out-of-plane deformation and bending mode with one order larger corrugation magnitude compared to the breathing mode. The out-of-plane deformation is caused by inhomogeneous interlayer coupling resulting from the periodic stacking order of the tBLG moiré superlattice. Instead of commonly observed screw dislocation in tBLG, we demonstrate a slip-induced helical dislocation network in the bending mode tBLG. We show that bending mode deformation is more stable at low twist angles, as the energy savings due to interface energy exceeds the energy penalty due to the strain energy caused by the large out-of-plane deformation. Our work provides a detailed picture of a new helical dislocation structure in tBLG and establishes a direct connection between the dislocation and deformation. Therefore, understanding the dislocation mechanics of tBLG may open up the possibility to control the corrugation, and reveal an opportunity to tune the intriguing physical properties of twisted bilayer graphene.

Topological field-effect transistor with quantized on/off conductance of helical/chiral dislocation states

Topology is a key ingredient driving the emergence of quantum devices. The topological field-effect transistor (TFET) has been proposed to outperform the conventional field-effect transistor by replacing the on state with topology-protected quantized conductance, while the off state has the same normal insulating characteristics and hence bears similar drawbacks. Here, we demonstrate a proof-of-concept TFET, having both on and off quantized conductance, by switching between helical and chiral topological screw dislocation (SD) states in three-dimensional topological insulators. A pair of SDs are configured, with one acting as a channel and the other as a gate controlled by a local magnetic field. Reversible field switching is achieved with the on and off conductance of $2{e}^{2}/h$ and ${e}^{2}/h$, respectively, as shown by tight-binding quantum transport calculations. Furthermore, $\mathrm{BaBi}{\mathrm{O}}_{3}$ is shown as one candidate material that has the desired topological SD states, based on first-principles calculations. Our findings open a new route to high-fidelity topological quantum devices.

Insights into electron wind force by a helical dislocation reconfiguration

Many efforts for decades have been made to explore electron wind force, produced by electric current itself under electropulsing treatment. However, the clear evidence of this force is hard to separate from Joule heating. Here we study a helical dislocation within quenched Al-Cu-Li alloy when subjected to a pulsed current. Such a helical configuration is quite suited for uncoupling this force from Joule heating effect because, contrary to general dislocations, it can take a unique reconfiguration under a driving force parallel to its Burgers vector. We find that within the pulsed samples, an initial helix happens to reconfigure, evolving into a line morphology. Therefore, it is this electron wind force Few, which parallel to the Burgers vector, would result in such novel helix reconfiguration when compared to the absence of this force. This is the first study to verify electron wind force by a helical dislocation reconfiguration.

Strain Effects in Twisted Spiral Antimonene.

Van der Waals (vdW) layered materials exhibit fruitful novel physical properties. The energy band of such materials depends strongly on their structures, and a tremendous variation in their physical properties can be deduced from a tiny change in inter-layer spacing, twist angle, or in-plane strain. In this work, a kind of vdW layered material of spiral antimonene is constructed, and the strain effects in the material are studied. The spiral antimonene is grown on a germanium (Ge) substrate and is induced by a helical dislocation penetrating through few atomic-layers of antimonene (β-phase). The as-grown spiral is intrinsically strained, and the lattice distortion is found to be pinned around the dislocation. Both spontaneous inter-layer twist and in-plane anisotropic strain are observed in scanning tunneling microscope (STM) measurements. The strain in the spiral antimonene can be significantly modified by STM tip interaction, leading to a variation in the surface electronic density of states (DOS) and a large modification in the work function of up to a few hundreds of millielectron-volts (meV). Those strain effects are expected to have potential applications in building up novel piezoelectric devices.

Formation of helical dislocations mediated by interstitials in ion irradiated FeCrAl alloy

Unraveling the interaction of the pre-existing dislocations with irradiation defects is crucial to understanding the microstructural evolution and the strength-ductility trade-off of iron-based alloys under service in an irradiation environment. In this work, the formation of helical dislocations was analyzed by combining an in-situ experiment of 400 keV Fe+ irradiated Fe-13.2Cr-5.6Al alloy at 823 K with molecular dynamics (MD) simulations. A mechanism that irradiation induced interstitial defects would mediate the pre-existing screw dislocation to produce the helical dislocations was revealed. The in-situ formation of helical dislocations and the absence of dislocation loop in the vicinity of helical dislocations were observed. The helical dislocations more likely form in the region with higher pre-existing screw dislocation density, but it will be suppressed by the dislocation loops nucleating near the screw dislocations.

Influence of dislocation morphology on T1 precipitation of an Al-Cu-Li alloy

It has been known for decades that dislocation density can significantly affect the precipitation of the strengthening phases for the age-hardening alloys. However, an understanding of the relationship between dislocation morphology and their ageing precipitation is still far limited. Here we study the impacts of especially helical dislocation morphology on the T1 precipitation of an Al-Cu-Li alloy. We find that this “opened out” helical dislocation line, tailored by pulsed currents, can markedly accelerate T1 precipitation during ageing of 155 °C. It is also revealed that a new and beneficial dislocation configuration, formed by these heavily deformed helices, can effectively decrease the field size of interacting among T1 precursors, and the resultant enhancement in nucleation and growth of T1 phases. This study is the first to propose an innovative method, tailoring helical dislocation morphology, to accelerate ageing precipitation process.

Dislocation reconfiguration during creep deformation of an Al-Cu-Li alloy via electropulsing

• Our findings proved another dislocation reconfiguration responsible for the electroplastic behaviour. • Synergy of the climb force F c and electron wind force F ew on the reconfiguration of helical dislocations. • Athermal component of electropulsing is proven to be more important during the dislocation reconfiguration. Creep mechanism was well-known to be mainly dominated by the dislocation sliding and climbing during creep deformation. Here we study the creep deformation of an Al-Cu-Li alloy with the assistance of electropulsing and subsequent microstructural observations. We find that creep strain increased drastically under electropulsing and was almost twelve times as much as that of the non-pulsed sample. Microstructural observations confirmed that dislocation reconfiguration happens via electropulsing, namely helical dislocations being opened rapidly. This opened dislocation structure can possess a much higher mobility than the initial helical dislocation, which mostly responsible for the greatly increased creep strain. Our results revealed a new mechanism accountable for the distinctly electroplastic creep deformation.

Scattering of plane-wave and twisted photons by helical media

By using quantum electrodynamics in a dispersive medium, we describe scattering of plane-wave and twisted photons by a slab made of a helical medium, the helix axis being normal to the slab plane and the medium being not translation invariant in this plane, in general. In the particular cases, the permittivity tensor of a helical medium corresponds to cholesteric liquid crystals, C*-smectics, biaxial chiral nematics and smectics, Q-plates, chiral sculptured thin films, and helical dislocations. Both perturbative and nonperturbative approaches are considered. The explicit expressions for scattering amplitudes, probabilities, and Stokes parameters of photons are found taking into account the form of the photon wave packet. The selection rules are established showing that the helical medium transfers the momentum and the angular momentum to scattered photons. This property can be employed for production of twisted photons with large projection of the total angular momentum. We describe the device for shifting the projection of the total angular momentum of a photon and the principal scheme for signal coding in terms of twisted photons.

Effect of Cooling Rates on the Characteristics of Carbides during Solidification of D2 Cold‐Work Die Steel

In article 2200137, Li, Liu, and Fu show here deeply corroded carbides in D2 cold work die steel, hexagonal hollow bar M7C3 carbides, the formation process of carbides, and bar M7C3 carbides subsequently grow within a lateral envelope of helical dislocations.

Discovery of Double Helix and Impact on Nanoscale to Mesoscale Crystalline Structures.

Screw dislocations play a significant role in the growth of crystalline structures by providing a continuous source of surface steps which represent available sites for crystal growth. Here, we show that pure screw dislocations can become helical from the absorption of defects (e.g., vacancies) and develop an attractive interaction with another helical dislocation to form a double helix of screw dislocations. These single and double helices of screw dislocations can result in the formation of interesting nanostructures with large Eshelby twists. We have previously proposed the formation of a double helix of screw dislocations to explain large Eshelby twists in crystalline nanostructures (Mater. Res. Lett.2021, 9, 453−457). We now show direct evidence for the formation of a double helix during thermal annealing of screw dislocations. The large Burgers vectors associated with these dislocations are used to explain the presence of large Eshelby twists in PbSe and PbS (NaCl cubic structure) and InP and GeS (wurtzite hexagonal structure) nanowires. These single- and double-helix screw dislocations can also combine to create even larger super Burgers vectors. These large effective Burgers also unravel the mechanism for the formation of nanopipes and micropipes with hollow cores and nanotubes with Eshelby twists in technologically important materials such as SiC, GaN, and ZnO that are utilized in a variety of advanced solid-state devices.