Screw Dislocations Research Articles

Growth, structure, and morphology of van der Waals epitaxy Cr1+δTe2 films

The preparation of two-dimensional magnetic materials is a key process to their applications and the study of their structure and morphology plays an important role in the growth of high-quality thin films. Here, the growth, structure, and morphology of Cr1+δTe2 films grown by molecular beam epitaxy on mica with variations of Te/Cr flux ratio, growth temperature, and film thickness have been systematically investigated by scanning tunneling microscopy, reflection high-energy electron diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy. We find that a structural change from multiple phases to a single phase occurs with the increase in growth temperature, irrespective of the Cr/Te flux ratios, which is attributed to the desorption difference of Te atoms at different temperatures, and that the surface morphology of the films grown at relatively high growth temperatures (≥ 300 °C) exhibits a quasi-hexagonal mesh-like structure, which consists of nano-islands with bending surface induced by the screw dislocations, as well as that the films would undergo a growth-mode change from 2D at the initial stage in a small film thickness (2 nm) to 3D at the later stage in thick thicknesses (12 nm and 24 nm). This work provides a general model for the study of pseudo-layered materials grown on flexible layered substrates.

Some remarks on scalar particles under the influence of noninertial effects in a spacetime with a screw dislocation

Some remarks on scalar particles under the influence of noninertial effects in a spacetime with a screw dislocation

Effect of Si on the dislocation state within martensite of ultra-high strength hot-rolled medium Mn steel with good ductility

Medium Mn steels are promising candidates for the third-generation advanced high-strength steels due to their good combination of ultimate tensile strength (UTS) and total elongation after fracture (TEL). Most investigations of medium Mn steels focused on the volume fraction of retained austenite (RA) and stability of RA; however, the strengthening phase martensite was paid little attention to. The present study systematically investigated the effect of Si on the deformation behavior of martensite in 0.2C–6.8Mn-(0, 2, 4)Si (wt. %) hot-rolled medium Mn steels without any heat treatment. The UTS and TEL of the 4Si hot-rolled medium Mn steel are 2163 MPa and 11.7%, respectively, which is rare in the existing hot-rolled medium Mn steels. All three hot-rolled steels had practically equal fraction and mechanical stability of RA, so that the influence of Si on the evolution of martensite fine structure and the nature of the ultra-high UTS and good ductility during deformation could be revealed. By combining the X-ray and neutron diffraction techniques, it was found that the addition of Si increased the dislocation density, especially screw dislocations, and decreased the crystallite size within martensite. The ultra-high UTS originates from the enhanced strain hardening rate caused by the Si alloying, high dislocation density and possibly increased number of martensite lath boundaries and dislocation cell formation. The good ductility mainly stems from the formation of dislocation cell structure due to the addition of Si causing dislocations to be mostly of screw type. The dislocation cells can also improve the strain hardening rate. This work opens a new pathway towards tailoring the dislocation state within martensite for substantially enhanced the mechanical properties of hot-rolled medium Mn steels.

Pre-stored edge dislocations-enabled pseudo-toughness in chromium

Pre-deformation is an effective mean to tune the ductile-to-brittle transition (DBT) temperature of body-centered cubic metals. However, due to complex microstructural evolution during pre-straining, altering not only dislocation density but also grain size, the mechanism of reduction in DBT temperature remains intriguing. Here, we designed two types of chromium samples, with similar grain size but different initial dislocation density, to uncover the underlying mechanisms that govern the variation in DBT. Small-punch, quasi-in situ crack propagation, and in-situ nanomechanical tests conducted over a wide temperature range revealed that pre-stored edge dislocations only can make pseudo-toughness, i.e., increasing deformability but the sample is still intrinsically brittle-prone to fragile cracking. Activation of the easy glide pre-stored edge dislocations could enhance the crack resistance at the initial stage of mechanical loading, which makes a left-shift of the DBT temperature, while the toughness/brittleness of chromium is still controlled by the relative mobility of screw versus edge dislocations.

Influence of Grain Size on Mechanical Properties of a Refractory High Entropy Alloy under Uniaxial Tension

There is a growing interest in High Entropy Alloys (HEAs) due to their outstanding mechanical properties. Most simulation studies have focused on face-centered cubic (fcc) HEAs; however, bcc HEAs can offer a larger elastic modulus and plastic yielding, thus, becoming possible candidates for the next generation of refractory materials. In this work, we focus on molecular dynamics (MD) simulations of bcc HfNbTaZr nanocrystalline samples, with a grain size (d) between 5 and 17 nm, deformed under tension at 300 K. The elastic modulus increases with the grain size and reaches a plateau near 10 nm. We find the typical inverse Hall–Petch (HP) behavior with yield strength, ultimate tensile stress (UTS), and flow stress increasing with d. Up to 12 nm, there are contributions from dislocations and twins; however, grain boundary (GB) activity dominates deformation. For the 5 nm grains, the GB disorder extends and leads to extensive amorphization and grain size reduction. For d>10 nm, there is a HP-type behavior with dislocations and twinning controlling deformation. For this regime, there is hardening at large strains. Compared to bcc single metal samples, the HP maximum of this HEA appears at a lower grain size, and this could be related to the chemical complexity facilitating dislocation nucleation. We use machine learning to help understand deformation regimes. We also compare our results to a single crystal (SC) HfNbTaZr HEA deformed along [001] and find that the single crystal is weaker than the nanocrystalline samples. The single crystal deforms initially by twinning and then rapidly by dislocation multiplication, leading to strong hardening. It has been proposed that edge dislocations play a major role in bcc HEA plasticity, and we also analyze the relative contributions of edge versus screw dislocations during deformation for both single crystal and nanocrystalline samples.

Oriented growth of cobalt electrodeposits assisted by an interfacial flow of bubbles

High-quality Co products are desirable for a variety of applications due to its unique properties. Herein, an interfacial field of flowing bubbles was used to modify the microstructure of Co deposits in the electrowinning process. The effect of flowing bubbles on morphology, crystallographic structure and impurities of Co deposits were investigated. The nucleation and growth of Co deposits were discussed based on chronoamperometric experiments and sampling experiments. The results indicated that the microstructure of Co deposits was effectively modified by an external field of flowing bubbles. The Co deposits prepared in a quiescent electrolyte were porous and inhomogeneous, while the deposits prepared by using the flowing bubbles were compact and uniform. The nucleation and growth modes of Co deposits were also altered by the flowing bubbles. Preferentially oriented grains in the (110) direction were formed by instantaneous nucleation and screw dislocation growth of the deposits. Improvements in microstructure and crystallographic orientation were able to enhance the purity of Co deposits.

A fully discrete Peierls model for the screw dislocation in Ta

ABSTRACT The discrete Peierls model is generalised to be able to study the nonplanar dislocations as well as the planar dislocations. In the discrete model, the six-fold screw dislocation that is widely studied in numerical simulation and analytical theory splits into two different cores: the six-fold B core and the six-fold O core. The six-fold B core has the lowest energy and is the most stable while the six-fold O core has about 38 meV more energy in a period length. Because local dislocation hopping can only occur along the folds and hopping to sites out of the folds is not allowed, the dislocation movement is realised by transforming itself into a planar one in the model. The Peierls stress associated with this transition tunnel is , which is much higher than that obtained in simulations. Limitations of the model can be removed by using the two-dimensional energy landscape that is a natural consequence of the concept of the nonplanar dislocation. In the leading terms approximation of the two dimensional energy landscape, the non-Schmid behaviour of the screw dislocation in Ta is quantitatively predicted based on the model.

Comprehensive analysis of current leakage at individual screw and mixed threading dislocations in freestanding GaN substrates

The electrical characteristics of Schottky contacts on individual threading dislocations (TDs) with a screw-component in GaN substrates and the structures of these TDs were investigated to assess the effects of such defects on reverse leakage currents. Micrometer-scale platinum/GaN Schottky contacts were selectively fabricated on screw- and mixed-TD-related etch pits classified based on the pit size. Current–voltage (I–V) data acquired using conductive atomic force microscopy showed that very few of the screw TDs generated anomalously large reverse leakage currents. An analysis of the temperature dependence of the I–V characteristics established that the leakage current conduction mechanisms for the leaky screw TDs differed from those for the other screw and mixed TDs. Specifically, anomalous current leakage was generated by Poole–Frenkel emission and trap-assisted tunneling via distinctive trap states together with Fowler–Nordheim tunneling, with the mechanism changing according to variations in temperature and applied voltage. The leaky TDs were identified as Burgers vector b = 1c closed-core screw TDs having a helical morphology similar to that of other screw TDs generating small leakage currents. Based on the results, we proposed that the atomic-scale modification of the dislocation core structure related to interactions with point defects via dislocation climbing caused different leakage characteristics of the TDs.

Cosine-type apodized spiral zone plate to handle the topological charge of a vortex beam

By taking advantage of spiral petal-like zone plates, we have recently demonstrated that a spiral element’s geometric shape impacts its charge. Keeping it in mind, we aim to control the topological charge of a vortex using phase modification of a spiral zone plate without transforming its geometrical shape. This is achieved by apodizing a spiral zone plate having p spiral zones with a radial grating with spatial frequency m. Having implemented some gratings with different m, we realized that an optical vortex is produced whose topological charge and its handedness depend on p and m. Consequently, a constant phase replaces on-axis screw dislocation under the situation that p takes odd multiples of m. Studies are carried out for large and small pairs of (p, m) and various gaps between the couple.

Atomic structure, stability, and dissociation of dislocations in cadmium telluride

Dislocation plays a crucial role in many material properties of semiconductors, ranging from plastic deformation to electronic transport. But the dislocation structures and reactions remain controversial in many semiconductors. Here, we systematically examine the dislocation properties of cadmium telluride (CdTe), a prototype of II-VI compound semiconductors, using molecular statics and molecular dynamics simulations with a machine-learning force field. We find that dislocation cores in CdTe are not reconstructed along the dislocation line due to the significant ionic bonding characteristics. Moreover, the undissociated screw dislocation in the shuffle set is more stable than that in the glide set, and the glide dislocation tends to move to the shuffle set, followed by random movement rather than dissociation, arising from its low Peierls stress. For 60° perfect dislocation, it also prefers to locate in the shuffle set, but the 60° glide dislocation is dissociated into a pair of 30° and 90° partial dislocations connected by a stacking fault with a width of ∼9.03 nm at low temperatures through reconfiguring atomic bonds in the core. This work provides an atomic-level understanding of dislocation core properties in CdTe, laying a basis for interpreting the mechanical and electronic performance of CdTe and other II-VI semiconductors.

Calculation of dislocation binding to helium-vacancy defects in tungsten using hybrid ab initio-machine learning methods

Calculations of dislocation-defect interactions are essential to model metallic strength, but the required system sizes are at or beyond ab initio limits. Current estimates thus have extrapolation or finite size errors that are very challenging to quantify. Hybrid methods offer a solution, embedding small ab initio simulations in an empirical medium. However, current implementations can only match mild elastic deformations at the ab initio boundary. We describe a robust method to employ linear-in-descriptor machine learning potentials as a highly flexible embedding medium, precisely matching dislocation migration pathways whilst keeping at least the elastic properties constant. This advanced coupling allows dislocations to cross the ab initio boundary in fully three dimensional defect geometries. Investigating helium and vacancy segregation to edge and screw dislocations in tungsten, we find long-range relaxations qualitatively change impurity-induced core reconstructions compared to those in short periodic supercells, even when multiple helium atoms are present. We also show that helium-vacancy complexes, considered to be the dominant configuration at low temperatures, have only a very weak binding to screw dislocations. These results are discussed in the context of recent experimental and theoretical studies. More generally, our approach opens a vast range of mechanisms to ab initio investigation and provides new reference data to both validate and improve interatomic potentials.

Evident thickness effect on structure and mechanical properties of molybdenum films

Magnetron sputtering was used to prepare nanocrystalline Molybdenum (Mo) films with a thickness (h) ranging from 200 to 1000 nm. Transmission Electron Microscope, Scanning Electron Microscope, X-Ray Diffraction, and Atomic Force Microscope were used to examine the micro-structure of as-deposited films. Using nanoindentation, the mechanical properties of Mo films were investigated. The micro-structure and mechanical properties were found to be dependent on h. With increasing h, the tensile residual stress increased from 88 to 435 MPa; the average roughness increased from 1.87 to 2.93 nm; the critical shear stress (τc) to initiate plastic deformation ranged between 10.8 and 12.3 GPa; and the hardness decreased from 6.70 to 4.72 GPa, as described by the Hall-Petch equation. The mechanism for plastic deformation was hypothesized to be screw dislocation nucleation and motion. Using Hertz contact theory, the theoretical τc matched the experimental results quite well. With decreasing h, the blocking effect of the grain boundary on dislocation motion was deemed to be the strengthening mechanism.

氮掺杂单层石墨烯上无中间层沉积高质量氮化镓

GaN on graphene/Al2O3 substrates grown via van der Waals epitaxy compensates for the deficiencies and defects caused by metal-organic chemical vapor deposition (MOCVD) on substrates with significant mismatches to GaN. However, the absence of dangling bonds on graphene leads to insufficient nucleation sites; hence, a thin layer of AlN or ZnO nanowalls should be deposited on graphene as an intermediate layer. In this work, high-quality GaN crystals with a low biaxial compressive stress of 0.023 GPa and low screw dislocation density of 9.76 × 107 cm−2 were successfully synthesized by MOCVD on nitrogen-doped graphene without a buffer layer. First-principles calculations demonstrated significant improvement in the adsorption energy of the Ga atom on the surface of nitrogen-doped graphene compared with that of pristine graphene, in agreement with the experimental observations of nucleation. In most cases, GaN films were obtained by forming C—Ga—N and N—Ga—N configurations via atomic nitrogen pretreatment on monolayer graphene. Therefore, it is hoped that the efficient method of atomic modulation of high-quality GaN films grown on nitrogen-doped graphene via interface manipulation used in this work will promote the industrial development of innovative semiconductor devices.

Nitride-reinforced HfNbTaTiV high-entropy alloy with excellent room and elevated-temperature mechanical properties

Nitride-reinforced (HfNbTaTiV)90N10 high-entropy alloy aiming at high-temperature applications is designed in this paper. Abundant FCC nitride phases are formed in situ in theBCC matrix by arc melting technique, without complex deformation or heat treatment. The (HfNbTaTiV)90N10 alloy exhibits a remarkable yield strength of 2716 MPa and ultimate compressive strength of 2833 MPa with a plastic strain of 10% at room temperature. Besides, the alloy remains a high yield strength of 279 MPa at 1400 °C. The nitride phases play an essential role in maintaining the excellent strength-ductility combination at room temperature and enhancing the high-temperature softening resistance. Alternating BCC and FCC phases possess the semi-coherent interface, which not only strengthens the BCC matrix but also promotes the compatible deformation of the duplex microstructure. The lattice coherency structure of the semi-coherent interface is conducive to the slip transfer of partial dislocations through the interface, which facilitates the accommodation of plastic deformation. The cross-slip of the screw dislocations effectively eliminates stress concentration and leads to good ductility of the dual-phase alloy. The results demonstrate that the nitride phases achieve coordinate deformation with the matrix without deteriorating the ductility of the (HfNbTaTiV)90N10 alloy.

The Effect of Nitridation on Sputtering AlN on Composited Patterned Sapphire Substrate.

Here, we report on the epitaxial growth of GaN on patterned SiO2-covered cone-shaped patterned sapphire surfaces (PSS). Physical vapor deposition (PVD) AlN films were used as buffers deposited on the SiO2-PSS substrates. The gallium nitride (GaN) growth on these substrates at different alternating radio frequency (RF) power and nitridation times was monitored with sequences of scanning electron microscopy (SEM) and atomic force microscopy (AFM) imaging results. The SEM and AFM show the detail of the crystalline process from different angles. Our findings show that the growth mode varies according to the deposition condition of the AlN films. We demonstrate a particular case where a low critical alternating current (AC) power is just able to break SiO2 covalent bonds, enabling the growth of GaN on the sides of the patterns. Furthermore, we show that by using the appropriate nitridation condition, the photoluminescence (PL) integral and peak intensities of the blue light epi-layer were enhanced by more than 5% and 15%, respectively. It means the external quantum efficiency (EQE) of epitaxial structures is promoted. The screw dislocation density was reduced by 65% according to the X-ray diffraction (XRD) spectra.

Mobility of carbon-decorated screw dislocations in bcc iron

We investigate the mobility of screw dislocations decorated by carbon solutes in body-centred cubic iron based on density functional theory (DFT) calculations and transmission electron microscopy (TEM) observations of in-situ straining experiments. We focus on the high-temperature domain, between 500 and 800 K, where plasticity is controlled by the slow glide of decorated screw dislocations with a mobility similar to the Peierls mechanism existing below 300 K, at variance with the athermal regime observed at intermediate temperatures. We propose that due to the strong pinning of reconstructed dislocation lines by carbon solutes, dislocation glide occurs by the formation and migration of kinks, both controlled by the jumps of carbon atoms. We calculate the associated kink-pair nucleation and formation energies as well as the migration energies of isolated kinks and use these DFT values to predict the dislocation velocity as a function of temperature, applied stress and dislocation length. The validity of the mobility law is assessed by the comparison with dislocation velocities measured in TEM observations at different temperatures and local shear stresses. The quantitative agreement between the experimental measurements and the analytical model supports the proposed glide mechanism and the DFT values obtained for kink energies.

Position-dependent mass Schrödinger particles in space-like screw dislocation: associated degeneracies and magnetic and Aharonov–Bohm flux fields effects

We consider non-relativistic position-dependent mass (PDM) Schödinger particles moving in an elastic medium with space-like screw dislocation. Within the cylindrical coordinates $$\left( r,\phi ,z\right) $$ , we study and report the effects of screw dislocation as well as PDM settings on the energy levels of some PDM-quantum mechanical systems. In so doing, we use a power-law type positive-valued dimensionless scalar multiplier $$ f(r)=\lambda r^{\sigma }$$ (which manifestly introduces the metaphoric notion of PDM-Schödinger particles). Next, we subject such PDM particles to magnetic and Aharonov–Bohm flux fields. We report exact or conditionally exact eigenvalues and eigenfunctions for $$V(r)=0$$ and $$V(r) =a+b\,r+c\,r^{2}$$ .

Nanoindentation avalanches and dislocation structures in HfNbTiZr high entropy alloy

High temperature nanoindentation was performed on a refractory HfNbTiZr high-entropy alloy to probe the unique dislocation behaviors. Anomalous nanoindenation avalanche occurs once the load reaches a critical value of 10 mN and exceeds the elastic limit of the alloy. The depth of the plastic zone under indentation is much deeper in the sample with avalanches, and continues to deepen with the increase of testing temperature. A large fraction of mixed dislocations is formed after nanoindentation, instead of forming the long screw dislocations as in common refractory metals. Nanoindentation avalanche is induced by the sudden movement of a group of usually sluggish dislocations, which have a low mobility due to local chemical fluctuations and large lattice distortions. These results manifest a weak temperature-dependence of dislocation mobility in body-centered cubic high-entropy alloy.

Atomic-scale insight into interaction mechanism between screw dislocation and HCP phase in high-entropy alloy

The face-centered cubic (FCC)/hexagonal close-packed (HCP) dual-phase structure is a new design strategy proposed in recent years to achieve high strength and excellent plasticity of high-entropy alloys (HEAs). Here, the effect of HCP phase thickness, strain rate, and temperature on the interaction mechanism between screw dislocation and the HCP phase in the FCC structured CoCrFeMnNi HEAs is investigated by molecular dynamics simulation. The results show that there are two types of interaction modes between dislocations and the HCP phase: one is the dislocation passing through the HCP phase, that is, the penetration mechanism, and the other is the dislocation being absorbed by the HCP phase, that is, the absorption mechanism. The generation of these two mechanisms mainly depends on the relative ability of the HCP phase to prevent dislocation slip, which is closely related to the HCP phase thickness, strain rate, and temperature. When the relative ability of the HCP phase to block dislocation is large, the interaction between dislocations and the HCP phase presents an absorption mechanism; otherwise, it presents a penetration mechanism. The research can provide theoretical guidance for the development and design of new high-performance HEAs to achieve high strength and high ductility of materials.

Microstructure and deformation behavior of MoReW in the temperature range from 25°C to 1500°C

Refractory alloys which retain high strength above 1000 °C and, at the same time, have excellent malleability, are in high demand for high temperature structural applications. Here we report the microstructure and mechanical properties of an equiatomic MoReW alloy, which has demanding combination of strength and ductility in the temperature range of 25 °C to 1500 °C. The alloy was prepared by arc melting followed by hot isostatic pressing at 1400 °C, 207 MPa for 5 h and it has a single-phase BCC structure with the average grain size of ∼170 μm. The alloy yield stress is relatively low (555 MPa) at 25 °C, but it has weak temperature dependence resulting in remarkably good yield stress values at high temperatures, e.g. 402 MPa at 1000 °C and 247 MPa at 1400 °C. The alloy shows strong strain hardening at 25–1000 °C, with the true flow stress tripled at 25 °C and doubled at 1000 °C after true strain of 0.4. The microstructural analysis of the deformed specimens reveals extensive deformation twinning activity at the beginning of deformation and activation of other deformation modes at later deformation stages in this temperature range. This is unusual behavior as generally activation of twinning occurs at high stresses, after extensive dislocation activity, and deformation twinning in refractory alloys has never been reported above 400 °C. To understand and explain the observed behaviors of the alloy, a detailed analysis of strengthening mechanisms associated with screw and edge dislocation glide, as well as with twin nucleation and propagation, has been conducted and reasonable qualitative models of yielding, strain hardening and ductility of the studied MoReW alloy have been proposed.