Lattice Curvature Research Articles

Two-level Nanostructural States in Metallic BCC-Materials after High-pressure Torsion in the Bridgman Anvils

The results of an electron-microscopy investigation of two-level nanostructural states (submicrocrystals, measuring about 100 nm in size with the inner nanostructure and nanocrystal size about 10 nm, and crystal lattice curvature of hundreds of deg/μm) formed in the specimens of unalloyed Ta and V and Mo–Re-based alloys in the course of high-pressure torsion in the Bridgman anvils are generalized. A mechanism of their formation is proposed – quasiviscous motion of nanodipoles of partial disclinations, which is controlled by the flows of nonequilibrium point defects in the fields of high local pressure gradients. It is shown that the microstructure evolution with the increasing deformation degree consists in an increase in the volume fraction of a two-level nanostructural state, which results in a 3–4-fold increase in the microhardness of the deformed specimens, its maximum values being in the range Hμ ≈ (E/27–E/32). The principal physical factors and conditions of formation of these states upon plastic deformation of submicro- and nanocrystals are discussed.

Influence of GaN/sapphire contact area on bowing of GaN wafer grown by the Na-flux method with a sapphire dissolution process

Although almost all freestanding GaN crystals exhibit concave bowing despite the elimination of a seed substrate, in the Na-flux multipoint-seed technique, a flat GaN substrate is stably obtained. Unlike other hetero-epitaxial growth techniques, this method allows the GaN/sapphire contact area to be minimized, and this in turn can lead to low curvature. However, the detailed mechanism underlying this effect on curvature is unclear. Therefore, in this study we investigated the relationship between the curvature and the GaN/sapphire contact area of freestanding GaN crystals with the use of a sapphire dissolution technique to eliminate thermal stress. As a result, the radii of the lattice curvatures of the grown crystals increased with a decrease in the contact area, and a GaN substrate with a large diameter (>2 in) and low curvature (>40 m) was obtained. These results may be attributed to grain coalescence on a flat seed.

Role of Recovery and Recrystallization on the Post Cold Work Corrosion Performance in a Super Duplex Stainless Steel

Cold rolled super duplex stainless steel (SDSS) were subjected to recovery (300 °C) and recrystallization (1020 °C) annealing. Annealing related microstructural changes were quantified with microtexture and phase-specific micro-hardness. Subsequent electrochemical characterization included potentiodynamic anodic polarization tests and Mott-Schottky analysis. It was established, for both single and two-phase specimens, that extensive recovery did not affect the electrochemical behavior, while the shortest recrystallization led to significant improvements in corrosion resistance. Improvements in corrosion performance of cold rolled SDSS was thus possible only through recrystallization, and associated removal of large lattice curvatures in the metallic substrate and reduced defect densities in the protective oxide film.

Disparity in Recrystallization of α- & γ-Fibers and its Impact on Cube Texture Formation in Non-Oriented Electrical Steel

An investigation into the recovery and recrystallization of the two major texture fibres during annealing, namely α and γ, yielded subtle differences between the two. It is reported that with thermal activation static recovery occurs, where dislocation free sub-grains are formed and tend to grow, coalesce, or bulge out. This can form new strain-free recrystallisation nuclei, with the coalesce or bulging phenomenon depending on stored energy and geometrically dislocation density (GND). Due to low lattice curvature or otherwise, it was found α-fiber has low stored energy and GND values which favours bulging into neighbouring deformed grains as opposed to subgrain coalescement. In contrast, γ-fiber tends to undergo rapid subgrain coalescement due to high lattice curvature, i.e., GND and stored energy. The newly formed grains from both texture fibers were also found to typically differ in size, as γ-fiber has a much higher nucleation rate with rapid subgrain coalescence. Furthermore, it was discovered that Cube texture component though nucleating in higher rates within α-fiber, nucleates in all regions of high dislocation densities but will only survive in regions with a low recovery and nucleation rates, typically as in α-fiber state condition. Moreover, Goss texture component was found to preferentially nucleate from γ-fiber.

Single-molecule imaging of HIV-1 envelope glycoprotein dynamics and Gag lattice association exposes determinants responsible for virus incorporation

The HIV-1 envelope glycoprotein (Env) is sparsely incorporated onto assembling virus particles on the host cell plasma membrane in order for the virus to balance infectivity and evade the immune response. Env becomes trapped in a nascent particle on encounter with the polymeric viral protein Gag, which forms a dense protein lattice on the inner leaflet of the plasma membrane. While Env incorporation efficiency is readily measured biochemically from released particles, very little is known about the spatiotemporal dynamics of Env trapping events. Herein, we demonstrate, via high-resolution single-molecule tracking, that retention of Env trimers within single virus assembly sites requires the Env cytoplasmic tail (CT) and the L12 residue in the matrix (MA) domain of Gag but does not require curvature of the viral lattice. We further demonstrate that Env trimers are confined to subviral regions of a budding Gag lattice, supporting a model where direct interactions and/or steric corralling between the Env-CT and a lattice of MA trimers promote Env trapping and infectious HIV-1 assembly.

Dislocation density distribution at slip band-grain boundary intersections

We study the mechanisms of slip transfer at a grain boundary, in titanium, using Differential Aperture X-ray Laue Micro-diffraction (DAXM). This 3D characterisation tool enables measurement of the full (9-component) Nye lattice curvature tensor and calculation of the density of geometrically necessary dislocations (GNDs). We observe dislocation pile-ups at a grain boundary, as the neighbour grain prohibits easy passage for dislocation transmission. This incompatibility results in local micro-plasticity within the slipping grain, near to where the slip planes intersect the grain boundary, and we observe bands of GNDs lying near the grain boundary. We observe that the distribution of GNDs can be significantly influenced by the formation of grain boundary ledges that serve as secondary dislocation sources. This observation highlights the non-continuum nature of polycrystal deformation and helps us understand the higher order complexity of grain boundary characteristics.

The role of nanoscale strain-induced defects in the sharp increase of low-temperature toughness in low-carbon and low-alloy steels

In the paper it is studied how the modes of thermal treatment of low-carbon steels alloyed with manganese (09G2S) and with manganese, vanadium, and niobium (10G2FBYu) affect their hierarchical structure and toughness in the low-temperature region from +20 down to −70 °C. A fine-grained structure, quasi-homogeneous lattice curvature, and nanoscale mesoscopic structural states arising due to radial-shear rolling at 850 °C are shown to be responsible for the formation of a nonequilibrium nanoscale bainitic structure, being a highly effective damping factor in a deformed material. The pearlitic steel structure is equilibrium and is formed within a translation-invariant crystal lattice, while the bainitic structure results from nanoscale mesoscopic structural states in interstices of lattice curvature zones. A spatial change in the lattice curvature is accompanied by a synchronous transformation of the nanoscale mesoscopic structural states and structural geometry of the bainitic phase. That is why the toughness of the steel with bainitic structure is very high down to the temperature −70 °C. Scratch testing is used to estimate the possibility of elastic recovery of bainitic phases in deformed steels.

Curvature and Torsion of the Crystal Lattice in Deformed Polycrystalline Alloys

Curvature and Torsion of the Crystal Lattice in Deformed Polycrystalline Alloys

Nanodipoles of Partial Disclinations and Strain Localization Mechanism of Nanostructured Materials in the Elastic Region

In the course of severe plastic deformation of nickel in the Bridgman anvils, a phenomenon of strain localization is observed in the region of elastic distortions, which is accompanied by the formation of reorientation nanobands with an elastic crystal lattice curvature of hundreds of deg/μm and nanodipoles of partial disclinations as the defects of elastically deformed medium and carriers of elastic shears and rotations at the front of nanoband propagation. A theoretical analysis is performed of the elastically stressed state and energy of these defects, including the case of their transformation into more complex ensembles of interconnected disclinations. Using the results obtained and the data of an MD investigation of dynamic vortex defects formed in the region of elastic distortions, the mechanisms of strain localization in this region in the stages of nucleation and propagation of these nanobands are proposed.

Особенности массопереноса при ионной имплантации

Исследованы продукты реакции, полученные при ионной имплантации азота, кислорода и фосфора в вольфрамовую мишень. Выяснены условия создания новых соединений при высокодозной имплантации, приводящей к процессам быстрого массопереноса ионов в материал мишени. Установлено, что ионная имплантация азота в вольфрам приводит к возникновению фазы внедрения W2N с ГЦК-структурой; имплантация кислорода — к образованию фазы с плотноупакованной ГЦК-структурой, близкой по структуре и параметрам к W20; фосфора — к образованию фосфида вольфрама W2P с орторомбической структурой после дополнительного отжига при температурах, превышающих 800-900 0С. Сделана оценка локального давления потока ионов на поверхность мишени, которое приводит к локальному искривлению кристаллической решетки, благодаря чему возможен быстрый массоперенос имплантированных ионов в материале мишени.
 Образование фаз внедрения при ионной имплантации является результатом локализации сдвиговой деформации в условиях ускоренного массопереноса. В условиях высокодозной ионной имплантации формируются высоконеравновесные пересыщенные твердые растворы, а также высокодефектные структурно-неравновесные состояния с высокой кривизной кристаллической решетки.

Diffraction-based misorientation mapping: A continuum mechanics description

As the typical intragranular misorientation parameters, Kernel Averaged Misorientation (KAM) and Grain Reference Orientation Deviation (GROD) are widely used in diffraction-based misorientation mapping, while their evolution laws under various conditions and physical meanings in continuum mechanics description are rarely investigated systematically. Therefore, we designed several comparative experiments considering the influences of grain boundaries (single-crystalline vs. poly-crystalline), sample geometries (smooth vs. notched) and plastic strain modes (tension vs. buckling) on KAM & GROD evolution, and captured the intragranular misorientation, dislocation density, and material distortion synchronously based on coupled EBSD-ECCI-DIC mapping in this research. Meanwhile, we also discussed the physical meanings of KAM & GROD based on continuum mechanics description and provided the theoretical explanations to phenomena observed in the above experiments. KAM results from three in-surface invariants (ρGNDI, ρGNDII & ρGNDIII) of GND density tensor ρGND induced by plastic strain distribution incompatible with the activated slip systems in unloaded elastic-plastic condition, which cause the same lattice curvature effects as three elastic strain modes with non-zero curl (buckling, in-surface bending & torsion) in purely elastic condition. GROD reflects neither local plastic strain nor local material rotation alone, but its “V-type” distribution near the neutral surface reflects the buckling curvature of single-crystal. Besides, KAM¯ & GROD¯ averaged over multiple grains can be used to estimate the nominal plastic strain applied in poly-crystal.

Effect of Nanoscale Mesoscopic Structural States Associated with Lattice Curvature on the Mechanical Behavior of Fe-Cr-Mn Austenitic Steel

The paper explores the effect of high-temperature cross rolling followed by cold rolling on the internal structure of metastable Fe-Cr-Mn austenitic stainless steel, formation of nonequilibrium martensite e and α′ phases in it, dynamic rotations on fracture surfaces, fatigue life under alternating bending, and wear resistance. Scratch testing revealed a strong increase in the damping effect in the formed hierarchical mesoscopic substructure that promotes the formation of a nanocrystalline grain structure, formation of hcp e martensite and bcc α′ martensite in grains, formation of a vortical filamentary substructure on the fracture surface, and an increase in the high-cycle fatigue properties and wear resistance. These processes are associated with a high density of nanoscale mesoscopic structural states that arise in lattice curvature zones during high-temperature cross rolling followed by cold rolling with smooth rolls. The described effects are explained by the self-consistent mechanical behavior of hcp e martensite laths in fcc austenite grains and bcc α′ martensite laths formed in cold rolling of the steel after high-temperature cross rolling.

Modeling of Localized Inelastic Deformation at the Mesoscale with Account for the Local Lattice Curvature in the Framework of the Asymmetric Cosserat Theory

In the paper, inelastic strain localization in homogeneous specimens and mesovolumes of a polycrystalline material is modeled based on the asymmetric theory of an elastoplastic Cosserat continuum in a two-dimensional formulation for plane strain. It is assumed that rotational deformation in loaded materials occurs due to the development of localized plastic deformation as well as bending and torsion of the material lattice at the micro- and nanoscale levels. For this reason, the parameters of the micropolar model are considered as functions of inelastic strain for each local mesovolume of the continuum. It is shown that the observed parabolic hardening can be attributed to a large extent to the development of rotational deformation modes, bending and torsion, and appearance of couple stresses in the loaded material. The modeling results indicate that if rotational deformation is stopped in the loaded material, its accommodation capacity decreases, the local and macroscopic inelastic strains sharply increase, leading to a much more rapid formation of fracture structures. Conversely, the formation of meso- and nanoscale substructures with high lattice curvature in materials promotes the activation of rotational deformation modes, reduction of localized strains, and relaxation of stress concentrators.

The Structure and Properties of a Weld-Deposited Layer onto Steel Hardox 450 Using a Boron-Containing Wire

By using the methods of modern physical material science, the structure-phase states and properties of the layers formed on low-alloyed steel Hardox 450 with the use of welding wires with a boron content amounting to 4.5 and 6.5 wt % have been analyzed. In the initial state, steel Hardox 450 has the structure of tempered martensite in the bulk and along the boundaries of crystals in which cementite particles are located. The particles located in the bulk are needle-shaped and those located along boundaries are mainly round-shaped. The revealed extinction bend contours indicate a torsion curvature of the crystal lattice in this area of the material. They begin and finish at the interfaces of the martensite crystals. The scalar density of chaotically arranged dislocations forming a reticulate-type substructure is 6.2 × 1010 cm–2. The weld-deposited layer onto steel Hardox 450 has more than two-fold microhardness than that of the base metal. The analysis of state diagrams for the Fe–C, Fe–B, and B–C systems and of polythermal cross-sections in the Fe–C–B system has shown that a rapid cooling of Fe23C6–Fe23B6 alloys from the liquid state facilitates the formation of multiphase structural states. It has been established by means of transmission electron diffraction microscopy that the reasons for the high microhardness level of the surface layers consist in the following factors: the formation of iron borides and the crystals of ultrafine packet martensite (up to 100 nm) having a high level (~1011 cm–2) of scalar dislocation density; the presence of nanosized particles of iron and boron carbides in the bulk and along the boundaries of martensite crystals; and a high torsion curvature level in the crystal lattice of iron borides and α-phase grains caused by internal stress fields along interphase boundaries (the interface between iron boride crystals and α-phase grains) and intraphase boundaries (the interface between iron borides and martensite crystals in the packet). Increasing boron concentration from 4.5 to 6.5% is accompanied by a sufficient (1.2–1.5-fold) increase in the hardness of the weld-deposited layer. It is caused by a 1.5–2.0-fold increase in the size and relative content of iron boride areas.

Two beam toy model for dislocation contrast in ECCI

Dislocation contrast in the SEM, as observed though electron channelling contrast imaging (ECCI), is commonly treated analogously to the contrast in the TEM. This perception is based on early studies done for dislocations parallel with the surface where the surface relaxation is negligible. However, for threading dislocations (TD) that interact with the surface (normal or inclined), as is the case for nitrides materials, g b type invisibility criteria are no longer fully applicable to ECCI, especially in forward geometry [1]. Dislocations change locally the lattice curvature and Bragg diffraction conditions in the crystal, affecting the form and diffracting behaviour of the electron wavefunction in that region. More explicitly, Howie and Whelan [2] had shown that dislocation contrast is the result of interband transitions between Bloch waves states which, in turn, are caused by the change in the displacement field, u(r), around the dislocation or local “strain”. Dynamical models have been used successfully to both predict and characterise dislocations in ECCI [3]. Nevertheless, the behaviour of dislocation contrast in ECCI in particular and diffraction contrast in the SEM in general remains somewhat opaque. In the work we investigate the behaviour of contrast causing strain as a means of insight into this problem.

Correlating dynamic strain and photoluminescence of solid-state defects with stroboscopic x-ray diffraction microscopy

Control of local lattice perturbations near optically-active defects in semiconductors is a key step to harnessing the potential of solid-state qubits for quantum information science and nanoscale sensing. We report the development of a stroboscopic scanning X-ray diffraction microscopy approach for real-space imaging of dynamic strain used in correlation with microscopic photoluminescence measurements. We demonstrate this technique in 4H-SiC, which hosts long-lifetime room temperature vacancy spin defects. Using nano-focused X-ray photon pulses synchronized to a surface acoustic wave launcher, we achieve an effective time resolution of ~100 ps at a 25 nm spatial resolution to map micro-radian dynamic lattice curvatures. The acoustically induced lattice distortions near an engineered scattering structure are correlated with enhanced photoluminescence responses of optically-active SiC quantum defects driven by local piezoelectric effects. These results demonstrate a unique route for directly imaging local strain in nanomechanical structures and quantifying dynamic structure-function relationships in materials under realistic operating conditions.

Formation of Gradient Structure–Phase States in the Surface Layers of 100-m Differentially Quenched Rails

Abstract—The evolution of the structural-phase states in the surface layers of the head of differentially quenched DT350 rails to a depth of 10 mm are studied by optical microscopy and transmission electron diffraction microscopy along the central axis after the passage of a tonnage of 691.8 mln t gross in the experimental ring of AO VNIIZhT (proof ground for railway transport). The formation of a gradient structure is revealed. In this structure, the scalar and excess dislocation densities, the lattice curvature–torsion amplitude, and the degree of deformation-induced transformation of lamellar pearlite change gradually. The mechanisms of fracture of cementite lamellae are considered.

Evolution of the Curvature and Torsion of a Crystal Lattice upon Deformation in Different Substructures of Copper-Based Alloys

The results are presented from an electron microscopic study of the curvature/torsion (χ) of the crystal lattice that occurs upon the deformation of polycrystalline solid FCC solutions. Alloys Cu + 0.4 at % Mn and Cu + 19 at % Mn with mean grain size 〈d〉 = 10–240 μm are investigated. Samples are deformed by tension at room temperature. Causes of curvature/torsion found. It is found that the highest values of χ are characteristic of the joints of grains and grain boundaries.

Representation of Nye’s Lattice Curvature Tensor by Log Angles

Representation of Nye’s Lattice Curvature Tensor by Log Angles

Growth of GaN and improvement of lattice curvature using symmetric hexagonal SiO2 patterns in HVPE growth

We implemented facet and flattening (FF) growth via hydride vapor phase epitaxy, and applied hexagonal SiO2 mask patterns to minimize the final threading dislocation density (TDD). We found that the FF growth technique with hexagonal masks significantly mitigates the lattice curvature. Following multiple growth steps, an isotropic curvature could be observed owing to the planar symmetry of the mask patterns; the radius of curvature was 55 m. Furthermore, TDD was significantly reduced to 6.8 × 105 cm−2. These results indicate that the hexagonal mask technique is effective for improving both lattice curvature and crystalline quality of a GaN substrate.