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Nanocrystals (NCs), representing a zero-dimensional system, are an ideal platform for exploring quantum phenomena on the nanoscale, and are expected to play a major role in future electronic and photonic devices. Here we review recent progress in the growth, characterization and utilization of some group-IV semiconductors (Si and Ge), metal and high-k NCs for silicon planar technology compatible light-emitting and floating gate memory devices. We first introduce the size-dependent electrical and optical properties of Si and Ge NCs. We outline some of the schemes to achieve light emission from indirect band gap Si and Ge NCs embedded in different high band gap oxide matrices. In particular, special emphasis is given on the review of the advances in Ge NCs because of some of their intriguing electronic and optical properties. We then describe the use of semiconductor and metal NCs as floating gates for non-volatile memory devices to achieve high data retention and faster program/erase speeds. The exploitation of high-k oxides with tunable and variable injection barriers for improved charge storage devices is discussed. Finally, the integration of single and multilayer metallic NCs and multilayer high-k oxides as floating gates is explored by the fabrication and testing of memory transistors. [ABSTRACT FROM AUTHOR]

We report on molecular dynamic calculations of combined tension-torsion of chiral single-wall nanotubes. We work within the framework of objective structures that exploits symmetry groups to enable torsion of chiral nanotubes, in addition to non-equilibrium extension. We apply the method to study the mechanical response and failure of nanotubes of various chiralities. We find that three distinct regimes exist for nanotubes under twist: distorted but unbuckled, reversible torsional buckling and irreversible torsional buckling. When twisted but unbuckled nanotubes are subject to tension, there is minimal change in failure strain, whereas reversibly buckled nanotubes have substantially reduced failure strain and load. We also observe the evolution of the twisting moment during the elongation process while keeping the twist angle fixed. This evolution has two interesting and potentially useful features: first, some nanotubes 'unbuckle' in the process of extension, and second, there is a clear correlation between extrema in the evolution of the twisting moment and impending nanotube failure. Given the sensitivity of electrical properties in carbon nanotubes to torsion, and the recent demonstrations of measuring torsion-induced changes, the latter feature suggests the possibility of real-time diagnostics to detect critical mechanical events. [ABSTRACT FROM AUTHOR]

The ability to manipulate the twisting topology of van der Waals structures offers a new degree of freedom through which to tailor their electrical and optical properties. The twist angle strongly affects the electronic states, excitons and phonons of the twisted structures through interlayer coupling, giving rise to exotic optical, electric and spintronic behaviours 1-5 . In twisted bilayer graphene, at certain twist angles, long-range periodicity associated with moiré patterns introduces flat electronic bands and highly localized electronic states, resulting in Mott insulating behaviour and superconductivity 3,4 . Theoretical studies suggest that these twist-induced phenomena are common to layered materials such as transition-metal dichalcogenides and black phosphorus 6,7 . Twisted van der Waals structures are usually created using a transfer-stacking method, but this method cannot be used for materials with relatively strong interlayer binding. Facile bottom-up growth methods could provide an alternative means to create twisted van der Waals structures. Here we demonstrate that the Eshelby twist, which is associated with a screw dislocation (a chiral topological defect), can drive the formation of such structures on scales ranging from the nanoscale to the mesoscale. In the synthesis, axial screw dislocations are first introduced into nanowires growing along the stacking direction, yielding van der Waals nanostructures with continuous twisting in which the total twist rates are defined by the radii of the nanowires. Further radial growth of those twisted nanowires that are attached to the substrate leads to an increase in elastic energy, as the total twist rate is fixed by the substrate. The stored elastic energy can be reduced by accommodating the fixed twist rate in a series of discrete jumps. This yields mesoscale twisting structures consisting of a helical assembly of nanoplates demarcated by atomically sharp interfaces with a range of twist angles. We further show that the twisting topology can be tailored by controlling the radial size of the structure.

The mechanical performance of miniaturized Si components depends on both microstructure and geometry. Nanoscale polysilicon beams with both fixed-fixed and portal-frame-type beam geometries have been experimentally deformed and modelled using FEA. It is shown here that the combination of both geometry of a beam support and nanoscale dimensions results in a significantly different mechanical response of beams to that predicted using bulk properties. In a fixed-fixed configuration, beams exhibit limited elasticity followed by rapid failure occurring at the beam mid-point. However, by allowing the beam to have a less rigid connection to the substrate as in the case for a portal-frame geometry, a beam of similar dimensions can undergo significantly more deformation (∼five times more), without failure. The increased flexibility of the support connection results in lower stresses particularly at critical positions along the beam and a lower maximum tensile stress of ∼35-40% less than for a fixed-fixed beam geometry. We also show that for cyclic deformation, a build-up in residual beam deformation is related to changes in microstructure, which can relax back to a less deformed state without continuously applied high deformation levels. [ABSTRACT FROM AUTHOR]

Employing the ab initio total energy method based on the density functional theory with the generalized gradient approximation, we have systematically investigated the theoretical mechanical properties of copper (Cu). The theoretical tensile strengths are calculated to be 25.3 GPa, 5.9 GPa, and 37.6 GPa for the fcc Cu single crystal in the [001], [110], and [111] directions, respectively. Among the three directions, the [110] direction is the weakest one due to the occurrence of structure transition at the lower strain and the weakest interaction of atoms between the (110) planes, while the [111] direction is the strongest direction because of the strongest interaction of atoms between the (111) planes. In terms of the elastic constants of Cu single crystal, we also estimate some mechanical quantities of polycrystalline Cu, including bulk modulus B, shear modulus G, Young's modulus Ep, and Poisson's ratio v. [ABSTRACT FROM AUTHOR]

InGaN/GaN epilayers, which are grown on sapphire substrates by the metal-organic chemical-vapour deposition (MOCVD) method, are formed into nanorod arrays using inductively coupled plasma etching via self-assembled Ni nanomasks. The formation of nanorod arrays eliminates the tilt of the InGaN (0002) crystallographic plane with respect to its GaN bulk layer. Photoluminescence results show an apparent S-shaped dependence on temperature. The light extraction efficiency and intensity of photoluminescence emission at low temperature of less than 30 K for the nanorod arrays are enhanced by the large surface area, which increases the quenching effect because of the high density of surface states for the temperature above 30 K. Additionally, a red-shift for the InGaN/GaN nanorod arrays is observed due to the strain relaxation, which is confirmed by reciprocal space mapping measurements. [ABSTRACT FROM AUTHOR]

We demonstrate the strong influence of strain on the morphology and In content of InGaN insertions in GaN nanowires, in agreement with theoretical predictions which establish that InGaN island nucleation on GaN nanowires may be energetically favorable, depending on In content and nanowire diameter. EDX analyses reveal In inhomogeneities between the successive dots but also along the growth direction within each dot, which is attributed to compositional pulling. Nanometer-resolved cathodoluminescence on single nanowires allowed us to probe the luminescence of single dots, revealing enhanced luminescence from the high In content top part with respect to the lower In content dot base. [ABSTRACT FROM AUTHOR]

The calculated effects of interstitial hydrogen on the elastic properties of α-iron from our earlier work are used to describe the H interactions with homogeneous strain fields using ab initio methods. In particular, we calculate theHsolublility in Fe subject to hydrostatic, uniaxial and shear strain. For comparison, these interactions are parametrized successfully using a simple model with parameters entirely derived from ab initio methods. The results are used to predict the solubility ofHin spatially varying elastic strain fields, representative of realistic dislocations outside their core. We find a strong directional dependence of the H-dislocation interaction, leading to strong attraction of H by the axial strain components of edge dislocations and by screw dislocations oriented along the critical (1 1 1) slip direction. We further find a H-concentration enhancement around dislocation cores, consistent with experimental observations. [ABSTRACT FROM AUTHOR]

The study of critical phenomena and universal power laws has been one of the central advances in statistical mechanicsduring the second half of the past century, explaining traditional thermodynamic critical points, avalanche behaviour near depinning transitions and a wide variety of other phenomena. Scaling, universality and the renormalization group claim to predict all behaviour at long length and timescales asymptotically close to critical points. In most cases, the comparison between theory and experiments has been limited to the evaluation of the critical exponents of the power-law distributions predicted at criticality. An excellent area for investigating scaling phenomena is provided by systems exhibiting crackling noise, such as the Barkhausen effect in ferromagnetic materials. Here we go beyond power-law scaling and focus on the average functional form of the noise emitted by avalanches-the average temporal avalanche shape. By analysing thin permalloy films and improving the data analysis methods, our experiments become quantitatively consistent with our calculation for the multivariable scaling function in the presence of a demagnetizing field and finite field-ramp rate. [ABSTRACT FROM AUTHOR]

An overview of recent work on the interaction of elastic waves with dislocations is given. The perspective is provided by the wish to develop nonintrusive tools to probe plastic behavior in materials. For simplicity, ideas and methods are first worked out in two dimensions, and the results in three dimensions are then described. These results explain a number of recent, hitherto unexplained, experimental findings. The latter include the frequency dependence of ultrasound attenuation in copper, the visualization of the scattering of surface elastic waves by isolated dislocations in LiNbO3, and the ratio of longitudinal to transverse wave attenuation in a number of materials. Specific results reviewed include the scattering amplitude for the scattering of an elastic wave by a screw, as well as an edge, dislocation in two dimensions, the scattering amplitudes for an elastic wave by a pinned dislocation segment in an infinite elastic medium, and the wave scattering by a sub-surface dislocation in a semi-infinite medium. Also, using a multiple scattering formalism, expressions are given for the attenuation coefficient and the effective speed for coherent wave propagation in the cases of anti-plane waves propagating in a medium filled with many, randomly placed screw dislocations; in-plane waves in a medium similarly filled with randomly placed edge dislocations with randomly oriented Burgers vectors; elastic waves in a three-dimensional medium filled with randomly placed and oriented dislocation line segments, also with randomly oriented Burgers vectors; and elastic waves in a model three-dimensional polycrystal, with only low angle grain boundaries modeled as arrays of dislocation line segments. [ABSTRACT FROM AUTHOR]

This paper employs a first-principles total-energy method to investigate the theoretical tensile strengths of bcc and fcc Fe systemically. It indicates that the theoretical tensile strengths are shown to be 12.4, 32.7, 27.5 GPa for bcc Fe, and 48.1, 34.6, 51.2 GPa for fcc Fe in the [001], [110] and [111] directions, respectively. For bcc Fe, the [001] direction is shown to be the weakest direction due to the occurrence of a phase transition from ferromagnetic bcc Fe to high spin ferromagnetic fcc Fe. For fcc Fe, the [110] direction is the weakest direction due to the formation of an instable saddle-point 'bct structure' in the tensile process. Furthermore, it demonstrates that a magnetic instability will occur under a tensile strain of 14%, characterized by the transition of ferromagnetic bcc Fe to paramagnetic fcc Fe. The results provide a good reference to understand the intrinsic mechanical properties of Fe as a potential structural material in the nuclear fusion Tokamak. [ABSTRACT FROM AUTHOR]

During the past decade, computer simulations based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei have developed an increasingly important impact on solid-state physics and chemistry and on materials science—promoting not only a deeper understanding, but also the possibility to contribute significantly to materials design for future technologies. This development is based on two important columns: (i) The improved description of electronic many-body effects within density-functional theory (DFT) and the upcoming post-DFT methods. (ii) The implementation of the new functionals and many-body techniques within highly efficient, stable, and versatile computer codes, which allow to exploit the potential of modern computer architectures. In this review, I discuss the implementation of various DFT functionals [local-density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, hybrid functional mixing DFT, and exact (Hartree-Fock) exchange] and post-DFT approaches [DFT + U for strong electronic correlations in narrow bands, many-body perturbation theory (GW) for quasiparticle spectra, dynamical correlation effects via the adiabatic-connection fluctuation-dissipation theorem (AC-FDT)] in the Vienna ab initio simulation package VASP. VASP is a plane-wave all-electron code using the projector-augmented wave method to describe the electron-core interaction. The code uses fast iterative techniques for the diagonalization of the DFT Hamiltonian and allows to perform total-energy calculations and structural optimizations for systems with thousands of atoms and ab initio molecular dynamics simulations for ensembles with a few hundred atoms extending over several tens of ps. Applications in many different areas (structure and phase stability, mechanical and dynamical properties, liquids, glasses and quasicrystals, magnetism and magnetic nanostructures, semiconductors and insulators, surfaces, interfaces and thin films, chemical reactions, and catalysis) are reviewed. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [ABSTRACT FROM AUTHOR]

The III nitrides have been intensely studied in recent years because of their huge potential for everything from high-efficiency solid-state lighting and photovoltaics to high-power and temperature electronics. In particular, the InGaN ternary alloy is of interest for solid-state lighting and photovoltaics because of the ability to tune the direct bandgap of this material from the near-ultraviolet to the near-infrared region. In an effort to synthesize InGaN nitride, researchers have tried many growth techniques. Nonetheless, there remains considerable difficulty in making high-quality InGaN films and/or freestanding nanowires with tunability across the entire range of compositions. Here we report for the first time the growth of single-crystalline InxGa1−xN nanowires across the entire compositional range from x=0 to 1; the nanowires were synthesized by low-temperature halide chemical vapour deposition and were shown to have tunable emission from the near-ultraviolet to the near-infrared region. We propose that the exceptional composition tunability is due to the low process temperature and the ability of the nanowire morphology to accommodate strain-relaxed growth, which suppresses the tendency toward phase separation that plagues the thin-film community. [ABSTRACT FROM AUTHOR]

The deformation microstructure evolution of single crystal copper wires produced by OCC method has been studied with the help of TEM, EBSD and OM. The results show that there are a small number of dendrites and twins in the undeformed single crystal copper wires. However, it is difficult to observe these dendrites in deformed single crystal copper wires. The structure evolution of deformed single crystal copper wires during drawing process can be divided into three stages. When the true strain is lower than 0.94, macroscopic subdivision of grains is not evident, and the microscopic evolution of deformed structure is that the cells are formed and elongated in drawn direction. When the true strain is between 0.94 and 1.96, macroscopic subdivision of grains takes place, and the number of microbands located on {111} and cell blocks is much more than that with the true strain lower than 0.94. When the true strain is larger than 1.96, the macroscopic subdivision of grains becomes more evident than that with the true strain between 0.94 and 1.96, and S-bands structure and lamellar boundaries will be formed. From EBSD analysis, it is found that part of 〈100〉 texture resulting from solidifying is transformed into 〈111〉 and 〈112〉 due to shear deformation, but 〈100〉 texture component is still kept in majority. When the true strain is 0.94, the misorientation angle of dislocation boundaries resulting from deformation is lower than 14°. However, when the true strain arrives at 1.96, the misorientation angle of some boundaries will be greater than 50°, and the peak of misorientation angle distribution produced by texture evolution is located in the range between 25° and 30°. [ABSTRACT FROM AUTHOR]

The elastic T-stress and stress intensity factor K for quarter-elliptical corner cracks have been investigated in elastic plates by detailed three-dimensional finite-element calculations. The distributions of normalized K and T-stress have been obtained along the crack front with aspect ratios ( a/ c) of 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1.0, and far-field tension and the effect of Poisson's ratio have also been considered. The normalized K increases and the normalized T-stress decreases with the increase of Poisson's ratio v. For v= 0.3, the normalized K gradually increases in the range of crack-face angle φ≥ 22.5° and decreases in the range of φ≤ 22.5° with the increase of a/ c. The normalized T-stress increases in the beginning and then decreases with increasing φ except for a/ c= 0.2 and a/ c= 0.3. By fitting the numerical results with the least squares method, empirical formulae have been given for the convenience of engineering applications. Combining with the corresponding out-of-plane constraint factor Tz, the three-parameter K- T- Tz approach has been provided, which can accurately describe the stress field around the crack front. [ABSTRACT FROM AUTHOR]

High-strength steels are used to increase the load carrying capacity of components. However, to guarantee a safe design, it is also necessary to combine high strength with adequate fracture toughness. In this paper, fracture toughness of three high-strength steels with yield strengths ranging from 460 to 890 MPa has been studied at very low temperatures. Taking into account experimental evidence, a new mechanism of cleavage at very low temperatures is proposed. This mechanism considers the possibility of reaching the ideal strength (the stress at which the lattice of a single crystal losses its stability) in the immediate vicinity of the fatigue crack tip. Moreover, a computational model able to calculate the external load needed to produce a catastrophic failure of these steels has been developed. [ABSTRACT FROM AUTHOR]

The torsional buckling of an individual multi-walled carbon nanotube under two different loading conditions is studied in this article. The multiple shell model is adopted and the effects of van der Waals forces between adjacent nanotubes are taken into account. An examination with an individual double-walled carbon nanotube shows that the effect of the change of interlayer spacing on the torsional buckling force can be neglected if only the innermost radius is larger than a certain value. Under this condition, single buckling equations are derived and explicit formulas for the critical torsional loads in terms of the buckling modes are obtained. It is found that the critical torsional load of a multi-walled carbon nanotube with torque exerted on the outermost tube is higher than that of the same multi-walled carbon nanotubes under the torques being proportionally applied to each individual layer of the multi-walled carbon nanotubes. For thin multi-walled carbon nanotubes with large radii, the critical torque linearly scales with its thickness, but the critical shear force (per unit length) of the multi-walled carbon nanotubes uniformly twisted along the cross-section does not increase as its layer number (thickness) increases, which is due to the interlayer slips between adjacent nanotubes. [ABSTRACT FROM AUTHOR]

Carbon nanotubes can be distinctly metallic or semiconducting depending on their diameter and chirality. Here we show that continuously varying the chirality by mechanical torsion can induce conductance oscillations, which can be attributed to metal–semiconductor periodic transitions. The phenomenon is observed in multiwalled carbon nanotubes, where both the torque and the current are shown to be carried predominantly by the outermost wall. The oscillation period with torsion is consistent with the theoretical shifting of the corners of the first Brillouin zone of graphene across different sub-bands allowed in the nanotube. Beyond a critical torsion, the conductance irreversibly drops due to torsional failure, allowing us to determine the torsional strength of carbon nanotubes. Carbon nanotubes could be ideal torsional springs for nanoscopic pendulums, because electromechanical detection of motion could replace the microscopic detection techniques used at present. Our experiments indicate that carbon nanotubes could be used as electronic sensors of torsional motion in nanoelectromechanical systems. [ABSTRACT FROM AUTHOR]

In nanoscale contact experiments, it is generally believed that the shear stress at the onset of plasticity can approach the theoretical shear strength of an ideal, defect-free lattice, a trend also observed in idealized molecular dynamics simulations. Here we report direct evidence that plasticity in a dislocation-free volume of polycrystalline aluminium can begin at very small forces, remarkably, even before the first sustained rise in repulsive force. However, the shear stresses associated with these very small forces do approach the theoretical shear strength of aluminium (∼2.2 GPa). Our observations entail correlating quantitative load–displacement measurements with individual video frames acquired during in situ nanoindentation experiments in a transmission electron microscope. We also report direct evidence that a submicrometre grain of aluminium plastically deformed by nanoindentation to a dislocation density of ∼1014 m−2 is also capable of supporting shear stresses close to the theoretical shear strength. This result is contrary to earlier assumptions that a dislocation-free volume is necessary to achieve shear stresses near the theoretical shear strength of the material. Moreover, our results in entirety are at odds with the prevalent notion that the first obvious displacement excursion in a nanoindentation test is indicative of the onset of plastic deformation. [ABSTRACT FROM AUTHOR]

Ultra-small-angle X-ray scattering data were obtained from deformed single-crystal aluminium samples. These data are consistent with recent theoretical predictions of scattering from dislocation walls, allowing quantitative microstructural parameters to be extracted. [ABSTRACT FROM AUTHOR]

The polygonization of 200 nm rutile crystals during dry ball-milling at 10 g was monitored in detail by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD). The TEM results showed how to modify the Williamson–Hall method for a successful evaluation of crystal size and microstrain from XRD profiles. Macrostrain development was determined from the minute shift of the most intense reflection. In addition, changes in pycnometrical density were monitored. Accordingly, the primary crystal is disintegrated during milling into a mosaic of 12–35 nm pieces where the grain boundaries induce up to 1.2% microstrain in a lower layer of 6 nm thickness. Macrostrain in the interior of the crystals rises to 0.03%. The pycnometrical density, reflecting the packing density of atoms in the grain boundary, decreases steadily by 1.1%. The results bear relevance to our understanding of plastic flow and the mechanism of phase transitions of metal oxides during high-energy milling. [ABSTRACT FROM AUTHOR]

The growth of III–V semiconductors on silicon would allow the integration of their superior (opto-)electronic properties with silicon technology. But fundamental issues such as lattice and thermal expansion mismatch and the formation of antiphase domains have prevented the epitaxial integration of III–V with group IV semiconductors. Here we demonstrate the principle of epitaxial growth of III–V nanowires on a group IV substrate. We have grown InP nanowires on germanium substrates by a vapour–liquid–solid method. Although the crystal lattice mismatch is large (3.7%), the as-grown wires are monocrystalline and virtually free of dislocations. X-ray diffraction unambiguously demonstrates the heteroepitaxial growth of the nanowires. In addition, we show that a low-resistance electrical contact can be obtained between the wires and the substrate. [ABSTRACT FROM AUTHOR]

We examine a simple one-dimensional (1D) model of dislocation activity, including a stress-activated source and mutually interacting dislocations. We demonstrate, through numerical and analytical steps, that the dislocations emitted from a 1D stress-activated source evolve towards a distribution which is self-similar in time, and we derive the power-law forms and distribution function. We show that the asymptotic distribution is a step function, and the dislocation front moves out linearly in time. The spacing between dislocations in the asymptotic distribution is uniform and increases logarithmically in time. The number of dislocations increases as t/ln (t), and the strain increases as t²/ln (t). [ABSTRACT FROM AUTHOR]

The microstructure origin of the elastic-plastic response of a Cu substrate during nanoindentation is studied using molecular dynamics simulation. The elastic response is found to deviate from the Hertzian solution observed experimentally. The departure can be traced to the small tip radius used in the simulation. Further penetration sees the development of an inhomogeneous microstructure. Even at the same strain rate, different parts of the contact surface deform via different mechanisms: some elastically, some via the dislocation bow-out and some via the nucleation and growth of Shockley partials that sometimes interact to form stair-rod locks. The resultant effect produces the observed quasi-elastic behaviour on the load-displacement curve, characterized by interspersed minor yields. The present computer simulation shows in some detail the corresponding dislocation structure development. The stair-rod lock formation is found to provide a more satisfactory explanation to the experimentally observed time-delayed occurrence of pop-in below the spontaneous pop-in load. [ABSTRACT FROM AUTHOR]

Quantitative measurement and analysis of structural parameters has shown for a variety of metals and processes that the microstructural evolution follows a universal path of grain subdivision down to the nanoscale. This behaviour has allowed an analysis of the formation and evolution of graded nanoscale structures produced by sliding. Transmission electron microscopy studies and scaling analyses of such structures show the dominating role of dislocations in the development deformation microstructures on multiple length scales. The crucial role of dislocations has been documented by high-resolution electron microscopy analysis, which has revealed the presence of a large number of glide dislocations in layers between geometrically necessary boundaries with individual spacing as fine as 5 nm. [ABSTRACT FROM AUTHOR]

In this study, the evolution of the flow stress for grain sizes ranging from about 11 to 1 µm under shear deformation was examined using two-dimensional discrete dislocation dynamics. The grain boundaries were assumed to be both the only sources for nucleation of the dislocations and also the only obstacles to the dislocation motion. The analyses were confined to a single-slip system within each grain, with various orientations with respect to the slip system of neighbouring grains. The simulations were carried out for two sets of system sizes. In the first set of simulations the grain morphology was kept constant and the simulation unit cell size varied from 25 µm × 25 µm to 2.5 µm × 2.5 µm. In the second set of simulations the simulation unit-cell size was kept at 25 µm × 25 µm and the grain size was varied. For the grain-size ranges considered, an inverse relationship between the grain size and 0.2% offset flow stress in the form of the Hall-Petch relationship with a d -1/2 dependence was observed, although there is some uncertainty in the exponent. The evolution of flow stress follows a narrow band when expressed as a function of dislocation density divided by the dislocation source density and hence suggests a scaling with the grain size, as seen in an earlier study. [ABSTRACT FROM AUTHOR]

A tensile test in ferromagnetic iron for loading in [001] and [111] directions is simulated by ab initio electronic structure calculations using all-electron full-potential linearized augmented-plane-wave method within the generalized gradient approximation. The theoretical tensile strengths and Young's moduli of ferromagnetic iron are determined and compared with those of other materials. The magnetic and elastic behaviours of iron under uniaxial tensile loading are discussed in detail and compared with the results for isotropic tension (i.e. for negative hydrostatic pressure). Marked anisotropy of theoretical tensile strength in [001] and [111] direction is explained in terms of higher-symmetry structures present or absent along the deformation paths. [ABSTRACT FROM AUTHOR]

The microstructures of cryogenically ball-milled Cu were investigated by high-resolution electron microscopy. It was found that the grain-size reduction is a dislocation-controlled continuous process which consists of the formation of small-angle grain boundaries (GBs), a gradual increase in misorientations as a result of accumulation of more dislocations and, finally, the formation of large-angle GBs. The GBs were generally curved, wavy or faceted, and heavily strained, which are typical characteristics of nanostructured materials. In addition, extrinsic dislocations were found in many GBs, indicating that most are in a high-energy non-equilibrium configuration, which is consistent with observations in equal-channel angular pressing processed Cu, Ni, and Al-Mg, repetitive corrugation and straightening processed Cu and room-temperature ball-milled Cu. These results support a still-disputed concept that GBs in nanostructured metals processed by severe plastic deformation are mostly in non-equilibrium states. [ABSTRACT FROM AUTHOR]

Studies periodic image effects in dislocation modelling. Mathematical subtlety involved in dealing with the periodic dislocation arrays; Regularization of the lattice sums.

The size-dependent mechanical response of a simple model microstructure is investigated using continuum dislocation-based, Cosserat and strain-gradient models of crystal plasticity. The governing equations and closed-form analytical solutions for plastic slip and lattice rotation are directly compared. The microstructure consists of a periodic succession of hard (elastic) and soft (elastoplastic single-crystal) layers, subjected to single glide perpendicular to the layers. In the dislocation-based approach, inhomogeneous plastic deformation and lattice rotation are shown to develop in the soft channels, either because of bowing of dislocations or owing to pile-up formation. The generalized continuum non-local models are found to be able to reproduce the plastic slip and lattice rotation distribution. In particular, a correspondence was found between the generalized-continuum results and line tension effects; the additional or higher-order balance equations introduced in the non-local models turn out to be the counterparts of the equilibrium equation for bowed dislocations. The relevance and possible physical interpretation of additional or higher-order interface conditions responsible for the inhomogeneous distribution of plastic slip and lattice rotations are discussed. [ABSTRACT FROM AUTHOR]

In this work, we reported the high ductility of an extruded AZ61 magnesium alloy tube achieved by electropulsing current-assisted tension. The elongation of the alloy reached up to about 45%, which is largely superior to the majority of AZ61 wrought Mg alloys. We found that the hardening capacity of the alloy seemed to slightly increase as the electropulsing frequency increased. Furthermore, electropulsing can arouse the serrated flow phenomenon. Here we proposed an equation describing the correlation between the average amplitude and frequency: Aa = C − 6 × 10−3f, where Aa is the average amplitude, f is the frequency, and C is the constant. In addition, introducing electropulsing current pronouncedly reduced the tendency of twinning, but the twinning fraction seemed to fail depending on the electropulsing frequency. Based on microstructure analysis, we concluded that the outstanding ductility of the studied alloy was mainly due to the combined role of the thermal effects from Joule heating, the athermal effects from electron wind, and the magnetic effects from the electropulsing current. The serrated flow phenomenon occurred along stress–strain curves after electropulsing treatment, and the underlying reasons also were uncovered. [ABSTRACT FROM AUTHOR]