Regions Of Different Orientation Research Articles

Mitigating the Recrystallization of a Cold-Worked Cu-Al2O3 Nanocomposite via Enhanced Zener Drag by Nanocrstalline Cu-Oxide Particles.

The strength of metals and alloys at elevated temperatures typically decreases due to the recovery, recrystallization, grain growth, and growth of second-phase particles. We report here a cold-worked Cu-Al2O3 composite did not recrystallize up to a temperature of 0.83Tm of Cu. The composite was manufactured through the internal oxidation process of dilute Cu-0.15 wt.% Al alloy and was characterized by transmission electron microscopy to study the nature of oxide precipitates. As a result of internal oxidation, a small volume fraction (1%) of Al2O3 particles forms. In addition, a high density of extremely fine (2-5 nm) Cu2O particles has been observed to form epitaxially within the elongated Cu grains. These finely dispersed second-phase Cu2O particles enhance the Zener drag significantly by three orders of magnitude as compared to Al2O3 particles and retain their original size and spacing at elevated temperatures. This limits the grain boundary migration and the nucleation of defect-free regions of different orientations and inhibits the recrystallization process at elevated temperatures. In addition, due to the limited grain boundary migration, a bundle of stacking faults appears instead of annealing twins. This investigation has led to a better understanding of how to prevent the recrystallization process of heavily deformed metallic material containing oxide particles.

Defects in motion

Crystalline defects appear when the perfect order of the lattice or ideal arrangements of atoms, molecules, or ionic groups is destroyed, which is inevitable during crystal growth, thereby impacting material functionalities, either in a reinforcing or unwanted way. For functional ferroic materials, natural interfaces called domain walls form, which separate regions of different orientations of a specific order (such as magnetic, ferroelastic, or ferroelectric) in the material. These, including phase boundary, grain boundary, and/or domain boundary, can be reckon as two-dimensional defects. During functioning of ferroic materials, phase transformation and/or dynamic motion of domain walls occur with external stimuli like electric fields or stress. Therefore, domain engineering, phase boundary construction, and grain engineering has long been the most considered effective strategies to enhance the performance of ferroic materials. Meanwhile, lower-dimensional defects, including point defects, defect dipole, and line defects, are another crucial dimension to be considered to tune functionality, since their motion greatly interact with the domain wall dynamics under external stimuli. In all, defect engineering (here we refer to one- and two-dimensional defects) and its coupled motion with order parameters is an interesting topic that has attracted significant attention for functional oxides.

Modeling the processes of atom structure formation of a superconducting spin valve

The paper considers the modeling of a multilayer nanocomposite, the combination of elements of which gives rise to a spin valve effect. The relevance and importance of effects in the field of spintronics and related materials and devices are described. We study the composition and atomic structure of individual layers of a multilayer nanocomposite, as well as the composition and morphology of the interface of nanocomposite layers. We analyzed a sample with a periodic superconductor-ferromagnet structure consisting of more than 20 alternating layers of niobium and cobalt. The deposition process took place in a deep vacuum. The simulation was carried out by the molecular dynamics method using the potential of the modified immersed atom method. The formation of layers was carried out in a stationary mode. The temperature was adjusted using the Nose-Hoover thermostat. The deposition of each nanofilm ended with a relaxation stage for the necessary stabilization and restructuring of the formed nanocomposite. Three deposition temperature regimes were considered: 300 K, 500 K, and 800 K. For these modes, we analysed the atomic structure of nanofilms and transition regions (interface) formed between the layers. A study of the atomic structure of nanofilms showed that niobium is formed by crystalline regions of different orientations. A cobalt nanofilm is characterized by a structure close to amorphous. The structural features of the interface between the superconductor-ferromagnet layers largely depend on a relief of the surface onto which the deposition is made. The smallest variation in atomic composition is observed in the first niobium-cobalt contact zone, since the formation of the first nanofilm occurs on an even plane of the substrate. An analysis of the influence of the temperature regime during the formation of the nanosystem shows the dependence of the processes of formation of multilayer nanofilm formation, the interface of nanolayers, as well as the composition and morphology of heterostructures on the temperature at which a nanocomposite is manufactured. An increased temperature leads to the formation of a more rarefied structure of nanolayers and an increase in the zones of the interface of nanolayers due to the diffusion of atoms of the sprayed materials.

Atomic-Scale Study of Cation Ordering in Potassium Tungsten Bronze Nanosheets.

It has long been accepted that the formation of superlattices in hexagonal‐based potassium tungsten bronzes is attributed to K vacancies only, together with small displacements of W cations. Here, the superlattices within potassium tungsten bronze nanosheets both structurally and spectroscopically at atomic resolution using comprehensive transmission electron microscopy techniques are studied. The multidimensional chemical analyses are realized by energy‐dispersive X‐ray spectroscopy, electron energy‐loss spectroscopy, and X‐ray photoelectron spectroscopy, the atomic‐scale structures are characterized using aberration‐corrected scanning transmission electron microscopy with high‐angle annular‐dark‐field detector. The observed superstructures are mainly attributed to small amount of W vacancies within single atomic layer, which would recover to more uniform distributions of W vacancies with lower concentrations at higher temperature. The band regions of different orientation from the matrix tend to regulate the superstructures to be pinned along the same direction, forming domains of highly ordered structures. The characterization of cation ordering and recovery processes of nanostructures from chemical and structural point of view at atomic resolution enables rational design of optoelectronic devices with controlled physical properties.

Mechanisms of anisotropic friction in nanotwinned Cu revealed by atomistic simulations

The nature of nanocrystalline materials determines that their deformation at the grain level relies on the orientation of individual grains. In this work, we investigate the anisotropic response of nanotwinned Cu to frictional contacts during nanoscratching by means of molecular dynamics simulations. Nanotwinned Cu samples containing embedded twin boundaries parallel, inclined and perpendicular to scratching surfaces are adopted to address the effects of crystallographic orientation and inclination angle of aligned twin boundaries cutting the scratching surface. The transition in deformation mechanisms, the evolution of friction coefficients and the friction-induced microstructural changes are analyzed in detail and are related to the loading conditions and the twinned microstructures of the materials. Furthermore, the effect of twin spacing on the frictional behavior of Cu samples is studied. Our simulation results show that the crystallographic orientation strongly influences the frictional response in different ways for samples with different twin spacing, because the dominant deformation mode varies upon scratching regions of different orientations. A critical inclination angle of 26.6° gives the lowest yield strength and the highest friction coefficient, at which the plasticity is dominated by twin boundary migration and detwinning. It is demonstrated that the anisotropic frictional response of nanotwinned Cu originates from the heterogeneous localized deformation, which is strongly influenced by crystallographic orientation, twin boundary orientation and loading condition.

Very low angle annular dark field imaging in the scanning transmission electron microscope: A versatile tool for micro- and nano-characterization

The author shows how very low angle annular dark field (VLAADF) imaging in the scanning transmission electron microscope (STEM) is used to delineate regions of different orientation or structural phase in the same material. First, the author describes the settings used to obtain images in which only very low indexed beams contribute to the image formation. These settings can be implemented easily on any commercial STEM/TEM by choosing the appropriate camera length and objective aperture. The author then demonstrates how VLAADF was used to perform detailed grain size studies in Cu interconnects, and show how the technique can be used to outline the amorphous region within the crystalline matrix in a phase change memory device. The author finishes by addressing and discussing the dependence of the “orientation contrast” on the main imaging parameters.

Disorientations in dislocation structures

During plastic deformation, dislocation structures develop with dislocation boundaries separating regions of different orientations. The intrinsic origins and evolution of these disorientations and the effect of the related excess dislocations on work-hardening are discussed. For statistical trapping of dislocations, the developing average disorientation angle follows a square root dependence on strain and the flow stress is governed by the classical Palm Voce behavior of stage III. A constant deterministic contribution to the accumulation of excess dislocations leads to a linear increase of average disorientation angle and flow stress characteristic for the work-hardening in stage IV. The modelled evolution of the average disorientation angles and the value of the constant work-hardening rate at large strains are in agreement with experimental observations.

Morphological features and mechanical behavior of one- and two-phase polymeric materials simulated by molecular dynamics

Single phase amorphous polymeric materials and two-phase polymer liquid crystals (PLCs) have been created on the computer and their behavior simulated using molecular dynamics. An external force was applied on the material and its response computed along time. The influence of several parameters was investigated, such as the concentration of the rigid LC second phase and the existence of regions of different orientation across the thickness of the material.A simplified 3-region model, such as that used to model the skin-core structure resulting from injection molding, was used. The influence of the relative size of each region with different properties was determined. Thicker skin regions increase the rigidity of the material, due to their higher orientation in the direction of force application. The concentration of the reinforcing LC second phase has a similar effect, also resulting in a more brittle behavior. The simulations have provided a better understanding of these phenomena.A method for calculating the true stress during simulation of computer-generated materials (CGMs) is proposed. The true stress behavior was found to differ qualitatively from the engineering stress when the structure of the material allows for considerable changes in cross-sectional area at large-scale deformation.

The evolution of disorientations for several types of boundaries

During plastic deformation dislocation boundaries appear separating regions of different orientation. A model for the occurrence of disorientations across these boundaries is proposed and discussed with emphasis on several types of boundaries. For incidental dislocation boundaries a statistical origin of disorientations is considered, additional deterministic contributions arising from geometrical reasons are taken into account for geometrically necessary boundaries. The resulting diversity in the modelled boundary behaviour explains the experimentally observed differences in the dependence of the average disorientation angle on plastic deformation.

Disorientations in dislocation boundaries: formation and spatial correlation

During plastic deformation dislocation boundaries appear separating regions of different orientation. The occurrence of disorientations across these boundaries is discussed with emphasis on several types of boundaries. For incidental dislocation boundaries a statistical origin of disorientations is considered, additional deterministic contributions arising from geometrical reasons are taken into account for geometrically necessary boundaries. The resulting diversity in the modelled boundary behaviour explains the experimentally observed differences in the dependence of the average disorientation angle on plastic strain. Spatial correlations between disorientation angles of neighbouring boundaries are investigated.

Correlations between disorientations in neighbouring dislocation boundaries

During plastic deformation, dislocation structures develop with regions of different orientations separated by dislocation boundaries. Experimental investigations by means of electron back-scattering diffraction or rocking curves from X-ray diffraction give some evidence for the existence of correlations between the disorientations in neighbouring boundaries. Based on a straightforward statistical model an occurrence of anti-correlations between disorientation angles is predicted which can be traced to the limited free path of mobile dislocations carrying the plastic deformation.

Deformation banding and copper-type rolling textures

It is shown by a crystallographic etching technique applicable to copper that deformation banding is an important deformation mode in f.c.c. metals and alloys. In a cold rolled coarse grain copper, deformation banding forms in a three dimensional manner dividing the grain on average into over 600 regions of different orientations. The influence of this important, but long ignored deformation mode, is studied by incorporating it into the Taylor model. The predicted textures from the new model are better than those from other existing models in mainly two respects. Firstly, the DB model predicts the co-existence of the three major f.c.c. rolling texture components, namely {123}〈634〉 or S component, >112>〈111〉 or C and >110>〈112〉 or {f}B{/f}. The existing models are deficient in that they predict either C and S or B, but not their co-existence. The second point is that textures predicted by the existing models are always too sharp compared to the experimental textures. The DB model predicts texture peaks with larger spread and hence more realistic texture sharpness. Another feature of the model is that only two independent slip systems, instead of five, are required to accomodate the imposed shape change, which agrees with experimental observation.

Bulk and surface structure of epitaxial YBa 2Cu 3O 7−δ thin films grown on Mg 2TiO 4 (001) substrate layers Part I: Bulk structure investigated by TEM

Epitaxial YBa 2Cu 3O 7−δ thin films have been grown by pulsed laser deposition onto single crystal Mg 2TiO 4 (001) substrate layers. The Mg 2TiO 4 layers have been prepared by a topotaxial solid-state reaction between MgO (001) single crystal surfaces and a TiO 2 vapour. The latter was obtained by electron beam evaporation of anatase powder targets. The Mg 2TiO 4 layers have been shown to be genuinely single crystalline, well-oriented and free of defects except cationic antiphase boundaries forming a regular network. The bulk structure of the YBa 2Cu 3O 7−δ films grown under various growth conditions has been investigated by transmission electron microscopy and selected area electron diffraction. Films grown under non-optimum conditions displayed a non-homogeneous bulk structure characterized by film regions of different orientation and by a high density of droplets. Under optimum growth conditions the YBa 2Cu 3O 7−δ films are epitaxially oriented throughout, with the [001] axis perpendicular to the film plane and with good in-plane alignment. The number of droplets is low in these films, which are superconductive at T c(onset) ≈ 88.5 K. These films also showed thickness variations, which can be attributed to the spiral growth hills detected by STM and described in part II.

THE EFFECTS OF PLASTIC DEFORMATION ON MAGNETIC PROPERTIES OF POLYCRYSTALLINE METALS

Most of the laws of the ferromagnetic behavior of metals and alloys were derived from studies of single crystals. In order to apply these laws to polycrystalline metals the effect of the crystallographic composition of the polycrystalline body has to be taken into account. For this purpose the concept of a "closed flux shell" as a fundamental quality of polycrystalline metal is put forward on the basis of the following trend of thought:One of the most important factors determining the size and arrangement of ferromagnetic domains in a single crystal is the tendency to close the magnetic flux throughout the specimen by a suitable arrangement of these domains. The size of the crystal defines the limits within which the flux can be closed. In a polycrystalline metal, a "crystal" is defined as a region of one crystallographic orientation separated by crystal boundaries from neighboring regions of different orientation. The corresponding unit, as far as ferromagnetic properties are concerned, will be a closed flux shell which may contain many domains but is always a region of closed magnetic flux. Neighboring regions, having different crystallographic orientation, will each also consist of a closed magnetic flux shell. Crystal boundaries are regarded as lattice discontinuities which separate regions of different orientation and at the same time separate different closed flux shells.The consequences of this model and experimental evidence supporting it are discussed. An explanation of the effect of plastic deformation on magnetic properties is suggested by considering the changes in the size of the closed flux shell which will occur during plastic deformation.

An X–Ray Criterion for Distinguishing between Lattice Curvature and Fragmentation

ASTERISM in Laue photographs and the arc–shaped smears in monochromatic X–ray photographs show that, in plastically deformed crystals, regions of different orientation are present. This may be due either to local curvatures of the lattice1,2, or to its disintegration into a mosaic of small fragments the orientations of which are independent of those of their neighbours3,4. It has been considered that X–ray methods could only reveal the existence of regions of different orientation, but could not give information as to how these regions were joined together, and thus could not tell which of the two alternatives, curvature or fragmentation, was present.