Crystal Lattice Orientation Research Articles

Studying grain boundary regions in polycrystalline materials using spherical nano-indentation and orientation imaging microscopy

In this article, we report on the application of our spherical nanoindentation data analysis protocols to study the mechanical response of grain boundary regions in as-cast and 30% deformed polycrystalline Fe–3%Si steel. In particular, we demonstrate that it is possible to investigate the role of grain boundaries in the mechanical deformation of polycrystalline samples by systematically studying the changes in the indentation stress–strain curves as a function of the distance from the grain boundary. Such datasets, when combined with the local crystal lattice orientation information obtained using orientation imaging microscopy, open new avenues for characterizing the mechanical behavior of grain boundaries based on their misorientation angle, dislocation density content near the boundary, and their propensity for dislocation source/sink behavior.

Analysis of particle induced dislocation structures using three-dimensional dislocation dynamics and strain gradient plasticity

Investigations of precipitation hardening are performed in term of analysis of distributions of geometrically necessary dislocations (GND) surrounding particles. The dislocation microstructures are computed from three dimensional discrete dislocation dynamics (DDD) and strain gradient plasticity (SGP) models. DDD simulations of spherical particle embedded in a single crystal matrix undergoing single slip provide the GND structures and the associated work-hardening. A 3D periodic arrangement of particles with cubic symmetry is considered. It is found that a network of slip and kink deformation bands develops, which is strongly dependent on the crystal lattice orientation of the matrix with respect to the particle array. For some relative orientations, the strain hardening is increased by the distributions of GND which act as additional barrier against slip. Some of these features are also captured with the SGP model in contrast to conventional continuum crystal plasticity.

Probing the Chain and Crystal Lattice Orientation in Polyethylene Thin Films by Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy

The chain and the crystal unit cell orientation of linear low density polyethylene (LLDPE) were measured with near edge X-ray absorption fine structure (NEXAFS) spectroscopy. A strongly attractive substrate, silicon, and a weakly attractive substrate, mica, were used. For a 100 nm thick LLDPE film on the silicon substrate, the crystals exhibit an edge-on lamellar morphology, with the chains predominantly parallel to the substrate, and the orthorhombic unit cell ⟨a, b, c⟩ in the following approximate orientation: b and c are in the film plane with b along the crystal fibril direction and c perpendicular to the fibril direction and a perpendicular to the film plane. On the mica substrates, LLDPE films with thickness below 180 nm completely dewet the surface and form isolated droplets, while a film 366 nm thick crystallizes as spherulites with most of the chains perpendicular to the substrate before annealing and with a twisted lamellar structure after isothermal crystallization at 60 °C. The results demonst...

Fabrication of GaN epitaxial thin film on InGaZnO 4 single-crystalline buffer layer

Epitaxial (0001) films of GaN were grown on (111) YSZ substrates using single-crystalline InGaZnO 4 (sc-IGZO) lattice-matched buffer layers by molecular beam epitaxy with a NH 3 source. The epitaxial relationships are (0001) GaN//(0001) IGZO//(111) YSZ in out-of-plane and [112¯0] GaN//[112¯0] IGZO//[11¯0] YSZ in in-plane. This is different from those reported for GaN on many oxide crystals; the in-plane orientation of GaN crystal lattice is rotated by 30° with respect to those of oxide substrates except for ZnO. Although these GaN films showed relatively large tilting and twisting angles, which would be due to the reaction between GaN and IGZO, the GaN films grown on the sc-IGZO buffer layers exhibited stronger band-edge photoluminescence than GaN grown on a low-temperature GaN buffer layer.

Polycrystalline behavior analysis of pure magnesium by the homogenization method

In this study, mechanical behaviors of pure magnesium polycrystals are numerically investigated. The homogenization method, which combines the crystal and macroscopic scales, is introduced to include the effect of crystalline scale behaviors. The polycrystal plasticity model modified for pure magnesium, in which twinning is considered as asymmetric slip-like deformation, is utilized as a constitutive equation. Within this framework, numerical convergence analyses are conducted, and a representative volume element to present realistic deformation of pure magnesium is investigated. Second, polycrystalline behaviors of pure magnesium are investigated. The present approach is shown to reproduce the typical phenomena induced by crystalline scale structure in pure magnesium: nonuniform strain distribution, asymmetric crystal lattice orientation, strength differential effect, and strongly anisotropic initial and subsequent yield surfaces.

Surface Structure of LiNi0.8Co0.2O2: a New Experimental Technique Using in Situ X-ray Diffraction and Two-Dimensional Epitaxial Film Electrodes

Surface and bulk structural changes in LiNi0.8Co0.2O2 were observed during electrochemical reactions using synchrotron X-ray scattering and a restricted reaction plane of two-dimensional (2D) epitaxial-film electrodes. The bulk structural changes confirmed lithium diffusion through the (110) surface, which is perpendicular to the 2D edges of the layered structure. No (de)intercalation reaction was observed through the (003) surface in the voltage range of 3.0−5.0 V. However, intercalation proceeded below 3.0 V, which indicates unusual three-dimensional lithium diffusion in the 2D structure in the overlithiated state. Structural changes at the electrode surface were different from those in the bulk. Contact with the liquid electrolyte caused an abrupt deformation of the surface crystal lattice, followed by restructuring upon applying a cell voltage. These slow reconstruction processes were dependent on the crystal lattice orientations. The reaction mechanism of the intercalation electrodes for lithium batt...

Measurement of the local mechanical properties in polycrystalline samples using spherical nanoindentation and orientation imaging microscopy

This paper describes the development of novel experimental protocols and data analysis procedures for extracting meaningful indentation stress–strain curves from spherical nanoindentation on polycrystalline samples, and correlating these measurements to the local crystal lattice orientation measured by orientation imaging microscopy at the indentation site. In particular, we demonstrate that it is possible to estimate the percentage increases in the local slip resistance in deformed samples of polycrystalline cubic metals from their fully annealed condition. These novel procedures are demonstrated on Fe–3% Si samples in the as-cast, 30% deformed and 80% deformed conditions.

Aerosol assisted chemical vapour deposition of ZnO films on glass with noble metal and p-type dopants; use of dopants to influence preferred orientation

The use of aerosol assisted chemical vapour deposition (AACVD) for the formation of zinc oxide and doped ZnO films with control of preferred orientation on glass is reported. Undoped and doped ZnO host matrix films were highly transparent with visible transmission >85%. The Cu (CuO/Cu 2O) doped ZnO thin films were highly coloured and opaque. Undoped and noble metal doped ZnO films had mainly spherical morphology. Aluminium oxide/ZnO composite films had a range of morphologies from spherical to cubic. The XRD patterns for both doped and undoped ZnO films grown at substrate temperatures less than 500 °C showed strong preferred (0 0 2) crystal lattice orientation. The undoped ZnO film at 500 °C exhibited a random crystal orientation pattern for hexagonal ZnO. The introduction of small amounts of Al 2O 3 within the ZnO to form a composite significantly altered the preferred crystal orientation to (1 0 1).

A model of ultrafine microstructure evolution in materials deformed by high-pressure torsion

The proposed crystal plasticity model outlines a possible mechanism of a material response under severe plastic deformation as observed in high-pressure torsion experiments. A simplified version of the model based on an assumption of uniform deformation of plane-strain double slip reveals rotations of slip systems caused by the imposed shear strain. An axial compression and a shear stress of twist govern this process. The accompanied continuous reconstruction of the deformation substructure is probably one of the main reasons for the observed strengthening. Local variations in the crystal lattice orientation are responsible for the microstructure fragmentation.

Giant faults in deformed Gum Metal

We have experimentally characterized and theoretically described plastic flow localization in Gum Metal, a special titanium alloy with high strength, low Young’s modulus, excellent cold workability and low resistance to shear in certain crystallographic planes. The electron transmission microscopy experiments demonstrate that plastic flow is localized in giant faults – macroscopic planar defects carrying very large plastic strains (thousand percent or more) – in deformed Gum Metal. Also, regions with highly inhomogeneous elastic strains and varying crystal lattice orientation are experimentally observed in the vicinity of giant faults. A theoretical model is suggested describing the generation of giant faults as a process resulting from generation and evolution of nanodisturbances (nanoscopic planar areas of local shear) in Gum Metal. It is shown that giant faults can effectively nucleate and evolve in Gum Metal, and their intersection with grain boundaries produces both elastic strain accumulation and inhomogeneities of crystal lattice orientation. This behavior of giant faults is expected to be essential for excellent cold ductility of high-strength Gum Metal.

Periodic crystal lattice rotation in microband groups in a bcc metal

Fe–3mass%Si specimens with a columnar grain structure were deformed in compression at ambient temperature. Groups of microbands were found in the vicinity of a grain boundary. These microband groups are characterized by shear and a periodic change of crystal lattice orientation within distances of 1–2 μm. Furthermore, new grains with a size of 200–300 nm were formed in the microband groups during deformation up to 36% strain. A local lattice rotation model is given that explains the experimental observations.

Garnet ferrite films with two thermostable frequencies of ferromagnetic resonance

The problem of thermal stabilization of the frequency of magnetization autooscillations in anisotropic cubic ferrite films is considered. The variable parameters are the magnetic field direction and the crystal lattice orientation relative to the film plane. It is shown that both parameters can be chosen so as to provide the thermal stability of two ferromagnetic resonance frequencies.

On the possibility of formation of domains with {110}〈001〉 orientation during cold deformation of commercial Fe-3% Si alloy

Crystallographic analysis of special misorientations of the Σ9 type with an “octahedral” orientation {111}〈112〉 has been carried out. A hypothesis about the origination of the special misorientation Σ9 as a result of secondary twinning is suggested. A “rule of selection” has been proposed, according to which the formation of secondary twins with a crystal-lattice orientation close to {110}〈001〉 is possible upon rolling. The hypothesis allows us to explain the known fact of a small deviation of Goss grains from an ideal orientation in real electrical-sheet anisotropic steel (EAS) and a number of other experimental data.

Structural Matching between the Polymeric Nucleating Agent Isotactic Poly(vinylcyclohexane) and Isotactic Polypropylene

Poly(vinylcyclohexane) (PVCH) is a patented polymeric nucleating agent for isotactic polypropylene (iPP) and induces its alpha crystal modification (αiPP). The structural interactions at the root of this activity are investigated by analyzing the transcrystallization of single crystals of αiPP on single crystals of PVCH. Electron microscopy results indicate (i) a quasi-perfect two-dimensional lattice match in the contact plane, with (100)PVCH facing (110)αiPP or (1̄10)αiPP planes, and (ii) a consistent link between the PVCH single crystals' internal structure and the orientation of αiPP overgrowth. The two polymers chain axes are parallel. The 3-fold (iPP) and 4-fold (PVCH) helices share the same chain axis repeat (≈6.5 Å), and repeat distances normal to the chain axis are ≈21.9 Å for (110)αiPP and (100)PVCH planes. In chain axis projection, the PVCH helices are tilted relative to the a and b axes of the tetragonal unit cell. The tilt generates an “asymmetric sawteeth” surface profile that differs for clock- or counterclockwise tilts. Similar surface profiles exist in the (110)αiPP or (1̄10)αiPP contact planes. Matching of the PVCH and iPP sawteeth profiles is selective and induces a single orientation of the αiPP crystal lattice in the overgrowth. The orientation of the αiPP overgrowth thus becomes also a marker of the local clock- or counterclockwise setting of PVCH helices in the growth face.

Sm–Co and Nd–Fe–B thin films with perpendicular anisotropy for high-density magnetic recording media

Sm–Co and Nd–Fe-B thin films have been prepared by sputtering. Particular efforts are concentrated on the perpendicular texture growth of the films. The Sm–Co and Nd–Fe–B thin films with perpendicular magnetic anisotropy can be prepared on Cu and W underlayer, respectively. Those underlayers play an important role to prevent oxidation and improve crystal lattice orientation. Perpendicular coercivity and remanent squareness ratio are higher than 10 kOe and almost 1.0, respectively, in both films prepared under optimum conditions.

Spectral calibration of crystal plasticity models

A new and improved spectral framework are presented to capture efficiently the predictions for the stresses, the lattice spins, and the strain hardening rates in individual crystals from the currently used crystal plasticity models as a function of the crystal lattice orientation. The proposed methodology has been successfully applied to two classes of crystal plasticity models that incorporate very different types of interactions between the individual crystals in the polycrystal. The models used in this study include: (1) a Taylor-type fully constrained model; and (2) a micromechanical finite element crystal plasticity model where each crystal is assumed to experience an average interaction with all other crystals in the polycrystal. Although the proposed method requires a one-time high computational cost in evaluating the relevant Fourier coefficients, it is expected to result in dramatic savings in computational time and effort in all subsequent computations.

Lattice Orientation Imaging of C60-Fullerene Single Crystal with Microbeam-Scanning-Type X-ray Scattering Topography Using Charged-Coupled Device Detector

The technique of X-ray scattering topography (XST) is useful for imaging imperfect single crystals that cannot be properly observed by ordinary X-ray topography and has been successfully applied to the mapping of crystal-lattice orientation. Using scanning-type XST with a finely collimated X-ray beam and a pinhole slit and applying the technique of Laue orientation imaging using a charged-coupled device (CCD) camera, gnomonic projection, and Hough transform data processing, we have taken pictures of the scattering topographs of a C60-fullerene single crystal with a CCD camera that enable the relative angular difference of the zone axis to be observed. C60-fullerene fcc single crystal samples were grown from vapour employing a double temperature gradient. The results indicate that crystal growth began at the central region and proceeded to the periphery. These observations provide valuable information about crystal growth.

A crystal plasticity based estimate for forming limit diagrams from textural inhomogeneities

An alternative method to predict forming limits for sheet metal forming is presented. Deformation instabilities are considered as the natural result of strain field fluctuations caused by textural inhomogeneities. The biaxial stretching of a sheet material configuration is analysed using the finite element method in plane stress conditions. For the description of the material behaviour at the microscopic level, a viscous crystal plasticity model is used. To accommodate the simulation of polycrystallinity, each individual integration point of the finite element mesh is assumed to be composed of a discrete number of single crystals. The single crystals are coupled according to a Taylor interaction approach. The initial texture inhomogeneity is considered as a natural microstructure imperfection, which is present in any real metal, potentially triggering deformation instabilities. The occurrence of a nonuniform deformation field is the result of the textural inhomogeneity only; no other imperfections are introduced to generate the instabilities observed. The structural instability criterion is used as an approximate localisation criterion. Proportional loading of a metal sheet with an initially random distribution of crystal lattice orientations has been analysed for different uniform strain paths. The results demonstrate the ability of the method to describe the development of strain localisation bands, from which the formability can be assessed.

Creep constitutive laws for rocks with evolving structure

Abstract Zones of localized strain are common geological features at all spatial scales, are persistent in time and have important implications for rock strength. Technical advances now allow mechanical tests to be carried out to high strain, and, thus, there is both need and the opportunity to formulate constitutive laws for rocks as their structure (i.e. fabric, texture and mineralogy) changes. This paper reviews some attempts to describe quantitatively the mechanical behaviour of calcite rocks deformed to large strains. To include the effect of evolving structure on strength, three or more interlinked laws are needed: an evolution equation describing the rate of change for each structural variable; a kinetic equation relating mechanical and thermodynamic loading and the structural variables to the rate of strain; and a kinematic equation involving a time integral of inelastic strain rate. Structure variables may be explicitly identified or implicitly determined without identification. Appropriate explicit state variables might include aspects of the dislocation microstructure or the grain size, as are common in studies of plasticity in metals. But because natural tectonic situations are more complex, a much broader class of state variables will be needed. Among these additional variables might be crystal lattice preferred orientation, progress variables for metamorphic reactions, solid-solution chemistry, porosity and pore fluid fugacity.

Ar ion milling process for fabricating CoCrPt patterned media using a self-assembled PS-PMMA diblock copolymer mask

Etching the damage of a patterned media fabricated with an artificially assisted self-assembled mask is estimated. CoCrPt thin films were etched by ion milling into aligned dots with a diameter of 40 nm. The milling condition was optimized for reducing the etching damage. As a result, the damage to the crystal lattice and crystal orientation was estimated to be slight by transmission electron microscope (TEM) and scanning electron microscope (SEM) analyses. Though strong perpendicular anisotropy was induced by the patterning process, magnetic measurements and Landau-Lifshitz-Gilbert simulation revealed that the magnetic anisotropy energy was almost unchanged throughout this etching process.