Deformation textures produced in diamond anvil experiments, analysed in radial diffraction geometry

Diamond anvil cells may not only impose pressure upon a sample but also a compressive stress that produces elastic and plastic deformation of polycrystalline samples. The plastic deformation may result in texture development if the material deforms by slip or mechanical twinning, or if grains have a non-equiaxed shape. In radial diffraction geometry, texture is revealed by variation of intensity along Debye rings relative to the compression direction. Diffraction images (obtained by CCD or image plate) can be used to extract quantitative texture information. Currently the most elegant and powerful method is a modified Rietveld technique as implemented in the software package MAUD. From texture data one can evaluate the homogeneity of strain in a diamond anvil cell, the strain magnitude and deformation mechanisms, the latter by comparing observed texture patterns with results from polycrystal plasticity simulations. Some examples such as olivine, magnesiowuestite, MgSiO(3) perovskite and ε-iron are discussed.     

Selective role of bainitic lath boundary in influencing slip systems and consequent deformation mechanisms and delamination in high-strength low-alloy steel

We elucidate here the deformation behaviour and delamination phenomenon in a high-strength low-alloy bainitic steel, in terms of microstructure, texture and stress evolution during deformation via in situ electron back-scattered diffraction and electron microscopy. Furthermore, the selective role of bainitic lath boundary on slip systems was studied in terms of dislocation pile-up and grain boundary energy models. During tensile deformation, the texture evolution was concentrated at {1 1 0}<1 1 1> and the laths were turn parallel to loading direction. The determining role of lath on the deformation behaviour is governed by length/thickness (l/t) ratio. When l/t > 28, the strain accommodates along the bainite lath rather than along the normal direction. The delamination crack initiated normal to (0 1 1) plane, and become inclined to (0 1 1) plane with continued strain along (0 1 1) plane and lath plane. This indicated that the delamination is not brittle process but plastic process. The lack of dimples at the delaminated surface is because of lack of strain normal to the direction of lath. The delaminated (0 1 1) planes were associated with cleavage along the (1 0 0) plane.     

Anisotropic interface magnetoresistance in Pt/Co/Pt sandwiches

We report on an effect of reduced dimensionality on the magnetotransport in cobalt layers sandwiched by platinum. In a current in-plane geometry it is found that the resistivity depends on the magnetization orientation within the plane perpendicular to the current direction. The resistivity shows a symmetry adapted cos(2) dependence on the angle to the surface normal, with the maximum along the surface normal. The Co thickness dependence of the effect in Pt/Co/Pt sandwiches clearly points out that the mechanism behind this effect originates at the Co/Pt interfaces and is disparate to the texture induced geometrical size effect.     

Effect of initial orientation on the tensile properties of commercially pure titanium

Effect of crystallographic texture on uniaxial tensile deformation of commercially pure titanium was studied using in situ as well as post-mortem electron backscatter diffraction and elastoplastic self-consistent simulations. Correlation of mechanical properties and strain hardening response with deformation micromechanisms like different modes of slip and twinning was established. Tensile specimens were machined along rolling direction in the plane perpendicular to normal and transverse direction (sample A and C, respectively) as well as along transverse direction in the plane normal to rolling direction (sample B) to obtain different initial texture from cold rolled and annealed plate of commercially pure titanium. Sample B showed higher strength but lower strain hardening rate and ductility than the orientations A and C. It showed extension twinning with lateral thickening while the other samples showed coexistence of extension and contraction twinning. Schmid factor accounted for most of the observed twinning although some contraction twinning in sample A is attributed to the effect of internal stresses. A combination of in situ tensile test in a field emission gun scanning electron microscope with electron backscatter diffraction facility and elastoplastic self-consistent simulations aid in obtaining high-fidelity Voce hardening parameters for different slip and twinning systems in commercially pure titanium. The variation in tensile properties can be explained on the basis of propensity of twinning which tends to provide strain hardening at lower strain but contributes to failure at higher strain.     

Quantitative Rietveld texture analysis of CaSiO(3) perovskite deformed in a diamond anvil cell

The Rietveld method is used to extract quantitative texture information from a single synchrotron diffraction image of a CaSiO(3) perovskite sample deformed in axial compression in a diamond anvil cell. The image used for analysis was taken in radial geometry at 49?GPa and room temperature. We obtain a preferred orientation of {100} lattice planes oriented perpendicular to the compression direction and this is compatible with [Formula: see text] slip.     

体心立方金属W薄膜晶体取向的膜厚尺寸效应及其表面映射

体心立方W膜(110)织构系数T₁₁₀的变化存在非单调的厚度尺寸效应，这依赖于薄膜中晶粒形核和长大时表面能和应变能的相互作用，薄膜表面结构演变反映了两者的竞争过程.应用小波变换结合分形几何描述薄膜表面结构各向异性行为，用此法构建了薄膜织构系数T₁₁₀与表面结构各向异性的关系，表明薄膜晶体取向存在表面映射. 关键词： 金属薄膜 晶体取向 膜厚 表面形貌     

Simulation of deformation and fracture of coated material with account for propagation of a Lüders-Chernov band in the steel substrate

The paper studies deformation and fracture of boride-coated steel. The dynamic boundary-value problem is solved in the plane strain statement by the finite difference method. The geometry of the coating-substrate interface corresponds to those experimentally observed and is explicitly specified in the calculations. The mechanical response of the steel substrate is described by an elastoplastic model of isotropically hardening material with relations for slow flows. The peculiarities of plastic strain localization and fracture during the propagation of a Lüders-Chernov band in the steel substrate are investigated under tension.     

Computational mesomechanics of titanium surface-hardened by ultrasonic treatment

This paper studies plastic strain localization and stress-strain evolution in commercial titanium specimens with an ultrasonically treated surface. A dynamic plane strain boundary-value problem is numerically solved by the finite difference method. The microstructure and mechanical properties of the composition are specified in the calculations based on microhardness measurements, mechanical tensile tests, and metallographic studies. The dependences of the plastic flow localization characteristics on the geometry and mechanical properties of ultrasonically treated surface layers have been established. Plastic strain localization is found to depend on the geometry and mechanical properties of ultrasonically treated surface layers.     

Effect of initial orientation on twinning in commercially pure titanium

Abstract

The effect of initial orientation on twinning micro-mechanisms during tensile deformation of commercially pure titanium has been studied using micro focus X-ray diffraction and electron back scatter diffraction (EBSD) in a scanning electron microscope. Three orientations A, B and C obtained from a rolled and annealed block of commercially pure titanium were deformed in uniaxial tension till failure and the tested specimens were characterised with regard to bulk texture, microstructure and crystal orientation mapping using EBSD. Orientation B along the transverse direction in ND-TD plane exhibits higher strength and lower strain hardening compared to orientations A and C along the rolling direction in TD-RD and ND-RD plane, respectively. This is attributed to different texture of sample B compared to samples A and C leading to dissimilar twinning micro-mechanisms and characteristic variation in nature of twinning. It is observed that limited twin nucleation and prominent lateral growth plays a dominant role in orientation B while multiple twin nucleation with significant non-Schmid behaviour is dominant for the other two orientations. It is proposed from this study that conventional factors associated with twin formation like Schmid factor play a main role in twin nucleation and propagation, however, growth or lateral thickening of the twins is explained by elastic stiffness variation across twins and their parent grains.     

Relative anisotropies of plane waves and guided modes in thin orthorhombic plates: Implication for texture characterization

The angular dependences of the long wavelength velocities of S₀ and SH₀ modes of orthorhombic (orthotropic) plates are compared to those of the velocities of corresponding plane waves. To first order in the anisotropy, many of the phenomena are as expected. The absolute velocities and anisotropy of the SH₀ plate modes are identical to those for plane SH waves and the absolute velocities of the S₀ modes propagating along symmetry axes are reduced from the longitudinal plane wave velocities by an amount explained by the change from the plane strain to the plane stress condition. However, for certain classes of materials such as metal polycrystals, the anisotropy of the S₀ mode can be substantially different from that of the longitudinal plane waves. This effect is explained through an expansion of the crystallite orientation distribution function in terms of generalized spherical harmonics. Implications of the results for the ultrasonic measurement of preferred grain orientation (texture) in polycrystals is indicated.     

Electrically controlled switching of light beams in the plane of liquid crystal layer

A simple experimental method and a set-up to control light beams in the plane of liquid crystal (LC) cell is developed. The main idea of the method is the reflection and the refraction of light beams propagating in a plane of LC layer from the boundary, which separates the regions of a liquid crystal cell with different director orientations and refractive indices. This boundary can be produced by electric field applied to the LC layer with a planar homogeneous orientation. The disclination lines that separate the extremely sharp changes of LC orientation can be also used for this purpose. The possibility of discrete deflection of light beam from the initial direction is shown. A computer simulation confirmed that the proposed experimental geometry was most suitable to control intensity of extraordinary rays propagating in the plane of liquid crystal layer.     

Three-dimensional X-ray diffraction in the diamond anvil cell: application to stishovite

Three-dimensional X-ray diffraction can be used for characterizing the orientation, position, and strain tensor of single grains in a polycrystalline aggregate. Here, we show how the method is well suited for diamond anvil cell data with heterogeneous grain sizes, with an application to two samples of stishovite at 15 and 26 GPa. For each grain, we obtain a well-defined orientation matrix and cell parameters. Center of mass position can also be adjusted to the experimental data, with errors in the present experiment. Finally, strain tensors are adjusted for the individual grains. The stress distribution obtained is in agreement with expectations from the diamond anvil cell geometry and previous measurements of stishovite strength. Advantages and potential for improvement of the method are then discussed.     

Bidimensional distortion in ferroelectric liquid crystals with strong anchoring in bookshelf geometry

In the last decade, it has been experimentally found that in certain conditions a periodic domain pattern arises in ferroelectric liquid crystals (FLC) in bookshelf geometry. Such a periodic texture appears after switching-off an external electric field, even in strong anchoring conditions. It has a static character and is bidimensional, being dependent on directions normal to both the smectic planes and the cell plates. Here a new model explaining this phenomenon is proposed, valid in the case of FLC with strong anchoring. The model is based on the coupling between the spontaneous polarization field in the first semi-period of its modulation along the cell plates and the same field in the second semi-period. This coupling competes with the FLC medium elasticity and is trapped by the anchoring. According to our model, in the ferroelectric state the biperiodic texture is favored by increasing the values of spontaneous polarization. The critical value of the spontaneous polarization for which the undeformed state becomes unstable is found. It is shown to be proportional to the square root of the ratio between the FLC elastic constant and the cell thickness. Moreover, it is inversely proportional to the sinus of the pre-tilt angle with respect to the cell walls.     

Ferromagnetic resonance in films with a plane anisotropy of electrical resistivity

High conductivity of metallic films leads to attenuation of an ultrahigh frequency field at the depth of the skin layer and, consequently, to a nonuniform distribution of the high-frequency magnetization component over the film thickness. This feature leads to a shift in the frequency and to widening of the ferromagnetic resonance line in a metallic film. In the present work, we study the effect of the plane anisotropy of electrical resistivity on the resonance behavior of polycrystalline magnetic films (Ni, Co, permalloy). Appearance of this anisotropy is related to the presence in the film of extended structural defects such as microgaps oriented along the chosen direction of the substrate texture (Lavsan, mica). In the geometry of the perpendicular magnetization and the normal incident angle of the ultrahigh frequency wave with respect to the film there are two resonance frequencies corresponding to the two orthogonal polarizations of the high-frequency field.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 10–13, June, 1985.     

A sample cell for in situ electric‐field‐dependent structural characterization and macroscopic strain measurements

When studying electro‐mechanical materials, observing the structural changes during the actuation process is necessary for gaining a complete picture of the structure–property relationship as certain mechanisms may be meta‐stable during actuation. In situ diffraction methods offer a powerful and direct means of quantifying the structural contributions to the macroscopic strain of these materials. Here, a sample cell is demonstrated capable of measuring the structural variations of electro‐mechanical materials under applied electric potentials up to 10 kV. The cell is designed for use with X‐ray scattering techniques in reflection geometry, while simultaneously collecting macroscopic strain data using a linear displacement sensor. The results show that the macroscopic strain measured using the cell can be directly correlated with the microscopic response of the material obtained from diffraction data. The capabilities of the cell have been successfully demonstrated at the Powder Diffraction beamline of the Australian Synchrotron and the potential implementation of this cell with laboratory X‐ray diffraction instrumentation is also discussed.     

Special features of strain-induced surface roughness formed in specimens with curvilinear geometry of the hardened surface layer-substrate interface

The paper studies the formation of strain-induced roughness on an initially flat surface of materials with a modified surface layer in tension. The dynamic boundary value problem of mechanics is solved numerically by the finite difference method for plane strain. The curvilinear surface layer-substrate interface is specified explicitly in the calculations. The mechanical response of the steel substrate and surface layer is described by elastoplastic models with isotropic hardening and elastic-brittle fracture models, respectively. The surface roughness shape and amplitude are found to depend on the thickness of both elastic and high-strength plastic layers with sinusoidal geometry of the hardened layer-substrate interface.     

Three-body coupling for electron impact ionization of helium in the perpendicular plane geometry

Dynamically screened three-Coulomb-wave (DS3C) model is applied to study single ionization of helium by 102 eV electron impact. Fully differential cross sections (FDCS) are calculated at different scattering angle of (8°, 10°, 15°, 20°) in the perpendicular plane asymmetric geometry. The results are compared with experimental data and theoretical predictions from a three-Coulomb wave function (3C) approach, convergent close-coupling calculation (CCC), as well as second-order distorted-wave Born method (DWB2). It is shown that three-body coupling effects are important for the perpendicular plane geometry.     

Two-dimensional hexagonal photonic crystal with new element geometry

A new type of the geometry of the elements of hexagonal-lattice photonic crystal shaped as the Wigner-Seitz cell is proposed. Dispersion curves of electromagnetic waves are calculated and the photonic crystal band gap characteristics are determined using the plane wave expansion technique. It is shown that the proposed geometry of the photonic crystal element leads to band gap widening. A simple method for fabricating prototypes using negative electron lithography is proposed.