Structure Of Polycrystalline Materials Research Articles

High Resolution EBSD-Based Dislocation Microscopy

Significant advances are reported in the application of HR-EBSD to the imaging of the dislocation structure of polycrystalline materials. The central assumption of the method is the compatibility of the total displacement field, which relates the (Nye) dislocation tensor to the (partially measurable) curl of the elastic displacement field. Two key challenges must be addressed, including: a) the fundamental limitation imposed by the electron-opacity of typical materials, which limits the measurement of gradients in the displacement field in the direction normal to the sample surface; and b) the inability of HR-EBSD to recover the spherical (elastic) distortions of the lattice. This second challenge can be overcome if a traction free boundary condition is applied. It is illustrated that consideration of the familiar stress equilibrium relations gives additional information, which may enable estimates of the missing components of the Nye tensor. An example of application of HR-EBSD to a Mg-Ce sample is presented.

Calculation of anisotropic properties of dental enamel from synchrotron data

Obtaining information about the intrinsic structure of polycrystalline materials is of prime importance owing to the anisotropic behaviour of individual crystallites. Grain orientation and its statistical distribution, i.e. the texture, have an important influence on the material properties. Crystallographic orientations play an important role in all kinds of polycrystalline materials such as metallic, geological and biological. Using synchrotron diffraction techniques the texture can be measured with high local and angular resolving power. Here methods are presented which allow the spatial orientation of the crystallites to be determined and information about the anisotropy of mechanical properties, such as elastic modulus or thermal expansion, to obtained. The methods are adapted to all crystal and several sample symmetries as well as to different phases, for example with overlapping diffraction peaks. To demonstrate the abilities of the methods, human dental enamel has been chosen, showing even overlapping diffraction peaks. Likewise it is of special interest to learn more about the orientation and anisotropic properties of dental enamel, since only basic information is available up to now. The texture of enamel has been found to be a tilted fibre texture of high strength (up to 12.5×). The calculated elastic modulus is up to 155 GPa and the thermal expansion up to 22.3 × 10(-6)°C(-1).

Nanoscale effects on oil migration through triacylglycerol polycrystalline colloidal networks

The following paper studies the effects of laminar shear on the nano- and meso-scale structure of polycrystalline materials (cocoa butter). Furthermore, the effect of laminar shear on the migration kinetics of stained triacylglycerol (TAG) oil through a polycrystalline colloidal TAG network was characterized and quantified. Several models were used to study this phenomenon. A low diffusion coefficient was observed in cocoa butter crystallized under shear. This suggested that the migration rate of liquid TAGs through a polycrystalline TAG network can be modified using external fields. A lower migration rate was observed in the shear-crystallized samples. These samples had a smaller crystallite size indicating that particles of this size will retard oil migration. The arrangement of the crystalline component also had a strong effect on the oil migration rate (OMR). Liquid oil diffusivity was decreased by about 4.6 to 30 times in samples that exhibited alignment of the crystalline material along a given axis. Moreover, the tortuosity of the diffusive path in the shear-crystallized samples was approximately 4 to 12.5 times higher than that of the static sample. Finally, the relationships between the structural factors (nanoparticle size, domain size, tortuosity and particle distribution) and the permeability coefficients suggest that shear crystallization can retard oil migration by reducing the network's permeability by about a factor of 100.

Using electron microscopy to complement X-ray powder diffraction data to solve complex crystal structures

High-resolution X-ray powder diffraction and electron microscopy techniques yield remarkably complementary information for crystal structure determination. By combining the two types of data, structures of polycrystalline materials that resist solution by more conventional methods can be tackled. In particular, crystallographic phase information extracted either from high-resolution transmission electron microscopy (HRTEM) images or from precession electron diffraction (PED) data has proven to be extremely useful in such cases. PED data can also be used to identify weak reflections in the diffraction pattern, and this information, in turn, can be exploited to improve the partitioning of the intensities of reflections that overlap in the powder diffraction pattern. The dual-space (diffraction and real space) structure determination programs Focus and pCF have been shown to be especially well-suited for combining the two different types of data. The power of this approach is illustrated with the details of the solutions of the structures of TNU-9, IM-5 and SSZ-74, the three most complex zeolite framework structures known.

EFFECTS OF CRYSTAL SIZE AND ORIENTATION OF SUBSTRATES ON CELL ADHESION: IMPLICATION FOR MEDICAL IMPLANTS

The aim of this study is to investigate the effect of atomic structure of polycrystalline materials on cell-substrate interactions. Samples are prepared from rods and sheets of Ti -6 Al -4 V substrates with predominately two distinct crystallographic orientations as well as nano-structured and annealed titanium fabricated through high-pressure torsion and heat treatment processes. The degree of preosteoblast attachment and rate of growth, which are regulated through the activity and interaction of proteins present in the extracellular matrix, are notably increased on the nano-structured titanium and substrate having predominant [Formula: see text] orientation. The improved cell activity is attributed to the nano-structured feature of these substrates consisting of ultra-fine crystals (< 50 nm) and specific atomic order of [Formula: see text] substrate which provide higher degree of surface wettability. These findings demonstrate the advantages of nano-structured titanium over the conventional and coated titanium implants, as both mechanical properties and cellular response are improved. Furthermore, crystal orientation of the substrates can influence cell responses and, therefore, substrate engineering can be used to improve and control cell-substrate interactions.

Migration of 45° [1 0 0] grain boundaries in an Fe–6 at.% Si alloy

The grain structure of polycrystalline materials is controlled by recrystallization processes which are strongly affected by the character of moving interfaces. To elucidate an anisotropy of grain boundary motion, the temperature dependence of migration of a 45° [1 0 0], {0 k l} symmetrical and a (0 0 1)/(0 1 1) asymmetrical tilt grain boundaries in an Fe–6 at.% Si alloy was studied by the reversed-capillary technique. The values of the migration characteristics – activation enthalpy and pre-exponential factor – were determined and are discussed from the point of view of anisotropic behavior and compared with previously obtained data.

Modeling elasto-plastic behavior of polycrystalline grain structure of steels at mesoscopic level

The multiscale model is proposed to explicitly account for the inhomogeneous structure of polycrystalline materials. Grains and grain boundaries are modeled explicitly using Voronoi tessellation. The constitutive model of crystal grains utilizes anisotropic elasticity and crystal plasticity. Commercially available finite element code is applied to solve the boundary value problem defined at the macroscopic scale. No assumption regarding the distribution of the mesoscopic strain and stress fields is used, apart the finite element discretization. The proposed model is then used to estimate the minimum size of polycrystalline aggregate of selected reactor pressure vessel steel (22 NiMoCr 3 7), above which it can be considered macroscopically homogeneous. Elastic and rate-independent plastic deformation modes are considered. The results are validated by the experimental and simulation results from the literature.

局所加熱法による純銅の結晶粒組織制御

The present work deals with a preferential grain growth process in a localized region utilizing local heating method in order to fabricate some unique microstructures different from those fabricated in the homogeneous way of microstructure evolution. A Monte Carlo simulation of grain growth under a heterogeneous temperature gradient, i.e. spot heating, was performed. Steep temperature gradient brought about a preferential grain growth in the higher temperature region, showing that the local heating was effective for the control of grain structure of polycrystalline materials. Such type of preferential grain growth became less significant under the mild temperature gradient. Local heating of pure copper foil with 0.2mm in thickness utilizing laser beam was performed by changing the irradiation conditions. In the case of 200W for laser power and 18mm/s for sweep velocity, some grains were observed to have larger grain sizes than their surrounding grains, suggesting a possibility of preferential grain growth in the localized region.

3D X-Ray Crystal Microscope

Penetrating, submicrometer X-ray beams probe the mesoscopic crystalline structure of polycrystalline materials in three dimensions. Crystalline morphology, strain, texture, and phase can now be studied non-destructively with a spatial resolution below the grain size of most materials. This new information will lead to a more complete understanding of the mesoscopic structure and dynamics of materials arid their relation to materials properties.

Mapping the Mesoscale Interface Structure in Polycrystalline Materials

Using the new method known as Orientation Imaging Microscopy (OIM), characterization of mesoscale aspect of the internal structure of polycrystalline materials in the two-dimensional section plane has become routine. However, in some applications which primarily focus either on the characterization of interface types or on their connectivity, OIM is rather inefficient since only the scan points that lie adjacent to the interfaces are used to determine interface character. When the OIM grid spacing is dictated by the precision with which the interfacial in-plane inclination must be determined in the section plane, the efficiency with which conventional OIM can harvest interfacial data is rather poor.In this paper a new approach and system of microscopy is described, called the Mesoscale Interface Mapping System (MIMS). MIMS overcomes the challenges associated with OIM by directing the beam to the vicinity of interfaces which have been identified by a microstructural contrast image.

Single-crystal-like diffraction data from polycrystalline materials

A method for solving structures from powder diffraction data was developed, and its validity was demonstrated on three complex structures. The method uses a textured sample and exploits the high intensity and parallel nature of synchrotron radiation. In principle, crystal structures as complex as those routinely solved by single-crystal methods can be determined with this approach. For example, the as-synthesized form of the zeolite UTD-1, with 69 nonhydrogen atoms in the asymmetric unit, could be solved directly. With this method, a larger range of structural complexity becomes accessible to scientists interested in the structures of polycrystalline materials that cannot be grown as single crystals.

Neutron diffraction using an electron linear accelerator

The Harwell electron linear acceleratorhelios has been used for studying the crystal structures of polycrystalline materials by neutron diffraction. Atomic positions and atomic vibrational amplitudes can be determined equally well with the pulsed method as with the more conventional method employing a high-flux nuclear reactor as source. The powder pattern can be indexed more easily with pulsed neutrons.