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Correlation between subgrains and coherently scattering domains

Crystallite size determined by X-ray line profile analysis is often smaller than the grain or subgrain size obtained by transmission electron microscopy, especially when the material has been produced by plastic deformation. It is shown that besides differences in orientation between grains or subgrains, dipolar dislocation walls without differences in orientation also break down coherency of X-rays scattering. This means that the coherently scattering domain size provided by X-ray line profile analysis provides subgrain or cell size bounded by dislocation boundaries or dipolar walls.

Twinning on pyramidal planes in hexagonal close packed crystals determined along with other defects by X-ray line profile analysis

A systematic procedure is developed to evaluate the frequency of {10.1}〈10.\overline 2〉 and {11.2}〈11.\overline 3〉 compressive twins and {10.2}〈10.\overline 1〉 and {11.1}〈\overline 1\overline 1.6〉 tensile twins together with dislocation densities, active slip systems and crystallite size in hexagonal close packed (hcp) metals. The effect of pyramidal twinning on X-ray line broadening in hcp metals is fundamentally different from the effect of twinning on close packed planes in face centred cubic (fcc) crystals. Therefore, the usual theoretical descriptions developed previously for fcc crystals cannot be used for pyramidal twinning in hcp crystals. The profile functions of sub-reflections for this type of twinning are derived to be the sum of a symmetrical and an antisymmetrical Lorentzian function. Sub-profile properties are parameterized and the parameter files are incorporated into the convolutional multiple whole profile (CMWP) procedure. The extended procedure, eCMWP, is applied to determine pyramidal twin frequencies together with dislocation densities, active slip systems and crystallite size in Mg deformed at different temperatures, in commercial purity Ti deformed at high temperature and in high-purity Ti deformed at room temperature.

Structure of misfit dislocations in niobium-sapphire interfaces and strength of interfacial bonding: An atomistic study

Abstract The formation of networks of misfit dislocations is investigated at the (0001) Al 2 O 3 ‖(111) Nb interface using a recently proposed approach (Vitek et al ., Phil. Mag. A , 1995, 71 , 1219) which employs a very simple pair-potential to describe interaction between the metal and the substrate that contains the strength of interfacial adhesion as a parameter. The calculations demonstrate how the strength of bonding between the two materials decides both the form of the network and the atomic structure of the cores of these dislocations. At the same time it reveals that diffusion is essential for the formation of the observed triangular network of 1/2〈111〉 dislocations. The calculated structures are then used to investigate related high resolution electron microscope (HREM) images using a multislice technique. In these simulations translational symmetry along the electron beam was not assumed but for each slice of material along the beam different sub-structures were used. This allowed us to investigate fully the effect of the dislocation intersections upon the images of the dislocation cores. Their effect is, indeed, considerable if an intersection is in the region producing the image but if not, the images of the cores of misfit dislocations are affected only marginally and HREM can capture fine details of the core structure. A direct comparison of an experimental observation in Mayer and co-workers ( Acta metall. mater. , 1992, 40 , S217; Scripta metall. mater. , 1994, 31 , 1097) with the present simulations demonstrates this ability.

A common theory of line broadening and rocking curves

X-ray diffraction peak broadening is discussed in terms of line broadening and rocking-curve broadening in a novel theoretical description. The nonlocal strain tensor is factorized by using the method of polar decomposition instead of the more conventional separation into symmetrical and antisymmetrical components. A number of X-ray line-broadening and rocking-curve experiments on the same single crystals or individual grains in bulk polycrystals prove that plastic deformation produces strained subgrains mutually rotated by rigid-body rotations. The novel theoretical description appropriately accounts for the rigid-body rotation and strain at the same time and provides straightforward separation of the two effects of line and rocking-curve broadening in the radial and normal directions of the diffraction vector. The mathematical results are discussed in terms of experiments of X-ray diffraction, Laue asterism and electron backscatter diffraction. From the experimental results it is shown that the simultaneous evaluation of line and rocking-curve broadening provides qualitative information about the redundant and geometrically necessary character of dislocations, not available if only one or the other is accessible.

A simple model for the vibrational modes in honeycomb lattices

The classical lattice dynamics of honeycomb lattices is studied in the harmonic approximation. Interactions between nearest neighbours are represented by springs connecting them. A short and necessary introduction of the lattice structure is presented. The dynamical matrix of the vibrational modes is then derived and its eigenvalue problem is solved analytically. The solution may provide deeper insight into the nature of the vibrational modes. Numerical results for the vibrational frequencies are presented. To show how effective our method for the honeycomb lattice is, we also apply it to triangular and square lattice structures. A few suggested problems are listed in the concluding section.

Nonuniversal behavior of the parity effect in monovalent atomic wires

We propose a mixed analytical–ab initio method for the accurate calculation of the conductance in monovalent atomic wires. The method relies on the most general formula for ballistic transport through a monovalent wire, whose parameters can be determined from first-principles calculations. Our central result is the demonstration of the highly nonuniversal behavior of the conductance, which depends on the fine details of the contacts to the leads. We are therefore able to reconcile a large number of the apparently contradictory results that have recently appeared in the literature.

Wetting Ability of Ag Based Molten Alloys on Graphite Substrate

The wetting angle (Θ) of molten Ag (Zn, Cu, Ga) drops on graphite substrate is investigated. A texture formation was found in the substrate/drop interphase region after solidification. The enrichment (segregation) of Ag atoms in the interface layer was observed in the Ag-Cu alloy, and unexpected mixing between graphite substrate and the Ag-Ga melt was also detected in the solidified structure.

Correlation between the average composition of coherent superlattice and the GMR properties of electrodeposited Co–Cu/Cu multilayers

Abstract In contrast to vacuum-deposited Co/Cu multilayers the electrodeposited ones do not exhibit any oscillation behaviour in magnetic resistance properties as a function of the thackness of non-magnetic Cu layers. According to the magnetic investigations they show ferromagnetic and superparamagnetic character as a consequence of the lack of antiparallel coupling of neighbouring magnetic layers. Magnetic resistance observed on electrodeposited Co– Cu/Cu multilayers is believed to originate from the randomly antiparallel oriented magnetic domains. In the present paper the evolution of magnetic resistance properties is studaed against the average Cu concentration of multilayers prepared under a Cu deposition potential of –0.25 and –0.62 V, and the best magnetic resistance properties were found in the films with composition of 52% and 60% Cu, respectively. The average concentration was determined by electron microscopic and X-ray diffraction measurements.

Geometry dependence of the conductance oscillations of monovalent atomic chains

Using a tight binding model we calculate the conductance of monovalent atomic chains for different contact geometries. The leads connected to the chains are modelled as semi-infinite fcc lattices with different orientations and couplings. Our aim is twofold: To check the validity of a three-parametric conductance formula for differently oriented leads, and to investigate the geometry dependence of the conductance oscillations. We show that the character of these oscillations depends strongly on the geometry of the chain-lead coupling.