Periodic Antiphase Boundaries Research Articles

Long period structure in the Cu-Sn system

High resolution electron microscopy and electron diffraction were used to study the Cu-Sn system in the composition range 3:1. Two different long period superstructures have been reported before as ordening phases with respect to the HCP structure. It was the purpose of this study to eliminate this ambiguity and investigate the influence of slight additions of a third element with respect to the Cu3Sn composition.Molten alloys were homogenized, quenched and annealed prior to electrolytical thinning of trepanned discs by jet polishing. The Cu3Sn-structures as described in the literature are derived from the Cu3Ti-orthorhombic basic structure which itself is a superstructure of the hexagonal close packed structure. As a confirmation of the structure model the derived phases should present the orientation and translation variants associated with the symmetry reduction fromfor the HCP structure tofor the Cu3Ti. From group theoretical considerations one expects three orientation variants and four translation variants;_the latter are then related by the three displacement vectors[100],[120] and[120].Our electron diffraction data and structure images confirm the structure proposal of Schubert and his description as a long period antiphase boundary structure with a periodicity of 10 times the b spacing of the orthorhombic cell.

The superstructure of plagioclase feldspars

The lattice imaging technique of high resolution electron microscopy has been applied on anorthite (An96) and labradorite (An52) to examine their antiphase boundaries (APBs) and superstructure relations. The effects of the structure details and superstructure relations on the lattice images change with the thickness of the specimens. The lattice images of the areas with the usual thickness (>150 A) generally represent not the structure details but the superstructure relations. In anorthite the type b-APBs are rather sharp and narrow with the displacement vector of 1/2[110]An. The type c-APBs have the displacement vector of 1/2[111]An. The existence of the I $$\bar 1$$ phase in the P $$\bar 1$$ matrix is not observed for both types in this study. In labradorite, the superstructure is coherent small-scale intergrowth of the anorthite-like and albite-like bands with the interval of 30 A corresponding to the t vector of the e 1-e 1 pairs. The thickness of both bands in the lattice images indicates that the anorthite-like and albite-like regions have approximate compositions of An80 and An5, respectively. The ordered arrangement of the subcells in each anorthite-like band, due to the reversal arrangement of Si and Al in the tetrahedral sites, is in antiphase relation with the next anorthite-like band separated by an albite-like band, resulting in a periodic antiphase structure. The type b-APBs in anorthite and the periodic APBs in labradorite are characterized with the displacement vector R=1/2[110]An=b 0.

T.E.M. study of high temperature precipitation in (Al 2O 3)n MgO spinels

Spinels (Al 2O 3)n MgO rich in alumina (n > 1) exhibit various solid phases at high temperature. We have studied some of them by T. E. M. They are produced directly inside the microscope by heating a thin foil by beam pulses. The crystallography of these new phases and their epitaxial relations with the matrix are deduced from the diffraction patterns while their chemical composition is evaluated with the help of microprobe attachments. Thus three phases have been characterized; all conserve the F. C. C. anionic lattice and are the physical result of a bulk diffusion process: 1. (i) the intermediate phase of type I corresponds to n ⋍ 15. It is monoclinic and had been already characterized in T. E. M. It is generaly assumed to be metastable. 2. (ii) the monoclinic θ Mg phase corresponds to n ⋍ 40. It is found to be imperfectly organised and some of its structural defects are imaged. 3. (iii) the tetragonal δ 2Mg phase corresponds to n ⋍ 8. It can be conveniently described on a geometrical point of view as a modification of the original spinel by a periodic crystallographic shear [or equivalently by a periodic antiphase boundary structure] plus a certain amount of ordered cation vacancies.

1232. Compositional change associated with periodic antiphase boundaries in the initial stages of ordering in Ni 3Mo. II. Elemental analysis : G van Tendeloo et al,Phys Stat Sol (a), 26 (1), 1974, 299–304

1232. Compositional change associated with periodic antiphase boundaries in the initial stages of ordering in Ni 3Mo. II. Elemental analysis : G van Tendeloo et al,Phys Stat Sol (a), 26 (1), 1974, 299–304

Compositional changes associated with periodic antiphase boundaries in the initial stages of ordering in Ni3Mo. II. Elemental analysis

Energy dispersive analysis of X-rays as an accessory to a transmission electron microscope has been used to investigate local compositional changes associated with the presence of periodic non-conservative antiphase boundaries in the initial stages of ordering in Ni3Mo. It is shown how a relative quantitative elemental analysis of the local alloy composition can be performed to a precision of better than 5%. A detailed analysis is described for 13.9 Å spaced periodic antiphase boundaries in Ni3Mo where a local excess in Ni could be determined, leading to a composition of Ni3.4Mo. The measurements confirm the proposed structural model. Es wird die Energiedispersionsanalyse von Röntgenstrahlen als Zusatz zu einem Transmissionselektronenmikroskop benutzt, um lokale Ånderungen der Zusammensetzung zu untersuchen, die mit periodischen nicht-konservativen Antiphasengrenzen im anfänglichen Ordnungszustand von Ni3Mo verbunden sind. Es wird gezeigt, wie eine relative, quantitative Elementanalyse der lokalen Legierungszusammensetzung bis zu einer Genauigkeit von besser als 5% durchgeführt werden kann. Es wird eine detaillierte Analyse für periodische Antiphasengrenzen mit 13,9 Å Abstand in Ni3Mo beschrieben, wobei ein lokaler Über schuß von Ni bestimmt werden konnte, der zu einer Zusammensetzung von Ni3,4Mo führt. Die Messungen bestätigten das vorgeschlagene Strukturmodell.

Electron Microscopy Study of the “Direct Resolution” of Periodic Antiphase Boundaries, with Special Reference to Ag3Mg

AbstractIn a long period ordered alloy, interference between superlattice and satellite beams allows to obtain fringe systems, the characteristics of which are analysed within the two beam kinematical theory of electron diffraction. Expressions for the intensity are given in the different cases and fit with electron microscopy observations performed on an Ag3Mg alloy of 22 at% Mg composition. These observations show conclusively that antiphase boundaries occur regularly on every second (100) plane for such a concentration.

Long range order in Ag3Mg

The long range order in a powder sample of Ag3Mg has been studied by X-ray diffraction as a function of quenching temperature below the critical ordering temperature of 665°K. By measuring the positions and integrated intensities of fundamental, superlattice and satellite reflections, the latter arising from the periodic anti-phase boundary structure, the variation with temperature of the overall long range order, antiphase boundary order, and anti-phase boundary periodicity have been determined. It appears that the anti-phase boundaries disorder slightly more with increasing temperature than the average superlattice, and that the boundary spacing, nominally equal to two (M = 2), increases towards that ideal value with increasing disorder below the critical point. These resalts are qualitatively interpreted by extending the reasoning of Sato & Toth on equilibrium long period superlattices to include the case where the Fermi surface becomes progressively more diffuse with increasing disorder.