Rhombohedral Planes Research Articles

In-Situ Observations of the Effect of Electron Irradiation on the Defect Microstructure of Alumina

Abstract The interaction of radiation produced point defects with a dislocation microstructure at high temperature is of considerable interest and careful high voltage microscopy experiments can provide valuable insight into the mechanisms. Veyssière and Westmacott carried out in-situ experiments monitoring the partial dislocation climbs in Ni3Al induced by thermal vacancies as well as by Frenkel pairs produced during irradiation.1 We report here the results of some preliminary experiments we performed on alumina (A12O3) single crystals with high dislocation densities to study the modification of the microstructure by electron irradiation at high temperature. The dislocation microstructures were produced by shock wave deformation using a high-velocity impact technique. The technique is capable of producing a very high density of defects consisting primarily of basal twins, and slips on the basal, pyramidal and rhombohedral planes in alumina. The dislocations are all of glide type, mostly forming shear bands. A typical microstructure prior to irradiation is shown in figure la.

Bismuth film coalescence determined by ellipsometry

The growth and coalescence of thin (1.5–100.0 nm) filmsof bismuth were examined with ellipsometry and atomic force microscopy. The Bi films were thermally evaporated onto oxidized, heated (343 K) Si substrates. An effective medium approximation was used with the ellipsometry data to determine that the films grew in an island growth mode out to a thickness of about 8.0 nm before coalescing into a continuous film. These results were confirmed usingatomic force microscopy which also indicated that the average grain size increased in an almost linear manner with film thickness up to thicknesses around 50.0 nm before levelling off. Surface steps of 0.4 and 0.8 nm suggested that the grains were mainly oriented with the (111) rhombohedral planes parallel to the surface.

A TEM investigation of shock metamorphism in quartz from the Vredefort dome, South Africa

The origin of the Vredefort structure in South Africa is still debated. Several causes have been discussed, namely asteroid impact, internal gas explosion or tectonic processes. Evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains, shatter cones, veins of pseudotachylite and occurrence of coesite and stishovite (high-pressure quartz polymorphs). A number of these characteristics is widely believed to support an impact origin. However, the planar features in quartz, which are generally considered as one of the strongest indicators of impact, are in the Vredefort case considered as anomalous when compared with those from accepted impact structures. We have investigated by optical and transmission electron microscopy (TEM) the defect microstructures in quartz grains from different lithologies sampled at various places at the Vredefort structure. Whatever the locality, only thin mechanical Brazil twin lamellae in the basal plane are observed by TEM. So far, such defects have only been found in quartz from impact sites, but always associated with sets of thin glass lamellae in rhombohedral planes 10−1 n with n = 1, 2, 3, and 4. At the scale of the optical microscope, Brazil twins in (0001) are easily detected in Vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them. Similar alignments of tiny fluid inclusions parallel to other planes are also detected optically, but at the TEM scale no specific shock defects are detected along their traces. If these inclusion alignments initially were shock features, they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae. Fine-grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen, but stishovite was not found, even in areas where its occurrence was previously reported. Finally, definite evidence of high-temperature annealing was observed in all the samples. These observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the Vredefort structure. Most of the original shock defects are now overprinted by an intense post-shock annealing episode. Only the thin mechanical twin lamellae in the basal plane have survived.

Slip systems in calcite single crystals deformed at 300–800°C

In an attempt to resolve questions recently raised regarding the principal high‐temperature slip systems in calcite, optical quality single crystals have been uniaxially compressed at 300–800°C and at a constant strain rate of ∼ 3×10−5 s−1. In addition, a single strain rate cycling test was performed at 650°C. The tests were all carried out with the compression direction parallel to , which lies at ∼30° to the c axis and makes angles of 52° and 23° with the poles to two rhombohedral (r) planes. Axial strains of 5–16% were achieved in the tests. The stress‐strain curves obtained showed three‐stage hardening behavior, while the strain rate cycling data showed flow stresses to be rather insensitive to strain rate with empirical power law fits yielding a conventional stress exponent of 22–30 at strains ≥ 5%. The active glide systems were identified by slip line analysis. Slip on r in the so‐called negative sense was found to be an important deformation mechanism across the entire range of temperatures investigated, while slip on a single f system in the negative sense became important at 650°C and above. The active f slip direction was of type, confirming the existence of a new set of f slip systems, namely, f , as opposed to the generally accepted f systems. In addition, clear evidence was found for significant slip on the basal (c) plane at temperatures above 600°C, probably in the 〈a〉 direction. By comparison with earlier data on crystals from the same batch, no evidence was found for significant strength asymmetry on the r and f slip systems in the positive and negative senses. Notably, the presently observed f and c sup systems have not been taken into account in previous modelling studies of plasticity and texture (i.e., crystallographic preferred orientation) development in calcite rocks, and r slip is usually taken as stronger in the positive than negative sense. Our results therefore imply a need for renewed modelling work on calcite, particularly regarding texture development at relatively high temperatures.

Formation mechanisms of planar deformation features in naturally shocked quartz

Shock waves induce peculiar defects in quartz: shock mosaics, high-pressure polymorphs (coesite and stishovite), diaplectic glass and ‘planar deformation features’ (PDFs). Together, these features are the indices of ‘shock metamorphism’, PDFs appear under the optical microscope as straight and narrow defects parallel to the { 10 1n } rhombohedral planes with n = 3 and 2 as most frequent values, although n = 1, 4 and ∞ (i.e. basal plane) are also found. We report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of PDFs in shocked quartz grains from a variety of sites (Slate Islands, La Malbaie and Manson in North America; the Vredefort complex in South Africa; the Ries Crater in Germany and the Cretaceous-Tertiary boundary layer at Raton Basin, Colorado). We distinguish four PDF fine structures: (1) bands of dislocations; (2) lamellae of mixtures in various proportions of amorphous silica and small crystallites; (3) thin Brazil twin lamellae; (4) short, parallel lamellae forming serrated ladder structures. The possible formation mechanisms of these PDFs are discussed in the second part of the paper. It is suggested that the dislocation bands are probably not original PDF structures, but stem from a later recrystallization stage. The Brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave. The other PDF structures, which consist of partial amorphization in the { 10 1n } planes, must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes. Theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 GPa. The growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave.

Morphology of Platelike Abnormal Grains in Liquid‐Phase‐Sintered Alumina

The microstructural features of platelike grains in liquid‐phase‐sintered Al2O3 were investigated. The flat boundaries of platelike grains wetted with a liquid phase were basal planes. After impingement of platelike grains, flat boundaries progressively changed to curved boundaries which consisted of both basal and rhombohedral planes as microscopic facets. Second‐phase particles were observed for most doped samples. Small‐angle subgrain boundaries inside platelike grains were also observed.

Dynamic etching of natural rhombohedral calcite cleavage planes

AbstractDynamic etching of natural rhombohedral calcite cleavage plane in low concentration of monocarboxylic acid (such as formic, acetic and propionic acids) is studied and interesting results are obtained. Eccentricity of rhombic etch pits increase indefinitely as stirring rate increase in low concentration of monocarboxylic acids, whereas peak value of eccentricity is obtained with stirring rate when there is a change in etch pit morphology. Change in etch pit morphology and change in eccentricity with stirring rate is explained with the help of lateral reaction rates along various directions. Eccentricity variation with stirring rate is compared with concentration variation with concentration of different monocarboxylic acids.

Electron irradiation induced crystalline to amorphous transition in boron carbide

Boron carbide is one of the current candidate materials for neutron absorbers in the control systems of various nuclear reactors because of its high neutron absorption cross-section over a broad spectrum of neutron energies. It is also one of the prime candidate materials for the first wall coating in fusion reactors because its low Z constituent atoms would lead to minimum plasma contamination. On the basis of this premise, a number of irradiation experiments have been carried out on boron carbide. Transmission electron microscope (TEM) observations of neutron irradiated boron carbide have revealed small disk-shaped cavities on /111/ and sometimes on /110/ and /100/ rhombohedral planes. These cavities have been identified as high-pressure He bubbles, since they are accompanied by surrounding strain fields. With increasing irradiation temperature, the size of these bubbles increases, while their density decreases. He bubbles induced by 100 keV He ion irradiation, on the other hand, were reported to have a spherical shape. Recently, it was established by the authors' group that a number of intermetallic compounds could be rendered amorphous by high energy electron irradiation as they could be by ion irradiation. This phenomenon is of interest not only from an engineering point of viewmore » but also from a scientific perspective to clarify the mechanism of the crystalline to amorphous (C-A) transition. As an extension of the work on the intermetallics, a systematic study of the electron irradiation induced C-A transition in ceramic materials was undertaken by the authors with some preliminary results showing the presence of an electron irradiation induced C-A transition in SiC. In the present paper, it is reported that boron carbide undergoes a C-A transition by electron irradiation at 2 MV, whereas BETA-rhombohedral boron remains crystalline during the irradiation.« less

Solubility and hydrothermal growth of calcite single crystal in nitrate solutions

The hydrothermal synthesis of calcite single crystals was studied using strong electrolyte solutions mineralizers. Alkali nitrate solutions (NaNO3, KNO3) were found to be effective solvents. The solubility of calcite in 3m (molarity) NaNO3 solutions was measured by the weight loss method and was around 2.2 g/l at 450°C and 100 MPa, which was twice as high as that in 3m NaCl solution. The (0001) plane tended to develop in NaNO3 hydrothermal solutions at higher temperatures, above 500°C, while rhombohedral (0011) planes developed predominately on crystals grown below 500°C.

Vacancy ordering in Sc 1− xS

This is a study of scandium vacancy ordering in slowly cooled Sc 1− x S using high-resolution electron microscopy. Previous X-ray analysis had shown vacancy segregation into alternate (111) planes of the rocksalt substructure. The present study reveals a further incommensurate ordering of the vacancies within the (111) c planes in approximately every third ( 110) rhombohedral plane. The incommensurate order was easily destroyed by the irradiation and heating of the electron beam. These observations are discussed in terms of the termination of the wave vector within the Brillouin zone and described in terms of a possible continuous transition to an incommensurate Sc atom population wave.

Evidence of ⟨a + c⟩ dislocations in synthetic quartz single crystals compressed along the c axis

When compressed along the c axis, quartz single crystals can accomodate deformation either by pure climb of ⟨c⟩ and ⟨a⟩ dislocations or by glide of ⟨c + a⟩ dislocations in rhombohedral and pyramidal planes {10[-1]1}, {11[-2]2} and {10[-1]1}. Detailled TEM investigations on samples deformed at high temperature clearly show ⟨c + a⟩ dislocations lying in {10[-1]1} and {11[-2]2} planes. The various possible mechanisms (climb versus glide) for this orientation are discussed.

Etch pits and dislocation in {1̄012} Czochralski sapphire wafers

The reliability of fused KOH for revealing dislocations in the (1̄012) rhombohedral plane in Czochralski-grown sapphire single crystals was investigated by using x-ray-transmission topography. It was found that the etch pits represent closely the dislocations in the (1̄012) sapphire, although a one-to-one correspondence was not established.

The tensile fracture of quartz crystals

Quartz crystals of five orientations have been fractured in tension at temperatures below 800 K in air and water-free environments. The radius of the initial crack nucleus determines the fracture stress which, in turn, controls the velocity-distance relationship. The unusual fractography is described in which fracture occurs predominantly along facets of the rhombohedral planes and requires a surface energy of 2 J m−2.

On Determining the Sign of Fault Vectors by X-Ray Topography

Abstract Experiments show that the excess or deficiency of diffracted intensity in the image of the first fringe in stacking-fault-type fringe patterns on X-ray projection topographs can give a clear indication of the sign of the fault vector even when μ t is no greater than unity (μ is the normal linear absorption coefficient and t is the crystal thickness) provided that the crystal structure is such as will exhibit fairly strong anomalous transmission (Borrmann effect) in the thick crystal case for the Bragg reflections used. A quite simple diffraction theory suffices to account for the observations. Sign determinations based on this theory agree with those expected at Brazil twin boundaries parallel to major rhombohedral planes in quartz.

Defects in boron carbide before and after neutron irradiation

Many crystalline compounds contain regular geometrical arrangements of atoms which are so stable that the presence and motion of defects such as dislocations are expected to leave these units more or less intact. In boron and most of its compounds, the stable units are icosahedral groups of twelve boron atoms and in boron carbide, which is rhombohedral with rhombohedral angle 65° 36′, the B 12 icosahedra are arranged in an array which is almost a f.c.c. lattice, as a result of which, fault surfaces analogous to stacking faults in close-packed metals are observed. The small departure from cubic symmetry is, however, sufficient to impose a distinction between rhombohedral planes, which are hosts for translation twin boundaries created by the dissociation of glide dislocations, and the basal plane, which is believed to contain Frank partial dislocation loops resulting from the agglomeration of point defects introduced during neutron irradiation. Another important consequence of the rhombohedral distortion is that the atomic displacements produced by the leading and trailing partials in an extended dislocation are different. In physical terms, both refinements of the close-packed cubic case arise from the presence of the carbon atoms. The analogy to close-packed metals is carried over to the relationship between translation twins and orientation twins; the dissociation of a glide dislocation in boron carbide is accompanied by rotation of C 3 chains which thread the slip plane and this rotation both may and may not be sufficient to render the translation twin boundary region identical to a thin orientation twin.

Inelastic neutron scattering investigation of spin waves and magnetic interactions in Cr 2O 3

The dispersion relation E( q) for spin waves in antiferromagnetic Cr 2O 3 has been measured in various directions within the rhombohedral (1⦶10) plane at 78 K. The observed E( q) is interpreted in terms of Heisenberg exchange interactions between the Cr 3+ ions. Only interactions between first and second neighbor ions are found to be strong.

Microwave resonance in the canted antiferromagnet FeF 3

Using microwave resonance in a nearly twin-free crystal of FeF 3, we have measured the six-fold anisotropy in the rhombohedral plane from 4.2 to 300°K. The resonance linewidth at 300°K increases almost linearly with frequency between 17 and 35 GHz.

Etch pits in flux-grown corundum

Procedures have been developed for chemically polishing and etching {0001}, {10¯11}, {10¯12}, {11¯20}, and {1¯100} planes in crystals of ruby and sapphire grown from a PbF2 flux. The shape and the orientation of the etch pits were found to be characteristic for each plane and the density of the pits was 102 to 104/cm2. Similar pits were produced in flame-fusion material, but the density was 106 to 108/cm2. Ruby and sapphire crystals grown by the same process behaved similarly. There is evidence that etch pits reveal dislocations which emerge normally to the basal or to the prismatic planes, since similar patterns of pits were produced after the removal of successive layers of material parallel to these planes, and a correlation was found between the pit patterns on opposite {0001} faces. Inconclusive evidence on this point was obtained for the rhombohedral planes.

Deformation twinning in corundum

Abstract Deformation twinning on rhombohedral planes was detected in corundum crystals fractured in bending at temperatures as low as − 196°c. The crystallographic parameters of this twinning were determined completely: s=0·202. Deformation markings, tentatively attributed to basal twinning, were detected on some crystals fractured above 1000°c. Twins were observed to be associated with fracture faces, and in some cases twin-sub-grain and twin-twin intersections, which probably nucleated fracture, could be found.

Rare findings of prism faces of natural quartz

Several hundreds of natural quartz crystals obtained from overseas mines and from the Pannala Hills, Amreli District (Gujarat State, India) have been examined under a metallurgical phase contrast microscope. First evidence of screw dislocations and spirals on prism faces of natural quartz crystals are reported. The slip step observed on the prism face is shown to be parallel to the trace of a major rhombohedral plane, which is a direction of the ‘partial cleavage’. The increasing straight slope of the slip step emerging from the face has been attributed to the screw dislocation. Rectangular and irregular spirals are illustrated and interpreted as growth spirals.