Rhombohedral Twinning Research Articles

Faceted Grain Boundaries in Al2O3

Different types of faceted grain boundaries in commercial poly‐crystalline Al2O3 were examined. Three distinct cases, namely the basal twin, the rhombohedral twin, and other special high‐angle grain boundaries, are discussed, comparing theoretical models and experimental observations. It is shown that the faceting of the boundaries can be associated with their migration. A mechanism for the separation of a σ= 13 grain houndary from a pinning void, small grain, or second‐phase particle is illustrated. It is also shown experimentally that faceting of high‐angle boundaries can occur on a scale of Σ 5 nm, and that, theoretically, faceting most likely also occurs on an atomistic scale. Throughout the theoretical analysis the basal plane is predicted to be a favored facet plane, which is confirmed by experimental observation.

Twinning Shear in Alumina

The homogeneous shear necessary to reorient a lattice into twin orientation can be calculated precisely from crystallographic lattice dimensions. Measurement of the surface tilt produced by rhombohedral twins in Al2O3 agrees well with the calculated value, indicating that significant additional shear from the motion of glide dislocations is not contributing to twin deformation.

Rhombohedral Twinning in Alumina

Alumina single crystals were deformed in compression along the c axes at T= 625 to 1373 K. Large‐scale deformation by rhombohedral twinning was observed at a constant resolved shear stress of only 12.6 MPa between ∼900 and 1373 K. Below 900 K, the twinning stress for specimens with ground surfaces rose rapidly to 227 MPa at 625 K. A model for twin growth in the absence of dislocation slip is proposed.

Twinning dislocations in haematite iron ore

Abstract Direct observation of twinning dislocations at rhombohedral twin interfaces in naturally deformed haematite ore is reported. An analysis of the coordination polyhedra produced by the operation of the observed twinning elements across {1012} composition planes leads to a simple description of the interface structure whereby Fo atoms occupy tetrahedrally coordinated sites. Simple displacive atomic movements allow for rapid propagation of the twinning dislocations. The structural results are also relevant to mechanical twinning of sapphire and other structures derived from hexagonal close-packing, including hexagonal metals and alloys.

Crystal growth of chemically reduced H.C.P. cobalt on F.C.C. single crystals

The growth of an h.c.p. cobalt layer on the surface of Cu and Ni single crystals has been studied over a large range of thickness. Thin deposits show martensitic transformations from the beginning; in medium thick deposits twinning occurs on {10.1} composition planes; in thick deposits the predominance of the bath orienting power is manifest. Rhombohedral twinning has been related to growth faults, and its occurrence is explored in the case of Co alloy deposition. The importance of hydrolyzed species in the chemical deposition process and of their competing role with hypophosphite in the layer near the reacting surface has also been recognized.

Abrasive Surface Deformation of Sapphire

The depth of slip and the size of deformation twins introduced into sapphire by abrasive grinding depend on surface orientation. Enhanced depths of slip minimize surface chipping and striations, and grinding can result in basal slip depths of 30 μm and twins 75 μm in size. Basal twins in nonbasal surfaces are largest for single‐point diamond abrasion directions parallel to a 〈1100〉 basal twin shear direction, whereas optically detectable rhombohedral twins are produced only in 30° and 60° surfaces.

Compressive Creep of Al2O3 Single Crystals

Aluminum oxide single crystals deformed by dislocation glide and deformation twinning during compressive creep at 1400° to 1700°C. The activation energy for basal slip was a function of the applied stress and agreed with activation energies previously measured by observation of yielding phenomena. The overcoming of a large Peierls‐Nabarro stress is the most probable rate‐controlling mechanism. Rhombohedral twinning, a significant deformation mode in creep, depends on surface damage for nucleation. The activation energy for rhombohedral twin growth, a function of the applied stress, is substantially lower than that for basal slip. When basal slip and rhombohedral twinning occur concurrently, creep by basal slip results, but the presence of twins can substantially reduce the creep rate.

High-Temperature Deformation of Alumina Double Bicrystals

Deformation and strength of alumina double bicrystals fabricated from verneuil-grown stock were studied as functions of temperature (1210°T1820°C) and strain rate (0.0005 ε 0.05 min-1). The oriented specimens had c axes parallel or perpendicular to the compressive stress axes; +a axes and c axes were matched and/or mismatched across the (1123) interfaces of the double bicrystals. Undoped (“pure”) double bicrystals in which the c axes and stress axis were coincident deformed by rhombohedral twinning and, at T1550°C, by slip. Double bicrystals containing crystal segments having c axes perpendicular to the stress axis deformed in part by basal kinking. Interfacial impurity additions of mullite or spinel thin films promoted grain-boundary fracture and sliding, whereas “pure” double bicrystals, even at stresses comparable to the ultimate strengths of single crystals in similar orientations, did not exhibit macrosliding of the grain boundary.

Scientific Serials

American Journal of Science, September.—The gas thermometer at high temperatures, by L. Holborn and A. L. Day. This is a further study of the nitrogen thermometer with platinum-indium bulb, which is superior to the porcelain bulb. The correction for expansion is 10° at 500°, 30° at 1000°, and 40° at 1150°. The authors make an elaborate comparison of the gas thermometer with the thermocouples, and determine anew the melting points of a number of metals. Those of silver and gold are 955° and 1064° respectively —Monazite, by O. A. Derby. A single granule of the mineral, no matter how minute, can be securely identified by moistening it with sulphuric acid on a slip of glass and burning off the sulphuric acid over a spirit lamp, when the residue shows the characteristic crystallisation of cerium in radiating needles or isolated crystals of the shape of cucumber seeds.—The spectra of hydrogen and the spectrum of aqueous vapour, by J. Trowbridge. When a condenser discharge is sent through a rarefied gas confined in a glass vessel, the gas cannot be considered dry, for aqueous vapour is liberated from the glass. The four-line spectrum of hydrogen in the solar atmosphere is an evidence of aqueous vapour, and therefore of oxygen in the sun. Conclusions in regard to the temperature of the stars exhibiting hydrogen spectra are misleading if purely based upon conditions of pressure and temperature, for electric dissociation plays a determining part. X-Ray phenomena produced by a steady battery current strongly suggest an electrical theory of the origin of the sun's corona.—A new effect produced by stationary sound-waves, by B. Davis. When a small cylinder, closed at one end, is placed in the stationary sound-wave of an organ pipe, it will not only arrange itself perpendicularly to the motion of the wave, but will move across the wave in a direction perpendicular to the stream-lines. When four such cylinders are mounted in the shape of an anemometer on a needle point, they rotate while the pipe is sounded.—Some interesting developments of ealeite crystals, by S. L. Penfield and W. E. Ford. The crystals described show a great diversity of habit, often on a single hand specimen, due to different methods of twinning, together with peculiarities in the development of certain crystal faces. Some peculiar cases of rhombohedral twinning are described.—Method of measuring surface tension, by J. S. Stevens. The surface tension is measured by floating an iron wire on the surface of the liquid, and suspending a piece of soft iron by it. The iron is pulled into a magnetising coil immersed in the liquid by currents which increase until the surface is broken through.