Pure Twist Boundaries Research Articles

On the role of texture in governing fatigue crack propagation behavior of 2524 aluminum alloy

The texture evolution of AA2524 cold-rolled sheet during annealing process and texture effect on fatigue crack propagation (FCP) behavior were investigated. Results showed the sample annealed at 400°C for 45min and solid solution-treated at 490°C for 20min had the strongest Goss and P textures, and possessed the lowest FCP rates. EBSD examination indicated Goss, P, Q, Cube, R and S grains induced great crack deflection to retard crack growth, when having large twist and tilt angle component boundary or pure twist boundary with neighboring grains. In the case of small angle boundary or relatively large angle tilt boundary with adjacent grains, Cube, Copper and R grains induced little crack deflection. Additionally, SEM results showed, in the presence of large twist angle component boundary or pure twist boundary, more severely plastic deformation presenting numerous slip bands occurred in Cube, R and S grains than that in Goss, P and Q grains during crack propagation. In contrast, no evident slip bands formed in Cube, Copper and R grains when having small angle boundaries or relatively large tilt angle boundary.

Frequencies of Tilt and Twist Boundaries among Random Grain Boundaries

There are a number of classifications of homophase grain boundaries. It is quite common to divide them into twist, tilt and general boundaries. As in the case of the classification into coincident lattice (CSL) boundaries and non-CSL boundaries, one may ask about the possible frequencies of incidence of tilt and twist boundaries in a set of “random” boundaries. The proba¬bilities of occurrence of these particular boundary types are clearly defined if small deviations from pure twist and tilt conditions are allowed. We estimated the probabilities numerically for the cases of cubic and hexagonal holohedries. For a given randomly generated boundary, a computer program searched for the nearest pure-tilt and pure-twist boundaries. All symmetrically equivalent representations of the random boundary were processed, and the smallest distance was taken as the result. The distance was based on both the difference in misorientations and the deviation between boundary inclinations. The findings concerning tilt boundaries turned out to be striking. For instance, if the allowed deviation from pure-tilt conditions is only 1°, then as many as 39.0% and 21.2% of random boundaries have near-tilt character for the cubic and hexagonal cases, respectively. If the limiting deviation is raised to 5°, the frequencies of near-tilt boundaries reach 98.6% and 77.0%, respectively.

Lamellar structure and twist boundary of NaV2O5 grown by flux method

Microstructures of NaV2O5 prepared using flux method were characterized. These structures were observed to be dominated by lamellae with an average thickness of ∼ 1 micron. These lamellae share (001) plane and stack along [001] direction. A small orientation difference among these lamellae was detected with electron diffraction to be within several degrees. A pure twist boundary with (001) plane as the boundary interface and high sigma value of 235 was determined. Within the lamellae there are numerous dislocations arranged parallel to the twist boundary. The twist boundaries and dislocations may introduce distortion in the layers of VO5 pyramids, which is believed to be unfavorable for Na atoms to position between these layers and may result in non-stoichiometry locally at the boundary.

Interfacial Structure and Lattice Mismatch in Lamellar TiAl Alloys

AbstractThe gamma lamellae in TiAI alloys are rotated relative to each other approximately by multiples of 60˚ so that close packed planes and directions nearly align. However, the deviations caused by the tetragonality of the lattice are important in determining the structure of the interfaces and the internal stresses in the lamellae. These, in turn, strongly affect the mechanical properties of the alloys. In this paper two new electron optical techniques are used to quantify the deviations from pure twist boundaries (1) selected area diffraction from two adjacent twin-related lamellae permits c/a to be measured with high precision, (2) convergent beam diffraction is used to measure tilt components of lamellar boundaries and also rotations of vectors within the lamellar boundaries. The results are discussed in connection with the weak beam observations of interface dislocation structures.

Study of energy vs misorientation for grain boundaries in gold by crystallite rotation method—II. Tilt boundaries and mixed boundaries

In Part I [ Acta metall. 33, 1113 (1985)] small gold crystallites were welded to thin film single crystal gold substrates at different controlled misorientations to produce pure twist boundaries in the neck regions which could be observed directly by transmission electron microscopy. In the present work these experiments were extended to include pure tilt and also mixed (tilt/twist) boundaries. Upon annealing, the crystallites untilted and untwisted into several particular misorientations in order to reduce the boundary energy. The untilting and untwisting occurred by the observed climb of edge grain boundary dislocations (GBDs) and glide of screw GBDs respectively. The directions of the rotations were generally consistent with a boundary energy vs tilt/twist “surface” constructed on the basis of the structural unit/GBD model for grain boundaries. Kinetic models for the untilting indicated that the climbing edge GBDs acted as efficient line sources/sinks and that the diffusional transport was controlled by either lattice diffusion or edge GBD core diffusion for low angle pure tilt boundaries and by grain boundary diffusion for high angle mixed boundaries.

Grain boundaries as point defect sources or sinks—Diffusional creep

The rate of emission and absorption of point defects at high angle grain boundaries in single phase materials is considered by means of an (intrinsic) boundary dislocation model. The emission (absorption) rate is found to depend on the boundary structure and the applied stress and/or point defect supersaturation. Ideal low energy boundaries act as point defect sources (sinks) only above a threshold stress that is proportional to the slope of the energy misorientation curve of the boundary. Boundaries deviating from ideal low energy misorientation relatíonships emit (absorb) point defects at all stresses with a rate that increases with increasing angular deviation from the ideal low energy misorientation relationship. At high stresses and/or large crystal sizes and also in the case of pure twist boundaries, extrinsic grain boundary dislocations may be important for the emission and absorption of vacancies. The results seem to agree with the experimental observations available.

Theory of simple two-phase interfaces

The coincidence-site-lattice theory of grain boundaries has been applied to simple two-phase boundaries. Symmetric and unsymmetric tilt boundaries, pure twist boundaries and unrotated and untwisted boundaries have all been considered. It has been shown that each type of boundary can be described in terms of a characteristic coincidence-site lattice. In addition, the dislocation content within the interphase boundaries has been defined in terms of Burgers circuits described with respect to the original crystal lattices in the new coincidence-site lattices.

The effect of torsional stress on pure twist boundaries

Abstract The twisting behaviour of pure [111] twist boundaries in zone-refined aluminium bicrystals has been studied as a function of torsional stress for two boundary misorientation angles. The bicrystals initially twisted or untwisted at a rapid rate, which steadily decreased with time. This hardening was non-recoverable even after reannealing the samples at 0-98 Tm or removing the surface layer. The effect of the variation of misorientation angle on the experimental results is examined and the implications discussed.

Technique for Measuring Grain Boundary Mobility and Its Application to Potassium Chloride

A simple technique for investigating boundary mobility is outlined and applied to the study of boundary migration in melt‐grown bicrystals of KCl containing pure twist boundaries. An unexpectedly high activation energy, 4 to 6 ev, is found for movement of the twist boundaries.

Electron Diffraction Studies of the Epitaxy of Cu Single Crystals. II. Early Stages of Epitaxy and Interfacial Dislocation Networks

The first stages of epitaxy were investigated with reflection HEED methods for continuous Cu, Cu2O, and Ag films on Cu single-crystal substrates. A new kind of diffraction pattern was obtained from the Cu{111} planes. It is produced by networks of interfacial dislocations. In case of the Cu layers, hexagonal arrangements of screw dislocations are formed in a pure twist boundary. The corresponding Burgers vectors are of the type ½〈110〉. For the Cu2O and Ag films, hexagonal and rhombohedric networks, respectively, of interfacial edge dislocations with Burgers vectors ⅙〈112〉 are probably present. The dislocation densities and modifications in the film lattice constants for the latter two epitaxial systems are in good agreement with values predicted by the theory of interfacial dislocations.

Dislocations and persistent slip bands in fatigued copper

Abstract Persistent slip bands in fatigued OFHC copper single crystals have been examined using both replica and transmission electron microscopy techniques. In general, the dislocation structure below persistent slip bands is found to consist of rows of elongated cells within a region extending at least 170 microns below the specimen surface. It is suggested that there is a one-to-one correspondence between these cells and intrusions and extrusions observed on the persistent slip bands. The longer cell walls are nearly pure twist boundaries lying in the primary slip plane, indicating that slip occurs in at least two directions in the primary plane. The cell structure at persistent slip bands is stable at an annealing temperature which causes the matrix structure to disappear.

A demonstration of a moiré pattern and a pure twist boundary model

A demonstration of a moiré pattern and a pure twist boundary model