Tensile strength and fracture of a tilt grain boundary in cubic SiC: A first-principles study

The ab initio tensile test has been applied to the non-polar interface of the {122}, Sigma = 9 tilt boundary in cubic SiC, where the tensile strength and mechanical behaviour at zero temperature are examined using the ab initio pseudopotential method based on the local density-functional theory. This interface is strong because of the reconstruction of interfacial bonds. The maximum tensile stress in the unaxial extension normal to the interface is about 42 GPa, which is about 80% of the theoretical and experimental values of the strength of bulk crystal along the <111> direction. Young's modulus and the fracture toughness are also comparable with the values of the bulk crystal. The back Si-C bond of the interfacial C-C bond is broken first because the C-C bond has a high strength and a short length like a diamond bond. Then the interfacial Si-C bonds are broken, and finally the Si-Si bond. The Si-C bonds are rapidly stretched and broken if the bond stretching exceeds about 20%, and the bond charge clearly disappears when the bond stretching exceeds about 30%. Changes in the electronic structure associated with the bond breaking are analysed.     

Stress concentration and crack nucleation at the end of a low-angle tilt boundary

A low-angle tilt boundary (LATB) is easily sheared by dislocations identical to those constituting the boundary. This shear results in the creation of semi-infinite low-angle tilt boundaries, which generate a stress at their tips large enough to cause fracture.     

Fracture mechanisms of symmetrical tilt grain boundaries

The mechanisms of failure of symmetrical tilt boundaries have been investigated using an atomistic technique. The fracture process advances in steps that are related to the structural unit of the symmetrical tilt grain boundary. These boundaries are shown to fail through a combined mechanism involving dislocation emission in certain regions of the structural unit, followed by microcleavage, and again dislocation emission. The overall toughness of the boundaries is given by the combination of the toughness of these two processes.     

Pinning of grain boundary migration by a coherent particle

We studied single-particle pinning of grain boundary (GB) migration during grain growth. A phase-field model was formulated to simulate the pinning by a coherent particle and validated quantitatively by comparison with analytical prediction. A study of GB migration velocity using this model revealed that second-phase coherent particles have a previously unknown restraining effect over the whole of the GB-particle interaction range, which is qualitatively different from the interaction between GB and incoherent particles.     

Phase-field simulations of the interaction between a grain boundary and an evolving second-phase particle

We performed phase-field simulations to analyse the interaction of a migrating grain boundary with an evolving second-phase particle. It is found that depending on the difference between the interfacial energies of the particle–matrix interface for the two grain orientations involved and the driving force for grain boundary movement, particles with a particle size well above the critical limit can dissolve due to passage of the boundary.     

On the structural unit model of grain boundary structure

Abstract

A critique of the structural unit model is presented. It is shown that the predictive capacity of the model is severely limited when it is applied to boundaries with relatively-high-index rotation axes. Recent computer simulations suggest that ‘high index’ means 〈221〉 and above. A new approach to grain boundary structure is needed for mixed tilt and twist boundaries.     

Experimental and simulated grain boundary groove profiles in tungsten

Grain-boundary grooving has been studied on polished surfaces of polycrystalline tungsten annealed at 1350°C. Atomic force microscopy images were taken in the same area for each groove after different annealing times. Secondary oscillations next to the main groove maxima (predicted for grooving by surface diffusion) were observed, to our knowledge for the first time. The agreement between experimental and calculated groove profiles (using the surface diffusion model of Mullins (1957, J. appl. Phys., 28, 333)) improved when grain-boundary fluxes were introduced.     

Nanoindentation creep behavior of grain boundary in pure magnesium

Nanoindentation creep tests were performed at the grain boundary and grain interior in pure magnesium. The grain boundary showed a high strain rate sensitivity exponent and was dominated by grain boundary sliding due to the high diffusion rate at the grain boundary. The grain boundary affected the deformation behavior of the area at a distance of 2 µm into the grain interior. On the other hand, the grain interior had a low strain rate sensitivity exponent, because its matrix was too large to be influenced by the grain boundary. The deformation mechanism in the grain interior was determined to be dislocation slip.     

Sebastian Rodrigues in Shusaku Endo's Silence as a Theologian of the Cross1

In his book, On Being a Theologian of the Cross, Gerhard Forde asserts, “[T]he theology of the cross is an offensive theology. The offense consists in the fact that unlike other theologies it attacks what we usually consider the best in our religion.” If causing an offense against Christian theologians and the populace in general is considered a criterion for this theology, Shusaku Endo surely sets forth the theology of the cross in his novel, Silence. Although he would not identify his thesis by such a term, Endo presents the theology of the cross challenging the conventional understanding of the Christian faith. This short article explores Endo's book, Silence, examines how it demonstrates an articulation of the theology of the cross, and argues that Sebastian Rodrigues, the main character of the novel, is a theologian of the cross.     

The atomic structure of the dislocation cores in a small-angle grain boundary in BaTiO3 thin films

The atomic structure of the cores of the dislocations forming a [110] tilt boundary of 10^o misorientation in a BaTiO₃ film is characterized by means of focal-series reconstruction. Along the small-angle grain boundary, facets with different inclinations are accompanied by different types of dislocation. The dislocation with a Burgers vector of a [001] appears as a perfect edge type. Those with a Burgers vector of a [111] are dissociated into three partials with a Burgers vector of (a/3)[111], leading to two segments of stacking faults. Structural models of these dislocation cores are suggested on the basis of the phase of the reconstructed wavefunction and tested by simulation of the wavefunction.     

Temporal characteristics of a comparator in adaptation to optical tilt

Two groups of eight Ss each and one group of seven Ss were exposed to optical tilts (T) of 50, 40, 30, 20, and 10 deg in succession. Exposure time at each tilt was 3, 15, and 27 min in Groups 1, 2, and 3, respectively. Trend analyses of the functions relating level of adaptation to T showed significant quadratic components for Group 1, quadratic and linear components for Group 2, and only linear components for Group 3. These results were consistent with derivations from a memory-comparator model of perceptual adaptation.     

Interactions of persistent slip bands with a grain boundary on the common primary slip plane in a copper bicrystal

In this letter, the dislocation patterns on the common primary slip plane in a fatigued \[1-34]-\[182-7] copper bicrystal with a Sigma=19b grain boundary (GB) have been investigated using the electron channelling contrast technique in a scanning electron microscope. The results show that the two-phase dislocation structure, such as veins and persistent slip band (PSB) walls, embedded within veins, can be clearly seen on the common primary slip plane. In particular, the interactions of PSBs with the GB are clearly revealed. It is found that there are three kinds of interaction mode between the GB and the dislocations during cyclic deformation, and those are discussed. It is suggested that the dislocations carried by PSBs cannot transfer through the GB continuously even though the bicrystal has a common primary slip plane and its surface slip bands are continuous across the GB.     

Mechanically driven grain boundary relaxation: a mechanism for cyclic hardening in nanocrystalline Ni

Molecular dynamics simulations are used to show that cyclic mechanical loading can relax the non-equilibrium grain boundary (GB) structures of nanocrystalline metals by dissipating energy and reducing the average atomic energy of the system, leading to higher strengths. The GB processes that dominate deformation in these materials allow low-energy boundary configurations to be found through kinematically irreversible structural changes during cycling, which increases the subsequent resistance to plastic deformation.     

Anomalous spiral aftereffects: a new twist to the perception of rotating spirals.

Stationary spirals viewed after inspecting rotating sectored disks appear to rotate and to expand or contract radially, even though the rotating disks contain no perceptible components of radial motion. Moreover, the relative directions of illusory rotation and radial motion observed in these instances are 'impossible' under the stimulus constraints normally imposed by the geometry of a spiral under rotation: the stationary spirals appeared to expand/contract in directions opposite to those normally observed under conditions of actual spiral rotation, and under conditions of illusory spiral rotation in classical spiral aftereffects.     

Decay of visual adaptation to tilt and displacement

Persistence in the dark following 48 min of visual adaptation to tilt and displacement was compared in two experiments to determine if same or different processes are involved in the two kinds of adaptation. Decay of tilt adaptation occurred rapidly, all within about 16 min. However, it was not complete and some residual tilt adaptation persisted for at least as long as 56 min. Decay of displacement adaptation occurred more slowly but was clearly complete after at most 56 min in the dark. Displacement adaptation appears to be entirely subject to decay, while tilt adaptation involves an additional, more long-term component. Results are interpreted in terms of independent systems for the perception of location and orientation.     

Spectral tilt as a cue to word segmentation in infancy and adulthood

Across a variety of tasks, adults respond differently to syllables with multiple stress cues than to syllables with only one cue to stress. This series of experiments was designed to explore how infants and adults use partial stress as a cue to word boundaries. In the first experiment, 9-month-old infants treated syllables with only one cue to stress (spectral tilt) as a strong cue to word boundaries. The second experiment shows that whereas adults treat syllables with multiple cues to stress as word onsets, they do not consider syllables marked only by spectral tilt to be strong indicators of word boundaries. Thethird experiment shows that 1-year-old infants are more adultlike than 9-month-olds in their use of stress cues. Taken together, these results suggest a rapid development of stress cue knowledge in infancy, perhaps due to infants' experience with word segmentation.     

Comparison of teaching methods to learn a tilt and balance wheelchair skill.

Often service professionals working with individuals who use wheelchairs find themselves inadequately prepared to give hands-on or verbal assistance in wheelchair maneuverability skills. The purpose of this study was to examine the effectiveness of two methods of teaching a tilt and balance wheelchair skill, evaluated by using an obstacle course. Subjects were 30 volunteers, having no past wheelchair experience. The Instruction group viewed an instructional video followed by structured wheelchair practice. The Instruction plus Biomechanics group experienced these same interventions and received a biomechanical explanation of the wheelchair skill. The Control group received no instruction but did receive additional practice time in a wheelchair. It was hypothesized that the first two groups would improve their maneuverability in the wheelchair over the Control group. A 2-way analysis of variance, with main effects for groups followed by a Tukey post hoc test, indicated that the two instruction groups had significantly better times on the obstacle course, demonstrating better maneuverability skills than the Control group. The repeated-measures portion of the analysis of variance showed the main effect for improving time from the pretest to posttest significant for all groups with the instruction groups improving times by 30 sec. over that of the Control group. The retention test was not significantly different from the posttest, and there were no significant interactions. Results indicated a need for wheelchair users to be taught maneuverability skills systematically.