Structural characterization of double stacking faults induced by cantilever bending in nitrogen-doped 4H-SiC

Defects in highly nitrogen-doped 4H-SiC deformed by cantilever bending at 550°C have been identified by weak-beam and high-resolution transmission electron microscopy techniques. The induced-defects consist of double stacking faults (DSFs) whose expansion produces a local 4H?→?3C phase transformation. Each DSF is bound by two identical 30° Si(g) partial dislocations which glide on two adjacent basal planes. The DSFs belong to three different populations which differ by their extension as a function of the applied-stress and the 30° Si(g) characteristics (line direction L , Burgers vector b , glide planes and glide direction). The external mechanical stresses are the main driving forces involved in the DSF expansion. However, extra driving forces such as thermodynamic or electronic forces are also likely to be involved.     

Irradiation-induced dissociation of an a ⟨100⟩ edge dislocation in SrTiO3

We report the dissociation of an a?100? dislocation core in SrTiO₃ into two (a/2)?100? partial cores during electron-beam irradiation in a high-voltage transmission electron microscope. The partial separation is approximately 1?nm. Based on the forces acting between the partials, a dissociation mechanism is proposed which includes a combination of climb and glide processes.     

Microstructure of hot-pressed α-Sic after creep experiments at high temperature

Abstract

Transmission electron microscopy analyses of hot-pressed α-Sic deformed at 1600°C have shown the activation of the basal plane with glide dislocations dissociated into Shockley partials. Loop nucleations have been frequently observed along the dislocation lines and a climb mechanism is proposed to explain, the experimental analyses.     

Mechanical spectroscopy of a high-Nb-bearing γ-TiAl-based alloy with near-gamma and fully lamellar microstructure

Intermetallic γ-TiAl sheet material of composition Ti–46?at.%Al–9?at.%Nb and two different microstructures (fine-grained near-gamma and coarse-grained fully lamellar) was studied by mechanical-loss (internal-friction) measurements using two frequency ranges: (I) 0.01–10?Hz and (II) around 2?kHz. The mechanical spectra in range I show (i) a loss peak of Debye type at T?≈?1000?K, which occurs only in fully lamellar samples; and (ii) a high-temperature damping background above?≈?1100?K. The values of the activation enthalpy H of the high-temperature background, 4.3?eV (near-gamma) and 4.2?eV (fully lamellar), which were determined from the frequency shift, are distinctly higher than those obtained for TiAl sheet material with low Nb content. The high-temperature damping background is assigned to diffusion-assisted climb of dislocations, and the 1000?K peak (H?=?2.9?eV) to local (reversible) glide of dislocation segments anchored between lamella interfaces. From measurements of the eigenfrequency in range II, the variation of Young's modulus in the temperature range 300–1000?K was determined.     

high-resolution electron microscopy at a (113) symmetric Thermally activated step motion observed by tilt grain-boundary in aluminium

Grain-boundary migration is demonstrated to proceed by lateral propagation of a small step in a (113), [110] symmetric Al tilt grain-boundary. In-situ high-resolution (transmission) electron microscopy (HREM) at 523K allowed the study of atomic-scale detail at video rates during the migration process. The grain-boundary translational states on both sides of the step are identical, which leads to a step dislocation. This defect can move laterally by a combination of climb and glide. Dynamic HREM images indicate considerable atomic agitation within the dislocation core. A detailed temporal analysis of the step movements shows small random displacements of the dislocation core.     

A study of thermal vacancies and dislocation structure in Fe–40 at.% Al

The densities of thermal vacancies and residual dislocations in bulk specimens of Fe–40?at.%?Al have been investigated using differential dilatometry and X-ray diffraction. A large quenched-in vacancy concentration at temperatures above about 873?K was apparent from the decrease in average lattice parameter. This was correlated to a lowering of the effective enthalpy of vacancy formation from about 91 to 42?kJ mol^?1, possibly caused by the presence of different types of point defect in lower- and higher- temperature regimes. The residual dislocations were found to have a major concentration on {100} planes at any given temperature. An increase in the dislocation density and a concurrent fall in the vacancy concentration was observed with a lowering of the quenching temperature from 1223 to 1073?K, indicating the possibility of vacancy annihilation at 1073?K.     

High-resolution characterization of the precipitation behavior of an Al–Zn–Mg–Cu alloy

The metastable particles in an Al–Zn–Mg–Cu alloy have been examined at atomic-resolution using high-angle annular dark field (HAADF) imaging. In under-aged conditions, thin η′ plates were formed with a thickness of seven atomic planes parallel to the {111}_Al planes. The five inner planes of the η′ phase appear to be alternatively enriched in Mg and Zn, with two outer planes forming distinct Zn-rich interfacial planes. Similar Zn-rich interfacial enrichment has also been identified for the η phase, which is a minimum 11-plane thick structure. In rare instances, particles less than seven planes were found indicating a very early preference for seven-layer particle formation. Throughout the aging, the plate thickness appears constant, while the plate radius increases and no particles between 7 and 11 planes were observed. Based on the HAADF contrast, our observations do not support the η′ models previously set forth by other authors. Clear structural similarities between η′ and η were observed, suggesting that drawing distinctions between η′ and η phases may not be necessary or useful.     

Dissociation of dislocations in MgAl2O4 spinel deformed at low temperatures

Abstract

When spinel is deformed in compression at 400°C along 〈110〉, the primary slip plane is found to be {111} with cross-slip occurring on a {001} plane. A comparison of weak-beam images of dislocations from both systems indicates that all dislocations which belong to the primary slip plane are dissociated out of the {111} plane independent of the character of the dislocation. It is proposed that deformation occurs by motion of dislocations in their dissociated state and that the partial dislocations actually glide on parallel glide planes. Movement of these dissociated dislocations is then accompanied by a concurrent migration of the stacking fault which takes place by a local shuffling of the cations. A stacking fault energy for conservative dissociation at 400°C on {001} of 530±90mJ m^?2 has been determined from weak-beam images of screw dislocations.     

Perceptual completion of a sound with a short silent gap

Listeners reported the perceptual completion of a sound in stimuli consisting of two crossing frequency glides of unequal duration that shared a short silent gap (40 ms or less) at their crossing point. Even though both glides shared the gap, it was consistently perceived only in the shorter glide, whereas the longer glide could be perceptually completed. Studies on perceptual completion in the auditory domain reported so far have shown that completion of a sound with a gap occurs only if the gap is filled with energy from another sound. In the stimuli used here, however, no such substitute energy was present in the gap, showing evidence for perceptual completion of a sound without local stimulation ('modal' completion). Perceptual completion of the long glide occurred under both monaural and dichotic presentation of the gap-sharing glides; it therefore involves central stages of auditory processing. The inclusion of the gap in the short glide, rather than in both the long and the short glide, is explained in terms of auditory event and stream formation.     

Mobility of c dislocations in Ti3, Al

Abstract

The rôle of dislocations with Burgers vectors, b, given by b = [0001] during deformation of samples of the intermetallic compound Ti₃Al has been assessed. At room temperature, the experimental evidence is consistent with these dislocations being sessile, their density and morphology being similar to that in undeformed samples. In samples deformed at 650°C and above, it is concluded that motion of these dislocations is effected by dislocation climb. The line directions of the various segments of dislocations with b= [0001] are shown to be perpendicular to planes that contain sheets of Ti atoms, with an expected tendency to exhibit a high Peierls stress.     

The Role of Natural Class Features in the Acquisition of Phonotactic Regularities

Previous research has shown that phonotactic regularities can be acquired through recent production or auditory experience (e.g., Dell et  al., Journal of Experimental Psychology: Learning, Memory, and Cognition, 26(6), 1355–1367, 2000; Onishi et al., Cognition, 83(1), B13–B23, 2002). However, little is known about the role of phonological natural classes in this learning process. This study addressed this question by investigating the acquisition of a contingency relationship between onsets and medial glides by Mandarin speakers. The experiments involved the manipulation of three types of phonotactic regularities. In the Laryngeal version, onsets that preceded the same glide shared a voicing feature. In the Place version, onsets that preceded same glide shared a place feature. In the Neither version, onsets associated with the same glide shared neither a voicing feature nor a place feature. Results showed the Place version and the Laryngeal version were more easily acquired than the Neither version in terms of the amount of exposure needed to acquire the experimentally manipulated phonotactic schema and the sustainability of the acquired schema. The results suggest that the statistical learning mechanism that guides our processing of speech input prefers phonological regularities that follow certain natural class features. This preference may account for the way natural languages are structured phonologically. An erratum to this article can be found at     

The flow stress contribution of the dislocation 'forest' in bcc lattices

The athermal flow stress contribution by junction reactions of a glide dislocation with a 'forest' of density rho is usually written as tau = alpha mu b rho ^1/2. The strength coefficient alpha has been computed for the bcc lattice by virtual displacement of triple nodes following the method of Puschl et al. (1982, Physica status solidi (a), 74, 211). In the isotropic approximation, the values for glide dislocations with 0, 30, 60 and 90^o character are as alpha = 0.18, 0.21, 0.19 and 0.22 respectively. A comparison with values calculated previously by Frydman is made, and good agreement is found when differences in the cut-off radii of the linear elastic solution are accounted for.     

Structural evolution during crystal nucleation in supercooled liquids with icosahedral short-range order

The structural evolution during crystal nucleation in supercooled Lennard–Jones liquids at a supercooling of 0.3T _m (T _m is the melting temperature) has been studied by molecular dynamic simulations. The icosahedral clusters are observed to compete with crystalline clusters in space, and rearrange before crystal nucleation. Both the stable face-centered-cubic and hexagonal-close-packed (hcp) structures and metastable body-centered-cubic (bcc) structures nucleate simultaneously, resulting in the formation of an incomplete bcc meso-layer in the nuclei. The nuclei form twinned crystal with five-fold axes through a successive twinning process bounded by planes with hcp atoms.     

Effects of glide slope, noise intensity, and noise duration on the extrapolation of FM glides through noise.

Listeners are quite adept at maintaining integrated perceptual events in environments that are frequently noisy. Three experiments were conducted to assess the mechanisms by which listeners maintain continuity for upward sinusoidal glides that are interrupted by a period of broadband noise. The first two experiments used stimulus complexes consisting of three parts: prenoise glide, broadband noise interval, and postnoise glide. For a given prenoise glide and noise interval, the subject's task was to adjust the onset frequency of a same-slope postnoise glide so that, together with the prenoise glide and noise, the complex sounded as "smooth and continuous as possible." The slope of the glides (1.67, 3.33, 5, and 6.67 Bark/sec) as well as the duration (50, 200, and 350 msec) and relative level of the interrupting noise (0, -6, and -12 dB S/N) were varied. For all but the shallowest glides, subjects consistently adjusted the offset portion of the glide to frequencies lower than predicted by accurate interpolation of the prenoise portion. Curiously, for the shallowest glides, subjects consistently selected postnoise glide onset-frequency values higher than predicted by accurate extrapolation of the prenoise glide. There was no effect of noise level on subjects' adjustments in the first two experiments. The third experiment used a signal detection task to measure the phenomenal experience of continuity through the noise. Frequency glides were either present or absent during the noise for stimuli like those use in the first two experiments as well as for stimuli that had no prenoise or postnoise glides. Subjects were more likely to report the presence of glides in the noise when none occurred (false positives) when noise was shorter or of greater relative level and when glides were present adjacent to the noise.     

Speed Modulation in Swimming Frogs

Swimming movements of 7 European green frogs (Rana esculenta) were studied, starting from the detailed analysis of the speed and timing of the propulsive, glide, and recovery phases of their intermittent swimming behavior. First, the authors identified the spatiotemporal factors used by the frogs to modulate their swimming behavior. None of the gait variables correlated strongly with average swimming speed, and no significant correlations were found between variables belonging to different phases. There did not seem to be an obvious control strategy. Instantaneous speeds at the transition of the different phases all increased significantly with average speed, however. The strong correlation between maximal speed at the end of propulsion and the speed averaged over a cycle might reflect the dominance of the propulsive phase in the determination of the overall swimming speed. The modulation of swimming speed thus seemed largely comparable with the regulation of jumping distance. That finding was confirmed in a mathematical model, in which the positive correlations between both glide and recovery speeds, on the one hand, and average speed, on the other, were shown to be only mathematical consequences of the strong impact of the propulsive phase on overall swimming performance. That finding suggests that the correlations did not result from an active control strategy.     

Deformation structure after fracture-toughness test of Mg–Al–Zn alloys processed by equal-channel-angular extrusion

Fracture toughness and deformation structures have been investigated using an AZ31 magnesium alloy processed by equal-channel-angular extrusion (ECAE). The ECAE-processed alloy (as-ECAE) was annealed at 573?K for 24?h (annealed-ECAE). The average grain sizes of as-ECAE and annealed-ECAE alloys were 4.0 and 16.3?µm, respectively. The plane-strain fracture toughness K _IC, obtained by stretched-zone analysis in as-ECAE and annealed-ECAE, were estimated to be 27.3 and 23.5?MPa/m^1/2, respectively. From optical microstructural observations in samples after the fracture-toughness tests, deformation twins were observed in annealed-ECAE. No deformation twins were observed in as-ECAE. In addition, dislocations on basal planes, as well as on non-basal planes, were activated in as-ECAE. It is concluded that the enhancement of the fracture toughness in the fine-grain structure was related to a reduction of deformation twins and dislocation movement in non-basal planes.     

Effects of glide slope,noise intensity,and noise duration on the extrapolation of FM glides through noise

Listeners are quite adept at maintaining integrated perceptual events in environments that are frequently noisy. Three experiments were conducted to assess the mechanisms by which listeners maintain continuity for upward sinusoidal glides that are interrupted by a period of broadband noise. The first two experiments used stimulus complexes consisting of three parts: prenoise glide, broadband noise interval, and postnoise glide. For a given prenoise glide and noise interval, the subject’s task was to adjust the onset frequency of a same-slope postnoise glide so that, together with the prenoise glide and noise, the complex sounded as “smooth and continuous as possible.” The slope of the glides (1.67, 3.33, 5, and 6.67 Bark/sec) as well as the duration (50, 200, and 350 msec) and relative level of the interrupting noise (0, ?6, and ?12 dB S/N) were varied. For all but the shallowest glides, subjects consistently adjusted the offset portion of the glide to frequencies lower than predicted by accurate interpolation of the prenoise portion. Curiously, for the shallowest glides, subjects consistently selected postnoise glide onset-frequency values higher than predicted by accurate extrapolation of the prenoise glide. There was no effect of noise level on subjects’ adjustments in the first two experiments. The third experiment used a signal detection task to measure the phenomenal experience of continuity through the noise. Frequency glides were either present or absent during the noise for stimuli like those used in the first two experiments as well as for stimuli that had no prenoise or postnoise glides. Subjects were more likely to report the presence of glides in the noise when none occurred (false positives) when noise was shorter or of greater relative level and when glides were present adjacent to the noise.     

Formation and structure of misfit dislocations at the La2Zr2O7-Y2O3 -stabilized ZrO2 (001) reaction front during vapour-solid reaction

La₂ Zr₂ O₇ (LZO)-based pyrochlore islands were grown on a Y₂ O₃ -stabilized ZrO₂ (YSZ) (001) single-crystal surface by the reaction between La₂O₃ vapour and the crystal. A network of interfacial edge dislocations with line directions [100] and [010] and Burgers vectors ( a_s/2)[101] and ( a_s/2)[011] respectively ( a_s being the lattice parameter of the YSZ) was observed at the moving LZO-YSZ reaction front. The interface-parallel component of the Burgers vectors accommodates the LZO-YSZ lattice mismatch of +5.0%, while the perpendicular component causes a slight tilt of the LZO lattice with respect to the YSZ lattice. The dislocation half-loops nucleate and glide on inclined {101} planes at the edges of the four corners of the growing islands.     

Illusory recouplings of onsets and terminations of glide tone components

We present a new auditory illusion, the gap transfer illusion, supported by phenomenological and psychophysical data. In a typical situation, an ascending frequency glide of 2,500 msec with a temporal gap of 100 msec in the middle and a continuously descending frequency glide of 500 msec cross each other at their central positions. These glides move at the same constant speed in logarithmic frequency in opposite directions. The temporal gap in the long glide is perceived as if it were in the short glide. The same kind of subjective transfer of a temporal gap can take place also when the stimulus pattern is reversed in time. This phenomenon suggests that onsets and terminations of glide components behave as if they were independent perceptual elements. We also find that when two long frequency glides are presented successively with a short temporal overlap, a long glide tone covering the whole duration of the pattern and a short tone around the temporal middle can be perceived. To account for these results, we propose an event construction model, in which perceptual onsets and terminations are coupled to construct auditory events and the proximity principle connects these elements.     

Dislocation motion in silicon: the shuffle-glide controversy revisited

Considering recently computed formation and migration energies of kinks on nondissociated dislocations, we have compared the relative mobilities of glide partial and shuffle perfect dislocations in silicon. We found that the latter should be more mobile over all the available stress range, invalidating the model of a stress driven transition between shuffle and glide dislocations. We discuss several hypotheses that may explain the experimental observations.