Properties of the Deformation Microstructure in Al-Rich γ-TiAl Deformed by Ordinary Dislocations

1999 ◽  
Vol 578 ◽  
Author(s):  
Fabienne Grjégori ◽  
Patrick VeyssiÈRe
Keyword(s):  
Flow Stress ◽  
Single Crystals ◽  
Peak Temperature ◽  
Clear Correlation ◽  
Microstructural Investigation ◽  
Screw Dislocations ◽  
Burgers Vector ◽  
Dislocation Pinning ◽  
The Relationship ◽  
Line Direction

AbstractIn the near vicinity of the [021] load orientation, γ-TiAl deforms via ordinary dislocations (Burgers vector b = 1/2<110]). As for deformation by <110] dislocations, the flow stress shows a peak at about 600°C. Results of an extensive microstructural investigation aimed at identifying the origin of this mechanical anomaly are presented. The analysis was conducted on single crystals oriented for single slip. It confirmed that ordinary dislocations tend to align themselves along the screw direction. This preferential line direction becomes gradually accentuated as the deformation temperature is raised up to the peak temperature. This effect is accompanied by a strong tendency towards forming cusps, but there is indication that the immobilisation along the screw direction takes place prior to dislocation pinning. In the vicinity of the peak temperature, screw dislocations gather in the form of bundles. No clear correlation is found between the temperature dependence of the flow stress and that of the density of pinning points. The relationship between these microstructural findings and the occurrence of a flow stress anomaly is discussed.

The Relationship Between Micropipes and Screw Dislocations in Pvt Grown 6H-Sic

1996 ◽  
Vol 423 ◽  
Author(s):  
Jennifer Giocondi ◽  
Gregory S. Rohrer ◽  
Marek Skowronski ◽  
V. Balakrishna ◽  
G. Augustine ◽  
...  
Keyword(s):  
Scanning Force Microscopy ◽  
Vapor Transport ◽  
Physical Vapor Transport ◽  
Growth Surface ◽  
Repeat Distance ◽  
Screw Dislocations ◽  
Burgers Vector ◽  
Force Microscopy ◽  
Scanning Force ◽  
The Relationship

AbstractThe growth surface of a 6H-SiC boule, grown by physical vapor transport, was examined using scanning force microscopy. The dimensions of surface/micropipe intersections and screw dislocation Burgers vectors have been determined from topographic data. All micropipes are positioned along the lines of super screw dislocations with a Burgers vectors of at least 4 times the c-axis repeat distance (15.2 Å). Perfect c-axis screw dislocations with Burgers vectors of only 15.2 Å are stable and do not have open cores. Measurements show that micropipe core radii, determined indirectly from the width of the craters formed at the surface/micropipe intersections, increase with the square of the dislocation Burgers vector.

The relationship between flow stress and dislocation behaviour in [111] aluminium single crystals

1982 ◽  
Vol 46 (5) ◽  
pp. 801-816 ◽  
Author(s):  
Teizo Tabata ◽  
Hiroshi Fujita ◽  
Masa-Aki Hiraoka ◽  
Seika Miyake
Keyword(s):  
Flow Stress ◽  
Single Crystals ◽  
Dislocation Behaviour ◽  
The Relationship

The Deformation Microstructure in NI3Si Polycrystals Strained Over the Range of Temperature of Flow Stress Anomaly

1988 ◽  
Vol 133 ◽  
Author(s):  
B. Tounsia ◽  
P. Beauchamp ◽  
Y. Mishima ◽  
T. Suzuki ◽  
P. Veysslière
Keyword(s):  
Flow Stress ◽  
Slip System ◽  
Room Temperature ◽  
Peak Temperature ◽  
Effect Of Temperature ◽  
Screw Dislocations ◽  
Thermally Activated ◽  
Cube Plane ◽  
Octahedral Slip ◽  
Increasing Temperature

ABSTRACTIn order to correlate the flow stress anomaly of Ni3Si with dislocation properties, a weakbeam study ofpolycrystalline samples deformed between ambient and the peak temperature was carried out. Samples with two extreme Ni/Si ratios were tested.The most frequently activated slip system changes progressively from octahedral to cubic with increasing temperature. The transformation of superdislocations into Kear-Wilsdorf configurations gives rise to screw dislocations that are rectilinear only after deformation at room temperature. The effect of temperature is to gradually promote bending of Kear-Wisdorf configurations in the cube plane, from a few nanometers at 230°C to several tenths of micrometers at intermediate temperature. Cube slip begins to be massively activated a little below the peak temperature. It is suggested that the flow stress anomaly is controlled by progressive exhaustion of octahedral slip by thermally-activated expansion of superdislocations on the cube cross-slip plane.

scholarly journals Modeling Collective Dislocation Dynamics in Ice Single Crystals

1999 ◽  
Vol 578 ◽  
Author(s):  
M.-Carmen Miguel ◽  
A. Vespignani ◽  
S. Zapperi
Keyword(s):  
Acoustic Emission ◽  
Single Crystals ◽  
Dislocation Dynamics ◽  
Dynamic Critical Phenomena ◽  
Burgers Vector ◽  
Avalanche Dynamics ◽  
Dislocation Pinning ◽  
Line Defects ◽  
Experimental Findings ◽  
Power Law Distributions

AbstractWe propose a model to study the plasticity of ice single crystals by numerical simulations. The model includes the long-range character of the interaction among dislocations, as well as the possibility of mutual annihilation of these line defects characterized by its Burgers vector. A multiplication mechanism representing the activation of Frank-Read sources due to dislocation pinning is also introduced in the model.With our approach we are able to probe the dislocation patterns, which result from the dislocation dynamics. Furthermore, our results exhibit features characteristic of driven dynamic critical phenomena such as scaling behavior, and avalanche dynamics. Some of these results account for the experimental findings reported for ice single crystals under creep deformation, like the power-law distributions of the acoustic emission intensity observed sistematically in experiments.

A Weak-Beam Study of the Microstructure of β-CUZN Deformed in the Domain of Flow Stress Anomaly

1988 ◽  
Vol 133 ◽  
Author(s):  
G. Dirras ◽  
P. Beauchamp ◽  
P. Veyssière
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Flow Stress ◽  
Single Crystals ◽  
Room Temperature ◽  
Burgers Vector ◽  
Weak Beam ◽  
Transmission Electron ◽  
Edge Component ◽  
Imaging Conditions

ABSTRACTβ-brass single crystals oriented along <001> were deformed between room temperature and 300°C. The deformation microstructure and dissociation properties were studied by transmission electron microscopy under weak-beam imaging conditions.Whatever the deformation temperature, superdislocations with <111> Burgers vector and strong edge component dominate within the microstructure. In addition, below the temperature of the flow stress peak (≈ 250°C), the density of screw relative to mixed superdislocations decreases as straining temperature increases. Dissociation does not always occur on the slip plane neither does it proceed exclusively by glide, even in samples deformed at 100°C.

Synchrotron White Radiation X-Ray Topographic Investigation of Dislocation Configurations Developed in Indium Antimonide Single Crystals by Plastic Bending

1993 ◽  
Vol 307 ◽  
Author(s):  
Jun Wu ◽  
M. Dudley
Keyword(s):  
Single Crystals ◽  
Indium Antimonide ◽  
Plastic Behavior ◽  
Dislocation Loops ◽  
Screw Dislocations ◽  
X Ray ◽  
Three Point Bending ◽  
Brittle To Ductile Transition ◽  
Plastic Deformation Process ◽  
The Relationship

ABSTRACTSynchrotron white beam x-ray topography has been used to study dislocation configurations induced in InSb single crystals by three point bending at temperatures above the brittle to ductile transition point. Semi-hexagonal dislocation loops with one long screw segment and two outcropping 60° segments, single 60° B(g) dislocation kinks on screw dislocations, and screw dislocation dipoles were observed. The relationship between these observed defect structures and the mobilities of A(g), B(g), and screw dislocations in InSb, which appear to control the plastic behavior of crystals with the sphalerite structure, is discussed. Dislocation interactions and their role in the plastic deformation process are also addressed.

Determination of Burgers Vector of Screw Dislocations in 6H-SiC Single Crystals by Synchrotron White Beam X-Ray Topography

1996 ◽  
Vol 437 ◽  
Author(s):  
W. Si ◽  
M. Dudley ◽  
C. Carter ◽  
R. Glass ◽  
V. Tsvetkov
Keyword(s):  
Single Crystals ◽  
Dislocation Core ◽  
Line Defect ◽  
Screw Dislocations ◽  
Burgers Vector ◽  
X Ray ◽  
White Beam ◽  
Lattice Planes ◽  
Vector Magnitude

AbstractIndividual screw dislocations along the [0001] axis in 6H-SiC single crystals have been characterized by means of Synchrotron White Beam X-ray Topography (SWBXT). The magnitude of the Burgers vector was determined from: (1) the diameter of circular diffraction-contrast images of dislocations in back-reflection topographs, (2) the width of bi-modal images associated with screw dislocations in transmission topographs, (3) the magnitude of the tilt of the lattice planes on both sides of dislocation core in projection topographs, and (4) also the magnitude of the tilt of the lattice planes in section topographs. All of the four methods showed reasonable consistency. The sense of the Burgers vector can also be deduced from the abovementioned tilt of the lattice planes. Results revealed that in 6H-SiC a variety of screw dislocations can be found with Burgers vector magnitude ranging from 1c to 7c (c is the lattice constant along [0001] axis). This work demonstrates that SWBXT can be used as a quantitative technique for detailed analyses of line defect configurations.

Dislocation structures around SiO2 Particles in aged Cu-Cr-SiO2

1978 ◽  
Vol 36 (1) ◽  
pp. 594-595
Author(s):  
N.J. Long ◽  
M.H. Loretto ◽  
C.H. Lloyd
Keyword(s):  
Flow Stress ◽  
Single Crystals ◽  
Room Temperature ◽  
Thin Foil ◽  
Age Hardening ◽  
Dispersion Strengthening ◽  
Accurate Picture ◽  
The Matrix ◽  
Very High ◽  
Good Agreement

IntroductionThere have been several t.e.m. studies (1,2,3,4) of the dislocation arrangements in the matrix and around the particles in dispersion strengthened single crystals deformed in single slip. Good agreement has been obtained in general between the observed structures and the various theories for the flow stress and work hardening of this class of alloy. There has been though some difficulty in obtaining an accurate picture of these arrangements in the case when the obstacles are large (of the order of several 1000's Å). This is due to both the physical loss of dislocations from the thin foil in its preparation and to rearrangement of the structure on unloading and standing at room temperature under the influence of the very high localised stresses in the vicinity of the particles (2,3).This contribution presents part of a study of the Cu-Cr-SiO2 system where age hardening from the Cu-Cr and dispersion strengthening from Cu-Sio2 is combined.

scholarly journals Infecting dose and severity of typhoid: analysis of volunteer data and examination of the influence of the definition of illness used

1995 ◽  
Vol 115 (1) ◽  
pp. 23-30 ◽  
Author(s):  
J. R. Glynn ◽  
R. B. Hornick ◽  
M. M. Levine ◽  
D. J. Bradley
Keyword(s):  
Symptom Score ◽  
Salmonella Typhi ◽  
Peak Temperature ◽  
Challenge Dose ◽  
Severity Of Disease ◽  
Definition Of ◽  
The Relationship

SummaryData from volunteers challenged with Salmonella typhi were reanalysed to explore the relationship between challenge dose and severity of disease. Among 120 ill volunteers who received between 105 and 109 organisms, dose was weakly correlated with peak temperature (r = 0·22, 95% CI 0·04–0·39), duration of temperature above 103 °F (39·4 °C: r = 0·13, 95% CI –0·03 to 0·55) and symptom score (r = 0·27, 95% CI 0·09–0·43). The association with symptom score was lost after adjusting for year, and the findings depended on the definition of illness used: with stricter definitions the associations with temperature were also lost. The study shows the need for caution in interpreting the relationship between dose and severity of disease.

Generation of Defects and Strain by Ion Implantation in Ge (100) Single Crystals, and in Pseudomorphic GexSi1-x Films Grown on Si (100)

1992 ◽  
Vol 262 ◽  
Author(s):  
D. Y. C. Lie ◽  
A. Vantomme ◽  
F. Eisen ◽  
M. -A. Nicolet ◽  
V. Arbet-Engels ◽  
...  
Keyword(s):  
Ion Implantation ◽  
Single Crystals ◽  
Room Temperature ◽  
Defect Concentration ◽  
Double Crystal ◽  
X Ray ◽  
Positive Strain ◽  
Independent Coefficient ◽  
The Relationship ◽  
Si Ion Implantation

ABSTRACTWe have studied the damage and strain produced in Ge (100) single crystals by implantation of various doses of 300 keV 28Si ions at room temperature. The analyzing tools were x-ray double-crystal diffractometry, and MeV 4He channeling spectrometry. The damage induced by implantation produces positive strain in Ge (100). The maximum perpendicular strain and maximum defect concentration rise nonlinearly with increasing dose. These quantities are linearly related with a dose-independent coefficient of ∼ 0.013 for Ge (100) single crystals implanted at room temperature. The results are compared with those available for Si (100) self-implantation. We have also monitored the strain and defects generated in pseudomorphic Ge0.1Si0.9/Si (100) films induced by room temperature 28Si ion implantation. It is found that the relationship between the strain and defect concentration induced by ion implantation is no longer a simple linear one.

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