Effects Of Externally Generated Dislocations On Brittleness/Ductility Of Crystals

1995 ◽  
Vol 409 ◽  
Author(s):  
Sinisa DJ. Mesarovic
Keyword(s):  
Dislocation Density ◽  
Slip Plane ◽  
Crack Velocity ◽  
Crack Tip ◽  
Atomic Scale ◽  
Burgers Vector ◽  
Density Dependent ◽  
Crack Trapping ◽  
Trapping Process ◽  
Temperature Crack

AbstractDirect interactions of externally generated dislocations with a moving, non-emitting crack tip, have been investigated. Dislocations of the appropriate Burgers vector, initially residing in a strip ahead of the incoming crack tip, are funneled toward the tip, with their motion restricted to the slip plane. Once drawn into the vicinity of the tip, these dislocations inevitably cause local, atomic scale blunting of the tip. The resulting crack trapping process is modelled, resulting in the temperature, crack velocity, and dislocation density dependent, tip toughness.

scholarly journals Twinning in Czochralski-Grown 36°-RY LiTaO3 Single Crystals

Crystals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1009
Author(s):  
Yutaka Ohno ◽  
Yuta Kubouchi ◽  
Hideto Yoshida ◽  
Toshio Kochiya ◽  
Tomio Kajigaya
Keyword(s):  
Single Crystals ◽  
Slip Plane ◽  
Edge Dislocation ◽  
Lithium Tantalate ◽  
Initiation Site ◽  
The Other ◽  
Burgers Vector ◽  
Partial Dislocations ◽  
Dislocation Arrays ◽  
Extension Direction

The origin of twinning during the Czochralski (CZ) growth of 36°-RY lithium tantalate (LiTaO3) single crystals is examined, and it is shown that lineages composed of dislocation arrays act as an initiation site for twinning. Two types of lineages expand roughly along three different {12¯10} planes and two different {11¯00} planes. The former lineages and some latter lineages are composed of two types of mixed-dislocations with different Burgers vectors, while the other lineages are composed of only one type of edge-dislocation. All the dislocations have the Burgers vector of ⟨12¯10⟩ type with the compression side at the +Z side. Twin lamellae on {101¯2} are generated at a lineage during the CZ growth. We have hypothesized that dislocations in the lineage with b = 1/3⟨12¯10⟩ change their extension direction along a slip plane of {101¯2}, and they dissociate into pairs of partial dislocations with b = 1/6⟨22¯01⟩and 1/6⟨02¯21¯⟩ forming twin lamellae on {101¯2}.

Distribution of Dislocation Density in Tubes from Zr-Based Alloys by X-Ray Data

2005 ◽  
Vol 105 ◽  
pp. 89-94 ◽  
Author(s):  
Margarita Isaenkova ◽  
Yuriy Perlovich
Keyword(s):  
Dislocation Density ◽  
Domain Size ◽  
Ray Method ◽  
Coherent Domain Size ◽  
Texture Measurement ◽  
Burgers Vector ◽  
X Ray ◽  
Pole Figures ◽  
Coherent Domain

As applied to tubes from Zr-based alloys, the X-ray method was developed to determine the dislocation density distribution in a-Zr depending on the orientation of Burgers vector. The method consists in registration of X-ray line profiles by each successive position of the sample in the course of diffractometric texture measurement using reflections of two orders, the following determination of coherent domain size and lattice distortion by means of the Warren-Averbach method for each orientation of reflecting planes, separate calculation of the density of c- and a-dislocations with all possible orientations of Burgers vector and presentation of results in generalized pole figures. Obtained data testify that the dislocation density varies within very wide intervals of several orders of magnitude depending on the grain orientation both in as-rolled and annealed tubes. Features of the constructed dislocation distributions are closely related to the crystallographic texture of studied tubes.

Modelling Dynamic Recovery and Recrystallization of Metals by a New Non-Equilibrium Thermodynamics Approach

2007 ◽  
Vol 558-559 ◽  
pp. 517-522
Author(s):  
Ming Xin Huang ◽  
Pedro E.J. Rivera-Díaz-del-Castillo ◽  
Sybrand van der Zwaag
Keyword(s):  
Steady State ◽  
High Temperature ◽  
Flow Stress ◽  
Dislocation Density ◽  
Dynamic Recovery ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
Equilibrium Thermodynamics ◽  
Burgers Vector ◽  
Non Equilibrium

A non-equilibrium thermodynamics-based approach is proposed to predict the dislocation density and flow stress at the steady state of high temperature deformation. For a material undergoing dynamic recovery and recrystallization, it is found that the total dislocation density can be expressed as ( )2 ρ = λε& b , where ε& is the strain rate, b is the magnitude of the Burgers vector and λ is a dynamic recovery and recrystallization related parameter.

The Influence of Creep-Fatigue Interaction on High Temperature Crack Growth in 316 Steel Weldments

2009 ◽  
Author(s):  
C. M. Davies ◽  
David W. Dean ◽  
A. N. Mehmanparast ◽  
K. M. Nikbin
Keyword(s):  
High Temperature ◽  
Crack Growth ◽  
Static Load ◽  
Growth Models ◽  
Creep Crack Growth ◽  
Crack Tip ◽  
Parent Material ◽  
Creep Crack ◽  
Static Creep ◽  
Temperature Crack

High temperature crack growth in weldments is of great concern and generally occurs along the boundary between the heat affected zone (HAZ) and parent material (PM) of welded components in high temperature plants. Static creep and low frequency cyclic crack growth tests have been performed on compact tension, C(T), specimens manufactured from sections taken from a 316 steel weldment at 550 °C, where the crack tip was located along the fusion line within the HAZ. The data has been analyzed in terms of both the creep-brittle and creep-ductile crack tip parameters, K and C*, respectively. The cyclic test results have been compared to static creep crack growth tests on 316 steel weldments and homogeneous parent material specimens, and to crack growth models. The cracking rates of the cyclic crack growth tests are found to be higher than that of the of the static load creep crack growth tests on weldments. The data may be bounded by the high frequency fatigue and the static load creep crack growth predictions. However, further work is required to establish the fatigue and subsequently the creep component of the cyclic load crack growth tests on weldments.

The Weighted Burgers Vector: A Quantity for Constraining Dislocation Densities and Types Using Electron Backscatter Diffraction on 2D Sections through Crystalline Materials

2012 ◽  
Vol 715-716 ◽  
pp. 732-736 ◽  
Author(s):  
John Wheeler ◽  
Elisabetta Mariani ◽  
Sandra Piazolo ◽  
David J. Prior ◽  
P.J. Trimby ◽  
...  
Keyword(s):  
Dislocation Density ◽  
Coordinate System ◽  
Lattice Distortion ◽  
Local Lattice Distortion ◽  
Burgers Vector ◽  
Density Tensor ◽  
The Third ◽  
Third Dimension ◽  
Dislocation Density Tensor ◽  
Local Misorientation

The Weighted Burgers Vector (WBV) is defined as the sum, over all types of dislocations, of [(density of intersections of dislocation lines with a map) x (Burgers vector)]. It can be calculated, for any crystal system, solely from orientation gradients in a map view, unlike the full dislocation density tensor, which requires gradients in the third dimension. No assumption is made about gradients in the third dimension and they may be non-zero. The only assumption involved is that elastic strains are small so the lattice distortion is entirely due to dislocations. Orientation gradients can be estimated from gridded orientation measurements obtained by EBSD mapping, so the WBV can be calculated as a vector field on an EBSD map. The magnitude of the WBV gives a lower bound on the magnitude of the dislocation density tensor when that magnitude is defined in a coordinate invariant way. The direction of the WBV can constrain the types of Burgers vectors of geometrically necessary dislocations present in the microstructure, most clearly when it is broken down in terms of lattice vectors. The WBV has five advantages over other measures of local lattice distortion. 1. It is a vector and hence carries more information than any scalar measure of local misorientation. 2. It has an explicit mathematical link to the individual Burgers vectors of dislocations. 3. Since it is derived via tensor calculus, it is not dependent on the map coordinate system, in contrast to existing measures of local misorientation which are not only scalar but dependent on the coordinate system used. 4. Calculation involves no assumptions about energy minimisation. 5. The numerical differentiation involved in calculating the WBV may introduce errors, but there is a direct mathematical link to a contour integral. The net Burgers vector content of dislocations intersecting an area of a map can be simply calculated by an integration round the edge of that area, a method which is fast and complements point-by-point WBV calculations. Errors in orientation measurement will have a much smaller effect here, and dislocations can be detected which are otherwise lost in the noise of any local calculation.

WORK HARDENING IN EASY GLIDE

1967 ◽  
Vol 45 (2) ◽  
pp. 765-775 ◽  
Author(s):  
P. M. Hazzledine
Keyword(s):  
Flow Stress ◽  
Slip Plane ◽  
Edge Dislocation ◽  
Work Hardening ◽  
Slip Line ◽  
Opposite Sign ◽  
Experimental Measurements ◽  
Line Pattern ◽  
Burgers Vector ◽  
Single Slip

A theory of single-slip work hardening is proposed, in which the flow stress is controlled by the stress required for one group of dislocations to pass another group with the same Burgers vector but with opposite sign on a parallel slip plane. The strain and the details of the slip-line pattern are calculated as a function of stress from the known properties of edge-dislocation multipoles. The predictions of the theory are compared with experimental measurements on Mg and are found to be in fair agreement.The theory presented is a highly simplified first version, and various possible developments of it are discussed.

scholarly journals INSTABILITY IN A DISLOCATION ENSEMBLE AT PLASTIC DEFORMATION IN METALS

2021 ◽  
Vol 83 (2) ◽  
pp. 198-206
Author(s):  
G.F. Sarafanov
Keyword(s):  
Plastic Deformation ◽  
Dislocation Density ◽  
Kinetic Equations ◽  
Elastic Field ◽  
Instability Criterion ◽  
Homogeneous Distribution ◽  
Homogeneous State ◽  
Screw Dislocations ◽  
Burgers Vector ◽  
Dislocation Ensemble

A problem related to the development of instability of a homogeneous state in an ensemble of screw dislocations under plastic deformation of metals is considered . The study of the development of instability and structure formation in the dislocation ensemble is carried out on the basis of the method developed for charged particles in plasma and associated with the correlation interaction of electrons and positively charged ions. Accordingly, the screw dislocation ensemble is represented as a system of dislocations with an opposite Burgers vector, i.e., as a plasma-like medium with opposite dislocation charges. The total dislocation charge of the dislocation ensemble is equal to zero due to the law of conservation of the Burgers vector. In this situation, the elastic field of dislocations is “cut off”. The stress field of a single dislocation is shielded by a uniformly distributed dislocation “background” and is characterized by a certain effective potential. It is found that at long distances it decreases exponentially. Therefore, the value in the argument of the falling potential can be considered as the radius of screening of the elastic field of dislocations. It is shown that the screening radius is equal to the correlation radius, which makes it possible to construct a two-particle correlation function and find the energy of the correlation interaction of dislocations. A system of kinetic equations for a dislocation ensemble is formulated, taking into account the elastic and correlation interaction of dislocations, as well as the processes of their generation and annihilation. The criterion of instability of the homogeneous distribution of dislocations for the formulated system of equations is established. The instability criterion is met under the condition that the dislocation density exceeds a certain critical value that depends on the square of the flow stress and material constants (such as the Burgers vector modulus and shear modulus, as well as indirectly, the packing defect energy). In the framework of linear analysis, it is shown that when one system of sliding screw dislocations is taken into account, a one – dimensional periodic dislocation dissipative structure is formed at the moment of instability occurrence, and when multiple sliding is taken into account, solutions appear in the form of various variants of polyhedral lattices (cellular structures). It is established that the characteristic size of the cellular structure coincides with the experimental dependence both qualitatively and quantitatively ( the cell size is proportional to the square root of the dislocation density, and the proportionality coefficient is about ten). It is shown that the origin of spatially inhomogeneous dislocation structures, based on correlation instability, depends mainly on the features of the elastic interaction of dislocations and is not critical to the choice of the mechanisms of their kinetics (i.e., the mechanisms of generation, annihilation, and runoff of dislocations).

Twinning in Crack Tip Plasticity of Two-Phase Titanium Aluminides

2000 ◽  
Vol 646 ◽  
Author(s):  
Fritz Appel
Keyword(s):  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Crack Tip ◽  
High Resolution Electron Microscopy ◽  
Strength Properties ◽  
Mechanical Twinning ◽  
Atomic Scale ◽  
Two Phase ◽  
Design Concepts ◽  
Crack Tip Plasticity

ABSTRACTIntermetallic titanium aluminides based on γ(TiAl) are prone to cleavage fracture on low index lattice planes. Unfavourably oriented grains may therefore provide easy crack paths so that the cracks can rapidly grow to a length which is critical for failure. The effect of crack tip plasticity on crack propagation in γ(TiAl) was investigated by conventional and high-resolution electron microscopy. Crack tip shielding due to mechanical twinning was recognized as toughening mechanism, which occur at the atomic scale and apparently is capable to stabilize fastly growing cracks. The potential of the mechanism will be discussed in the context of novel design concepts for improving the strength properties of γ-base titanium aluminide alloys.

Inhomogeneous Distribution of Dislocation Density as Manifestation of Multiscale Structure in Tubes from Zr - Based Alloys

2003 ◽  
Vol 779 ◽  
Author(s):  
Yuriy Perlovich ◽  
Margarita Isaenkova
Keyword(s):  
Dislocation Density ◽  
Domain Size ◽  
Nuclear Industry ◽  
Ray Method ◽  
Coherent Domain Size ◽  
Texture Measurement ◽  
Burgers Vector ◽  
X Ray ◽  
Pole Figures ◽  
Coherent Domain

AbstractAn X-ray method was developed to determine the dislocation density in metal materials as a distribution depending on the orientation of Burgers vector. The method includes registration of X-ray line profiles by each successive position of the sample in the course of diffractometric texture measurement using reflections of two orders, the following determination of coherent domain size and lattice distortion by means of the Warren-Averbach method for each orientation of reflecting planes, separate calculation of the density of c- and a-dislocations with all possible orientations of Burgers vector and presentation of results in the generalized pole figures. The method was used to determine the dislocation density in tubes of Zr-based alloys for nuclear industry. Obtained data show, that the dislocation density varies within very wide interval of several orders of magnitude depending on the grain orientation both in as-rolled and annealed tubes. Features of the dislocation distribution in tubes are closely related to their crystallographic texture.

Micromechanical modeling of crack propagation in weak snow layer

2020 ◽  
Author(s):  
Gregoire Bobillier ◽  
Alec van Herwijnen ◽  
Bastian Bergfeld ◽  
Johan Gaume ◽  
Jürg Schweizer
Keyword(s):  
Mechanical Properties ◽  
Crack Propagation ◽  
Crack Velocity ◽  
Stress Component ◽  
Crack Tip ◽  
Micromechanical Modeling ◽  
Snow Layer ◽  
Mechanical Field ◽  
Fracture Dynamics ◽  
Micromechanical Approach

<p>Improving the prediction of snow avalanches requires a detailed understanding of the fracture behavior of snow, which is intimately linked to the mechanical properties of the snow layers (strength, elasticity of the weak and slab layer). While the basic concepts of avalanche release are conceptually relatively well understood, understanding crack propagation and fracture propensity remains a great challenge. About 15 years ago, the propagation saw test (PST) was developed. The PST is a fracture mechanical field test that provides information on crack propagation propensity in weak snowpack layers. It has become a valuable research tool to investigate processes and mechanical parameters involved in crack propagation.</p><p>Here, we use the discrete element method (DEM) to numerically simulate PST and therefore analyze fracture dynamics based on micromechanical approach. Using cohesive and non-cohesive ballistic deposition, we numerically reproduce the basic required layers for dry-snow avalanche: a highly porous and brittle weak layer covered by a dense cohesive slab.</p><p>The results of these numerical PTSs reproduce the main dynamics of crack propagation observed in the field. We developed different indicators to define the crack tip and therefore derive the crack velocity. Our results show that crack propagation on flat terrain reaches a stationary velocity if the snow column in long enough. The length of the snow column to reach stationary crack velocity depends on snowpack parameters. On sloped terrain our results show a transition in the local failure mode, this transition can be visualized from the crack tip morphology and from the main stress component.</p><p>Overall, our results lay the foundation for a comprehensive study on the influence of the snowpack mechanical properties on these fundamental processes for avalanche release.</p>

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