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).

Investigation of shock-wave deformation history from recovered structure

1986 ◽  
Vol 44 ◽  
pp. 434-435
Author(s):  
M.A. Mogilevsky ◽  
L.S. Bushnev
Keyword(s):  
Shock Wave ◽  
Plastic Deformation ◽  
Dislocation Density ◽  
Shock Waves ◽  
Shock Front ◽  
Single Crystals ◽  
Residual Deformation ◽  
Deformation Structure ◽  
Deformation History ◽  
Deformation Velocity

Single crystals of Al were loaded by 15 to 40 GPa shock waves at 77 K with a pulse duration of 1.0 to 0.5 μs and a residual deformation of ∼1%. The analysis of deformation structure peculiarities allows the deformation history to be re-established.After a 20 to 40 GPa loading the dislocation density in the recovered samples was about 1010 cm-2. By measuring the thickness of the 40 GPa shock front in Al, a plastic deformation velocity of 1.07 x 108 s-1 is obtained, from where the moving dislocation density at the front is 7 x 1010 cm-2. A very small part of dislocations moves during the whole time of compression, i.e. a total dislocation density at the front must be in excess of this value by one or two orders. Consequently, due to extremely high stresses, at the front there exists a very unstable structure which is rearranged later with a noticeable decrease in dislocation density.

Computation on continuous grain refinement in AA1050 aluminum during repetitive tube expansion and shrinking technique

2015 ◽  
Vol 232 (4) ◽  
pp. 307-318
Author(s):  
H Jafarzadeh ◽  
K Abrinia
Keyword(s):  
Finite Element Method ◽  
Plastic Deformation ◽  
Grain Size ◽  
Finite Element ◽  
Dislocation Density ◽  
Severe Plastic Deformation ◽  
Grain Refinement ◽  
Half Cycle ◽  
Tube Expansion ◽  
Element Method

The microstructure evolution during recently developed severe plastic deformation method named repetitive tube expansion and shrinking of commercially pure AA1050 aluminum tubes has been studied in this paper. The behavior of the material under repetitive tube expansion and shrinking including grain size and dislocation density was simulated using the finite element method. The continuous dynamic recrystallization of AA1050 during severe plastic deformation was considered as the main grain refinement mechanism in micromechanical constitutive model. Also, the flow stress of material in macroscopic scale is related to microstructure quantities. This is in contrast to the previous approaches in finite element method simulations of severe plastic deformation methods where the microstructure parameters such as grain size were not considered at all. The grain size and dislocation density data were obtained during the simulation of the first and second half-cycles of repetitive tube expansion and shrinking, and good agreement with experimental data was observed. The finite element method simulated grain refinement behavior is consistent with the experimentally obtained results, where the rapid decrease of the grain size occurred during the first half-cycle and slowed down from the second half-cycle onwards. Calculations indicated a uniform distribution of grain size and dislocation density along the tube length but a non-uniform distribution along the tube thickness. The distribution characteristics of grain size, dislocation density, hardness, and effective plastic strain were consistent with each other.

Simulation of Grain Growth of Materials Produced by Severe Plastic Deformation

2011 ◽  
Vol 409 ◽  
pp. 597-602
Author(s):  
Yuichi Mizuno ◽  
Kenji Okushiro ◽  
Yoshiyuki Saito
Keyword(s):  
Plastic Deformation ◽  
Grain Size ◽  
Dislocation Density ◽  
Severe Plastic Deformation ◽  
Size Distribution ◽  
Grain Size Distribution ◽  
Boundary Migration ◽  
Interface Energy ◽  
Diffusion Reaction ◽  
Field Methods

Grain boundary migration in materials under severe plastic deformation was simulated by the phase field methods. The interface energy and dislocation density on growth kinetics were simulated on systems of 2-dimensional lattice. .In inhomogeneous systems grain size distributions in simulated grain structures were binodal distributions. The classification of the solution of differential equations based on the mean-field Hillert model describing temporal evolution of the scaled grain size distribution function was in good agreement with those given by the Computer simulations. Effect of dislocation on thermodynamic stability was taken into consideration. Dislocation density distribution was calculated by a equation based on the diffusion-reaction equation.. Scaled grain size distribution was known to be affected by the dislocation.

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.

scholarly journals Breakdown Degradation Associated With Elementary Screw Dislocations In 4H-SiC P+N Junction Rectifiers

1997 ◽  
Vol 483 ◽  
Author(s):  
P. G. Neudeck ◽  
W. Huang ◽  
M. Dudley
Keyword(s):  
Video Camera ◽  
Initial Study ◽  
Hollow Core ◽  
Power Devices ◽  
Screw Dislocations ◽  
Burgers Vector ◽  
X Ray ◽  
Current Voltage ◽  
The Impact ◽  
Sic Wafer

AbstractIt is well-known that SiC wafer quality deficiencies are delaying the realization of outstandingly superior 4H-SiC power electronics. While efforts to date have centered on eradicating micropipes (i.e., hollow core super-screw dislocations with Burgers vector > 2c), 4H-SiC wafers and epilayers also contain elementary screw dislocations (i.e., Burgers vector = Ic with no hollow core) in densities on the order of thousands per cm2, nearly 100-fold micropipe densities. This paper describes an initial study into the impact of elementary screw dislocations on the reverse-bias current-voltage (I-V) characteristics of 4H-SiC p+n diodes. First, Synchrotron White Beam X-ray Topography (SWBXT) was employed to map the exact locations of elementary screw dislocations within small-area 4H-SiC p+n mesa diodes. Then the high-field reverse leakage and breakdown properties of these diodes were subsequently characterized on a probing station outfitted with a dark box and video camera. Most devices without screw dislocations exhibited excellent characteristics, with no detectable leakage current prior to breakdown, a sharp breakdown I-V knee, and no visible concentration of breakdown current. In contrast devices that contained at least one elementary screw dislocation exhibited a 5% to 35% reduction in breakdown voltage, a softer breakdown I-V knee, and visible microplasmas in which highly localized breakdown current was concentrated. The locations of observed breakdown microplasmas corresponded exactly to the locations of elementary screw dislocations identified by SWBXT mapping. While not as detrimental to SiC device performance as micropipes, the undesirable breakdown characteristics of elementary screw dislocations could nevertheless adversely affect the performance and reliability of 4H-SiC power devices.

scholarly journals Nonequilibrium evolution thermodynamic of poly- and two-components alloys affected by severe plastic deformation

2021 ◽  
Vol 2052 (1) ◽  
pp. 012026
Author(s):  
L Metlov ◽  
M Gordey
Keyword(s):  
Plastic Deformation ◽  
Severe Plastic Deformation ◽  
Grain Boundaries ◽  
Kinetic Equations ◽  
Copper Matrix ◽  
Structural Defects ◽  
Two Component ◽  
Diffusion Phase ◽  
Qualitative Phase ◽  
And Diffusion

Abstract The nonequilibrium evolutionary thermodynamics approach is generalized to the case of alloys prone to structural martensitic and diffusion phase transitions in them. A system of kinetic equations is written out to describe the evolution of the density of structural defects, grain boundaries, dislocations and point defects, as well as for the order parameter in the processing of these alloys by the severe plastic deformation way. The approach is illustrated by the numerical experiments results on a specific example of two-component copper-based alloys. Kinetic curves of the evolution of the grain boundaries, dislocations and atoms dissolved in a copper matrix are obtained, qualitative phase diagrams are constructed.

Plastic Deformation of Approximants: Dislocations vs. Phason Lines

1998 ◽  
Vol 553 ◽  
Author(s):  
H. Klein ◽  
M. Feuerbacher ◽  
P. Schall ◽  
K. Urban
Keyword(s):  
Plastic Deformation ◽  
High Temperature ◽  
Single Crystals ◽  
Periodic Lattice ◽  
Length Scale ◽  
Relative Importance ◽  
Line Lattice ◽  
Burgers Vector ◽  
Mechanisms Of Plastic Deformation

AbstractDeformation experiments were performed on single crystals of the ξ-AIPdMn approximant in bending geometry at high temperature. Two different mechanisms of plastic deformation are shown to exist in this phase: one based on dislocations and another novel mechanism based on the motion of phason lines. Burgers vector and line directions of dislocations were determined. Phason lines are shown to build a periodic lattice. The interaction of a dislocation with the phason line lattice results in dislocations on another length scale. This meta-dislocation in the periodic phason line lattice has a Burgers vector of magnitude 165 Å. The relative importance of phason lines and dislocations for the plastic deformation is discussed as a function of the orientation of the sample with respect to the bending geometry.

TEM Characterization of a 7042 Aluminum FSW Joint

2012 ◽  
Vol 186 ◽  
pp. 331-334
Author(s):  
Mateusz Kopyściański ◽  
Stanislaw Dymek ◽  
Carter Hamilton
Keyword(s):  
Plastic Deformation ◽  
Aluminum Alloy ◽  
Friction Stir Welding ◽  
Dislocation Density ◽  
Severe Plastic Deformation ◽  
Base Material ◽  
Friction Stir ◽  
Tem Characterization ◽  
Equiaxed Grains

This research characterizes the changes in microstructure that occur in friction stir welded extrusions of a novel 7042 aluminum alloy. Due to the presence of scandium the base material preserved the deformation microstructure with elongated grains and fairly high dislocation density. The temperature increase with simultaneous severe plastic deformation occurring during friction stir welding induced significant changes in the microstructure within the weld and its vicinity. The weld center (stir zone) was composed of fine equiaxed grains with residual dislocations and a modest density of small precipitates compared to the neighbouring thermomechanically and heat affected zones where the density of small precipitates was much higher.

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