Dislocation creep
Encyclopedia
Dislocation creep is a deformation mechanism
in crystalline materials. Dislocation creep involves the movement of dislocation
s through the crystal lattice of the material. It causes plastic deformation of the individual crystal
s and in the end the material itself.
Dislocation creep is highly sensitive to the differential stress
on the material. At relatively low temperatures it is the dominant deformation mechanism in most crystalline materials.
. To allow the movement, all ionic bond
s along the plane have to be broken. If all bonds were broken at once, this would require so much energy that dislocation creep would only in theory be possible. When it is assumed that the movement takes place step by step, the breaking of bonds is immediately followed by the creation of new ones and the energy required is much lower. Calculations of molecular dynamics and analysis of deformed materials have shown that deformation creep can be an important factor in deformation processes, under certain circumstances.
By moving a dislocation step by step through a crystal lattice a linear lattice defect is created between parts of the crystal lattice, which is called a dislocation. Two types of dislocations exist. Edge dislocations form the edge of an extra layer of atoms inside the crystal lattice. Screw dislocations form a line along which the crystal lattice jumps one lattice point. In both cases the dislocation line forms a linear defect through the crystal lattice, the crystal can be perfect on all sides of the line.
Edge dislocation move in a direction perpendicular to the dislocation line, screw dislocations move parallel to the dislocation line. In both cases this causes a part of the crystal to move relative to other parts. Meanwhile the dislocation itself moves further on along a glide plane. The crystal system
of the material (mineral
or metal
) determines how many glide planes are possible, and in which orientations. The orientation of the differential stress then determines which glide planes are active and which are not. The Von Mises criterion states that to deform a material, movement along at least five different glide planes is required. A dislocation will not always be a straight line and can thus move along more than one glide plane. Where the orientation of the dislocation line changes, a srew dislocation can continue as an edge dislocation and vice versa.
The length of the displacement in the crystal caused by the movement of the dislocation is called the Burgers vector
. It equals the distance between two atoms or ions in the crystal lattice. Therefore each material has its own characteristic Burgers vectors for each glide plane.
(the boundary between two crystals). When it reaches a grain boundary, the dislocation will disappear. In that case the whole crystal has shear
ed a little. There are however different ways in which the movement of a dislocation can be slowed or stopped. When a dislocation moves along several different glide planes, it can have different velocities in these different planes, due to the anisotrope nature of some materials (anisotrope means the material properties are not the same in each direction). Dislocations can also encounter other defects in the crystal on their ways, such as other dislocations or point defects. In such cases a part of the dislocation could slow down or even stop moving altogether.
In alloy design, this effect is used to a great extent. on adding a dissimilar atom or phase, such as a small amount of carbon
to iron
, it is hardened
, meaning deformation of the material will be more difficult (the material becomes stronger). The carbon atoms act as interstitial particles (point defects) in the crystal lattice of the iron, and dislocations will not be able to move as easily as before.
point of view reduce the amount of free energy
in the system. Therefore, parts of a crystal that have more dislocations will be relatively unstable. By recrystallisation the crystal can heal itself. Recovery of the crystal structure can also take place when two dislocations with opposite displacement meet each other.
A dislocation that has been brought to a halt by an obstacle (a point defect) can overcome the obstacle and start moving again by a process called dislocation climb. For dislocation climb to occur, vacancies have to be able to move through the crystal. When a vacancy arrives at the place where the dislocation is stuck it can cause the dislocation to climb out of its glide plane, after which the point defect is no longer in its way. Dislocation climb is therefore dependent from the velocity of vacancy diffusion
. As with all diffusion processes, this is highly dependent on the temperature. At higher temperatures dislocations will more easily be able to move around obstacles. For this reason, many hardened materials become exponentially weaker at higher temperatures.
To increase the free energy in the system, dislocations can tend to concentrate themselves in certain zones, so that other regions will stay free of dislocations. This leads to the formation of 'dislocation walls', planes in a crystal where dislocations localise. Edge dislocations form so called tilt walls, while screw dislocations form twist walls. In both cases the increasing localisation of dislocations in the wall will increase the angle between the orientation of the crystal lattice on both sides of the wall. This leads to the formation of subgrains. The process is called subgrain rotation
(SGR) and can eventually lead to the formation of new grains when the dislocation wall becomes a new grain boundary.
Another way in which new dislocations can form are so called Frank-Read sources
. These form when a dislocation is stopped at two places. The part of the dislocation in between will move along, causing the dislocation line to curve. This curving can continue until the dislocation curves over itself to form a circle. In the centre of such a circle the source will produce a new dislocation, and this process will produce a sequence of dislocations on top of each other. Frank-Read sources are also created when screw dislocations double cross-slip (change glide planes
twice), as the jogs
in the dislocation line pin the dislocation in the 3rd plane.
Notes:
Literature:; 1976: Plasticité à haute température des solides cristallins, Eyrolles, Paris., 2000: Structural Geology, W.H. Freeman & co (6th ed.), ISBN 0-7167-2252-6
Deformation mechanism
In structural geology, metallurgy and materials science, deformation mechanisms refer to the various mechanisms at the grain scale that are responsible for accommodating large plastic strains in rocks, metals and other materials.-Mechanisms:...
in crystalline materials. Dislocation creep involves the movement of dislocation
Dislocation
In materials science, a dislocation is a crystallographic defect, or irregularity, within a crystal structure. The presence of dislocations strongly influences many of the properties of materials...
s through the crystal lattice of the material. It causes plastic deformation of the individual crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s and in the end the material itself.
Dislocation creep is highly sensitive to the differential stress
Differential stress
Differential stress is the difference between the greatest and the least compressive stress experienced by an object. For both the geological and civil engineering convention \sigma_1 is the greatest compressive stress and \sigma_3 is the weakest,...
on the material. At relatively low temperatures it is the dominant deformation mechanism in most crystalline materials.
Principles
Dislocations and glide planes in crystals
Dislocation creep takes place due to the movement of dislocations through a crystal lattice. Each time a dislocation moves through a crystal, part of the crystal moves one lattice point along a plane, relative to the rest of the crystal. The plane that separates both parts and along which the movement takes place is called a glide planeGlide plane
In crystallography, a glide plane is symmetry operation describing how a reflection in a plane, followed by a translation parallel with that plane, may leave the crystal unchanged....
. To allow the movement, all ionic bond
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
s along the plane have to be broken. If all bonds were broken at once, this would require so much energy that dislocation creep would only in theory be possible. When it is assumed that the movement takes place step by step, the breaking of bonds is immediately followed by the creation of new ones and the energy required is much lower. Calculations of molecular dynamics and analysis of deformed materials have shown that deformation creep can be an important factor in deformation processes, under certain circumstances.
By moving a dislocation step by step through a crystal lattice a linear lattice defect is created between parts of the crystal lattice, which is called a dislocation. Two types of dislocations exist. Edge dislocations form the edge of an extra layer of atoms inside the crystal lattice. Screw dislocations form a line along which the crystal lattice jumps one lattice point. In both cases the dislocation line forms a linear defect through the crystal lattice, the crystal can be perfect on all sides of the line.
Edge dislocation move in a direction perpendicular to the dislocation line, screw dislocations move parallel to the dislocation line. In both cases this causes a part of the crystal to move relative to other parts. Meanwhile the dislocation itself moves further on along a glide plane. The crystal system
Crystal system
In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...
of the material (mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
or metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
) determines how many glide planes are possible, and in which orientations. The orientation of the differential stress then determines which glide planes are active and which are not. The Von Mises criterion states that to deform a material, movement along at least five different glide planes is required. A dislocation will not always be a straight line and can thus move along more than one glide plane. Where the orientation of the dislocation line changes, a srew dislocation can continue as an edge dislocation and vice versa.
The length of the displacement in the crystal caused by the movement of the dislocation is called the Burgers vector
Burgers vector
The Burgers vector, named after Dutch physicist Jan Burgers, is a vector, often denoted b, that represents the magnitude and direction of the lattice distortion of dislocation in a crystal lattice....
. It equals the distance between two atoms or ions in the crystal lattice. Therefore each material has its own characteristic Burgers vectors for each glide plane.
Dislocation movement
A dislocation can ideally move through a crystal until it reaches a grain boundaryGrain boundary
A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material...
(the boundary between two crystals). When it reaches a grain boundary, the dislocation will disappear. In that case the whole crystal has shear
Shearing (physics)
Shearing in continuum mechanics refers to the occurrence of a shear strain, which is a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress in the material...
ed a little. There are however different ways in which the movement of a dislocation can be slowed or stopped. When a dislocation moves along several different glide planes, it can have different velocities in these different planes, due to the anisotrope nature of some materials (anisotrope means the material properties are not the same in each direction). Dislocations can also encounter other defects in the crystal on their ways, such as other dislocations or point defects. In such cases a part of the dislocation could slow down or even stop moving altogether.
In alloy design, this effect is used to a great extent. on adding a dissimilar atom or phase, such as a small amount of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
to iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, it is hardened
Hardening (metallurgy)
Hardening is a metallurgical and metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain...
, meaning deformation of the material will be more difficult (the material becomes stronger). The carbon atoms act as interstitial particles (point defects) in the crystal lattice of the iron, and dislocations will not be able to move as easily as before.
Dislocation recovery
Dislocations are imperfections in a crystal lattice, that from a thermodynamicThermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
point of view reduce the amount of free energy
Thermodynamic free energy
The thermodynamic free energy is the amount of work that a thermodynamic system can perform. The concept is useful in the thermodynamics of chemical or thermal processes in engineering and science. The free energy is the internal energy of a system less the amount of energy that cannot be used to...
in the system. Therefore, parts of a crystal that have more dislocations will be relatively unstable. By recrystallisation the crystal can heal itself. Recovery of the crystal structure can also take place when two dislocations with opposite displacement meet each other.
A dislocation that has been brought to a halt by an obstacle (a point defect) can overcome the obstacle and start moving again by a process called dislocation climb. For dislocation climb to occur, vacancies have to be able to move through the crystal. When a vacancy arrives at the place where the dislocation is stuck it can cause the dislocation to climb out of its glide plane, after which the point defect is no longer in its way. Dislocation climb is therefore dependent from the velocity of vacancy diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
. As with all diffusion processes, this is highly dependent on the temperature. At higher temperatures dislocations will more easily be able to move around obstacles. For this reason, many hardened materials become exponentially weaker at higher temperatures.
To increase the free energy in the system, dislocations can tend to concentrate themselves in certain zones, so that other regions will stay free of dislocations. This leads to the formation of 'dislocation walls', planes in a crystal where dislocations localise. Edge dislocations form so called tilt walls, while screw dislocations form twist walls. In both cases the increasing localisation of dislocations in the wall will increase the angle between the orientation of the crystal lattice on both sides of the wall. This leads to the formation of subgrains. The process is called subgrain rotation
Continuous sub-grain rotation type dynamic recrystallization
In Metallurgy, Materials Science and Structural geology continuous sub-grain rotation type dynamic recrystallization is recognized as an important mechanism for dynamic recrystallisation. It involves the rotation of initially low-angle sub-grain boundaries until the mismatch between the crystal...
(SGR) and can eventually lead to the formation of new grains when the dislocation wall becomes a new grain boundary.
Origin of dislocations
When a crystalline material is put under differential stress, new dislocations form at the grain boundaries, and begin moving through the crystal.Another way in which new dislocations can form are so called Frank-Read sources
Frank-Read Source
A Frank-Read Source is a mechanism explaining the generation of multiple dislocations in specific well spaced slip planes in crystals when they are deformed. It was proposed by and named after Sir Charles Frank and Thornton Read. When a crystal is deformed, slip is found to occur only on certain...
. These form when a dislocation is stopped at two places. The part of the dislocation in between will move along, causing the dislocation line to curve. This curving can continue until the dislocation curves over itself to form a circle. In the centre of such a circle the source will produce a new dislocation, and this process will produce a sequence of dislocations on top of each other. Frank-Read sources are also created when screw dislocations double cross-slip (change glide planes
Glide plane
In crystallography, a glide plane is symmetry operation describing how a reflection in a plane, followed by a translation parallel with that plane, may leave the crystal unchanged....
twice), as the jogs
Jog (dislocations)
Jog is a term related to the concept of dislocation. It is used to describe the turns of a dislocation line inside a crystal structure. A dislocation line is rarely uniformly straight as in the figure, often containing many curves and/or steps to facilitate movement through the crystal in...
in the dislocation line pin the dislocation in the 3rd plane.
See also
- diffusion creepDiffusion creepDiffusion creep refers to the deformation of crystalline solids by the diffusion of vacancies through their crystal lattice. Diffusion creep results in plastic deformation rather than brittle failure of the material....
Notes:
Literature:; 1976: Plasticité à haute température des solides cristallins, Eyrolles, Paris., 2000: Structural Geology, W.H. Freeman & co (6th ed.), ISBN 0-7167-2252-6