Pinning points
Encyclopedia
In a crystalline material, a dislocation
is capable of traveling throughout the lattice when relatively small stresses are applied. This movement of dislocations results in the material plastically deforming. Pinning points in the material act to halt a dislocation's movement, requiring a greater amount of force to be applied to overcome the barrier. This results in an overall strengthening of materials
.
s within a material that disallow traveling dislocations to come into direct contact. Much like two particles of the same electric charge feel a repulsion to one another when brought together, the dislocation is pushed away from the already present stress field.
ing atom is by nature a point defect, thus it must create a stress field when placed into a foreign crystallographic position, which could block the passage of a dislocation. However, it is possible that the alloying material is approximately the same size as the atom that is replaced, and thus its presence would not stress the lattice (as occurs in cobalt alloyed nickel). The different atom would, though, have a different elastic modulus
, which would create a different terrain for the moving dislocation. A higher modulus would look like an energy barrier, and a lower like an energy trough – both of which would stop its movement.
of a second phase within the lattice of a material creates physical blockades through which a dislocation cannot pass. The result is that the dislocation must bend (which requires greater energy, or a greater stress to be applied) around the precipitates, which inevitably leaves residual dislocation loops encircling the second phase material and shortens the original dislocation.
, there is a lattice mismatch, and every atom that lies on the boundary is uncoordinated
. This stops dislocations that encounter the boundary from moving.
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...
is capable of traveling throughout the lattice when relatively small stresses are applied. This movement of dislocations results in the material plastically deforming. Pinning points in the material act to halt a dislocation's movement, requiring a greater amount of force to be applied to overcome the barrier. This results in an overall strengthening of materials
Strengthening mechanisms of materials
Methods have been devised to modify the yield strength, ductility, and toughness of both crystalline and amorphous materials. These strengthening mechanisms give engineers the ability to tailor the mechanical properties of materials to suit a variety of different applications. For example, the...
.
Point defects
Point defects (as well as stationary dislocations, jogs, and kinks) present in a material create stress fieldStress field
A stress field is a region in a body for which the stress is defined at every point. Stress fields are widely used in fluid dynamics and materials science....
s within a material that disallow traveling dislocations to come into direct contact. Much like two particles of the same electric charge feel a repulsion to one another when brought together, the dislocation is pushed away from the already present stress field.
Alloying elements
The introduction of atom1 into a crystal of atom2 creates a pinning point for multiple reasons. An alloyAlloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
ing atom is by nature a point defect, thus it must create a stress field when placed into a foreign crystallographic position, which could block the passage of a dislocation. However, it is possible that the alloying material is approximately the same size as the atom that is replaced, and thus its presence would not stress the lattice (as occurs in cobalt alloyed nickel). The different atom would, though, have a different elastic modulus
Elastic modulus
An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force is applied to it...
, which would create a different terrain for the moving dislocation. A higher modulus would look like an energy barrier, and a lower like an energy trough – both of which would stop its movement.
Second phase precipitates
The precipitationPrecipitation (chemistry)
Precipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
of a second phase within the lattice of a material creates physical blockades through which a dislocation cannot pass. The result is that the dislocation must bend (which requires greater energy, or a greater stress to be applied) around the precipitates, which inevitably leaves residual dislocation loops encircling the second phase material and shortens the original dislocation.
Grain boundaries
Dislocations require proper lattice ordering to move through a material. At grain boundariesGrain 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...
, there is a lattice mismatch, and every atom that lies on the boundary is uncoordinated
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....
. This stops dislocations that encounter the boundary from moving.