What Is a Transmission Line?
Transmission lines are lines of misaligned atoms (defects) in metals.
Although it has been believed that all atoms in metals are regularly aligned, it has become clear that there are actually disarrangements in the alignment of atoms in some places.
When a group of atoms on a plane shifts in the same direction, the plane is called a slip plane, but it requires a considerably large amount of energy to shift a group of atoms at once. In reality, the metal begins to deform with very little force as a portion of the atoms are displaced in sequence along the slip surface. This theory was proposed by Taylor, Olowan, and others in 1934.
Uses of Transmission Lines
Dislocation lines are deformed by external forces caused by metal forging or plastic forming. Whenever deformation occurs, transmission lines are generated and move inside the metal. In order to strengthen metals, it is necessary to suppress the movement of dislocation lines.
Methods to suppress transmission lines include work hardening, precipitation strengthening, solid solution strengthening, and grain refinement.
Transmission lines are used to measure dislocation density by X-ray analysis. Dislocation density is the total length of dislocation lines. By measuring dislocation density, one can evaluate the degree to which a material has been strengthened. A greater dislocation density will create a greater strength material.
Other Information on Transmission Lines
1. Characteristics of Transmission Lines
Dislocation lines, which are linear lattice defects, are generated during plastic deformation of crystals and propagate to neighboring atoms, eventually reaching the surface of the crystal and displaying a step of one atom. Plastic deformation occurs inside the crystal as dislocation lines, which move to neighboring atoms step by step, so that no step is suddenly formed on the crystal surface.
An external shear force is required to cause dislocation migration, and this force is called the Peierls force. The Peierls force can be obtained from the spacing of slip surfaces, Burgers vector, Poisson’s ratio, and stiffness ratio.
2. Classification of Transmission Lines
The Burgers vector is a measure of the size or direction of dislocations. Transmission lines are classified into edge dislocations, helical dislocations, and mixed dislocations according to the relationship between transmission lines and Burgers vectors.
Edge Dislocations
This is an image of an atomic plane with one extra blade cut into the top or bottom half of the atomic plane in a row of many atomic planes. If there are slip planes at the top and bottom and a blade dislocation in the top half, the lattice is compressed at the top and expanded at the bottom, the condition is called “positive dislocation” and the opposite is called “negative dislocation”.
Helical Dislocation
When a line is defined in a metal, a 360° rotation around that line results in a displacement of the atomic layers in one plane. Depending on the direction of displacement, it is distinguished as right-handed or left-handed. In a blade dislocation, the Burgers vector and transmission lines are perpendicular to each other, while helical dislocations are parallel.
Mixed Dislocation
A dislocation is neither right-angled nor parallel and is a mixture of blade dislocations and helical dislocations.