Line defects in crystalline materials where atoms deviate from their ideal lattice positions, fundamentally affecting material properties and behavior.

Dislocations

Dislocations are fundamental crystal defects that occur when the regular arrangement of atoms in a crystal lattice is disrupted along a line. These imperfections play a crucial role in determining the mechanical properties of crystalline materials.

Types of Dislocations

Edge Dislocations

Characterized by an extra half-plane of atoms inserted into the crystal structure
Movement occurs perpendicular to the dislocation line
Creates localized stress field around the defect
Often visualized as a "T" shaped distortion in the lattice

Screw Dislocations

Formed by shear displacement parallel to the dislocation line
Creates a spiral ramp around the dislocation line
Movement occurs parallel to the applied stress
Results in a helical arrangement of atomic planes

Formation and Motion

Dislocations can form through various mechanisms:

Crystal growth processes
Plastic deformation
Phase transformation
Thermal stress cycling

Their motion through the crystal is fundamental to:

Impact on Material Properties

Mechanical Properties

Lower yield strength than theoretical perfect crystals
Enable ductility through slip mechanisms
Contribute to strain hardening behavior
Affect fracture mechanics

Other Effects

Influence electrical conductivity in semiconductors
Impact crystal growth processes
Affect diffusion rates in solids

Engineering Applications

Understanding and controlling dislocations is crucial for:

Detection and Characterization

Modern techniques for studying dislocations include:

Historical Development

The concept of dislocations was first proposed by Taylor in 1907, but their significance in explaining crystal plasticity was not recognized until the theoretical work of Taylor, Orowan, and Polanyi in the 1930s. This understanding revolutionized our comprehension of material behavior and enabled numerous technological advances in materials engineering.

The study of dislocations continues to evolve with new applications in: