Dislocations move primarily due to stress applied to the crystal lattice in which they exist.
Mechanisms of Dislocation Motion
Several mechanisms contribute to dislocation movement within a crystalline material:
-
Applied Stress: The most fundamental driver. When a stress (force per unit area) is applied to a crystal, it exerts a force on the dislocation line. This force, known as the Peach-Koehler force, causes the dislocation to glide (move along a slip plane) and climb (move perpendicular to the slip plane).
-
Glide (Slip): Glide is the primary mode of dislocation motion. It occurs when the dislocation moves along a crystallographic plane (slip plane) in the direction of the Burgers vector. The Burgers vector represents the magnitude and direction of the lattice distortion caused by the dislocation. Glide is relatively easy and requires less energy compared to climb.
-
Climb: Climb involves the movement of a dislocation perpendicular to its slip plane. This process requires diffusion of atoms to or from the dislocation core, which is typically a slower and more temperature-dependent process than glide. Climb allows dislocations to overcome obstacles that are not easily bypassed by glide.
-
Thermal Activation: The movement of dislocations, particularly climb and overcoming obstacles, is often thermally activated. This means that atoms need to gain enough thermal energy to break bonds and move, allowing the dislocation to proceed. Higher temperatures generally lead to easier dislocation motion.
-
Presence of Other Dislocations and Obstacles: The presence of other dislocations, grain boundaries, precipitates, or solute atoms can impede dislocation motion. These obstacles create stress fields that interact with the moving dislocation, requiring it to overcome these barriers through increased applied stress, thermal activation, or other mechanisms like cross-slip.
Factors Influencing Dislocation Movement
Several factors affect the ease with which dislocations move:
- Material Properties: The crystal structure, bond strength, and presence of defects in the material all influence dislocation mobility.
- Temperature: Higher temperatures generally facilitate dislocation motion, especially climb.
- Stress State: The magnitude and type of stress (tensile, compressive, shear) applied to the material significantly affect the driving force for dislocation movement.
- Grain Size: Smaller grain sizes typically impede dislocation motion due to the increased density of grain boundaries, which act as obstacles.
Consequences of Dislocation Movement
Dislocation movement is the fundamental mechanism responsible for plastic deformation in crystalline materials. Plastic deformation refers to the permanent change in shape of a material under applied stress. By moving and multiplying, dislocations allow the material to deform without fracturing. However, the accumulation and entanglement of dislocations also lead to strain hardening, where the material becomes stronger and more resistant to further deformation.
