Atomic Mechanism
Discrete movements of a dislocation line across a lattice resistance barrier occur through the formation of local loops. The process of kink-pair nucleation involves the creation of two opposing steps on a defect line that subsequently spread apart. This action allows the defect to advance by one lattice spacing.
Thermal Activation
Energy fluctuations within the crystal provide the necessary impetus for these small-scale structural changes. When stress is applied, the energy barrier for kink-pair nucleation decreases, making the event more likely at a given temperature. This mechanism dominates the plastic behavior of materials with high lattice friction.
The probability of an event depends on the ratio between the activation energy and the thermal energy available in the system.
Expansion Rate
Separation of the newly formed steps determines the overall speed of the dislocation. Once kink-pair nucleation completes, the segments move rapidly along the line until they meet an obstacle or another kink. The frequency of these events sets the fundamental limit on the strain rate.
Boundary Condition
Low temperatures or extremely high strain rates alter the efficiency of this movement. In such regimes, kink-pair nucleation becomes the rate-limiting step for all plastic flow. Materials exhibiting this behavior often show a sharp transition from ductile to brittle states.
