Plastic deformations occur when a material is subjected to stress beyond its elastic limit, causing it to undergo permanent shape changes. This happens due to the motion of dislocations within the material’s crystal lattice. The mechanisms behind plastic deformation can be described as follows:
Dislocations are line defects in the crystal structure of materials. Under applied stress, these dislocations move, which causes the material to deform plastically.
There are two primary types of dislocations:
Edge dislocations: The dislocation line is perpendicular to the direction of applied stress.
Screw dislocations: The dislocation line is parallel to the direction of applied stress.
When stress is applied, dislocations move through the material along a specific plane, called the slip plane, and in a specific direction, called the slip direction. The movement of dislocations leads to permanent changes in the material’s shape.
Slip occurs when dislocations move along the most favorable planes (the planes with the highest atomic density) and in the direction of the applied stress.
Twinning is another plastic deformation mechanism, where part of the crystal structure becomes oriented in a mirror image of the original structure. This occurs under high stress or low temperatures and is less common than slip in many materials.
As dislocations move and interact with one another, they create obstacles that make it more difficult for additional dislocations to move. This leads to an increase in the material’s strength and hardness, known as strain hardening or work hardening. As a result, further plastic deformation requires higher stress.
Dislocations can interact in several ways, such as:
Dislocation annihilation: When two dislocations of opposite types meet, they cancel each other out, reducing the overall dislocation density.
Dislocation multiplication: Under high stresses, dislocations can multiply, creating more dislocations, which leads to greater deformation.
In polycrystalline materials, grain boundaries act as barriers to dislocation motion. When dislocations encounter grain boundaries, they can either:
Be blocked or pinned by the boundary, causing the material to harden.
Cross the boundary if the stress is high enough, resulting in plastic deformation across grains.
High temperatures generally facilitate plastic deformation as dislocation motion becomes easier.
At low temperatures, the material may undergo more brittle deformation since dislocations are less mobile.
In certain conditions, materials may undergo slow plastic deformation over time under constant stress, known as creep. This mechanism is important at high temperatures or under long-term load conditions.