Understanding the Fascinating Mechanism of Plastic Deformation

Mechanism of plastic deformation
Have you ever wondered why some objects regain their shape on bending, while others don't? This is because of a theory called 'plastic deformation', which is described in this Buzzle post with the help of its definition and mechanism.
Did You Know?
Plastic deformation ensures that, under the right temperature/pressure, even solid rocks can bend and fold to form geological folds.
Deformation is the change in the shape and size of an object. This change is usually caused when pressure or heat is applied on it. While this term is mostly used for metals and thermoplastics (recyclable polymers), the fact is, many other materials can undergo such changes. These include bones, concrete, and even rocks. Indeed, the phenomenon of deformation has played a vital role in shaping our planet, as its rock structure was sculpted over billions of years under enormous pressure. Depending on how pressure/heat is applied, the type of material, and its structure, deformation can be of various types. Of these, let us understand the process of plastic deformation, with its definition and mechanism.
Plastic Deformation: Definition
In material science and engineering, plastic deformation is a process which causes a permanent change in the shape and size of a solid object. This is the opposite of elastic deformation, where an object returns to its original shape and size, after the force which caused the deformation is removed.
When a force is applied on an object, this can cause a change in its shape and size. However, if the force is below a certain limit (called the 'elastic point'), the object will regain its original dimensions after force is removed. Such temporary change is called elastic deformation. However, if stress applied on the solid exceeds its elastic limit, it produces a permanent change in its dimensions, which cannot be reversed even after the stress is removed. This is called plastic deformation.
If the stress is still applied on the solid even after irreversible deformation occurs, then its cross-sectional area reduces at a certain point (called 'necking') and the solid breaks (called 'fracturing'). Plastic deformation is usually a property of ductile materials, such as gold, silver, and steel, and not of brittle ones like glass and cast iron.
Mechanism
Plastic deformation types
Slipping
This is the most important and probable type of plastic deformation in crystalline solids. In such solids, the atoms are tightly packed into symmetrical shapes, called crystals. Each crystal has atoms arranged in different layers or planes.
As the name suggests, slip deformation occurs when atomic layers in a crystal 'slip' over one another when it is subjected to stress. Each atom in a crystal plane moves by a few inter-atomic distances. This is analogous to the movement of a deck of cards, when pushed from one end. Slipping occurs when the shear forces (unaligned forces) that are exerted exceed a particular limit.
The movement of individual planes of a crystal leads to the formation of 'steps', where a plane protrudes from the crystal. However, different planes still have the same orientation to each other, because all atoms move by the same distance. Moreover, the 'slip' between different layers occurs on 'slip planes', which are regions where the most atoms are concentrated. This slip occurs because atoms move to fill in 'dislocations', i.e., microscopic defects in the crystal, such as vacancies in a crystal layer because of missing atoms.
Twinning
Twinning is a mechanism of deformation where the orientation of a crystal changes. The deformed part of the crystal forms a reversed duplicate, or mirror image of the undeformed part. Hence, the name of the process. The plane dividing the deformed and undeformed parts is called the 'twin plane'.
Twinning causes the elongation of the crystal in the direction of the tensile (stretching) force, which is usually higher than the force required for slip deformation. For this reason, twinning occurs when slip deformation is hindered. This type of plastic deformation causes all planes between the two parallel twin planes of the crystal to slip in the same direction, making their orientation different to the rest of the crystal.
The crystal orientation below and above the twin plane is different, whereas, the orientation above and below slip planes (in the former process) are always similar. This is why polishing a deformed metal does not remove the twin, while it can remove the 'steps' formed in slipping where there is no change in orientation. This difference also ensures that future deformation, in the form of slipping, occurs more easily.

In twinning, atoms move by a length proportional to their distance from the twin plane. This is usually a small fraction of the inter-atomic distance, meaning, the overall amount of deformation that takes place is usually small as compared to slipping.
The phenomenon of plastic deformation finds a wide range of industrial applications. It is used in metal-forming processes like extrusion and forging, where heat and pressure is used simultaneously to deform metals into their desired shapes or tools.