What is plastic deformation?

Plastic deformation can be interpreted as the type of deformation in which invariable changes in the orientation of an element occurs due to an externally employed force. Some of the elements in which plastic deformation can be observed are concrete, rocks, metals, solids, and many others.

The methods of plastic deformation are as follows:

• Equal channel angular pressing (ECAP)
• Constrained groove pressing (CGP)
• High-pressure torsion (HPT)
• Accumulative roll bonding (ARB)
• Twist extrusion
• Multi-directional forging

What are metal single crystals?

A single metal crystal is a kind of element that contains continual as well as unimpaired crystal structure up to its corners. In metal crystals, the subatomic particles are kept collectively with the help of a metallic bond. Some of the common types of single crystals are listed below:

• Cadmium
• Bismuth
• Antimony
• Iron
• Lead

The following are various applications of single crystals.

• Single crystals are used in turbine blades
• It plays an essential role in the semiconductor industry
• It is used in optical applications and electrical conductors

Types of plastic deformation in single metal crystals

There are generally two types of plastic deformation that take place in single metal crystals.

• Slip
• Twinning

Slip

Slip is one of the most common plastic deformations found in single metal crystals. Suppose shear stress is employed on the crystal. In that case, the sliding of any certain section of the metal crystal happens within the direction of the crystal plane corresponding to the adjacent part. Dislocation motion is the main factor that causes a slip, and in order to reduce the resistance in dislocations, several stresses can be exerted on the crystal.

The diagram of slip deformation in a face-centered cubic crystal is as follows:

Twinning

In a cubic metal crystal, twinning deformation happens only if a section of the crystal takes up orientation in a symmetrical way corresponding to the other sections of the crystal structure. Any crystal lattice under high-stress rate conditions will experience twinning deformation, and it occurs a lot quicker than slip deformation. Generally, in face-centered cubic crystals, metal alloys, and other materials, twinning deformations take place.

The diagram of twinning deformation in a face-centered cubic crystal is as follows:

Difference between slip and twinning deformation

Below are some of the differences between slip deformation and twinning deformation.

• Slip deformation happens in discontinuous multiples of atomic spacing, whereas in twinning deformation, the motion of atoms is less.
• In twinning deformation, there is the possibility of variation in orientation around the plane, but in slip, the orientation remains the same and doesn't get affected by deformation.
• In slip deformation, critical resolved shear stress acts. On the other hand, critical shear stress doesn't exert for twinning.
• A slip in a single crystal is considered as line defect. Twinning is considered as a surface defect or boundary defect.
• When observed from a microscope, thin lines are observed for slip, and broad lines are observed for twinning.
• After the deformation, the axis of orientation remains unchanged for a slip, but it alters in the case of twinning.

Plastic deformation of polycrystals

Polycrystals are expressed as materials that are comprised of several kinds of crystallites separated by boundaries. All the crystallites do have different directions and inclinations with each other. The deformation in polycrystals is less as compared to single crystals because the organizations of grains are not symmetric as well as regular. The crystal grains in the polycrystalline materials are deformed in groups with force, and because of the uneven deformation, internal stresses are developed in the material.

The following are the structural diagrams of crystallites on polycrystals.

Some of the essential points about polycrystals are listed below.

• Ultrasonic boundary scattering is present in Polycrystals.
• The strength of polycrystals reduces by increasing the size of the crystals grain.
• The ductility of the materials rises by decreasing the size of the grain.
• Polycrystals are used in semiconductor devices.
• It is also used for large scale photovoltaic devices.
• Polycrystalline are less efficient as compared to monocrystalline.

Elastic deformation

When a surface undergoes a deformation caused by outside applied force (tension or compression) and regains its original orientation after releasing the force, then such sort of deformation is described as elastic deformation. Generally, elastic deformation is found in ceramics or metals at low strains. In order to achieve high elasticity, some softening is blended with materials under certain pressure. Some of the softening performance is created by the localization of the breakdown mechanism.

Difference between elastic deformation and plastic deformation

The below points represent the difference between plastic deformation and elastic deformation.

• Elastic deformation is recognized as temporary deformation, and plastic deformation is known as permanent deformation.
• Subatomic particles in the materials are dislocated for some time and return to their previous location in elastic deformation. On the other hand, the subatomic particles present in materials don't return to their previous position.
• Elasticity and plasticity are the properties characterized by elastic and plastic deformation, respectively.
• The requirement of the amount of force (tension or compression) is low in elastic deformation, whereas it is high in plastic deformation.
• In elastic deformation, Hooke's law can be employed, but it can't be used in plastic deformation.
• Mechanical properties don't vary due to elastic deformation, but it alters considerably for plastic deformation.
• Chemical bonds of elements stretch or bend in elastic deformation; on the other hand, the chemical bonds go through wreckage under plastic deformation.

The following is the stress-strain curve which explains the mechanism of elastic and plastic deformation.

The relationship between elastic and plastic deformation can be explained from the above graph between stress and strain. A point in the middle of the curve expressed elastic limit. The elastic region lies below this point, and the plastic region lies above this point.

Advantages of plastic deformation

• The ductility and strength of materials do vary with the extent of the grain, and these properties can be changed by altering grain size.
• Fatigue and creep behaviour
• Improvement in thermal stability of low carbon steels
• Rapid bulk formation
• Overall materials characteristics enhanced.

Disadvantages of plastic deformation

• Expensive equipment and fatigue resistance
• Precision and accuracy
• Less flexibility in surfaces as compared to machining
• Maintenance cost is high

Common mistakes

• Student makes mistakes between ductile and brittle materials because both of them are associated with plastic deformation, whereas only one of them show deformation under external load which is ductile.
• One of the critical points to remember from the stress-strain curve is that the fracture in the material does not happen immediately after crossing the elastic limit.
• Always use standard international units for the calculation of stress or strain.

Context and Application

The topic plastic deformation is very much significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

• Bachelors of Science in Physics
• Bachelors of Technology in Mechanical Engineering
• Bachelors of Technology in Civil Engineering
• Masters of Technology in Mechanical Engineering
• Doctor of Philosophy in Mechanical Engineering
• Diploma in Mechanical Engineering

Related concepts

• Crystalline imperfections
• Crystal lattice
• Theory of deformation
• Mechanical and metallurgical properties
• Deformation in solids

Practice problems

Q1. What is the point at which the engineering stress-strain curve is equivalent to true stress strain curve?

1. Plastic limit
2. Yield point
3. All of the above
4. None of these

Correct answer: (b)

Q2. In materials, from where does the high elastic modulus occurs?

1. Weak bonds
2. High strengths of bonds
3. mixture of bonds
4. All of these

Correct answer: (b)

Q3. Any particular materials ability to absorb energy with plastic limit is?

1. Malleability
2. Ductility
3. Fatigue strength
4. Toughness

Correct answer: (d)

Q4. In cemented carbides, which element is used?

1. Cobalt
2. Lithium
3. Potassium
4. Sodium

Correct answer: (a)

Q5. Which of the element increases the tensile strength?

1. Carbon
2. Manganese
3. Nickel
4. None of the above

Correct answer: (b)