FMP 221 Lecture 2

Crystal defects

The missing and lacking of atoms or ions in an ideal or imaginary

crystal structure or lattice and the misalignment of unit cells in real

crystals are called crystal defects or solid defect.

Types of crystal defects

These imperfections result from deformation of the solid, rapid cooling from high temperature, or high-energy radiation striking the solid.

Located at single points, along lines, or on whole surfaces in the solid, these defects influence its mechanical, electrical, and optical behaviour.

Types of defects

1. Point defects

2. Line defects

3. Surface defects

Point defectsPoint defects are where an atom is missing or

is in an irregular place in the lattice structure.

Point defects are localized imperfections in

crystals. There are three types of point defect.

Vacancies

Interstitial defects

Substitutional defects.

The simplest type of point defect is formed when atoms are missing from the lattice, leaving a hole, this is known as a vacancy.

The other two types of point defect arise from the presence of alloying elements in the host metal. .

Vacancies

Substitutional defectsAlloying elements can dissolve

in the basic metal in two ways.

They can replace host atoms in

the lattice, which creates a

substitutional solid solution.

The substitute or impurity atom

is often larger than the atoms

of the host material.

This means there are strains imposed on the lattice.

Interstitial defects

Alternatively they may fit in the small spaces between the atoms of the host material creating an interstitial solid solution.

Interstitial atoms are much smaller than the host material- however they are usually bigger than the interstitial site so the lattice must deform to accommodate them.

Line defects

The defects which takes place due to

dislocation of atoms along a line, in

some direction is called line defects.

1. Edge dislocation

2. Screw dislocation

Edge dislocation

Edge dislocation is an extra half

plane of atoms “inserted” into

the crystal lattice.

Edge dislocations are caused by the

termination of a plane of atoms in the

middle of a crystal.

The top and bottom of the crystal above and below the

line appears to be perfect.

If the extra half plane is inserted from the top, the

defect so produced is represented by inverted Tee.

Edge location- When enough force is applied from one side

of the crystal structure, this extra plane passes through planes of

atoms breaking and joining bonds with them until it reaches the

grain boundary.

Due to the edge dislocations metals possess high

plasticity characteristics: ductility and malleability.

Edge dislocation moves parallel to the direction of stress

Screw dislocation

Screw dislocation forms when one part of crystal lattice is shifted (through shear) relative to the other crystal part.

The screw dislocation will move upward in the image, which is perpendicular to direction of

the stress

Surface or Planar defects

The defects which takes place on the surface of a

material are known as surface defects or plane

defects.

Surface defects take pace either due to imperfect

packing of atoms during crystallization or

defective orientation of the surface.

Grain Boundary is a general planar defect that

separates regions of different crystalline

orientation within a polycrystalline solid.

A grain boundary is the interface between two grains in a

polycrystalline material.

Grain boundary

Types of grain boundary

• Depending on the rotational axis, direction, two ideal types of a grain boundary are possible:

1. Tilt Boundary

2.Twin Boundary

A Tilt Boundary, between two slightly misaligned grains appears as an array of edge dislocations.

TILT BOUNDARY

Twin Boundary happens when the crystals on either side of a plane are mirror images of each other.

TWIN BOUNDARY

Deformation is a change in shape due to an applied force. This can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting). Deformation is often described in terms of strain.

Deformation may be temporary, as a spring returns to its original length when tension is removed, or permanent as when an object is irreversibly bent or broken.

DEFORMATION

Depending on the type of material, size and geometry of the object, and the forces applied, various types of deformation may result.

1.Elastic deformation

2.Plastic deformation

Types of deformation

Elastic deformation

Elastic deformation is reversible. Once the forces are no longer applied, the object returns to its original shape.

Soft thermoplastics and metals have moderate elastic deformation ranges while ceramics, crystals, and hard thermosetting plastics undergo almost no elastic deformation.

Fig. The Change of the lattice Structure with deformation.a) Original stateb) Elastic deformation with vertical force appliedc) Elastic deformation with diagonal force appliedd) The beginning of plastic deformation upon a slip planee) Deformation Proceeding upon further slip plane.

Plastic deformationThe permanent change in shape of a metallic body as a result of forces acting in a metal even after the removal of the stress.

It is due this property that the metals may be subjected to various operations like forging, drawing, spinning etc.

There are two basic modes of plastic deformation

Deformation by slip

Deformation by twinning

A shear deformation which moves the atom through

many inter-atomic distances relative to their initial

positions is called as deformation by slip.

Deformation by slip

The atoms move only a fraction of an inter atomic space

and this leads to a rearrangement of the lattice structure.

Twinning is observed as wide bands under the

microscope.

Deformation by twinning

Strain hardening (also called work-hardening or

cold-working) is the process of making a metal

harder and stronger through plastic deformation.

When a metal is plastically deformed,

dislocations move and additional dislocations

are generated.

Strain hardening

When a metal is worked at higher temperatures

(hot-working) the dislocations can rearrange and

little strengthening is achieved.

The more dislocations within a material, they will

interact and become pinned or tangled.

This will result in a decrease in the mobility of the

dislocations and a strengthening of the material.

Recovery, Recrystallization and Grain growth

During cold working of metals, the various properties change and certain amount of work done on the metal is stored internally in the form of strain energy. This energy produces internal stress in a cold worked metal and leads to cracking of metals and metal is thermo dynamically unstable.

To bring the metal approximately to the same state as it was before deformation, the deformed metal is heated to a temperature below the melting point.

In doing so the metal losses its stored energy and comes back to almost the same state as it was before deformation.

The metal losses its stored energy in three stages (Recovery, recrystallization and grain growth).

Recovery

The process of removing internal stresses, in

a metal, by heating it to a relatively low

temperature. This temperature is usually

below the melting point.

Recrystallization

The term ‘recrystallisation’ is the process of

forming strain-free new grains, in a metal by

heating it to a temperature known as

recrystallisation temperature.

At a higher temperature, new, strain-free grains nucleate

and grow inside the old distorted grains and at the grain

boundaries.

These new grains grow to replace the deformed grains

produced by the strain hardening.

With recrystallization, the mechanical properties return to

their original weaker and more ductile states.

Grain growth

The term ‘grain growth’ may be defined as the process of

forming strain-free grains larger in size in metal by

heating it to a temperature above to that of

recrystallisation.

It may be noted that the recrystallisation produces strain-

free new grains.

These grains are smaller of size, but of equal shape.

When the temperature is increased above that of

recrystallisation, these grains grow in size.

The grain growth takes place even during

recrystallisation. But the growth rate is slow and

becomes rapid with the increase of temperature.

The grain growth takes place due to the combination of

individual grains, thereby reducing their boundary area.

As a result of this, the total energy decreases and the

grains become stable.

The factors which affect the growth rate are time of

heating, temperature, degree of cold work and addition

of impurities.