fmp 221 lecture 2
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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.