Plastic deformation of single and polycrystalline materials
1. Prepared By:
NEGESA BEKUMA
ID/No ..1300723
Post Graduate Program in Manufacturing Engineering
Course Title: Advanced Materials Technology
Course code:MEng 7032
presentation on:
Title: PLASTIC DEFORMATION OF SINGLE AND POLYCRYSTALLINE MATERIALS
Jan-3- 2022
Ethiopia
Dire Dawa
Submitted to: Getahun Aklilu (PhD)
4/6/2022 12:06 AM
2. Contents
Introduction
1. Deformation Of Materials
1.1 Elastic Deformation and Plastic Deformation
1.2 Lattice Defects
1.3 Mechanisms Of Plastic Deformation In Metals
1.4 Dislocation Theory
1.5 Dislocation Behavior In BCC, FCC And HCP
Crystal Structures
1.6 Strengthening Mechanisms In Metals
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3. PLASTIC DEFORMATION OF SINGLE AND
POLYCRYSTALLINE MATERIALS:
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Fig. 1. types of solid materials
4. 1. DEFORMATION OF MATERIALS
o The deformation of materials is divided into two types’ those are
Elastic deformation and Plastic deformation.
i. Elastic deformation:
Elastic deformation can be restored to its original shape or size
after the external force is removed.
Elastic deformation is reversible i.e. recoverable.
Up to a certain limit of the applied stress, strain experienced by
the material will be the kind of recoverable i.e. elastic in nature.
This elastic strain is proportional to the stress applied.
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6. ii. Plastic deformation
Plastic deformation of a substance (including fluids and solids) is
caused by the action of an external force under certain conditions.
After the external force is removed, the elastic deformation part
disappears and the part of the deformation that cannot be
recovered and remains is called plastic deformation.
When the stress applied on a material exceeds its elastic limit, it
imparts permanent non-recoverable deformation called plastic
deformation in the material.
The main difference between elastic deformation and plastic
deformation is that elastic deformation is reversible and plastic
deformation is irreversible.
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7. Deformation Types in Single Crystals
Materials:
In general, the plastic deformation of a single crystal can be
divided into two types: slip and twinning.
Slip refers to sliding of a part of the crystal on a certain crystal
plane (slip plane) along a certain direction (slip direction) relative
to another part under the action of shear stress.
The combination of the slip surface and the slip direction is called
the slip system.
Slip is caused by dislocation motion.
Slip occurs when the shear stress applied to the slip direction on
the slip surface reaches a certain critical value.
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8. Plastic Deformation in Polycrystalline Materials
Deformation and slip in polycrystalline materials is somewhat
more complex.
Because of the random crystallographic orientations of the
numerous grains, the direction of slip varies from one grain to
another.
Microscopically it can be said of plastic deformation involves
breaking of original atomic bonds, movement of atoms and the
restoration of bonds.
It is important that deformation of grains is constrained by grain
boundaries, which maintain their integrity and coherency (i.e.
typically do not come apart and open during deformation).
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9. 1.2. Lattice defects
Lattice defects are missing atoms (vacancies) or atom clusters
and lattice misalignments such as dislocations.
Lattice defects in the films can be reduced by increased
substrate heating during deposition or controlled concurrent
ion bombardment during deposition.
Generally high defect concentrations result in poor electro
migration properties.
Types of lattice defects based on dimensionality:
A. Point defect.
B. Line defects
C. Surface defects
D. Volume defects
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10. 1. Point defect
Point defects are lattice defects of zero dimensionality,
i.e., they do not possess lattice structure in any
dimension.
When atoms are missing or an atoms is in an irregular
place in the lattice structure.
In a pure metal two types of point defect are possible,
namely a vacant atomic site or vacancy and a self-
interstitial atom.
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12. Classifications point defects.
a. Vacancy: - An atom missing from regular lattice position.
Vacancies are present invariably in all materials.
b. Interstitial: - An atom trapped in the interstitial point (a
point intermediate between regular lattice points) is called
an interstitially.
c. Impurity: - An impurity atom at the regular or interstitial
position in the lattice is another type of point defect.
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13. 2. Line defects
The most important two-dimensional, or line defect is the
dislocation.
Dislocation is the region of localized lattice distortion
which separates the slipped and not yet slipped portion
of the crystal.
The two basic types of dislocation are the edge
dislocation and the screw dislocation.
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15. 3. Surface defects:
are the grain boundaries or planes that separate a material
into regions, with each region having the same crystalline
structure but a different orientation.
Most crystalline solids are an aggregate of several
crystals. Such materials are called polycrystalline.
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|Fig.5 . Surface defect
16. 4. Volume defects
are two dimensional defects known as Bulk defects.
These includes porosity, cracks, foreign inclusions and
other phases.
Inclusion of foreign particles or non- crystalline regions of
dimensions of at least 10-30Å also belong to the category
of volume defects.
Voids are small regions where there are no atoms and can
be thought of as clusters of vacancies.
Impurities can cluster together to form small region of a
different phase.
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17. 1.3. Mechanisms of plastic deformation in metals
There are two prominent mechanisms of plastic
deformation, namely slip and twinning.
I. DEFORMATION BY SLIP: -
The usual method of plastic deformation in metals is by
the sliding of blocks of the crystal over one another along
definite crystallographic planes, called slip planes.
Slip is the prominent mechanism of plastic deformation in
metals.
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18. II. DEFORMATION BY TWINNING:
In addition to slip, plastic deformation in some metallic
materials can occur by the formation of mechanical twins, or
twinning.
The second important mechanism by which metals deform
is the process known as twinning.
Creates a deformed portion of a grain which is just mirror
image of the rest of the parents grain.
Twinning results when a portion of the crystal takes up an
orientation that is related to the orientation of the rest of the
un-twinned lattice in a definite, symmetrical way.
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20. Deformation in most common metallic crystal structures
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Fig .7. common metallic crystal structures
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Deformation of BCC materials? single crystalline metallic
nanoparticles, which has been seldom addressed and is lack
of consensus as to the deformation mechanism.
Deformation of FCC materials
Because fee crystals have high symmetry and 12 potential slip
systems, there is a wide choice of slip systems.
The slip plane will not have to undergo much rotation before the
resolved shear stress becomes high on another {111} slip system.
Deformation of HCP materials
The HCP structure has a relatively complex deformation
mechanism in comparison with BCC and FCC structures.
In the HCP crystal structure a number of slip systems exist which
are rather difficult to activate.
cont’d…
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22. 1.4 . Dislocation theory
The concept of the dislocation was proposed independently by Taylor,
Orowan, and Polanyi1 in 1934, but the idea lay relatively undeveloped until
the end of World War II.
Taylor's dislocation is a linear crystallographic defect or irregularity within
a crystal structure that contains an abrupt change in the arrangement of atoms.
In elasticity theory, a dislocation is defined as the strong discontinuity of the
displacement field.
The most powerful method available today for the detection of dislocations in
metals is transmission electron microscopy of thin foils.
Thin sheet, less than 1 mm thick, is thinned after deformation by electro
polishing to a thickness of about 1,000 A (= 100 nm).
At this thickness the specimen is transparent to electrons in the electron
microscope.
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23. 1.5 Dislocation Behavior In BCC, FCC And HCP Crystal
Structures :
I. Dislocation in BCC Crystal: -
In body-centered cubic metals (e.g. iron, molybdenum,
tantalum, vanadium, chromium, tungsten, niobium, sodium
and potassium) slip occurs in BCC lattice <111> directions.
The shortest lattice vector.
The crystallographic slip planes are {110}, {112} and
{123}in iron.
Thus, if cross slip is easy it is possible for screw dislocations
to move in a haphazard way on different {110} planes or
combinations of {110} and {112} planes, etc.
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24. II. Dislocation in FCC Crystal
The primary slip planes in FCC lattice are (octahedral
planes) [111].
The stacking sequence in this system is ABC ABC,……
Dislocations in FCC metals are also highly mobile.
Their motion does not require kink mechanism and is less
likely to be dominated by the surface nodes.
All these effects reduce the likelihood of the self-
multiplication mechanism in FCC pillars.
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25. DISLOCATION CLIMB
Dislocation climb occurs by the diffusion of vacancies or
interstitials to or away from the site of the dislocation.
Since climb is diffusion-controlled, it is thermally activated and
occurs more readily at elevated temperature.
In positive climb atoms are removed from the extra half plane of
atoms at a positive edge dislocation so that this extra half plane
moves up one atom spacing.
In negative climb a row of atoms is added below the extra half
plane so that the dislocation line moves down one atom spacing.
Under certain conditions an edge dislocation can move out of the
slip plane onto a parallel plane directly above or below the slip
plane. This is the process of dislocation climb.
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26. The intersection of two dislocations produces a sharp break, a
few atom spacing’s in length in the dislocation line.
These breaks can be of two types.
A jog is a sharp break in the dislocation moving it out of the
slip plane.
A kink is a sharp break in the dislocation line which remains
in the slip plane.
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INTERSECTION OF DISLOCATIONS:-
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27. MULTIPLICATION OF DISLOCATIONS
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Thus, there must be a method of generating dislocations or of
multiplying the number initially present to produce the high
dislocation density found in cold-worked metal.
One of the original stumbling blocks in the development of
dislocation theory was the formulation of a reasonable
mechanism by which sources originally present in the metal
could produce new dislocations by the process of slip.
Moreover, if there were no source generating dislocations, cold-
work should decrease, rather than increase, the density of
dislocations in a single crystal.
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28. 1.6. Strengthening mechanisms in Metals
The ability of a metal to deform plastically depends on
the ability of dislocations to move.
Hardness and strength are related to how easily a
metal plastically deforms, so, by reducing dislocation
movement, the mechanical strength can be improved.
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Fig.8 . Strengthening mechanism
29. i. Grain boundary strengthening:
In a polycrystalline metal, grain size has a tremendous influence on
the mechanical properties.
Because grains usually have varying crystallographic orientations,
grain boundaries arise.
While undergoing deformation, slip motion will take place.
Grain boundaries act as an impediment to dislocation motion for the
following two reasons:
1. Dislocation must change its direction of motion due to
the differing orientation of grains.
2. Discontinuity of slip planes from grain one to grain two.
Grain boundaries are barriers to slip.
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30. ii. Strain ageing:
Strain ageing is due to the diffusion of carbon and/or nitrogen
atoms in solution to dislocations that have been generated by
plastic deformation.
Initially, an atmosphere of carbon and nitrogen atoms is formed
along the length of a dislocation, immobilizing it.
Extended ageing however, results in sufficient carbon and nitrogen
atoms for precipitates to form along the length of the dislocation.
Change of mechanical properties of a metal by aging induced by
plastic deformation.
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31. iii. Solid solution strengthening
For this strengthening mechanism, solute atoms of one element are
added to another, resulting in either substitutional or interstitial
point defects in the crystal.
There are two types of solid solutions.
If the solute and solvent atoms are roughly similar in size, the
solute atoms will occupy lattice points in the crystal lattice of the
solvent atoms. This is called substitutional solid solution.
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Fig. 9. types of solid solution strengthens
32. v. Fiber strengthening
Second phase material can also be introduced into matrix in
form of fibers to strengthen it.
Materials of high strength and especially high strength-to-
weight ratio, can be produced by incorporating fine fibers in a
ductile matrix.
Glass-fiber-reinforced polymers are the most common fiber-
strengthened materials.
Prerequisites are materials to be used as fibers include high
strength and/or high strength-to-weight ratio.
Fibers usually, thus have high strength and high modulus while
the matrix must be ductile and non-reactive with the fibers.
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33. vi. Strain hardening:
Strain hardening is used commercially to enhance the
mechanical properties of metals during fabrication procedures.
Sometimes it is also called work hardening, or, because the
temperature at which deformation takes place is “cold” relative to
the absolute melting temperature of the metal, cold working.
It is convenient to express the degree of plastic deformation as
percent cold work, defined as:
%CW = (
𝐴0−𝐴𝑑
𝐴0
) x100
Where, A0 is the original area of the cross section that
experiences deformation and Ad is the area after deformation.
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34. Vii. Martensite Strengthening:
The transformation of austenite to martensite by diffusion less
shear-type transformation in quenching of steel is one of the
most common strengthening processes used in engineering
materials.
The Martensite strengthening process, thus basically is a
diffusion less and displacive (not diffusive) reaction.
It occurs by a process of lattices shearing.
Martensite under microscope appears as lenticular plates
which divide and subdivide the grains of the parent phase.
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35. Viii. STRENGTHENING FROM FINE PARTICLES
Small second-phase particles distributed in a ductile
matrix are a common source of alloy strengthening.
In dispersion hardening the hard particles are mixed
with matrix powder and consolidated and processed
by powder metallurgy techniques.
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