Due to dislocations, it is no longer necessary to break all bonds between two atomic planes at once in order to shear off a lattice planes.
Rather, it is enough to overcome only one binding series at a time.
The dislocation line jumps step-by-step from atomic row to atomic row with little effort and finally emerges as a slip step on the surface of the material.
1. Dislocations and Strengthening Mechanisms
Mr. MANICKAVASAHAM G, B.E., M.E., (Ph.D.)
Assistant Professor,
Department of Mechanical Engineering,
Mookambigai College of Engineering,
Pudukkottai-622502, Tamil Nadu, India.
Email:mv8128351@gmail.com
Dr. R.Narayanasamy, B.E., M.Tech., M.Engg., Ph.D., (D.Sc.)
Retired Professor (HAG),
Department of Production Engineering,
National Institute of Technology,
Tiruchirappalli-620015, Tamil Nadu, India.
Email: narayan19355@gmail.com
2. Due to dislocations, it is no longer necessary to break all bonds between two atomic
planes at once in order to shear off a lattice planes.
Rather, it is enough to overcome only one binding series at a time.
The dislocation line jumps step-by-step from atomic row to atomic row with little effort
and finally emerges as a slip step on the surface of the material.
Role of the dislocations in the deformation process
4. Contd.
Due to the low-energy migration of the dislocations, the deformation process starts at already
much lower critical shear stresses than the theory predicts without considering dislocations!
This critical shear stress is referred to as Peierls stress.
Dislocations allow low-energy slippage of atomic blocks, so that deformation processes in real
crystals already occur at lower critical shear stresses than in ideal crystals!
The low-energy sliding of the atomic plane by a dislocation can be illustrated by moving a
carpet.
Moving a large and heavy carpet usually requires a very large force due to the friction between
the carpet and the floor.
If, however, wrinkles are struck in the carpet and these are then moved through the carpet, then
one achieves the same result with less effort.
The carpet can move in stages like a caterpillar.
6. By understanding the atomic mechanism of deformation and the central role of dislocations, specific
measures can now be taken to prevent deformation.
After all, many materials that are used under high load should not deform so easily.
Therefore they should be high-strength.
How measures for increasing the strength of metals can look like is explained in the following
section.
Contd.
7. By the term strengthening mechanisms one understands measures which aim to prevent the
deformation of a metal purposefully.
Thus, the highest possible strength of the corresponding material is achieved.
Since the primary cause of a deformation process is the migration of dislocations, ultimately all
strengthening mechanisms are based on blocking of this movement.
Strengthening Mechanisms
All measures with the aim of blocking dislocation movement are summerazied under
the term strengthening mechanisms!
8. The most important strengthening mechanisms are described in the following sections:
solid solution strengthening
precipitation hardening
grain boundary strengthening (Hall–Petch strengthening)
work hardening (strain hardening)
Contd.
9. The principle of solid solution hardening is based on the distortion of the lattice by foreign
atoms. These can be either substitutional atoms or interstitial atoms.
Due to their blocking of the dislocation movement, the lattice planes can consequently no longer
slide off so easily.
A deformation of the lattice thus occurs only at significantly higher critical shear stresses, since the
lattice distortion must also be overcome.
In this way, an increase in the strength of the material is ultimately achieved.
Solid Solution Strengthening
10. Figure: Principle of solid solution hardening
Contd.
With solid solution hardening, foreign atoms block the dislocation movement!
11. Precipitation Strengthening
Not only single foreign atoms can impede the dislocation movement, but
precipitates can also block the movement of dislocations.
This is called precipitation hardening or precipitation strengthening.
12. Contd.
Figure: Principle of precipitation hardening
In a precipitation hardening precipitates block the dislocation movement!
13. Contd.
This principle of precipitation strengthening is used in so-called hardenable aluminum alloys.
The aluminum alloy is first heated to a relatively high temperature, so that the foreign atoms
contained therein can completely dissolve in the aluminum lattice structure.
Note that solubility generally decreases with decreasing temperature, so high temperatures are
required for complete solubility.
If it is cooled rapidly (called quenching), then the foreign atoms, despite the lower solubility, remain
forcibly dissolved in the lattice.
14. Since the concentration of dissolved atoms in this state is above the actual solubility limit, one
speaks of a so-called supersaturated solid solution.
This state is not thermodynamically stable, so that the forcibly solved foreign atoms begin to
segregate from the lattice and form their own compounds (precipitates) within the metal.
To accelerate this process of so-called aging, the alloy is heated slightly, so that the diffusion
processes can proceed more quickly.
Contd.
15. Grain Boundary Hardening (Grain Refining)
The principle of grain boundary hardening is based on the complicated dislocation
movement across grain boundaries.
Grain boundaries are therefore no weak points in the material in this context but
contribute to a particular extent to the increase in strength!
16. Contd.
Figure: Principle of Grain boundary hardening
With a grain refining, grain boundaries block the dislocation movement!
17. Contd.
A high number of grain boundaries can be achieved by fine grains in the material.
Therefore, grain boundary strengthening is also called grain refinement (sometimes referred to
as Hall–Petch strengthening).
A small grain size can be achieved by targeted influencing of the melt during cooling (for example
by seeding or supercooling of the melt).
18. Contd.
The principle of grain refining is applied to so-called weldable fine grain steels in steel
building.
Although carbon has a strength-increasing effect in steel, it is undesirable in terms of
good weldability.
Carbon makes the steel hard and brittle due to the rapid cooling after welding.
Therefore, due to a good weldability (as is often required in steel building), it is necessary
to keep the carbon content as low as possible.
Nevertheless, in order to ensure a high strength, one is dependent on grain refining.
The diameters of the individual grains in fine grain steels are in the range of
approximately 20 μm.
19. Work hardening (strain strengthening)
The principle of work hardening is based on the introduction of additional
dislocations during plastic deformation.
In every deformation process, new dislocations are always introduced into the
material.
The dislocations thus hinder each other from moving, which results in a strength-
increasing effect.
20. Contd.
Figure: Principle of work hardening (strain hardening)
With work hardening additionally introduced dislocations block each other from
moving!
21. Contd.
The increase in strength during plastic deformation can also be comprehended using the
stress-strain diagram.
For this purpose, a tensile specimen is first stretched out beyond the yield strength Rp
(red line to point A) and therefore suffers a plastic deformation.
If the force is subsequently removed, the operating point returns to zero stress in a
direction parallel to Hooke’s straight line (point B).
Accordingly, the material has a permanent elongation.
23. If the tensile test is repeated again, the operating point first goes up the elastic straight line
again and only reaches plastic deformation at higher stress (blue curve)!
The comparison of both stress–strain curves makes it clear that the plastic deformation
now only occurs at higher stress.
This means that the work-hardened sample obviously has an increased yield
strength Rp! The tensile strength also increases accordingly.
Due to the strain a workpiece has suffered during plastic deformation, work hardening is
also referred to as strain strengthening.
Contd.
24. Work hardening is specifically brought about, for example, in the manufacture of cold-rolled sheets in
order to achieve a significantly higher strength compared to the hot-rolled condition.
Note that work hardening can not be done to any degree.
If too many dislocations are introduced by plastic deformation, the material is thereby locally
destroyed and ruptured.
This behavior is evident, for example, in the repeated bending back and forth of a wire.
This only works well until too many dislocations have been introduced and the wire eventually
breaks.
