7. Second important plastic deformation mechanism
which can occur in metals is twinning.
In this process a part of the atomic lattice is deformed
so that it forms a mirror image of the undeformed
lattice next to it.
Plastic Deformation by Twinning
8. The crystallographic plane of symmetry
between the undeformed and deformed parts of
metal lattice is called the twining plane.
Plastic Deformation by Twinning
9. Strain-Hardening or Work Hardening
Strain Hardening :- Strain hardening means increase
in hardenss and strength during plastic deformation
and more and more stress is required to continue
further plastic deformation.
10. As the dislocations are moved by applying stress,
their density increases (i.e. they regenerates) if
they come across any obstacle such as impurity
particle, precipitate particle, local stress field in
the lattice, grain boundary etc.)
In the presence of such obstacles, the dislocation
assumes the positions as shown in fig. with
increasing values of stress.
Strain-Hardening or Work Hardening
Dislocation Theory
12. Plastic Deformation in Polycrystals
In the presence of grain boundary. Dislocation
movement along the slip planes and elongation of
the grains make plastic deformation more and more
difficult.
Piled dislocations increase further resistance
to plastic deformation.
Thus grain boundaries increases strength and
hardness of polycrystalline material by plastic
deformation.
13. • Grain boundaries are barriers to slip.
• Smaller grain size: more barriers to slip.
• Hall-Petch Equation:
d
k
o
yield
Plastic Deformation in Polycrystals
14. Cold Working
Cold Working is defined as the working of metal and
alloys below their recrystallization temperature
Common forming operations change the cross
sectional area:
100
x
%
o
d
o
A
A
A
CW
-Rolling
roll
Ao
Ad
roll
15. • Yield strength (sy) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
As cold work is increased
Impact of Cold Working
16. • Yield strength (sy)
increases.
• Tensile strength (TS)
increases.
• Ductility (%EL or %AR)
decreases.
Impact of Cold Working
Impact on Mechanical Properties
17. Impact on Microstructure
During cold working, all grains get deformed,
distorted and elongated in the direction of cold
working.
Grain size slightly decreases.
Before Cold Working
235 mm
After Cold Working
rolling direction
Impact of Cold Working
18. Other Effects of Cold Working
• An appreciable decrease in electrical conductivity
(increased number of scattering centers)
• Small increase in the thermal coefficient of expansion
• Because of increased internal energy – chemical
reactivity is increased (decreased resistance to
corrosion)
• Corrosion resistance decreases
Impact of Cold Working
19. Definition:
Annealing is heating to an elevated temperature for
an extended time period and then slowly cooling
Steps for all anneals:
1. Heating
2. Holding or “soaking”
3. Cooling
Time is important for all 3 steps
Annealing
Temp Time
21. Annealing Stages
Annealing is easily divided into 3 distinct
processes:
Annealing
1. Recovery
2. Recrystallization
3. Grain Growth
22. Recovery
Defined as:
Restoration of physical properties of a cold
worked metal without any observable
change in microstructure
Driving force for recovery is the release of
stored strain energy
23. Recovery
Removal of grain curvature created during
deformation
Regrouping of edge dislocations into low angle
boundaries within grains
Reduces the energy of system by creating reduced
energy subgrains
25. Recovery
Effects:
Annihilation reduces dislocation density.
Lattice strain is reduced
Relieve residual stresses
Electrical conductivity increases
Corrosion resistance increases
Strength properties are not affected
26. Recrystallization
Recrystallization is:
The replacement of the cold worked structure
by the nucleation and growth of a new set of
strain free grains
Driving force for recrystallization is the release
of stored strain energy
28. Variables for Recrystallization
Six main variables influence recrystallization behavior:
The amount of prior deformation
Temperature
Time
Initial grain size
Composition
29. Final grain size depends:
mainly on the degree of deformation
annealing temperature.
The greater the deformation & the lower the
annealing temp., the smaller the recrystallized
grain size.
Variables for Recrystallization
30. Grain Growth
Grain growth refers to the increase in the average
grain size on further heating in annealing after all the
cold worked material has recrystallized
Driving force :-The driving force for grain growth is
the decrease in free energy resulting from a decreased
grain boundary area due to an increase in grain size.
Grain growth is also known as secondary
recrystallization
31. After 8 s, 580C After 15 min, 580C
GRAIN GROWTH
32. Small grains with sides less than 6 => concave inward
=> shrink small grains with sides larger than 6 =>
concave outward => grow
Thus the large grow larger and the small grow smaller.
This results in a tendency for longer grains to grain
at the expense of smaller grains
Stable
Shrink
120o
grow
GRAIN GROWTH
33. GRAIN GROWTH
The extent of grain growth is dependent to a large
degree on the following factors :
The annealing temperature
Annealing time
Degree of previous cold work
Presence of insoluble impurity atoms
34. Size of grains vs. temperature
G
R
A
I
N
S
I
Z
E
Temperature, deg.C
200 600
400
GRAIN GROWTH
35. Grain-Growth is not recommended mainly because:
Energy consumption
Excessive grain growth is not desirable
because it decreases the useful properties.
However, one of the useful application of
this is in the growth of single crystal.
GRAIN GROWTH
39. • Hot working is defined as deforming the
material at a temperature above the
recrystallization temperature
Hot Working ≡
Cold Working+ Annealing
Hot Working
40. Cold Working Vs Hot Working
Cold Working Hot Working
Working of metals and
alloys below their
recrystallisation temp.
Working of metals and
alloys above their
recrystallisation temp.
Strain hardening and
distorted grains are
produced
No strain hardening stress –
strain free equiaxed grains
are produced.
41. Cold Working Hot Working
Defect density
increases.
No change in defect
densities.
Very large
deformations are not
possible because of
strain hardening
Very large
deformations are
possible
Energy and stress
require for cold working
is high.
Energy and stress
require for hot working
is low.
42. Cold Working Hot Working
Surface finish is good. Surface finish is not
good because of surface
oxidation
Handling of materials is
easy.
Handling of materials is
difficult
Cost of cold working
plant is low
Cost of hot working
plant if high.
43. Unit-I- Deformation of Materials
• Types of fractures (brittle, ductile)
• Creep Failure
• Fatique Failure
44. : Some types of mechanical failure are
• Failure is the state or condition of not meeting a desirable or
intended objective.
• Excessive deflection,
• Buckling, ductile fracture,
• Brittle fracture, impact, creep, relaxation,
• Thermal shock
• Wear
• Corrosion
• Stress corrosion cracking
52. Creep
• Creep is the term used to describe the tendency of a
material to move or to deform permanently to relieve
stresses.
• Material deformation occurs as a result of long term
exposure to levels of stress that are below the yield or
ultimate strength of the material.
• Creep is more severe in materials that are subjected to
heat for long periods and near melting point.
64. Fatigue Cont..
• Fatigue is the progressive, localised, and permanent structural
damage that occurs when a material is subjected to cyclic or
fluctuating strains at nominal stresses.
• The fatigue life of a member or of a structural detail subjected
to repeated cyclic loadings is defined as the number of stress
cycles it can stand before failure.
• Stages of failures due to fatigue :
• I. Crack initiation at high stress points (stress raisers)
• II. Propagation (incremental in each cycle)
• III. final failure by fracture
• Nfinal = Ninitiation + Npropagation
65. Main parameters influencing fatigue life
• The fatigue life of a member or of a structural detail subjected
to repeated cyclic loadings is defined as the number of stress
cycles it can stand before failure.
• The stress difference, or as most often called stress range,
• The structural detail geometry,
• The material characteristics.
• The environment
66. Factors that affect fatigue life:
• Quality of the surface; surface roughness, scratches
• Material Type
• Surface defect geometry and location.
• Environmental conditions
• stress concentration
• overload, metallurgical structure
• The stress difference, or as most often called stress range,
• The structural detail geometry,
• The material characteristics.
• The environment
