2. 2
2.1 Cold Working /Strain Hardening
• Strain hardening is used for
hardening/strengthening materials that are not
responsive to heat treatment.
• The phenomenon where ductile metals become
stronger and harder when they are deformed
plastically is called strain hardening or work
hardening.
• Increasing temperature lowers the rate of strain
hardening.
3. 3
• The treatment is given, usually, at temperatures
well below the melting point of the material. Thus
the treatment is also known as cold working.
• Working is continued below re – crystallisation
temperature (0.3 to 0.5 Tm, where Tm is the
melting point in absolute degrees).
• There is no grain growth but only grain
disintegration and elongation.
• The consequence of strain hardening a material is
improved strength and hardness but material’s
ductility will be reduced.
4. 4
• It is convenient to express the degree of
plastic deformation as percent cold work,
defined as:
where
A0 is the original cross sectional area
that experiences deformation
Ad is the area after deformation
100
%
o
d
o
A
A
A
CW
5. 5
2.2 Recovery, Recrystallization and Grain
Growth
• The cold worked state is a condition of
higher internal energy than the un-
deformed metal.
• Cold worked dislocation cell structure is
mechanically stable, it is not
thermodynamically stable.
• With increase in temperature state
becomes more unstable, eventually
reverts to strain-free condition.
6. 6
• The process of heating to attain strain-free
condition is called annealing heat
treatment where effects of strain
hardening may be removed.
• Annealing process can be divided into
three distinct processes:
• Recovery,
• Recrystallization and
• Grain growth.
8. 8
• During cold working grain shape changes, while
material strain hardens because of increase in
dislocation density.
• Between 1-10% of the energy of plastic
deformation is stored in material in the form of
strain energy associated with point defects and
dislocations.
• On heating the deformed material to higher
temperatures and holding, material tends to lose
the extra strain energy and revert to the original
condition before deformation by the processes
of recovery and recrystallization.
9. 9
Recovery
• After cold working physical properties of the
cold-worked material are restored without any
observable change in microstructure.
• During recovery, which takes place at low
temperatures of annealing, some of the stored
internal energy is relieved by virtue of dislocation
motion as a result of enhanced atomic diffusion.
• Excess point defects that are created during
deformation are annihilated either by absorption
at grain boundaries or dislocation climbing
process.
• Stored energy of cold work is the driving force
for recovery.
10. 10
Recrystallization
• This stage of annealing follows after recovery
stage.
• After complete recovery, the grains are still in
relatively high strain energy state.
• This stage involves replacement of cold-worked
structure by a new set of strain-free,
approximately equi-axed grains.
• It is the process of nucleation and growth of new,
strain-free crystals to replace all the deformed
crystals.
11. 11
• It starts on heating to temperatures in the
range of 0.3-0.5 Tm, which is above the
recovery stage.
• This process is characterized by
recrystallization temperature which is
defined as the temperature at which 50%
of material recrystallizes in one hour time.
• Pure materials may recrystallizes around
0.3 Tm, while impure materials may
recrystallizes around 0.5-0.7 Tm.
13. 13
• During this stage newly formed strain-free
grains tend to grow in size.
• This grain growth occurs by the migration
of grain boundaries.
• Driving force for this process is reduction
in grain boundary energy i.e. decreasing in
free energy of the material.
14. 14
• As the grains grow larger, the curvature of
the boundaries becomes less.
• This results in a tendency for larger grains
to grow at the expense of smaller grains.
Boundary motion is just the short – range
diffusion of atoms from one side of the
boundary to the other.
16. 16
• The process of grain growth depends on
the following:
– Annealing temperature and time
– Rate of heating
– Insoluble impurities
– Alloying elements
– Rate of cooling
– Extent of initial deformation.
17. 17
2.3 Hot Working
• The cold-working process is carried out below
the recrystallization temperature whereas hot
working processes are carried out at
temperatures above the recrystallization range.
• In hot working, hardening due to deformation
and softening due to annealing occur
simultaneously.
• The work hardening process decreases when
the temperature is increased constantly due to
the active slip system.
18. 18
• If plastic deformation is carried out at a
higher rate than softening, the metal will
become harder.
• Temperature where hardening and
softening processes balance each other,
the rate of deformation is termed
temperature deformation.
• Temperatures above this will result in hot
working and below in cold working
19. 19
Advantages
• Porosity in the metal is largely eliminated due to the
pressing together of cavities, cracks and blow holes .
• Impurities are squeezed into fibres and distributed
throughout the mass.
• Grain refinement due to breaking up of grains into
smaller crystals.
• Mechanical properties like strength, ductility, elongation
percentage and impact resistance are improved.
• Due to the directional flow obtained in hot working, the
desirable directional properties are obtained from a fibre
structure.
20. 20
Limitations
• Due to rapid oxidation and scaling of the surface
of the metal at high temperatures, a poor surface
finish of the final product is obtained.
• Close dimensional tolerances cannot be
maintained due to poor surface texture.
• The equipment used to withstand high-
temperature operations like rolling, drop forging,
etc. is very expensive. Thus the process is
costly.
21. 21
2.4 Super Plasticity
• Superplasticity is a phenomenon that allows
materials such as metals and even ceramics to
undergo extreme plastic elongations, beyond its
usual breaking point.
• The world record for elongation is on the order
of 6,000%.
• Such a state is usually achieved at high
homologous temperature, typically half the
absolute melting point.
22. 22
• Superplastically deformed material gets thinner
in a very uniform manner, rather than forming a
'neck' (a local narrowing) which leads to fracture.
• Metals, ceramics, and intermetallics have been
made superplastic by judicious control of their
microstructure and deformation conditions.
• The hallmark of superplastic deformation is a
high strain-rate sensitivity (typically close to 0.5),
which stabilizes against localized necking and
results in high plastic elongation.
23. 23
Applications
• Superplasticity is being utilized to form parts in the
automotive and aerospace industries in addition to many
other smaller industries.
• Because flow stresses are relatively low in superplastic
forming, sheets of material can be formed by gas
pressure.
• The high elongation and complex shapes afforded by
superplasticity make this process amenable to forming
everything from aircraft doors to computer cases.
• Advantages of the process include reduced weight and
part count as well as lower die costs.
24. 24
2.5 Hume – Rothery’s Rules
• It is frequently desirable to increase the
strength of the alloy by adding a metal that
will form a solid solution.
• If an alloying element is chosen at
random, it is likely to form an objectionable
intermediate phase instead of a solid
solution.
• In the choice of such alloying elements, a
number of rules govern the formation of
substitutional work of Hume-Rothery.
25. 25
1) Chemical Affinity Factor: The greater the
chemical affinity of two metals two metals tend
to form an intermediate phase rather than a
solid solution.
2) Relative Valence Factor: A metal of high
valence can dissolve only a small amount of
lower valance metal, while the lower valence
metal may have good solubility for a higher
valence metal.
26. 26
3) Relative Size Factor: If the size of two metallic atoms
(given approximately by their constants) differs by less
than 15 percent, the metals are said to have a
favourable size factor for solid solution formation. If the
size factor is greater than 15%, solid solution formation
tends to be severely limited and is usually only a fraction
of one percent.
4) Lattice-Type Factor: Only metals that have the same
type of lattice (FCC for example) can form a complete
series of solid solutions. Also, for complete solid
solubility, the size factor must usually be less than 8
percent. Copper-nickel and silver-gold-platinum are
examples of binary and ternary systems exhibiting
complete solid solubility.
27. 27
Solution and Mixture
• Different components can be combined into a
single material by means of solutions or
mixtures.
• A solution (liquid or solid) is phase with
more than one component.
• A mixture is a material with more than one
phase.
• In mixtures, there are different phases, each with
its own atomic arrangement.
• It is possible to have a mixture of two different
solutions.
28. 28
Phase
• A pure substance, under equilibrium conditions,
may exist as either of a phase namely vapour,
liquid or solid, depending upon the conditions of
temperature and pressure.
• A phase can be defined as a homogeneous
portion of a system that has uniform physical
and chemical characteristics.
• It is a physically distinct from other phases,
chemically homogeneous and mechanically
separable portion of a system.
29. 29
• In other words, a phase is a structurally
homogeneous portion of matter.
• A single-phase system is termed as
homogeneous.
• Systems composed of two or more phases are
termed as mixtures or heterogeneous.
• Most of the alloy systems and composites are
heterogeneous.
30. 30
2.7. The Phase Rules/ Gibbs Phase Rule
• This states that the maximum number of phases
P which may co-exist under equilibrium
conditions is equal to the sum of the number of
components C and between the numbers of
degrees of freedom in the system (the number of
variable factors).
• It is expressed mathematically as follows:
P+F=C+ n
P+F=C+ 2
n = no of external factors
= 2 (temperature and pressure)
31. 31
• The number of degrees of freedom is the quantity of
independent external or internal variables, like
temperature, pressure and concentration
• In studying chemical equilibrium, temperature and
pressure are considered as external factors determining
the state of the system.
• In applying the phase rule of to metal systems the effect
of pressure is neglected, leaving only one variable
factor-temperature. The equation will then be:
F=C+ 1- P
• In equilibrium all factors have definite values; hence the
degree of freedom cannot be less than zero.
C-P+1≥ 0
then P ≤ C + 1
33. 33
• In a two-phase region, we can determine the
relative amount of each phase that is present
from the phase diagram, using a relationship
called the lever rule.
• To determine the relative amounts of two
phases, an ordinate or vertical line is erected at
a point on the composition scale which gives the
total composition of the alloy.
• The intersection of this ordinate with the given
isothermal line is the fulcrum of a simple lever
system.
• The ordinate KL intersects the temperature line
at a point M.
34. 34
• The relative lengths of lever arms OM and MP multiplied
by the amount of phases present must balance.
• The length MP shows the amount of liquid, whereas the
length OM indicates the amount of solid.
• The percentage of solid present =
• The percentage of liquid present =
• where OM + MP = OP = Total composition of alloy
between liquidus and solidus, say at tp.
• The isothermal (line OMP) can be considered a tie line
since it joins the composition of two phases in
equilibrium at a specific temperature tp.
100
OP
OM
100
OP
MP
35. 35
2.9. Equilibrium Phase Diagrams
• A diagram that depicts existence of different
phases of a system under equilibrium is termed
as Phase diagram.
• Equilibrium phase diagrams represent the
relationships between temperature and the
compositions and the quantities of phases at
equilibrium.
• It is sufficient to consider only solid and
liquid phases, thus pressure is assumed to
be constant (1 atm.) in most applications.
• It depicts information related to microstructure
and phase structure of a particular system in a
convenient and concise manner.
36. 36
Information from phase diagram
• To show phases are present at different
compositions and temperatures under slow
cooling (equilibrium) conditions.
• To indicate equilibrium solid solubility of one
element/compound in another.
• To indicate temperature at which an alloy starts
to solidify and the range of solidification.
• To indicate the temperature at which different
phases start to melt.
• Amount of each phase in a two-phase mixture
can be obtained.
37. 37
Classified based on the number of
components in the system.
• Unary diagrams - Single component systems
• Binary diagrams - Two-component systems have
• Ternary diagrams - Three-component systems
39. 39
• In these systems there is no composition change (C=1)
• Only variables are temperature and pressure. Thus in
region of single phase two variables (temperature and
pressure) can be varied independently.
• If two phases coexist then, according to Phase rule,
either temperature or pressure can be varied
independently, but not both.
• At triple points, three phases can coexist at a particular
set of temperature and pressure.
• At these points, neither temperature nor the pressure
can be changed without disrupting the equilibrium i.e.
one of the phases may disappear.
41. 41
• These diagrams constitutes two components,
– Two metals (Cu and Ni),
– A metal and a compound (Fe and Fe3C),
– Two compounds (Al2O3 and Si2O3), etc.
• Binary diagrams are usually drawn showing
variations in temperature and composition only.
• It is also to be noted that all binary systems
consist only one liquid phase i.e. a component is
completely soluble in the other component when
both are in liquid state.
• If both the components are completely soluble in
each other, the system is called isomorphous
system. E.g.: Cu-Ni, Ag-Au, Ge-Si, Al2O3-
Cr2O3.
42. 42
2.9.4 Monotectic System
• Monotectic reaction is in which a liquid phase transforms
into a solid phase and a liquid phase of different
composition.
• The monotectic reaction is of the form:
Liquid -1(L1) ↔ Liquid -2 (L2) + Solid (α)
• Example system for monotectic reaction: Cu-Pb at 954 ْ
C and 36%Pb. Another system is Fe-C, 0.51%C, 1495 ْ
C.
43. 43
2.9.5 Monotectoid System
• Monotectoid reaction in which a solid phase transforms
to produce two solid phases of different compositions.
• The monotectoid reaction is of the form:
Solid -1 (α1) ↔ Solid -2 (α2) + Solid -3 (β)
47. 47
• The Eutectoid reaction involves a solid-solid reaction or
represents equilibrium of a system whose components
are subjected to allotropic transformation.
• It indicates the decomposition of a solid solution into two
different solid phases.
• The eutectoid reaction is of the form:
Solid (α) ↔ Solid -1 (β) + Solid- 2 (γ)
49. 49
• In the peritectic reaction, two phases are used to
produce one different phase with reaction just the
opposite of eutectic reaction.
• The peritectic reaction is of the form:
Liquid (L) + Solid -1 (α) ↔ Solid-2 (β)
51. 51
• In Peritectoid reaction two solid phases react to
form a new solid phase.
• The Peritectoid reaction is of the form:
Solid -1 (α) + Solid -2 (β) ↔ Solid-3 (γ)
53. 53
• In Syntectic reactions two liquid phases react to form a
solid phase. The syntectic reaction is of the form
Liquid -1(L1) + Liquid -2 (L2) ↔Solid (α)
54. 54
2.10 Polymorphism
• Polymorphism is the ability of a metal to exist
in two or more crystalline forms
depending upon temperature and
composition.
• Polymorphism can potentially be found in any
crystalline material including polymers,
minerals, and metals, and is related to allotropy,
which refers to elemental solids.
• The complete morphology of a material is
described by polymorphism and other variables
such as crystal habit, amorphous fraction or
crystallographic defects.
55. 55
1. The alpha phase, from room temperature to 663°C
2. The beta phase, from 663°C to 764°C.
3. The gamma phase, from 764°C to its melting point of 1133°C.
57. 57
• α–ferrite (BCC) Fe-C solid solution
• γ-austenite (FCC) Fe-C solid solution
• δ-ferrite (BCC) Fe-C solid solution
• Fe3C (iron carbide) or cementite - an inter-
metallic compound
• Liquid Fe-C solution
Five phases that exist in the diagram
58. 58
Four invariant reactions that cause
transformations
• Peritectic reaction at 1495ْC and 0.16%C, δ-
ferrite + L ↔ γ-iron (austenite)
• Monotectic reaction 1495ْC and 0.51%C, L ↔ L
+ γ-iron (austenite)
• Eutectic reaction at 1147ْC and 4.3 %C, L
↔ γ-iron + Fe3C (cementite) [ledeburite]
• Eutectoid reaction at 723ْC and 0.8%C, γ-iron
↔ α–ferrite + Fe3C (cementite) [pearlite]
59. 59
Critical temperatures
• Upper critical temperature (point) A3 is the
temperature, below which ferrite starts to form as a result
of ejection from austenite in the hypoeutectoid alloys.
• Upper critical temperature (point) ACM is the
temperature, below which cementite starts to form as a
result of ejection from austenite in the hypereutectoid
alloys.
• Lower critical temperature (point) A1 is the
temperature ofthe austenite-to-pearlite eutectoid
transformation. Below this temperature austenite does
not exist.
• Magnetic transformation temperature A2 is the
temperature below which α-ferrite is ferromagnetic.
61. 61
• Isothermal transformation diagram, also
known as TTT diagram, measures the rate
of transformation at a constant
temperature
• It shows time relationships for the phases
during isothermal transformation.
• Information regarding the time to start the
transformation and the time required to
complete the transformation can be
obtained from set of TTT diagrams.
