A brief explanation Phase transformation of Re-crystallization, Phases of Re-crystallization,micro-structural transformation.

1. “RECRYSTALIZATION”
MECHANISM OF PLASTIC DEFORMATION
PRESENTED BY
MANOHAR H S
M.TECH(MACHINE DESIGN)
PESCE,MANDYA

2. PHASE TRANSFORMATION
 When one phase transforms to another phase it is called phase transformation.
 Often the word phase transition is used to describe transformations where there is
no change in composition.
 In a phase transformation we could be concerned about phases defined based on:
 Structure → e.g. cubic to tetragonal phase
 Property → e.g. ferromagnetic to paramagnetic phase
 Phase transformations could be classified based on:
 Kinetic: Mass transport → Diffusional or Diffusion less
 Thermodynamic: Order (of the transformation) → 1st order, 2nd order, higher
order.

3. PHASE TRANSFORMATION
 Often subtler aspects are considered under the preview of transformations.
E.g. (i) roughening transition of surfaces, (ii) coherent to semi-coherent transition
of interfaces.
 Phase transformations are associated with change in one or more properties.
 Hence for microstructure dependent properties we would like to additionally
‘worry about’ ‘subtler’ transformations, which involve defect structure and stress
state (apart from phases).
 Therefore the broader subject of interest is Microstructural Transformations.
PHASES
PHASES TRANSFORMATION
MICROSTRUCTURE
MICROSTRUCTURAL TRANSFORMATION

4. MICROSTRUCTURAL TRANSFORMATION
 We now introduce a ‘technical term’ called Cold Work. We will arrive at a formal
definition of the term at the end of this topic.
 For now we use a working definition of cold work as: Plastic deformation in the
temperature range (0.3 – 0.5) Tm → COLD WORK. We will refine this definition
soon.
 During cold work the point defect density (vacancies, self interstitials…) and
dislocation density increase. Typical cold working techniques are rolling, forging,
extrusion etc.
 Cold working is typically done on ductile metals and alloys (e.g. Al, Cu, Ni) and is
a standard method of increasing the strength of soft metals like Aluminum.
Cold work
↑ point defect density
↑ dislocation density

5. MICROSTRUCTURAL TRANSFORMATION
 Point defects and dislocations have strain energy associated with them.
 (1 -10) % of the energy expended in plastic deformation typically is stored in the
form of strain energy (in these defects)  The material becomes battery of energy!
 The cold worked material is in a microstructurally metastable state.
 Depending on the severity of the cold work the dislocation density can increase 4-
6 orders of magnitude or more. The material becomes stronger, but less ductile.
ANNEALED MATERIALS
ρ ~ (10^6 - 10^9)
COLD
WORK
STRONGER MATERIAL
ρ ~ (10^12 - 10^14)

6. MICROSTRUCTURAL TRANSFORMATION
 Due to cold work changes occur to almost all physical and mechanical properties.
 The cold worked material is stronger (harder), but is brittle.
 The electrical resistance of the material increases due to primarily the increase in
point defect density. (This is mostly reversed during recovery).
 Changes can also be noted in the X-Ray diffraction pattern.
► Laue patterns of single crystals show pronounced asterism → due to lattice
curvatures.
► Debye-Scherrer photographs show line broadening → Residual stresses +
deformations.
COLD WORK
↑ Strength
↑ Hardness
↑ Electrical resistance
↓ Ductility

7. MICROSTRUCTURAL TRANSFORMATION
 Heating the material (typically below 0.5 Tm) is and holding for sufficient time is a
heat treatment process called annealing.
 Depending on the temperature of annealing processes like Recovery (at lower
temperatures) or Recrystallization (at higher temperatures) may take place. During
these processes the material tends to go from a microstructurally metastable state
to a lower energy state (towards a stable state).
 Further ‘annealing’ of the recrystallized material can lead to grain growth.
COLD WORK
↑ point defect
density
↑ dislocation density
(Increase in strength
of the material)
Material tends to
lose the stored strain
energy
(Softening of the
material)
COLD WORK
(ANNEAL)
RECOVERY
(LOW
TEMPERATURE)
RECRYSTALIZATION
(HIGH
TEMPERATURE)

8. MICROSTRUCTURAL TRANSFORMATION
COLD WORK(ANNEAL)
Recovery
(Driving force is free energy stored
in point defects and dislocations)
Recrystallization
(Driving force is free energy stored
in dislocations)
Grain growth
(Driving force is free energy stored
in grain boundaries)

9. RECRYSTALIZATION
 During recrystallization, ‘strain free grains’ replace the ‘cold worked grains’.
 For recrystallization we can define a temperature: TRecrystallization (or Trx). Unlike the
usual definitions we encounter in materials science, the definition of Trx is a little
‘convoluted’ (it involves a percentage and time!).
 TRecrystallization is the temperature at which 50 % of the material recrystallizes in 1
hour
 The recrystallization temperature typically is in the range of 0.3-0.5 of the melting
point. Trecrystallization  (0.3 – 0.5) Tm
 Two processes contribute to the formation of strain free grains:
(i) “Nucleation” and growth of new strain free grains and (ii) migration of the grain
boundaries to a region of high dislocation density. Process (ii) does not involve the
nucleation of new grains and during the migration of grain boundaries to a region
of higher dislocation density, dislocation density reduces (grain boundaries
accommodate the excess dislocations).

10. RECRYSTALIZATION
 The driving force for recrystallization is the free energy difference between the
deformed and un deformed material.
G (recrystallization) = G (deformed material) – G (un deformed material)
 Increased deformation (cold work) leads to a decrease in recrystallization
temperature (Trx).
 If the initial grain size is smaller then the recrystallization temperature is lower.
 Higher amount of cold work + low initial grain size leads to finer recrystallized
grains.

11. RECRYSTALIZATION
 Higher temperature of working, lower strain energy stored, which will lead to a
higher recrystallization temperature
 The rate of recrystallization is an exponential function of temperature. But, as the
recrystallization process is a complex one (combination of many processes), the
activation energy for recrystallization can not be treated as a fundamental constant.
 The Trecrystallization is a strong function of the purity of the material.
 For very pure materials Trecrystallization is about 0.3 Tm
[Trecrystallization (99.999% pure Al) ~ 75oC ]
 For impure materials Trecrystallization ~ (0.5 – 0.6) Tm
[Trecrystallization (commercial purity) ~ 275oC].
 Impurity atoms tend to segregate to the grain boundary and retard their motion →
Solute drag (can be used to retain strength of materials at high temperatures).
 Second phase particles can also be used to pin down the grain boundary and impede
its migration.

12. CONCEPT OF RECRYSTALLIZATION TO DEFINE
HOT AND COLD WORKColdWorkHotWork
Recrystallization temperature (~ 0.4 Tm)
 Hot Work  Plastic deformation above Trecrystallization
 Cold Work  Plastic deformation below Trecrystallization.
When a metal is hot worked, the conditions of deformation are such
that the sample is soft and ductile.
The effects of strain hardening are negated by dynamic and static
processes (which keep the sample ductile).
The lower limit of temperature for hot working is taken as 0.6 Tm.
 The effects of strain hardening is not neglected. Recovery mechanisms
involve mainly motion of point defects.
 Upper limit  0.3 Tm.
0.1 Tm
0.2 Tm
0.3 Tm
0.4 Tm
0.5 Tm
0.6 Tm
0.7 Tm
0.8 Tm
0.9 Tm
Warm
working