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1. Recovery, Recrystallization & Grain Growth
Now let us consider some microstructural transformations
MATERIALS SCIENCE
&
ENGINEERING
Anandh Subramaniam & Kantesh Balani
Materials Science and Engineering (MSE)
Indian Institute of Technology, Kanpur- 208016
Email: anandh@iitk.ac.in, URL: home.iitk.ac.in/~anandh
AN INTRODUCTORY E-BOOK
Part of
http://home.iitk.ac.in/~anandh/E-book.htm
A Learner’s Guide

2.  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 Diffusionless
 Thermodynamic: Order (of the transformation) → 1st order, 2nd order, higher order.
 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.
We had noted the following in our introduction to phase transformations Phase Transformations
Microstructural Transformations
 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.
Microstructure
Microstructure
PhasesPhases
Microstructural Transformations
Microstructural Transformations
Phases Transformations
Phases Transformations
We now take up three microstructural transformations: Recovery, Recrystallization & Grain Growth

3.  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.
 Cold work is an important method to increase the strength of metals (& alloys), especially for
those, wherein other methods like precipitation hardening are not available.
Notes. Cold work can be used to augment other strengthening mechanisms. Cold working is not a good
strengthening mechanism for materials, wherein the service temperature is ‘high’. Cold work does not
involve change in composition and hence has its benefits. Strengthening due to cold work may be a ‘by
product’ of shaping of metals by deformation processing (like extrusion, forging, wire drawing, etc.) at
‘low temperatures’
 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. This leads to an increase in the internal energy of the material. 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 Aluminium.
Cold Work Click here to know more about strengthening mechanisms

4. Cold work
↑ dislocation density
↑ point defect density
 Point defects and dislocations have strain energy associated with them.
 (1 -15) % 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 amount of energy stored depends on the material, temperature, strain rate and type of
deformation, grain size, etc.
 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.
)1010(~)1010(~ 1412
ndislocatio
96
ndislocatio 
 
 
materialStrongermaterialAnnealed workCold

5. Cold work
↑ Hardness↑ Strength
↑ Electrical resistance
↓ Ductility
 Due to cold work changes occur to almost all physical and mechanical properties.
 The cold worked material is stronger (harder), but is brittle (as noted before).
 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 Recovery Recrystallization Grain growth
Tensile strength
Ductility
Electical conductivity
Internal stress

6. Cold work
↑ dislocation density
↑ point defect density
Anneal
Material tends to lose
the stored strain energy
Increase in strength
of the material
Softening of the material
Cold work
Recrystallization
Recovery
Low T
High T
Anneal
 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). Note again: this is not a phase transformation but a microstructural transformation.
 Further ‘annealing’ of the recrystallized material can lead to grain growth.

7. Cold work Anneal
Recrystallization
Recovery
Grain growth
Driving force is free energy stored in
point defects and dislocations
Driving force is free energy stored in dislocations
Driving force is free energy
stored in grain boundaries
Overview of processes taking place during annealing of cold worked material and the driving
force for these processes
 It should be noted that the driving force cited above is a global (thermodynamic) driving force.
If the process will actually takes place will depend on the ‘local’ conditions. I.e. both global
and local criteria have to be satisfied if these processes have to take place.
 Recovery and recrystallization may occur even during deformation (depending on the
temperature, strain rate, etc.) and in this case they are referred to as dynamic recovery and
dynamic recrystallization.
 Metal working reduces the ‘residual ductility’. Hence, it is preferable to carry out deformation
processing (like forging, rolling, etc.) in temperature and strain rate regime wherein dynamic
recrystallization occurs. This enables us to deform the material to large strains.
 Deformation processing maps can be used to locate such regimes.

8. Recovery
 Recovery takes place at low temperatures of annealing (after cold work).
 “Apparently there no change in microstructure” (i.e. if seen in an optical microscope, the
microstructure looks similar before and after recovery).
 Two processes which occur during recovery are:
 Reduction in point defect density (+ their reconfiguration),
 Annihilation of dislocations and their arrangement into low energy configurations.
 Note: not all point defects and dislocaitons participate in the above processes.
 It was noted that excess point defects are created during cold work. During recovery these
are absorbed by processes which include (there are other processes which also may be active):
► at surface or grain boundaries
► or by dislocation climb process.
 During recovery, random dislocations (statistically stored dislocations) of opposite sign
come together and annihilate each other. However, the overall reduction in the dislocation
density by this process is small.
 Dislocations of same sign arrange into low energy configurations.
► Edge dislocations ‘rearrange’ to form Tilt boundaries
► Screw dislocations ‘rearrange’ to form Twist boundaries.
 The formation of low angle tilt and twist boundaries is termed as POLYGONIZATION
(figure in next page).
 Hence, the overall reduction in dislocation density is small during recovery.

9. POLYGONIZATION
Bent crystal
Low-angle tilt grain boundaries
Statistically stored edge dislocations in a
crystal, which has undergone cold work.
Excess on sign of dislocations leads to a bent
crystal.
 During recovery polygonization takes place.
 In this process, dislocations of the same sign
arrange themselves in a low energy
configuration.
 This leads to the formation of sub-grain
boundaries or low-angle grain boundaries.
 In the example considered formation of low
angle tilt grain boundary is shown (staring with
an excess of positive edge dislocations).
 If we start with an excess of screw dislocations,
we will obtain a low-angle twist boundary.
 An interesting point is that during this process,
statistically stored (random) dislocations
‘become’structural dislocations.

10. Recrystallization
 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 tempearture 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).
Region of lower
dislocation density
Region of higher
dislocation density
Direction of grain
boundary migration

11. Further points about recrystallization
 The driving force for recrystallization is the free energy difference between the deformed and
undeformed material.
G (recrystallization) = G (deformed material) – G (undeformed 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.
 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 cannot 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. Hot Work and Cold Work  Hot Work  Plastic deformation above TRecrystallization
 Cold Work  Plastic deformation below TRecrystallization
ColdWorkHotWork
Recrystallization temperature (~ 0.4 Tm)
 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 negated. 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
Often the range is further subdivided into Hot, Cold and Warm working as in the figure
Knowing the concept of recrystallization we are in a position to define hot and cold work

13. Grain growth
 The growth of larger grains at the expense of smaller ones, leading to the increase in the
average grain size is termed as grain growth. (Obviously all the grains cannot grow!).
 This is also called ‘grain coarsening’.
 A related term to this is ‘Ostwald ripening’. Similar processes is observed in the case of
precipitation, wherein larger precipitates grow at the expense of smaller ones, leading to an
overall increase in the size of the precipitates (called precipitate coarsening).
 For grain growth to occur, both the global and the local criteria must be satisfied.
 The global criterion is easy to understand. Grain growth is Globally driven by reduction in
grain boundary energy (per unit volume).
 The local condition is explained in the next page. If we make ‘hexagonal grains’ as in the
figure below, the system will not coarsen.
 Grain growth will lead to a further drop in the strength of the material (i.e. after recrystallization has led
to a considerable drop). The strength of a material depends on the grain size via the Hall-Petch
relation (wherein larger grains imply a lower strength).
A conceptual model of hexagonal
grains, which will not coarsen, as the
local criterion will not be satisfied.

14. Bonded to
4 atoms
Bonded to
3 atoms
Direction of grain
boundary migration
JUMP
G1 G2
 Locally grain growth is driven by bond maximization (coordination number maximization).
This can be visualized as in the schematics as below. The smaller grains have a larger
curvature. Let us assume that a small grain G1 is in contact with a larger grain G2. Due to
higher curvature an atom at the grain boundary (from G1 side) is bonded to less number of
atoms (3 in the schematic), while a similar atom on the G2 side is bonded to more number of
atoms (4 in the schematic). The system can lower its energy by the jump of an atom from G1
to G2. Such jumps lead to the shift (migration) in the GB towards the smaller grain (G1 in
the current example). This leads to a shrinkage of the smaller grain (at the benefit of the
larger grain).