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MATERIALS SCIENCE AND TESTING
Recrystallization 1/7
Recrystallization
The physical properties and the grain structure of the materials, due to plastic cold forming,
are significantly altered because of the increase in the free energy of the material.
Microstructure-sensitive properties (for example strength, deformation etc) change to a
greater extent, while microstructural independent properties (for example electrical
conductivity) change to a lesser extent. Plastic cold forming has three major effects.
(1) The grain structure and the shape of the grains change (Figure 1). Deformation (e.g.
rolling) causes the originally polygonal grains to elongate.
Unformed grain structure Deformed (rolled) grain structure
Figure 1. Grain structure before and after plastic deformation.
(2) The density of dislocations increases (108 cm-2→1012 cm-2), due to the Frank-Reed
sources which start to form because of the energy introduced by cold forming.
(3) The increase of dislocation-density results in deformation hardening: strength
properties (yield strength, tensile strength) increase, while deformation characteristics (break
elongation, specific reduction in cross section area) decrease. The reason for this is that a large
number of dislocations significantly hinder each other in the free movement. Figure 2 shows
the change of these characteristics in general. It should be noted that the increased free-
energetic state can occur not only due to plastic cold forming, but also neutron irradiation or
rapid cooling.
Recrystallization 2/7
Figure 2. Effect of cold working on mechanical properties.
Much of the energy introduced by the plastic formation is converted to heat (~ 90%)
while the remaining fraction (~ 10%) is stored in the material (formation of new vacancies,
formation of dislocations). The material, like every thermodynamic system, seeks to reduce
its energy. This aspiration in solid bodies at low temperatures (due to the relatively strong
location of the ions) is not happens or only to a small extent. At higher temperatures, however,
with the higher thermal energy of the ions, the free energy-reducing changes are easier to
complete. The processes involved in this energy reduction are referred to as recrystallization
as a summary name. The phenomenon of recrystallization has many important aspects. During
the cold forming progresses, the material's deformation capability is gradually depleting.
However, many industrial technologies require high deformation ability (for example: wire
drawing, die-casting), therefore these technological operations are carried out at a higher
temperature (hot forming) or heat treatment is inserted in the technological steps, in order to
prevent depletion of deformation capability. In other cases, increasing strength is the goal, via
reducing grain size (Hall-Petch equation). By recrystallization, assuming proper preliminary
cold forming, the reduction of the average grain size can be induced to reach fine grain
structure. The temperature of recrystallization (Trecryst) also plays an important technical role
in plastic forming technologies.
The forming processes under the recrystallization temperature (T < Trecryst) are called
cold forming (cold work), above the recrystallization temperature (T > Trecryst) are called hot
forming (hot work) and the forming processes near the recrystallization temperature are semi-
hot forming (T≈Trecryst). Changed properties due to the plastic formation can be restored by
proper heat treatment to their original value at a predetermined temperature (above the
recrystallization temperature).
Recrystallization 3/7
Figure 3. Changes in mechanical properties due to cold forming, recrystallization
and grain growth.
The whole process involves three main stages: recovery, recrystallization and grain
growth (or coarsening) (Figure 3).
Recovery
When a strain hardened material is held at an elevated temperature an increase in atomic
diffusion occurs that relieves some of the internal strain energy. Remember that atoms are
not fixed in position but can move around when they have enough energy to break their
bonds. Diffusion increases rapidly with rising temperature and this allows atoms in severely
strained regions to move to unstrained positions. In other words, atoms are freer to move
around and recover a normal position in the lattice structure. This is known as the recovery
phase and it results in an adjustment of strain on a microscopic scale. Internal residual stresses
are lowered due to a reduction in the dislocation density and a movement of dislocation to
lower-energy positions. The tangles of dislocations condense into sharp two-dimensional
boundaries and the dislocation density within these areas decrease. These areas are called
subgrains. There is no appreciable reduction in the strength and hardness of the material but
corrosion resistance often improves.
Polygonization is the movement and location of dislocations during creep. For
dislocations with the same sign, the least energy position is when they are placed under each
other. These dislocations create the boundaries of the subgrains (Figure 4). The subgrains will
be the nucleation points of the newly emerging (nearly stress-free) grains, which grow during
recrystallization. Therefore, the completion of polygonization is a fundamental condition for
recrystallization.
Recrystallization 4/7
Figure 4. Process of polygonization.
Recrystallization
At higher temperature, new, strain-free grains nucleate and grow inside the old distorted
grains and at the grain boundaries. These new grains grow to replace the deformed grains
produced by the cold forming (Figure 5). The driving force behind recrystallization is actually
the difference in energy between the deformed and the newly formed grains. With
recrystallization, the mechanical properties return to their original values. Recrystallization
depends on the temperature, the amount of time at this temperature and also the amount of
strain hardening that the material was subjected to. The higher the strain hardening, the lower
the temperature will be at which recrystallization occurs. However, a minimum amount
(typically 2-20%) of cold work is necessary for any amount of recrystallization to occur. The
size of the new grains is partially depends on the amount of prior strain hardening. The higher
the stain hardening, the more nuclei for the new grains forms, and the resulting grain size will
be smaller (at least initially).
Figure 5. Effect of recrystallization to grain structure.
Grain growth, coarsening
If a specimen is left at the high temperature beyond the time needed for complete
recrystallization, the grains begin to grow in size. This occurs because diffusion occurs across
the grain boundaries and larger grains have less grain boundary surface area per unit of
volume. Therefore, the larger grains loose fewer atoms and grow at the expense of the smaller
grains (Figure 6). Larger grains will reduce the strength and toughness of the material.
dislocations before polygonization
dislocations after polygonization
Recrystallization 5/7
Figure 6. The process of grain growth (the right grain grows to the expense of the left grain).
Thus, recrystallization itself is a diffusion process and, as such, takes place over a given
period of time after the incubation time, like the diffusion phase transformations.
Factors influencing recrystallization
The amount of cold deformation affects the final grain size
Increasing the cold deformation (or reducing the deformation temperature), increases the
rate of nucleation faster than it increases the rate of growth. As a result, the final grain size is
reduced by increased deformation. The prior deformation applied to the material must be
adequate to provide nuclei and sufficient stored energy to drive their growth, we call this
critical deformation (Figure 7). Depending on the alloy, critical deformation is within the
deformation range of 2-20%. If the prior deformation is larger, many highly deformed regions
with high dislocation-density will emerge, polygonization will take place in a larger range and
many new crystals will be generated and therefore, recrystallized domain will be fine grained.
Figure 7. Critical deformation needed to recrystallization.
Recrystallization 6/7
Amount of cold deformation affects the critical temperature
Recrystallization requires a minimum temperature for the necessary atomic mechanisms to
occur, we call this critical temperature. This recrystallization temperature decreases with
annealing time. Increasing with the amount of prior deformation (in other words it is reducing
the deformation temperature), will increase the stored energy and the number of potential
nuclei. As a result, the recrystallization temperature will decrease with increasing
deformation. The recrystallization temperature can be estimated by the melting point by
several methods (Table 1).
Name Trecryst (°C) Tmelting (°C)
Pb -4 327
Sn -4 232
Zn 10 420
Al 99,99% 80 660
Cu 99,99% 120 1085
Brass (Cu60Zn40) 475 900
Ni 370 1455
Iron 450 1538
W 1200 3410
Table 1. Typical recrystallization temperatures and melting points
Duration of heat treatment affects the final grain size
After performing the critical deformation and adding the adequate heat treatment over
the recrystallization temperature, the newly developing average grain size increases
(approximately) linearly by increasing the duration of heat treatment.
Temperature of heat treatment affects the final grain size
At a temperature above the recrystallization temperature, the newly emerging average
grain diameter increases exponentially with increasing the temperature.
Alloying affects the time of recrystallization
The presence of the alloying or contaminating atoms usually slows the recrystallization
processes due to inhibited diffusion.
Recrystallization 7/7
References
[1] William D. Callister Jr.: Fundamentals of Materials Science and Engineering,
An Interactive e-Text, John Wiley and Sons. Inc., 2001.
[2] William D. Callister Jr.: Materials Science and Engineering: An Introduction, John Wiley
and Sons. Inc., 2007.
[3] NDT Education Resources: Introduction to Materials and Processes, Strengthening and
Hardening Mechanisms. www.nde-ed.org/EducationResources/CommunityCollege
/Materials/Structure/strengthening.htm (2017).
[4] Constitutive Modelling and Computational Materials Science: Recrystallization and
grain growth, 2014. http://www.hhallberg.com/?p=556
[5] Stuart Keeler: Why sheetmetal grain size is important, MetalForming Magazine, 2011.
http://www.metalformingmagazine.com/magazine/article.asp?aid=4457
[6] Departmental notes.
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