MATERIALS SCIENCE AND TESTING

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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.

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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).

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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.

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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

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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.

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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.

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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.