Recovery, Recrystallization and Grain Growth of Metals | Electricity

When a metal is deformed significantly, the grains become elongated and metal are in a non-equilibrium state. Now when the metal is heated to a temperature of about 0.3-0.5 T_m, T_m being melting temperature of metal, and held at this temperature.

The metal attempts to approach equilibrium through a series of interesting processes, namely:

(i) Recovery,

(ii) Recrystallization and

(iii) Grain growth.

The Fig. 3.16 shows a systematics of these processes with the variation of time. At time T₁, new grains begin to nucleate from cold worked grains; the nucleating of grains will continue and grow upto time T₂. By this time all the cold worked grains will nucleate to from new grains. The size of these new grains increases at slower rate at time T₃.

1. Recovery:

It is a low temperature phenomenon which results in the restoration of the physical properties without any observable change in microstructure. The recovery is important for releasing internal stresses in forging, welded and fabricated equipment, without decreasing the strength acquired during and working.

Mechanisms of Recovery:

The mechanism operating at low temperature is vacancy motion. This involves (i) migration of point defects to grain boundaries and dislocations, and (ii) combination of point defects.

At intermediate temperature the mechanism is dislocation movement without climb. This involves (i) arrangement of dislocations within tangles (ii) annihilation of dislocation and (iii) grain growth.

At high temperature, the mechanism is dislocation movement with climb, which involves (i) dislocation climb (ii) disappearance of boundary between two sub-grain, known as coalescence (iii) polygonization (a state between recovery and recrystallization).

In recovery process, the cold worked metals are heated at low temperature; mobile imperfections (vacancies, interstitials and dislocations) undergo rearrangement in the lattice. Vacancies and interstitials are eliminated first and then some dislocations of opposite sign are annihilated. However, majority of dislocations are not removed by usual recovery treatments. The minor structural changes during recovery have pronounced effect on residual stresses and on electrical properties.

The characteristics of recovery process are shown in Fig. 3.18. It can be observed from the figure; (i) The rate of recovery is fast initially and drops off with time, (ii) The amount of recovery increases with increasing temperature. In metals, the individual properties, recover at different rates.

By recovery, stresses are relieved from cold worked alloys which prevent stress corrosion cracking. Stress relieving can be achieved without much affecting the mechanical properties. For complete removal of residual stresses, high recovery temperature is required. This high temperature treatment is useful for casted or welded parts.

2. Recrystallization:

It is a process by which distorted grains of cold worked metal are replaced by new strain free grains during heating above a minimum temperature called recrystallization temperature. During recrystallization, there is a sharp decrease in hardness and strength and an increase in ductility.

Mechanisms of Recrystallization:

Two mechanisms have been observed depending upon metal and degree of deformation. The deformed metal has two types of interface (a) Pre-existing grain boundaries (b) Sub-grain boundaries resulting from deformation.

(a) Growth of Pre-Existing Grain Boundaries:

The boundary between a grain of high dislocation density and a grain of low dislocation density suddenly grows. Thus, the nucleation is essentially a growth phenomenon. The nucleation by this growth mechanism will occur at boundaries having grain boundary mobility, e.g., high-angle boundaries.

(b) Growth of Sub-Grain Boundaries:

The sudden growth may be due to either by coalescence mechanism or by grain boundary migration. High mobility boundary forms a high angle boundary, which suddenly grows out, occurs on atomic scale.

Primary Recrystallization:

It occurs when cold worked metal is heated. It is defined as the nucleation and growth of strain-free grains, from the matrix of cold worked metal. When primary recrystallization occurs, there is some degree of recovery and sub-grain formation. Primary recrystallization is of much importance because the properties of an alloy, after primary recrystallization will be same as the properties it has before cold working.

Consider a cold working operation of an alloy such as deep drawing. During cold working, it becomes hard and less ductile. It becomes difficult to continue the forming operation. A partial forming operation is done first and then the alloys (job) are given a recrystallization (annealing) treatment. This partially formed alloy (job) regains its original ductility and hardness. Now the job can be given further forming operation for deep drawing.

The factors which control recrystallization are:

(a) Nucleation of strain free grains.

(b) Growth of these nuclei to surround the whole specimen.

Generally nucleation occurs at highly deformed portion of grains. Due to random thermal vibration, the cluster of atoms in highly strained lattices transforms spontaneously into strain-free nucleus. The rate of nucleation increase with increasing deformation and increasing temperature. The alloying elements present in solid solution decrease the rate of nucleation.

The creation and growth of nuclei are difficult to study. But there is some critical size, below which the cluster of atoms cannot form a stable nucleus. Once the nucleus is formed, it grows by motion of boundary.

The rate of growth during recrystallization is independent of time but increases with the degree of deformation and with annealing temperature. The presence of impurities decreases the rate of growth, i.e., recrystallization rate. When all the cold worked metal grains have been replaced by grains of strain free crystal structure, the growth stops and recrystallization is said to be complete.

Laws of Recrystallization:

1. A minimum deformation is necessary for recrystallization.

2. The smaller the degree of deformation, the higher the temperature required to initiate recrystallization.

3. Increase in annealing time decreases the temperature required for recrystallization.

4. The final grain size depends chiefly on the degree of deformation and lesser on annealing temperature. When degree of deformation is more and annealing temperature is less, the recrystallized grain size will be smaller.

5. The lesser the original grain size, the greater the amount of deformation required to give equivalent temperature and time.

6. The amount of deformation required to produce equivalent recrystallization behaviour increases with increased temperature of working.

7. New grains do not grow into deformed grains.

8. Continued heating after primary recrystallization causes grains size to increase.

Secondary Recrystallization or Coarsening:

When annealing of a deformed sample is continued beyond the primary recrystallization, or the sample is heated at higher temperature, the usual grain growth is interrupted. Some grains suddenly experience very rapid growth. The dimensions of these rapidly grown grains may be of the order of few centimeters, while the rest of the grains small. These grains expand at the cost of other grains. This is called secondary recrystallization.

The mechanism involved, is the rapid migration of boundaries of a few primary recrystallized grains; thus most of the small primary grains are annihilated and large secondary grains are formed. Secondary grains are more perfect than initial recrystallization grains.

The process has the following characteristics:

(i) The large grains are not freshly nucleated; there are only particular grains of primary structure which are grown larger.

(ii) It is not yet clear that which primary grains will grow in size, but the grains which will grow in size will be larger than the mean primary grain size.

(iii) Initially the growth of larger grains is slow. Secondary recrystallization occurs after some period of time.

(iv) The texture of secondary structure always differs from the previous primary texture.

3. Grain Growth:

It refers to the increase in the average grain size on further annealing after material has recrystallized. Large grains have lower free energy than small grains. The atoms in the smaller crystals, which have higher energy, tend to become a part of larger crystal. This tendency leads to grains growth.

This can be achieved, when the material is held for longer times at a temperature above crystallization temperature. The increase in grain size decreases the hardness and strength but increases the ductility.

Factors Controlling Grain Size:

(a) Degree of Prior Deformation:

The increased amount of prior deformation favours nucleation and decreases final grain size. A minimum amount of deformation is required before recrystallization takes place. This is generally 2.8% of deformation. When deformation is small (but above the minimum deformation), the grain size is coarse because small number of nuclei are formed. As the deformation increases, the number of distorted points also increases and thus grain size decreases.

(b) Temperature:

There is some minimum temperature below which recrystallization does not take place. Above this temperature, the grain size increases slowly.

(c) Heating Time:

The effect of time of heating on grain size depends upon temperature at which the recrystallization is taking place. A certain amount of time is required in which recrystallization is to be completed but this amount of time decreases as temperature increases. The shorter the time of annealing, the finer is the grain size. For longer time of annealing, the grain is coarse. Slow heating will form new nuclei, favouring grain growth and hence the grain will be coarse.

(d) Impurities:

With greater amount and finer distribution of impurities, finer grain size will be there. The impurities increase nucleation and also act as barrier for grains to grow.

The effect of various phenomena, on mechanical and physical properties are summarized in the following Fig. 3.21.

Grain Size Determination:

The properties of the polycrystalline materials are greatly affected by the grain size, so it is important to determine the grain size. In this context, there exist a number of various techniques by which the size can be specified in terms of average grain volume, diameter, area etc. Often, grain size is estimated by using an intercept method, which follows. Straight lines (all of equal length) are drawn through photomicrographs that show the grain structure.

The grains intersected by each line segment are counted; the line length is then divided by an average of the number of grains intersected, taken over all the line segments. The average grain diameter is found by dividing this result by the linear magnifications of the photomicrographs. The American Society for Testing and Materials (ASTM) has assigned the grain sizes in materials as grain size number V ranging from 1 to 10. The average number of grains (N) per square inch observed on a photomicrograph at a magnification of 100 X is given by-

N = 2^{n – 1}

or n = log N/log 2 + 1

Thus, the grain size number ASTM 1 corresponds to one grain per square inch (645 mm²) at magnification of 100X i.e. 10⁴/645 = 15.5 grains/mm² without magnification. This is approximated to a grain diameter of 1/√15.5 mm. Similarly, grain size number ASTM 2 corresponds to 2 × 15.5 grain/mm² and hence the average grain diameter for ASTM 2 is .

In general, one can write the average grain diameter ‘d’ as-

Based on the average number of grains and average grain diameters, the grains may be classified as:

(a) Coarse grain for n ≤ ASME 3

(b) Medium grain for ASME 4 ≤ n ≤ ASTM 6

(c) Fine grains for ASTM 7 ≤ n ≤ ASTM 9

(d) Ultrafine grain for n ≥ ASTM 10.