In this article we will discuss about the structure of recrystallisation diagram for electrolytically refined iron.
It is difficult to show the effect of amount of cold work and annealing temperature (for a given time of annealing) on the grain size of the metal or alloy by simple two dimensional graphs. Thus, the recrystallisation diagrams as shown in Fig. 7.43 are drawn. Such a diagram guides to select annealing conditions to obtain a desired structure in a metal.
A second maximum in grain size (first was at critical deformation) was seen in aluminium and copper, etc. at large cold work and high temperature of annealing. Secondary recrystallisation may be the cause of such large grains obtained after obtaining a perfect texture at deformations higher than 80%. Fig. 7.44 illustrates this too.
Annealing Twins:
A twin is a straight-sided band which runs across a grain, and is a mirror image of the neighbouring region across the twinning plane, as discussed while describing the mechanical twinning. Most, cold worked and annealed, FCC metals also show such parallel bands under optical microscope. These are called annealing twins. The boundary of the twins has been seen to coincide with a {111} twinning plane. Fig. 7.45 illustrates twins in brass.
In FCC metals, a change in the long-range stacking-sequence results in the formation of a twin, if this change occurs over many atom-spacings. This change can occur if a properly oriented grain boundary moves. Suppose by its movement an additional (111) plane is to be deposited on the FCC-sequence of…ABCABCA, but this new layer falls into wrong position to result in a stacking sequence of ABCABCAC i.e., instead of ‘B’ positions, the atoms occupy ‘C’ positions. This becomes the first layer of a twin. Further growth can then occur in the reverse order to give a sequence of ABCABCACBACBA …. This growth can continue till there occurs again a wrong positioning of a layer of atoms.
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That then, becomes the last layer of the twin band such as:
The occurrence of such a change in stacking-sequence does not change the coordination number, and thus can take place if the interface energy in that metal is low. Copper invariably shows annealing twins as its interface energy {ϒtwin/ ϒgb) is low, whereas aluminium rarely shows twins as this energy is high.
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Annealing twin-bands of wider widths are more clearly seen if extensive grain-growth has occurred, i.e., grain boundary has moved greater distances. Fig. 7.46 illustrates the steps in grain growth. If the orientation of grain D is close to the twin orientation of grain C, then nucleation of an annealing twin at the grain boundary, Fig. 7.46 (d) occurs as it decreases the total boundary energy.
This happens because the interface OQ has about 5% of the normal grain boundary energy (which it replaces); the energies of interface PQ is almost equal to the original interface PQ (between grains C and A), it has replaced; the extra interface OP has only a very low energy as then only annealing twins form in that metal (such as copper).
The number of twins per unit grain boundary area depends on the number of new-grain-contacts made during the process of grain-growth, because the twins nucleate there. Fig. 7.47 illustrates the nucleation of a twin at the corner of a grain. The twin then grows in width till a change in stacking sequence occurs again to complete the twin band formation.
Growth twins may form in a crystal whether it is growing from a vapour, a liquid or a solid. But normally annealing twins are not seen in cast metals as enough grain boundary motion does not occur. Coarse grained metals often contain twins, which are many times wider than any grain that was present shortly after recrystallisation but has increased the width during grain growth.
