Annealing twins

6.8.5 Annealing twins

A prominent feature of the microstructures of most annealed fcc metals and alloys is the presence of many straight-sided bands that run across grains. These bands have a twinned orientation relative to their neighboring grain and are referred to as annealing twins (see Chapter 3). The parallel boundaries

Mechanical properties I 359

Figure 6.57 Formation and growth of annealing twins (from Burke and Turnbull, 1952).

A A A Twin A C D C D C D C D B B B B

Figure 6.58 Nucleation of an annealing twin during grain growth.

usually coincide with a (1 1 1) twinning plane with the structure coherent across it, i.e. both parts of the twin hold a single (1 1 1) plane in common.

As with formation of deformation twins, it is believed that a change in stacking sequence is all that is necessary to form an annealing twin. Such a change in stacking sequence may occur whenever a properly oriented grain boundary migrates. For example, if the boundary interface corresponds to a (1 1 1) plane, growth will proceed by the deposition of additional (1 1 1) planes in the usual stacking sequence ABCABC. . . . If, however, the next newly deposited layer falls into the wrong position, the sequence ABCABCB is produced, which constitutes the first layer of a twin. Once a twin interface is formed, further growth may continue with the sequence in reverse order, ABCABC |BACB. . . until a second accident in the stacking sequence completes the twin band, ABCABCBACBACBABC. When

a stacking error, such as that described above, occurs the number of nearest neighbors is unchanged, so that the ease of formation of a twin interface depends on the relative value of the interface energy. If this interface energy is low, as in copper, where γ twin <

20 mJ m −2 twinning occurs frequently while, if it is high, as in aluminum, the process is rare. Annealing twins are rarely (if ever) found in cast metals because grain boundary migration is negligible during casting. Worked and annealed metals show considerable twin band formation; after extensive grain growth a coarse-grained metal often contains twins which are many times wider than any grain that was present shortly after recrystallization. This indicates that twin bands grow in width, during grain growth, by migration in a direction perpendicular to the (1 1 1) composition plane, and one mechanism whereby this can occur is illustrated schematically in Figure 6.57. This shows that

a twin may form at the corner of a grain, since the grain boundary configuration will then have a lower interfacial energy. If this happens the twin will then be able to grow in width because one of its sides forms part of the boundary of the growing grain. Such a twin will continue to grow in width until a second mistake in the positioning of the atomic layers terminates it; a complete twin band is

then formed. In copper and its alloys, γ twin /γ gb is low and hence twins occur frequently, whereas in aluminum the corresponding ratio is very much higher and so twins are rare. Twins may develop according to the model shown in Figure 6.58 where, during grain growth, a grain contact is established between grains C and D. Then if the orientation of grain D is close to the twin orientation of grain C, the nucleation of an annealing twin at the grain boundary, as shown in Figure 6.58d, will lower the total boundary energy. This follows because the twin/D interfaces will

be reduced to about 5% of the normal grain boundary energy, the energies of the C/A and twin/A interfaces will be approximately the same, and the extra area of interface C/twin has only a very low

360 Physical Metallurgy and Advanced Materials

Figure 6.59 Combination of transient and steady-state creep. energy. This model indicates that the number of twins per unit grain boundary area depends only on

the number of new grain contacts made during grain growth, irrespective of grain size and annealing temperature.