Engineering Concepts: Formation of Annealing Twins

Twin boundaries are usually flat and extend across an entire grain. These twin boundaries define a trace of a {111} atomic plane, i.e. the close packed planes in the FCC crystal structure. In some cases the twin is terminated within the grain and a stair step interface is observed consisting of a series of {111} planes.

The formation of a twin is accomplished by changing the atomic stacking sequence of these {111} planes. In the FCC structure, the {111} planes are stacked in a sequence ABCABCABC, which is equivalent to stacking billiard balls to form a pyramid. However, a choice of two positions is possible with the start of each new {111} layer.