Experiments on “annealing twins”, have been made with copper and other metals. These are produced by plastic deformation followed by heat treatment, during which recrystallisation and crystal growth occur. It is suggested that the principal cause of twinning is crystal growth. Deformation is an indirect cause of twinning, merely because if causes subsequent growth on annealing. From the crystallographic point of view, the mechanism of twinning has been discussed principally on metals crystallising in the face-centred cubic lattice. The capacity for forming twins appears to be closely related to the atomic arrangement in the crystal lattice of the metal. Those metals which crystallise in the face-centred cubic, tetrahedral cubic. body centred cubic and face-centred tetragonal lattices produce annealing twins after appropriate treatment, whereas metals possessing other atomic arrangements are not likely to produce them. Annealing twins are structually in a very stable state, being different in nature from what is called mechanical twins, strain-etch lines and Neumann banads.
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