A step-wise nucleation and growth mechanism, which was based upon coincidence site lattice theory, was proposed for deformation twinning in hexagonal close-packed structures. Lattice transformation during twinning was achieved via the coordinated movement of a large number of atoms from the matrix to twin positions, rather than by a layer-by-layer movement which involved twinning dislocations. The sideways propagation and thickening of a twin lamella proceeded in a step-wise manner, with lattice-matched planes being the coherent boundaries between the matrix and twins. This model predicted that twinning in hexagonal close-packed lattices could occur at a velocity which was close to the speed of sound. This was impossible according to twinning dislocation theories. The proposed mechanism was also consistent with other observations, such as a lack of a critical resolved shear stress for twinning, emissary dislocations, and an insensitivity to temperature. It was suggested that dislocation reactions could be involved in twinning, although they were not required at high stresses.

S.G.Song, G.T.Gray: Acta Metallurgica et Materialia, 1995, 43[6], 2325-37