Internal twinning in deformation twinning

Internal twinning, i.e. the formation of a pattern of secondary (or internal) twins within a primary deformation twin, has been observed in austenitic steel. The description of this phenomenon is given in the framework of both geometrical and transformation defect approaches. The geometric approach uses the notion of the phenomenological theory of martensite and reveals the geometry and the crystallography of the process. The mesoscopic defect approach uses the notion of disclinations and Somigliana dislocations and reveals the defect structure and energetics of the structured twins. It is found that as a result of internal twinning the habit plane (twin/matrix interface) moves away from the twinning plane, the crystal lattice inside the primary twin regions rotates with respect to the surrounding matrix, and the effective shear associated with the structured twin is reduced. It is predicted that no primary dislocations are trapped at the habit plane. It is argued that internal twinning is an essentially collective dislocation process resulting in the simultaneous development of primary and secondary twins.