Internal twinning in deformation twinning

Internal twinning, i.e. the formation of a pattern of secondary (or interna l) twins within a primary deformation twin, has been observed in austenitic steel. The description of this phenomenon is given in the framework of bot h geometrical and transformation defect approaches. The geometric approach uses the notion of the phenomenological theory of martensite and reveals th e geometry and the crystallography of the process. The mesoscopic defect ap proach uses the notion of disclinations and Somigliana dislocations and rev eals the defect structure and energetics of the structured twins. It is fou nd that as a result of internal twinning the habit plane (twin/matrix inter face) moves away from the twinning plane, the crystal lattice inside the pr imary twin regions rotates with respect to the surrounding matrix, and the effective shear associated with the structured twin is reduced. It is predi cted that no primary dislocations are trapped at the habit plane. It is arg ued that internal twinning is an essentially collective dislocation process resulting in the simultaneous development of primary and secondary twins. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. AN right s reserved.