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Mechanistic understanding of deformation twinning in ordered superlattice structures is reviewed, and the inter-relationships between twinning and generalized plastic flow or fracture toughness are discussed. While general discussions refer to all the fcc-based and bcc-based cubic and noncubic ordered intermetallic alloys, specific calculations of the energetic and kinetic aspects of deformation twinning are made for TiAl. The importance of the twin-slip conjugate relationship on high temperature mechanical properties is emphasized. Discussion is given of possible effects of macro- and micro-alloying on twinning propensity.

The important roles of (111)[112] twinning in cleavage fracture of single-phase γ-TiAl and TiAl3 are assessed on the basis of theoretically calculated elastic constants, shear fault energies, and cleavage energies. The importance of elastic anisotropy in both homogeneous twin nucleation theory and dislocation models for twin nucleation is emphasized. The twin-slip conjugate relationship makes an important contribution to the strain compatibility for localized plasticity at a crack tip of Mode-I type. The intrinsic brittleness of these aluminides is attributed to a combined effect of the low mobility of slip and twin dislocations and the relatively low cleavage strength. The effect of the resolved normal stress on the twin plane is to enhance twinning activity at a (110) crack tip, leading to transformation toughening of shear type.