Experimental determination of domain orientations and domain orientation relationships across lamellar interfaces in polysynthetically twinned TiAl crystals

Six domain orientations in an as-grown polysynthetically twinned (PST) TiAl crystal are unambiguously determined. Fifteen domain orientation relationships across lamellar interfaces (not including those across order-translational lamellar interfaces) are uniquely identified. The atomic arrangements of these 15 domain orientation relationships are investigated. Domains within a lamellar lath are related to each other by a 120°-rotation: either domains [I] to [III] or domains [IV] to [VI] are within a lamellar lath. Diffraction patterns across the lamellar interfaces show that a <1̄01] zone disordered twin diffraction pattern (the twin diffraction pattern without superlattice diffraction spots) taken parallel to the lamellar interfaces cannot unambiguously determine whether the interface is a true-twin related interface or a pseudo-twin related interface. A second different zone diffraction pattern is necessary to characterize the twin-related interfaces but <2̄11> zone diffraction patterns can not be used to determine the twin relationships since the diffraction spots from both crystals are coincident. A diffraction criterion for an unambiguous determination of domain orientations and domain orientation relationships across lamellar interfaces is proposed.