PHYSICAL CONSTANTS, DEFORMATION TWINNING, AND MICROCRACKING OF TITANIUM ALUMINIDES

Physical properties that are relevant to mechanical behavior of single -phase TiAl and Ti3Al and two-phase TiAI/Ti3Al alloys are summarized. By using planar-fault energies and temperature-dependent elastic const ants, dislocation dissociation reactions applicable to twin formation in TiAl are analyzed, and a pole mechanism based on a jogged [110]/2 o rdinary dislocation is proposed to explain the available experimental data on deformation twinning in gamma-TiAl single crystals. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) c rystals is attributed in part to the localized slip along lamellar int erfaces, thus lowering the yield stress for soft orientations. The exp erimental findings reported on cleavage habit planes of PST crystals a re discussed in terms of the calculated ideal work of adhesion and pos sible extrinsic factors.