Yield and fracture of lamellar [formula omitted] TiAl alloys

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Lamellar TiAl is formed from α 2 grains by Shockley dislocations sweeping alternate basal planes, thereby transforming the structure into the tetragonal γ phase which exhibits close-packed planes and directions aligned with the hexagonal substrate. Mismatches at the lamellar interfaces, of the order of 1%, give rise to internal stresses and interfacial dislocations. Both the flat plate geometry of the grains and the structure of the interfaces contribute to an extreme plastic anisotropy. The main trends evident in the plastic tensile properties are as follows: both the yield and fracture stresses are low when deformation occurs in the plane of the lamellae (soft mode) and are high when deformation occurs across the lamellae (hard mode); the ductility is high when the tensile axis lies close to the lamellar plane and is low when the tensile axis is nearly normal to the lamellar plane. The trends in the yield and fracture stresses and in the ductility may be understood by considering dislocations piled up at either lamellar (in hard mode) or domain (in soft mode) boundaries. Depending on the strength of the interface, which in turn depends on the type of interface and on the orientation of the adjacent grain, the stress field of the pile up may either cause dislocations to cross into the next grain (ductile, Hall-Petch behavior) or to nucleate cracks (semibrittle, Stroh behavior). The anisotropies of the yield and fracture stresses are explained principally by the effects of Schmid factors and anisotropies in the Hall-Petch (or Stroh) stresses resulting from the lamellar shapes.