ROOM-TEMPERATURE TENSILE DEFORMATION OF POLYSYNTHETICALLY TWINNED (PST) CRYSTALS OF TIAL

Tensile deformation behavior of polysynthetically twinned (PST) crystals of TiAl with a nearly stoichiometric composition was investigated as a function of the angle phi between the lamellar boundaries and tensile axis at room temperature. Tensile elongation to fracture strongly depends on the angle phi but its dependence on the angle phi is not symmetrical with respect to phi = 45-degrees. A tensile elongation as large as 20%, which is far larger than any other reported values on TiAl-based compounds, has been obtained for phi = 31-degrees. The yield stress also strongly depends on the angle phi but any particularly significant tension-compression asymmetry in yield stress has not been observed. Fracture has been found to occur in a brittle manner without showing any local contraction even after deformation to more than 10%. When the tensile axis is perpendicular or inclined to the lamellar boundaries, fracture occurs in a cleavage-like mode with a habit plane parallel to the lamellar boundaries while fracture occurs in a zigzag across the lamellar boundaries when the tensile axis is parallel to the lamellar boundaries.