TENSILE DUCTILITY OF EXTRINSICALLY TOUGHENED INTERMETALLICS

A theoretical analysis based on crack instability is presented to elucidate factors effecting the tensile ductility of intermetallics that are toughened by extrinsic mechanisms and resistance-curve effects. The analytical results indicate that extrinsic-toughening mechanisms are ineffective in imparting tensile ductility in brittle intermetallics. The plastic strain at the onset of unstable fracture is generally controlled by the initiation toughness, K(IC), except for materials with a high-tearing modulus, T(R). The consequence is that tensile ductility increases with the initiation toughness, K(IC), but is unrelated to the K value measured at the peak load or the T(R) value for materials with low- to intermediate-tearing resistance (e.g., T(R) < 18 for K(IC) = 10 MPa square-root m). Application of the model to a two-phase TiAl alloy reveals good agreement between theory and experiment. This finding indicates that tensile ductility in brittle intermetallics can be imparted more effectively by intrinsic-toughening mechanisms than by extrinsic ones.