An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture be havior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordere d B2 + orthorhombic intermetallic alloy is shown to have attractive combina tions of room-temperature ductility (11 to 14 pet), fracture toughness (60 to 92 MPa root m), and comparable fatigue crack growth resistance to IN718. Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic defo rmation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (similar to 9.6 pet of the 0.2 pet offset yield strength (YS)), The onset of bulk yield ing is shown to correspond to the spread of microplasticity completely acro ss the gage sections of the tensile specimen. Fatigue crack initiation is a lso postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the "unzippi ng" of cracks along slip bands that form ahead of the dominant crack tip. T he proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 degrees C are attributed to the shielding effects of oxide-in duced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallic s.