No abstract

The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb 3 Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa at room temperature. Fracture in these alloys occurs ͌m by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at roomtemperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa ), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, ͌m and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (ϳ700 ЊC to 750 ЊC) regime.

The results of an investigation of toughening mechanisms in NiAl composites reinforced with yttriapartially stabilized zirconia polycrystals are presented. Different yttria stabilization levels in the zirconia, between 0 and 6 mole pct are employed. It is shown that substantial improvements in fracture toughness are obtained in all the composites reinforced with partially stabilized zirconia particles. The phase contents and microstructures of the composite systems are characterized by X-ray diffraction and transmission electron microscopy (TEM) techniques. Crack tip deformation was also studied using crack tip TEM analysis, and laser Raman spectroscopy was used to estimate the size of the transformation zone. The results show that transformation toughening is significant only in the 2 mole pct yttria-stabilized zirconia composite. Toughening is also shown to occur via slip phenomena within the NiAl grains in the near-tip regions of the composites reinforced with 2, 4, or 6 mole pct yttria-stabilized zirconia particles.

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 ϩ orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPaΊm), 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 deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (ϳ9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the "unzipping" of cracks along slip bands that form ahead of the dominant crack tip. The 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 ЊC are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.