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

Abstract

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.

Toughness and strength characteristics of Nb-W-Si ternary alloys prepared by Arc melting

“…[1,2,3] Approaches to improving the high-temperature strength of Nb have involved solid-solution strengthening, or forming composites with Nb and intermetallics such as silicides [3][4][5][6][7][8][9][10][11][12] and aluminides. [13][14][15][16] Systematic development of Nb/intermetallic combinations, defined as in-situ composites, has been reported extensively; among them, Nb-Al and Nb-Si binaries are typical. The Nb-Al and Nb-Si binaries have equilibrium phases of Nb solid solution (Nb ss )/Nb 3 Al aluminide and Nb ss /Nb 5 Si 3 silicide, [17] respectively, in which the Nb ss offers room-temperature ductility, and the aluminides and silicides supply strength and creep resistance at elevated temperatures.…”

The role of Hf and TiC additions in the mechanical properties and microstructure of NbAlV alloys

“…Jiao et al [27] reported there is almost no tensile ductility in an as-cast Nb-15Al-20V alloy. The study of Soboyejo et al [28] showed that forged Nb-12Al-44Ti-1.5Mo alloy (partially ordered B2) has good room temperature ductility (11-14 pct). Our investigation [20] on forged Nb-Ti-Al alloys with A2 structure showed that adding 1 at.% C does not improve the strength, slightly impairs the ductility, but gives a slight increase in the toughness; a 10 at.% C addition dramatically decreases the ductility and the toughness, but significantly improves the strength.…”