An investigation of the fracture and fatigue crack growth behavior of forged damage-tolerant niobium aluminide intermetallics

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.

A computational approach to designing ductile Nb-Ti-Cr-Al solid-solution alloys

“…Specifically, experimental data of tensile ductility and fracture toughness of Nb, Nb-Ti-Al, Nb-Ti-Cr, and become more ductile with increasing Ti content, because the computed ␥ s /U P-N value becomes increasingly larger and Nb-Ti-Cr-Al alloys were collected from previous work [2][3][4][5][26][27][28] and from the literature. [11,[72][73][74] These materithan the critical value, which is 6.3. In contrast, both NbCr and Nb-Al alloys become less ductile with increasing Cr als contained low interstitial contents; the possible effects than the value of 3.5 or 6.3 suggested by Rice.…”

“…[3,27] The of a Griffith crack [39] from the crack tip. [40][41][42][43][44][45][46][47][48] The limiting parameter is usually the shear modulus, but Rice has used stage in the dislocation emission process can either be the the shear modulus in the [111] Burgers vector direction, nucleation of a dislocation at the crack tip [41] or, once nucleleading to slip ϭ (c 11 Ϫ c 12 ϩ c 44 )/3. [41] This factor ( slip ) ated, the motion of the dislocation away from the crack is computed from estimates of the elastic constants (c ij ) made tip.…”

“…[4] The ␥ s /␥ us ratio has been used by Waghmare et al [36] as the through a discrete, periodic lattice is the P-N barrier energy. [37,38] This energy parameter can be viewed as the basis for selecting ternary alloying additions to improve maximum resistance of the discrete lattice to dislocation [11] against the results for Nb by Duesbery and Vitek [63] suggested that L ϭ d /8, rather than L ϭ d. motion and can be used as a measure of dislocation mobility. Calculation of the lattice resistance was first made by For anisotropic solids, the P-N barrier energy is obtained from Eq.…”

“…cosh sinh [11] c ij ϭ ͚ nϭ7 nϭ0 p n q n [19] and ␥ us is a multiplier to the lattice term (cosh sinh) Ϫ1 in the P-N barrier energy. Comparison of Eqs.…”

A Study of the Kinetics of Phase Transformations in Nb-Ti-Al Alloys

“…These alloys have been extensively examined for their promising mechanical properties up to 800°C, with microstructures based on the ordered ß solid solution (ß o , B2) and orthorhombic Ti^AlNb (O, El a ) phases [7]. Numerous investigations have also been conducted on Nb-rich alloys related to the phase equilibria between the ß o and Nb 3 Al (5, A15) phases [8][9][10], as well as the mechanical properties of these alloys [11][12][13][14]. Little experimental knowledge exists, however, on the phase transformations and equilibria at high Nb concentrations along the psuedobinary involving the ß o , 8, a (Nb,Al, D8 b ) and Ti.AlNb phases [15,16], from which accurate determinations of the phase equilibria can be made towards the development of potential alloys.…”