Examination of solidification pathways and the liquidus surface in the Nb-Ti-Al system

Abstract

The solidification pathways, subsequent solid-state transformations, and the liquidus surface in the Nb-Ti-Al system have been examined as part of a larger investigation of phase equilibria in Nb-TiAl intermetallic alloys. Fifteen alloys ranging in composition from 15 to 40 at. pct Al, with Nb to Ti ratios of 4:1, 2:1, 1.5:1, 1:1, and 1:1.5, were prepared by arc melting and the as-cast microstructures were characterized by optical microscopy (OM), microhardness, X-ray diffraction (XRD), differential thermal analysis (DTA), backscattered electron imaging (BSEI), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The results indicate that the range of primary ␤ solidification is much wider than that indicated in previously reported liquidus surfaces, both experimental and calculated. Differential thermal analysis has identified the existence of a ␤ to ϩ ␥ transformation in three alloys where it was previously thought not to exist; confirmation was provided by hightemperature vacuum heat treatments in the single-phase ␤ region followed by rapid quenching. The location of the boundary between the ␤, , and ␦ primary solidification fields has been redefined. A massive ␤ → ␦ transformation, which was observed in the cast microstructure of a Nb-25Ti-25Al alloy, was repeatable through cooling following homogenization. A ␤ → ␦ ϩ eutectoid-like transformation in the 25 at. pct Al alloys, was detected by DTA and evaluated through microstructural analysis of heat-treated samples. Trends in the ␤ phase with variations in composition were established for both lattice parameters and microhardness. As a result of this wider extent of the primary ␤ solidification field, a greater possibility exists for microstructural control through thermal processing for alloys consisting of either ϩ ␥, ␤ ϩ , or ␤ ϩ ␦ phases.

The Al–B–Nb–Ti system

“…Compared to the thermodynamic descriptions by Kattner [4] and Servant and Ansara [6] the primary ␤ field extends further towards the Al-rich corner, while correspondingly the fields of primary ␣, and ␦ are less extended. The limit of the primary -phase field proposed by [54] to be at 20 at.% Ti is not fully confirmed by Fig. 14. The ␤-transus surface of the Al-Nb-Ti system projected on the Gibbs triangle, with isotherms given in K: within the shadowed area the ␤-phase is ordered, i.e.…”

“…The liquidus surface of the Al-Nb-Ti system showing (a) primary phase fields (points are experimental data of[25,27,38,[51][52][53][54] and present work (PW) and (b) projection of isotherms (in K) and univariant lines on the Gibbs triangle.…”

Al-Nb-Ti (Aluminum-Niobium-Titanium)

“…The updated version of the Al-Ti phase diagram [2005Rag] depicts a number of intermediate phases. TiAl 3 has two crystal modifications.…”

“…A complete liquidus projection of this ternary system was reported by [1989Kal], followed by more experimental results by [1990Per], a computed projection by [1992Kat], a partial projection by [1992Pav1], and a reinvestigation by [1995Zdz] and[2000Leo1]. [1990Per] arc-melted alloys under an Ar atmosphere.…”

Characterization of the invariant reaction involving the L, η, γ and σ phases in the Ti–Al–Nb system

“…The majority of the available experimental data for the high-temperature equilibrium was reported by Perepezko et al [14], Zdziobek et al [15] and Leonard et al [16,17]. Recently, we have experimentally demonstrated that the b phase field on the liquidus projection should be expanded by pushing the univariant L + b + c liquidus reaction to higher Al compositions [18].…”