Materials Science and Engineering A 424 (2006) 77–82 Liquidus projection of the Nb–Cr–Al system near the Al 3 (Nb,Cr) + Cr(Al,Nb) eutectic region S.A. Souza a , P.L. Ferrandini a , C.A. Nunes b , A.A. Coelho c , R. Caram a,∗ a DEMA/FEM, State University of Campinas, C.P. 6122, 13083-970 Campinas, SP, Brazil b FAENQUIL, Materials Engineering Department (DEMAR), Polo Urbo-Industrial Gleba AI-6, C.P. 116, 12600-970 Lorena, SP, Brazil c IFGW, State University of Campinas, C.P. 6165, 13083-970 Campinas, SP, Brazil Received 9 February 2006; accepted 21 February 2006 Abstract The system Nb–Cr–Al was investigated in the region near the Al 3 (Nb,Cr) + Cr(Al,Nb) eutectic and the liquidus projection of that region was determined based on the microstructural characterization of arc melted alloys. The characterization utilized scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), differential thermal analysis (DTA) and X-ray diffraction (XRD). The results allowed one to determine three primary solidification liquidus surfaces ((Cr,Al) 2 Nb, Cr(Al,Nb) and Al 3 (Nb,Cr)), that are originated from the binary systems Cr–Nb, Cr–Al and Al–Nb. It is proposed the occurrence of the invariant reaction L + (Cr,Al) 2 Nb ⇆ Al 3 (Nb,Cr) + Cr(Al,Nb) and of a point of minimum, which involves a three phase reaction, L ⇆ Al 3 (Nb,Cr) + Cr(Al,Nb). All alloys studied showed formation of the Al 3 (Nb,Cr) + Cr(Al,Nb) eutectic as the last solidification step with Al(Nb)Cr 2 precipitating from Cr(Al,Nb). © 2006 Elsevier B.V. All rights reserved. Keywords: Ternary alloy systems; Phase diagrams; Microstructure 1. Introduction During the 1950s and 1960s there was a high interest in the study of Nb alloys as structural materials for the aerospace industry [1]. In the middle 1980s the interest for these alloys as high temperature structural materials was renewed [2] and nowa- days, the niobium silicide-based composites seem to be partic- ularly interesting for replacing the nickel-based superalloys [3]. The niobium alloys present reasonable oxidation resistance [2] but undergo embrittlement under oxidizing environments [1]. According to Zhao et al. [4], Cr and Al additions to Nb alloys significantly improve their oxidation resistance and hence, there is an effort to obtain phase equilibrium information on several ternary systems involving Nb, among them the Nb–Cr–Al, for which there is little information on the literature [5]. Hunt et al. [6] proposed a 1000 ◦ C isothermal section for this system, which was reviewed by other authors [7,5,4]. Svechnikov et al. [8] proposed partial isothermal sections at 1200 and 1500 ◦ C and Argent [9] proposed a partial isothermal section at 1200 ◦ C, ∗ Corresponding author. Tel.: +55 19 37883314; fax: +55 19 37883314. E-mail address: rcaram@fem.unicamp.br (R. Caram). which is considered to present speculative phase solubility limits [10]. Among the Nb-based intermetallics, Al 3 Nb presents the low- est oxidation rate [11]. Kaufman and Nesor [12] observed the existence of a eutectic structure involving Al 3 Nb fibres within the Nb–Cr–Al system. This eutectic structure was stud- ied by Thomas [13], however, the information obtained on the eutectic features and the constituent phases are consid- ered incomplete and became more unsatisfactory when new invariant reactions were identified, mostly within the Al–Cr binary system [14]. The eutectic structure is formed by the Al 3 (Nb,Cr) and Cr(Al,Nb) phases. The ductile Cr(Al,Nb) phase undergoes a solid/solid transformation to Al(Nb)Cr 2 [15]. This work further investigates the Al 3 (Nb,Cr) + Cr(Al,Nb) eutectic. The microstructural analysis of the as cast alloys allowed the determination of the liquidus projection of the Nb–Cr–Al system near the eutectic region. The literature presents only one study on the determination of the liquidus projection of this system, obtained by computational methods [12]. However, the constitutive binary phase diagrams of the Nb–Cr–Al system, Al–Nb [16], Cr–Nb [17] and Al–Cr [14], differ considerably from those used for the calculated liquidus projection. 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.024