Effects of niobium on thermal stability and corrosion behavior of glassy Cu–Zr–Al–Nb alloys M.K. Tam, S.J. Pang, C.H. Shek * Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Received 14 March 2005; accepted 1 November 2005 Abstract The corrosion behavior of Cu 95Kx Zr x Al 5 (xZ40, 42.5 and 45 at.%) in 1 N HCl, 3 mass% NaCl and 1 N H 2 SO 4 solutions was studied. As Zr content increases, the corrosion resistance is slightly enhanced. In order to improve the corrosion resistance of the Cu–Zr–Al glassy alloy, Nb was selected to substitute Cu. Although the supercooled liquid region DT x of the Cu–Zr–Al glassy alloys decreases with increasing Nb content, the alloys still retain high glass-forming ability and bulk glassy samples with 1.5 mm diameter can be obtained when up to 5 at.% Nb was added. It is found that the addition of Nb results in improvement of the corrosion resistance of the glassy Cu–Zr–Al alloys. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Amorphous materials; C. Differential scanning calorimetry (DSC); D. Surface properties; D. Thermodynamic properties 1. Introduction Cu-based glassy alloys with high glass-forming ability (GFA) and tensile strength around 2000 MPa have attracted a lot of attention of researchers and engineers [1–3]. In order to use this type of glassy alloys as a practical engineering material, the resistance to environmental attacks should also be investigated. Without a high chemical stability in the service environment, their advantages in mechanical properties cannot be fully utilized. Investigations on the chemical stability of the Cu-based glassy alloys, such as Cu–Zr–Ti–Nb [4], Cu–Hf–Ti- (Mo, Ta and Nb) [5], Cu–Zr–Ti–Ni–Nb [6] and Cu–Zr–Ti- (Mo, Ta and Nb) [7] have been reported recently. The corrosion resistance of the Cu-based glassy alloys in acidic environments [4–7], especially those containing Cl K ion, is relatively low when compare with other alloy systems, such as Zr- [8] and Ni- [9] based glassy alloys. Recently, new Cu-based glassy alloys in Cu–Zr–Al system were formed, which also have high GFA and high strength [10]. Work on the corrosion behavior of these glassy alloys is rare and incomplete [11]. Therefore, it is important to investigate and improve the corrosion resistance of the Cu–Zr–Al glassy alloys by systematic experiments. This paper reports the corrosion behavior of the Cu–Zr–Al glassy alloys with various Zr content and the effects of the addition of strongly passivating element Nb, which substitute the low corrosion resistant Cu, on the thermal stability and corrosion behavior of the Cu–Zr–Al alloys. 2. Experimental procedure Alloy ingots with nominal compositions of Cu 95Kx Zr x Al 5 (xZ40, 42.5 and 45 at.%), Cu 55Kx Zr 40 Al 5 Nb x (xZ1, 3 and 5 at.%) and Cu 50Kx Zr 45 Al 5 Nb x (xZ1, 3 and 5 at.%) were prepared by arc melting the mixtures of the pure metals in an argon atmosphere. From the master alloys, bulk samples with a diameter of 1.5 mm were prepared by copper-mold casting and ribbon samples of 1 mm wide and approximately 0.02 mm thick were obtained by melt spinning in an argon atmosphere. The structure of the samples were examined by X-ray diffraction (XRD) using Cu K a radiation at tube voltage of 40 kV and tube current of 30 mA. Thermal stability with respect to glass transition and crystallization of the glassy samples was investigated with a Perkin–Elmer DSC-6 differential scanning calorimeter (DSC) operated at a heating rate of 0.67 K/s. Corrosion behaviors of the metallic glasses were evaluated by immersion tests and potentiodynamic polarization measure- ments. Before the tests, the ribbon specimens were mechani- cally polished in cyclohexane with a silicon carbide paper down to 1200 grit, degreased in acetone, dried in air and further exposed to air for 24 h. Electrolytes used were 1 N HCl, 3 Journal of Physics and Chemistry of Solids 67 (2006) 762–766 www.elsevier.com/locate/jpcs 0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.11.012 * Corresponding author. Tel.: C852 2788 7831; fax: C852 2788 7830. E-mail address: apchshek@cityu.edu.hk (C.H. Shek).