Cyclic oxidation response of multiphase niobium-based alloys
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Cyclic Oxidation Response of Multiphase Niobium-Based Alloys KWAI S. CHAN Cyclic oxidation tests were performed on multiphase Nb-based alloys containing silicide, Laves, and Nb solid solution phases. In particular, the oxidation resistance of six alloys with various compositions (Nb, Ti, Hf, Cr, Ge, and Si) and microstructures was characterized by thermal cycling from ambient temperature to a peak temperature that ranges from 900 °C to 1400 °C. Weight change data were obtained and the corresponding spalled oxides were collected and identified by X-ray diffraction. The results indicated that Nb-based alloys formed a mixture of CrNbO4, Nb2O5, and Nb2O5 TiO2, with possibly small amounts of SiO2 or GeO2. The oxidation resistance was improved when CrNbO4 formed instead of Nb2O5 and Nb2O5 TiO2. These results were used to assess the influence of microstructure and composition on the oxidation resistance of multiphase Nb-based alloys.
I. INTRODUCTION
EXTENSIVE work has been undertaken to develop niobium-based structural alloys for high-temperature applications. The early studies (e.g., References 1 through 9) focused mostly on the use of alloying addition to reduce the oxidation kinetics of Nb alloys by modifying the oxidation products. The early efforts, which were reviewed extensively by Stringer[10] and others,[11,12,13] were largely unsuccessful because of the formation of Nb2O5, a nonprotective oxide, which was too dominant to be altered by alloying addition. Subsequent studies[14–19] were concentrated on an aluminidebased system and the use of Al addition to improve oxidation resistance of Nb-based alloys through the formation of a protective alumina layer. While the formation of a protective alumina was feasible, the resulting alloys had low melting points and were too brittle to be used as structural materials.[16] Recent efforts have focused mostly on materials that contain substantial amounts of niobium silicides and Laves phases.[20–32] These new systems include Nb-Si, Nb-Ti-Si, Nb-Ti-Al-Si, Nb-Ti-Cr-Si, and, among others, Nb-Ti-Hf-Cr-Al-Si. In these Nb-based alloys, Ti and Hf additions are intended for improvement in fracture[24,33,34] and oxidation resistance.[32,35] The Cr, Al, and Ge additions[32,35] are intended to enhanc oxidation resistance, while Si is intended to provide oxidation and creep resistance[35] through the formation of various refractory-metal silicides (e.g., alloyed Nb5Si3, Nb3Si, Ti5Si3, and Ti3Si) and Laves phases (e.g., alloyed NbCr2). Often referred to as refractory metal intermetallic composites (IMCs),[28,29] the salient characteristics of these Nb-based alloys are multiphase microstructures comprised of silicides, Laves phases, and Nb solid solution. Hereafter, these Nb-based materials will be referred to either as multiphase alloys or in-situ composites in this article. The silicides and Laves phases are intended to provide high-temperature strength and oxidation resistance, while the “ductile” Nb solid solution is intended to provide tensile ductili
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