Nonequilibrium synthesis of nbai₃ and Nb-Ai-V Alloys by Laser Cladding: Part II. Oxidation Behavior

Abstract

In this study, the isothermal oxidation behavior of laser-clad NbAl₃ has been investigated in the temperature range between 800 °C and 1400 °C in air. The effect on oxidation of vanadium microalloying, used to increase the ductility of the otherwise brittle NbAl₃ and discussed in Part I, [1] has also been considered. Bulk and surface oxide chemistry has been investigated using X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Oxidation kinetics have been determined from weight gain data. The XPS and X-ray diffraction data show that NbAl₃ does not exclusively form a protective A1₂O₃ layer when oxidized in air. The oxidation products at 800 °C are a mixture of Nb₂O₅ and A1₂O₃, while at 1200 °C, a mixture of NbAlO₄, Nb₂O₅, and Al₂O₃ is formed. At 1400 °C, a mixture of NbAlO₄, A1₂O₃, NbO₂, NbO_2.432, and Nb₂O₅ forms. Upon addition of vanadium, the oxidation rate is found to dramatically increase and may be related to the formation of (Nb, V)₂O₅ and VO₂, which grows in favor of protective A1₂O₃. Consequently, although vanadium may be a good additive in terms of its potential for improving ductility in NbAl₃, it is not in terms of its deleterious effects on oxidation.