Surface segregation behaviors of amorphous Ni Nb alloy under 65 35 oxidation in O at various temperatures

Variations of surface morphology and chemical composition of amorphous Ni65Nb35 alloy after oxidation in 1 atm O2 at temperatures from room temperature to 773 K have been investigated using scanning electron microscopy SEM., X-ray photoelectron spectroscopy XPS.and Auger electron spectroscopy AES.depth profile methods. Inverse segregation behaviors have been observed at two oxidation temperature ranges. At the lower temperature range F373 K., Nb segregates at the surface as Nb2O5; while at the higher temperature range G523 K., Ni segregates at the surface as metallic Ni and NiO. Different oxidation mechanisms have been suggested to operate in the two oxidation temperature ranges, which may be concerned with different oxidation rates of Nb and diffusion speeds of O and Ni ions in the Nb2O5 layer at different temperatures. The affinities of Ni and Nb to oxygen may act as the driving force of the segregation. q1999 Elsevier Science B.V. All rights reserved.