OXIDATION-KINETICS OF AMORPHOUS, POLYCRYSTALLINE, AND NANOCRYSTALLINECO33ZR67 ALLOYS

The oxidation kinetics of amorphous, polycrystalline, and nanocrystall ine CoZr2 alloys have been studied under isothermal conditions in air atmosphere with a microbalance. DSC and X-ray diffraction were used to study the influence of oxidation on crystallization. The three alloys under investigation which have the same composition but differ in the microstructure showed noticeable differences in their oxidation behav iour: the oxidation of the amorphous alloy is characterized by a very fast initial oxygen absorption step, completed in less than 1-2 min, f ollowed by a slow oxide growth obeying a logarithmic time law at 400 d egrees C. The polycrystalline thermodynamically stable tetragonal allo y absorbs small amounts of oxygen and obeys a parabolic rate law in th e temperature range 500-800 degrees C. The activation energy of oxygen diffusion in tetragonal CoZr2 was determined from the temperature dep endence of the parabolic rate constant. The intermediate nanocrystalli ne phase with a local structure more closely packed and ordered than t hat of the precursor amorphous phase, shows an oxidation behaviour sim ilar to that of the tetragonal phase, but with a different time expone nt.