Oxidation of glassy and nanocrystalline Zr₇₀Pd₃₀ alloys

Glassy Zr₇₀Pd₃₀ as prepared by melt-spinning exhibits "catastrophic" oxidation behavior and deteriorates into powder for example at 240°C within a few hours; the scale consists of nanoscale ZrO₂ particles as well as large plate-like crystals of as yet unknown structure. TGA reveals an oxidation kinetics by orders of magnitude faster than in comparable Zr-Ni as well as related ternary or quaternary glasses. Detailed cross-sectional TEM combined with Auger depth profiling indicate an internal oxidation process probably as a result of fast oxygen diffusion into the binary glass. Further oxidation depends on the reaction temperature: Whereas oxidation at 180°C results in the formation of monoclinic and tetragonal ZrO₂ and nanocrystalline Pd, at higher temperatures the amount of tetragonal ZrO₂ increases and the Pd will be oxidized into PdO. As in the case of other Zr-based metallic glasses nanocrystallization leads to a significant improvement of the oxidation resistance. The lower free energy of the nanocrystalline alloy as compared to the amorphous precursor material is assumed to be responsible for the improved behavior.