Oxidation of amorphous Zr70Pd30 and coarse crystalline Zr2Pd

Zr-based bulk metallic glasses are of increasing interest due to their excellent properties, e.g. high elastic limit or catalytic activity. For some applications (golf clubs, hydrogen storage) a good oxidation resistance is necessary; in other cases (catalysis) fast oxidation is required. Zr-Pd melt-spun glasses are known to exhibit "catastrophic" oxidation even at temperatures far below their glass transition. However, the surface of as-cast amorphous Zr70Pd30 was found to be protected by a thin native ZrO2 layer, thus allowing only localized nucleation of the oxidation reaction at preferred nucleation sites activated by destruction of the native oxide layer. Detailed cross-sectional SEM and TEM revealed a rather complicated microstructure of the oxide islands consisting of a lamellar structure of tetragonal and moniclinic ZrO2 with different Pd contents and embedded nanocrystalline Pd. There is also evidence for a Pd enriched reaction zone between the oxide cone and the glassy matrix. In order to understand the influence of the structure on the oxidation, the behavior of the metallic glass was compared with that of a crystallized alloy or even coarse crystalline Zr2Pd. Coarse crystalline Zr2Pd was found to be rather stable against oxidation, but exhibit very fast oxygen diffusion along the grain boundaries followed by oxide formation deep inside the material. Grain boundaries in the crystallized Zr70Pd30 does not exhibit such a behavior.