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Oxidation of Zr-based metallic glasses and nanocrystalline alloys.

Köster U., Jastrow L.

Archives of Materials Science

2004

Vol. 25, nr 4

283-295

A number of Zr-alloys are among the most promissing systems for bulk glass formation; in addition, these glasses can be used as precursor material for nanocrystallization. In order to reveal the potential application of these materials their stability against oxidation is of utmost importance. In melt-spun Zr-based metallic glasses a native oxide layer was found to protect against further oxidation of the ribbon. Oxygen diffusion into the Zr-based metallic glasses ahead of the proceeding oxidation from seems to be too slow; any effect of increased oxygen content could be neglected. Oxidation kinetics can be assumed to be controlled by oxygen diffussion through the scale towards the ZrO2/glass interface and can be improved significantly By Be or small additions of the alloying elements. Two modes of oxidation were observed: In Zr69.5Cu12Ni11Al7.5 glasses, for example, growth of continuous scales consisting of modules of tetragonal ZrO2 occur; Cu, Ni and Al form nanocrystals between these modules. Zr-Au or Zr-Pd glasses, on the other hand, exhibit "catastrophic" oxidation even at temperatures far below their glass transition. Fast oxidation can be correlated to the formation of oxide cones with a lamellar structure of monoclinic and tetragonal ZrO2. The structure is assumed to allow very fast diffusion along the interfaces between the lamellar. Whereas nanocrystalization seems to improve the oxidation resistance of many Zr-based metallic glasses, a coarse crystalline structure exhibits fast oxidation along grain boundaries, at least in Zr2Pd. The reason for this behaviour is not understood as yet.