Thermal oxidation of amorphous Al0.44Zr0.56 alloys

The oxidation of amorphous Al0.44Zr0.56 alloys upon exposure to pure O2(g) at 500 and 560 °C (and pO2 = 1 × 105 Pa) was investigated by a combinatorial experimental approach using X-ray diffraction, spectroscopic ellipsometry, transmission electron microscopy and Auger electron spectroscopy. During the early stages of oxidation at 500 and 560 °C, an amorphous (Zr,Al)-oxide layer of homogeneous composition and uniform thickness is formed, which is enriched in Zr with respect to the alloy substrate. At 500 °C, both the alloy substrate and the oxide layer remain amorphous during continued oxidation, whereas at 560 °C, a crystalline tetragonal ZrO2 (t-ZrO2) phase nucleates after prolonged oxidation, while the alloy remains amorphous. The nucleation and growth of t-ZrO2 at 560 °C occurs exclusively close to the interface between the initially formed amorphous (Zr,Al)-oxide layer and the alloy, immediately underneath a region of Al enrichment in the substrate, as triggered by oxidation-induced compositional changes in the alloy below the reacting alloy/oxide interface and a favorable energy of the interface between t-ZrO2 crystallites and the amorphous alloy matrix. The growing t-ZrO2 oxide crystallites eventually laterally coalesce to form a continuous layer constituted of branches of dendrite-shaped t-ZrO2 phase crystallites surrounded by an Al-rich amorphous Al-Zr alloy matrix. The underlying mechanism of the oxidation process is discussed.