Oxidation kinetics of amorphous Al_xZr_1−x alloys

The oxidation kinetics of amorphous Al_xZr_1−x alloys (solid solution) has been studied as function of the alloy composition (0.26 ≤ x ≤ 0.68) and the oxidation temperature (350 °C ≤ T ≤ 400 °C; at constant pO₂ = 1 × 10⁵ Pa) by a combinatorial approach using spectroscopic ellipsometry (SE), Auger electron spectroscopy (AES) depth profiling, transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. Thermal oxidation of the am-Al_xZr_1−x alloys results in the formation of an amorphous oxide overgrowth with a thermodynamically preferred singular composition, corresponding to a constant Al^ox/Zr^ox ratio of 0.5. Both the solubility and the diffusivity of oxygen in the am-Al_xZr_1−x alloy substrate increase considerably with increasing Zr content, in particular for Zr contents above 49 at.% Zr. Strikingly, the oxidation kinetics exhibit a transition from parabolic oxide growth kinetics for Al-rich am-Al_xZr_1−x alloys (x ≥ 0.51) to linear oxide growth kinetics for Zr-rich am-Al_xZr_1−x alloys (x < 0.35). The underlying oxidation mechanism is discussed. It is concluded that the oxidation kinetics of the amorphous Al_xZr_1−x alloys for 0.26 ≤ x ≤ 0.68 and 350 °C ≤ T ≤ 400 °C are governed by: (i) the atomic mobilities of O and Al in the alloy substrate at the reacting oxide/alloy interface, (ii) the solubility of O in the substrate and (iii) the compositional constraint due to the thermodynamically preferred formation of an amorphous oxide phase of singular composition.