Thermodynamics of ultra-thin oxide overgrowths on binary Al-based alloys

Abstract

Thermodynamic analyses have been carried out to understand the growth of ultrathin oxide overgrowths on single-crystalline bare [AlMg] and [AlSi] alloys due to their dry, thermal oxidation. The parameters considered in this formalism are alloy composition at the alloy/oxide interface, growth temperature, oxide-film thickness and low index crystallographic surfaces of the substrate. Along with the bulk Gibbs free energies of the respective oxides, the role of energies at the alloy/oxide interface as well as at the oxide/ambient interface were also taken into the account. Finally, this model was then compared with the already existing thermodynamic analyses for the growth of corresponding amorphous oxides on these alloy substrates and are then validated with the available experimental data from literature. It is found that, for thegrowth of only crystalline oxides on [AlMg] alloy substrate, crystalline 2 3 [Al O ] forms for a combination of lower growth temperature, Mg alloying content at the alloy/oxide interface and oxide-fim thickness, beyond which crystalline [MgO] formation on this alloy substrate is thermodynamically stabilized. However, for the growth of a thickening oxide-film on bare [AlMg] alloy substrate, amorphous 2 3 {Al O } and crystalline [MgO] are found to be preferred thermodynamically at lower and higher oxide-film thicknesses respectively. Similarly, for the growth of only crystalline oxide overgrowths on [AlSi] alloy substrate, formation of crystalline 2 3 [Al O ] is thermodynamically preferred irrespective of Si alloying content at the alloy/oxide interface, growth temperature and oxide-film thickness. Overall, amorphous 2 {SiO } and amorphous 2 3 {Al O } are found to form at lower and higher oxide-film thicknesses respectively, followed by phase transformation of amorphous 2 3 {Al O } to crystalline 2 3 [Al O ] on further thickening of oxide-film due to dry, thermal oxidation of bare [AlSi] alloy substrate only at lower growth temperature and Si alloying content at the alloy/oxide interface. At higher growth temperature, amorphous 2 3 {Al O } was found to be formed without transforming to crystalline 2 3 [Al O ] due to increased contribution of positive strain energies. Further, at higher Si alloying content at the alloy/oxide interface, crystalline 2 3 [Al O] becomes thermodynamically preferred without forming an amorphous 2 3 {Al O } phase. Crystalline 2 [SiO ] was never found to be thermodynamically favorable for the parameters considered in this study. These thermodynamic predictions are found to be in agreement with the experimental findings.