Microstructural analysis of multi-phase ultra-thin oxide Overgrowth on Al–Mg Alloy by high resolution transmission electron microscopy

High-resolution transmission electron microscopy analyses are carried out to understand the microstructure of the ultra-thin oxide-film grown on a (native) amorphous Al2O3-coated Al-0.8 at.% Mg alloy substrate at T = 600 K for t = 2 h and at pO2 of 1 × 10−2 Pa. This oxide-film is found to be non-uniformly thick with thicknesses varying from 1.50 to 4.60 nm. Occasionally, this oxide is found to diffuse into the Al–Mg alloy substrate, forming oxide thicknesses up to 10.5 nm. Overall, this oxide-film is found to consist of a mixed amorphous, (poly) crystalline and an intermediate amorphous-to-crystalline transition regions, with crystalline regions consisting mostly of MgO and the diffused oxide regions into the Al–Mg alloy substrate coated with γ-Al2O3. These observations are then compared with the experimental results obtained using angle-resolved X-ray Photoelectron Spectroscopy analysis and thermodynamic predictions for the growth of an ultra-thin oxide-film due to dry, thermal oxidation of Al–Mg alloy substrates.by Narendra Bandaru, Darshan Ajmera, Krishna Manwani, Sasmita Majhi and Emila Pand

Stability of ultra-thin oxide overgrowths on binary Al–Si alloy substrate

This study presents a thermodynamic formalism to predict the type of ultra-thin oxide overgrowth due to dry, thermal oxidation of bare single-crystalline ⟨AlSi⟩ alloy substrate. The various oxide growth parameters considered in this formulation are Si alloying element content at the alloy/oxide interface, growth temperature, oxide-film thickness (up to 5 nm), and low-index crystallographic surfaces of the alloy substrate. Along with the bulk Gibbs free energies of oxide formation, this developed formalism also considered alloy/oxide interface energies and oxide surface energies. Further for estimating the alloy/oxide interface energies of the crystalline oxide overgrowths, chemical interaction energy and strain energy arising due to the anisotropic growth strain have been taken into account. Similarly, the alloy/oxide interface energies of the amorphous oxides considered contributions arising from chemical interaction, entropy, and enthalpy between the alloy substrate and oxide overgrowth. Overall, the model predicted the stability of amorphous {SiO2} and {Al2O3} at lower and higher oxide-film thicknesses, respectively, followed by phase transformation of amorphous {Al2O3} to γ−⟨Al2O3⟩ on further thickening of the oxide film. Moreover, crystalline ⟨SiO2⟩ 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.by Darshan Ajmera and Emila Pand