TECHNICAL PAPER Microstructural Analysis of Multi-phase Ultra-Thin Oxide Overgrowth on Al–Mg Alloy by High Resolution Transmission Electron Microscopy Narendra Bandaru 1 • Darshan Ajmera 1 • Krishna Manwani 1 • Sasmita Majhi 1 • Emila Panda 1 Received: 14 October 2015 / Accepted: 6 June 2016 / Published online: 23 June 2016 Ó The Indian Institute of Metals - IIM 2016 Abstract High-resolution transmission electron micro- scopy analyses are carried out to understand the microstructure of the ultra-thin oxide-ﬁlm grown on a (native) amorphous Al 2 O 3 -coated Al-0.8 at.% Mg alloy substrate at T = 600 K for t = 2 h and at pO 2 of 1 9 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 sub- strate, 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-crys- talline transition regions, with crystalline regions consist- ing mostly of MgO and the diffused oxide regions into the Al–Mg alloy substrate coated with c-Al 2 O 3 . These obser- vations are then compared with the experimental results obtained using angle-resolved X-ray Photoelectron Spec- troscopy analysis and thermodynamic predictions for the growth of an ultra-thin oxide-ﬁlm due to dry, thermal oxidation of Al–Mg alloy substrates. Keywords High-resolution transmission electron microscopy (HR- TEM)  Oxidation  Al 2 O 3  MgO  Ultra-thin 1 Introduction Microstructure of an ultra-thin ( \ 15 nm) oxide ﬁlm determines its functionality for various applications, like, tunnel junctions, gas sensors, model catalysts and (thin) diffusion barriers [1–9], hence an understanding of this microstructure with respect to the growth conditions is crucial. To this end, extensive investigations have been carried out in the literature [10–32]. However, most of these literature have focused discreetly on the overall microstructure of the oxide-ﬁlm grown at relatively higher oxidation temperature ( [ 600 K) and at atmospheric con- ditions on a natively grown oxide (due to aging for a few days at room temperature under atmospheric pressure) without presenting an in-depth analysis on the developed oxide microstructure [18–32]. Others are limited to the thin oxide ﬁlms grown at lower oxidation temperature of T \ 600 K on in vacuo sputter cleaned (using inert Ar ? ion beam to remove the surface contaminants and native oxide present on to the surface) pure metal/alloy substrate [10–17]. Establishing interrelationship between the microstructure of the thin oxide ﬁlm due to thermal oxi- dation of a native oxide coated pure metal/alloy substrate with respect to the growth conditions is extremely impor- tant as this will help understanding the role of a natively formed barrier oxide layer on the ﬁnal microstructure of the oxide-ﬁlm grown due to dry, thermal oxidation. In an independent study, cleaned (by means of Ar? ion sputtering) Al–Mg alloy substrates were oxidized by varying a range of experimental parameters (like, oxidation pressure, temperature, time and alloy composition) in a controlled environment in an ultra-high vacuum (UHV) chamber [15, 33]. Overall, these oxide-ﬁlms were found to be extremely thin ( \ 3 nm). The oxide-ﬁlms grown at lower oxide-ﬁlm thicknesses of B0.7 nm were found to consist of Narendra Bandaru and Darshan Ajmera have contributed equally to this work. & Emila Panda emila@iitgn.ac.in 1 Department of Materials Science and Engineering, Indian Institute of Technology, Gandhinagar, Ahmedabad 382424, India 123 Trans Indian Inst Met (2017) 70(5):1269–1275 DOI 10.1007/s12666-016-0920-x