Interface thermodynamics of ultra-thin, amorphous oxide overgrowths on AlMg alloys

The amorphous to crystalline transition of ultrathin (Al,Mg)-oxide films grown by thermal oxidation of AlMg alloys: A high-resolution transmission electron microscopy investigation

“…] This suggests that the oxygen anion arrangement of these amorphous (Al,Mg)-oxide films resembles a distorted close-packed oxygen sublattice with the Mg cations preferentially occupying the octahedral(-like) interstices (as in MgO) and the Al cations preferentially concentrated in the tetrahedral(-like) interstices (as in amorphous Al 2 O 3 23 ). As evidenced by the depth-resolved analysis of the oxide-film constitution by AR-XPS, as long as the oxide film is still predominantly amorphous, the Al 2 O 3like and MgO-like local chemical states of the Al and Mg cations are concentrated at the alloy/oxide interface and near the surface, respectively (in agreement with recent model predictions on the basis of interface thermodynamics 14 ).…”

“…In particular, if more than one of the alloy constituents has a strong affinity toward oxygen, such as for the Al-Mg system (in contrast to binary alloy systems such as Au-Cu, Ag-Cu, or Pt-Al, where one constituent is only moderately nobler than the other), the interrelationships between the microstructural evolution of the growing oxide film, the oxidation-induced compositional changes in the parent alloy substrate, and the oxidation conditions can be highly complicated. 3,14,15 The alloy constituents Al and Mg fall into the category of light metals and Al-Mg alloys therefore find manifold technological application in areas where weight reduction is a major concern (e.g., in automotive and aerospace industries). Commercially used Al-based Al-Mg alloys generally have a nominal Mg alloying content in the range of 0.5 to 12 at.%, which usually leads to an improvement of the castability, strength, and wet-corrosion resistance, which is also important for application in, e.g., food handling, cooking utensils, and marine uses.…”

“…The negatives of the recorded micrographs were digitized using a charge-coupled device camera for further quantitative evaluation using Digital Micrograph software (Gatan Inc., Pleasanton, CA). The length scale of each investigated micrograph was internally calibrated employing the known lattice parameter of the bulk alloy (i.e., 0.40547 and 0.40537 nm for the Al-1.1% Mg alloy or Al-0.8% Mg alloy, as determined by XRD in this work 14 ).…”

Role of Interface(s) for the Growth of Ultra‐Thin Amorphous Oxides on Al–Si Alloys: A Thermodynamic Analysis

“…It is evident that, both am ‐Al 2 O 3 and am ‐SiO 2 growths become thermodynamically stable with respect to their crystalline counterparts during oxidation of pure Al and pure Si, respectively . Furthermore, am ‐SiO 2 is thermodynamically stable up to exceptionally larger thicknesses (up to ~40–80 nm range), whereas am ‐Al 2 O 3 grows initially up to ~0.6–3.6 nm .…”

“…This interface can be between the metal/alloy substrate and the thin oxide film as well as between the thin oxide film and vacuum. However, most of these published works have emphasized on the growth of the ultra‐thin oxide films on pure metals, with only a few predicting the growth of these oxide films on metallic alloys . Furthermore, prediction of the growth of ultra‐thin amorphous oxide films on metallic alloy substrates can be complicated because of segregation of one or more bulk alloying species at the alloy/oxide interface and continuous compositional changes in the alloy surface and subsurface regions due to preferential oxidation of any of the bulk alloying species .…”