Atomistic simulation of the formation and fracture of oxide bifilms in cast aluminum

Abstract

The formation and entrainment of double layer oxides (bifilms) in aluminum casting is inevitable due to the high oxidation rate of liquid aluminum and particularly turbulence during the mold filling process. The final mechanical properties of the aluminum castings suffer from these inclusions but neither the formation process nor fracture mechanism is fully understood due to the difficulty of in-situ observation on nano-scale aluminum oxide thin film. To understand the impact of bifilms on the fracture mechanism at different bifilm formation stages and the aging processes, atomic level bifilm slab models were built according to their formation history. ReaxFF reactive forcefield-based molecular dynamics (MD) method was used to simulate the formation and deformation of different types of bifilms. The MD simulations showed that an incomplete “healing” process happened at the oxide/oxide interface during bifilm formation and the fracture occurred at the Al/oxide interface instead of the oxide/oxide interface. When the oxide transformed from amorphous to α-Al2O3 due to aging, the fracture energy increased from 0.43 J/m2 to 0.53 J/m2. With 30% coverage of hydroxyl group surface contamination, the –OH terminated oxide bifilm fractured at the oxide/oxide interface and the corresponding fracture energy dropped to 0.30 J/m2. This is most likely due to the H2 bubbles being trapped in the aluminum oxide bifilm interface. To facilitate multiscale modeling, such as casting process simulation and component durability analysis, the MD predicted oxide bifilms fracture energy and fracture strength were converted to cohesive zone parameters, via a simple size bridging relationship.