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

Aluminum is a silvery-white, lightweight metal. It is soft and malleable. As a result, it has found use in a wide variety of products including cans, foils, kitchen utensils, window frames, plane parts-among other. This metal has a high oxygen affinity, particularly in its molten form. As such, oxide inclusion often occurs during casting. This entails the formation of a few nanometer thick aluminum oxide film on the surface of liquid aluminum. This defect, i.e. the formation and entrainment of double layer oxides (bifilms) is inevitable due to the high oxidation rate of liquid aluminum and particularly turbulence during the mold filling process. Consequently, the final mechanical properties of the aluminum castings suffer from these inclusions.

Unfortunately, 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. Therefore, it is imperative that studies focused on resolving the aforementioned drawback be carried out so as to eliminate the defect.

Recently, Michigan State University scientists Dr. Jialin Liu and Professor Yue Qi together with Dr. Qigui Wang at Global Propulsion Systems, General Motors proposed the application of molecular dynamics (MD) simulations to understand the atomistic details of the formation and fracture of aluminum oxide bifilms. In particular, they focused on assessing the impact of structure and chemistry change during the aging process at the atomistic scale. Their work is currently published in the research journal, Acta Materialia.

The research team constructed three representative aluminum oxide bifilms. These nano-meter thick oxide bifilm structures were then subjected the uniaxial tensile simulations normal to the bifilm direction, in a bid to understand their fracture behaviors. All these processes, the formation and deformation of different types of bifilms, were simulated by using ReaxFF reactive forcefield-based molecular dynamics (MD) method.

The authors observed that 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. Additionally, they recorded that 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.

In summary, the study presented the application of a set of carefully designed reactive MD simulation procedures to predict the mechanical properties of the oxide bifilms formed in aluminum casting. The MD results obtained can be used to quantify oxide bifilm deformation and fracture properties at different aging stage and/or different chemical environments. Moreover, the predictions reported in their study can serve as inputs to a multiscale modeling framework to simulate the oxides distributions in the casting process and the fracture of the casted components during operation.