Aluminum Nitride in Al–Si–Mg Alloy

Abstract

When liquid aluminum is exposed to air, an oxide layer instantaneously forms on the surface of the melt, which, when enfolded, becomes a double oxide film defect (bifilm), producing entrained air pockets forming voids, cracks, and areas of stress. Studies of the reactions between the air trapped inside the bifilms and the surrounding melt have produced reaction products such as AlN. In this study, examination of the fracture surfaces of an Al–7Si–0.3Mg (2L99) as-cast alloy revealed AlN apparently of a different “nodular” morphology of approximately 15 μm in length. In contrast to the “feather-shaped” AlN morphology grown through holding in nitrogen or the addition of Mo to 2L99 observed in previous research, the observed nodular morphology in this study was thought to be less permeable. This confirmed the hypothesis of Nyahumwa et al. (AFS Trans 106:215–223, 1998), who suggested that the entrapped air in bifilms can react with the surrounding liquid melt by initially consuming oxygen to form oxides, such as spinel (MgAl2O4), followed by the consumption of the nitrogen to form AlN. The AlN observed has been proposed to reduce the size of bifilms and correspondingly improve the mechanical properties of the alloy. In this work, AlN was also grown by holding alloys in a controlled nitrogen gas environment at 850 °C at various holding times to reveal AlN morphologies.