Interface study by dual-beam FIB-TEM in a pressureless infiltrated Al(Mg)-Al₂O₃ interpenetrating composite

This paper considers the microstructures of an Al(Mg)-Al₂O₃ interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al-Al₂O₃ system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal-ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al-8 wt% Mg alloy and 15% dense alumina foams at 915°C in a flowing N₂ atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin-film samples, specifically produced across the Al(Mg)-Al₂O₃ interface, were prepared using a dual-beam focussed ion beam and subsequently observed using transmission electron microscopy. XRD scan analysis shows that Mg₃N₂ formed in the foam at the molten alloy-ceramic infiltration front, whereas transmission electron microscopy analysis revealed that fine AlN grains formed at the metal-ceramic interface and MgAl₂O₄ and MgSi₂ grains formed at specific points. It is concluded that it is the reactions between Al, Mg and the N₂ atmosphere that improve the wettability between molten Al and Al₂O₃ and induce spontaneous infiltration.