Formation of MgAI 2 O 4 at interface between a squeeze cast piston alloy and Saffil fibre reinforcement

Abstract

An investigation was carried out into the formation of the spinel MgAl₂0₄ phase on the surface of fibres in aluminium alloy metal matrix composites. Three castings were made by squeeze casting ceramic fibre preforms. The following preforms were used with piston alloy AE109: a Saffil fibre preform with a Si0₂ binder, a Saffil fibre preform deoxygenated before casting with a Si0₂ binder, and an aluminosilicate fibre preform with a CaF₂ binder. Interface analysis was performed by a high resolution scanning transmission electron microscope using a windowless, energy dispersive spectrometry detector and parallel electron energy loss spectrometry. It was found that the MgAI₂0₄ phase was formed initially during the casting process by a reaction between the alloy and the oxygen within the air spaces of the preform. The Si0₂ binder was not reduced during casting or during cooling even though diffusion of magnesium and aluminium atoms has been reported in the literature. The contact angle between the metal and the fibres was dependent on the thickness of the oxide layer formed on the molten metal and not on the interfacial energies of the metal and fibres. The oxide thickness was ∼30 nm, which is consistent with a contact angle of ∼150° in air.