Interfacial reaction chemistry in Al-based metal-matrix composites

Abstract

The role of Sr in promoting the formation of thin interfacial amorphous films has been studied in a series of Al 6061 alloys reinforced with Si₃N₄ or Al₁₈B₄O₃₃ whiskers. In the absence of Sr, extensive reaction products (principally MgAl₂O₄) are formed at the whisker-matrix interface during processing of the alloys by squeeze casting. The addition of 0.5 or 1 wt% Sr to the base alloy suppresses the formation of the spinel phase by forming a thin (1–2 nm thick) amorphous film at the interface. The film has been characterized by analytical electron microscopy, and shown to be an amorphous (Mg, Sr) silicate phase. The thermodynamic and kinetic factors that favour the formation of the amorphous films are discussed in the text; the key factor that emerges from this discussion is the important role that Sr must play in controlling the contact angle of the amorphous phase at the metal-ceramic interface. High-resolution electron microscopy observations show that the contact angle is zero. The amorphous films are remarkably uniform in thickness. The applicability of a model, first developed for intergranular amorphous films in ceramic systems, to the metal-ceramic case, is considered; although the model gives reasonable predictions for the equilibrium film thickness, further experimental evidence is needed to support this conclusion.