Interfacial reaction chemistry in Al-based metal-matrix composites

The role of Sr in promoting the formation of thin interfacial amorphous fil ms has been studied in a series of Al 6061 alloys reinforced with Si3N4 Or Al18B4O33 whiskers. In the absence of Sr, extensive reaction products (prin cipally MgAl2O4) are formed at the whisker-matrix interface during processi ng 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 character ized by analytical electron microscopy, and shown to be an amorphous (Mg,Sr ) silicate phase. The thermodynamic and kinetic factors that favour the for mation of the amorphous films are discussed in the text; the key factor tha t emerges from this discussion is the important role that Sr must play in c ontrolling the contact angle of the amorphous phase at the metal-ceramic in terface. High-resolution electron microscopy observations show that the con tact 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; althou gh the model gives reasonable predictions for the equilibrium film thicknes s, further experimental evidence is needed to support this conclusion.