REACTIONS AT THE MATRIX REINFORCEMENT INTERFACE IN ALUMINUM-ALLOY MATRIX COMPOSITES

In this paper, the nature of interface reactions between F332 aluminum alloy matrix and alumina and alumina-silica-based reinforcement fiber s in squeeze cast metal matrix composites (MMCs) is examined. The long range goal of the study is to evaluate Fiberfrax(R) fiber as a low co st alternative to Saffil fibers for reinforcement of squeeze cast meta l matrix composites, such as those used in diesel engine pistons. The fiber-matrix interface chemistry is investigated using high spatial re solution secondary ion mass spectrometry (SIMS). It is found that reac tions occur during fabrication of the composites between the silica bi nder, which is used in both types of fiber preforms, and the liquid al uminum alloy. In the ''as-cast'' and T5 heat treated condition, Mg fro m the alloy matrix reacts with the silica binder to form MgO and eleme ntal Si. The extent of the reaction during T5 heat treatment is indepe ndent of the fiber composition because the reactions are limited to th e silica binder that coats the fibers. T5 heat treated Fiberfrax(R) an d Saffil-fiber-reinforced materials provide comparable tensile and fat igue strength up to 400 degrees C. During T6 heat treatment and prolon ged treatment at 400 degrees C the silica binder reacts completely wit h Mg to form MgAl2O4 spinel; the fibers react strongly with the matrix as well. Possible mechanisms for these reactions are discussed.