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 fibers in squeeze cast metal matrix composites (MMCs) is examined. The long range goal of the study is to evaluate Fiberfrax® fiber as a low cost alternative to Saffil fibers for reinforcement of squeeze cast metal matrix composites, such as those used in diesel engine pistons. The fiber-matrix interface chemistry is investigated using high spatial resolution secondary ion mass spectrometry (SIMS). It is found that reactions occur during fabrication of the composites between the silica binder, which is used in both types of fiber preforms, and the liquid aluminum alloy. In the “as-cast” and T5 heat treated condition, Mg from the alloy matrix reacts with the silica binder to form MgO and elemental Si. The extent of the reaction during T5 heat treatment is independent of the fiber composition because the reactions are limited to the silica binder that coats the fibers. T5 heat treated Fiberfrax® and Saffil-fiber-reinforced materials provide comparable tensile and fatigue strength up to 400 °C. During T6 heat treatment and prolonged treatment at 400 °C the silica binder reacts completely with Mg to form MgAl2O4 spinel; the fibers react strongly with the matrix as well. Possible mechanisms for these reactions are discussed.