A calorimetric investigation of fiber/matrix reactions in fiber-reinforced aluminum alloy 339

The fiber/matrix reaction in aluminum composites reinforced with discontinuous ceramic fibers is critical to the formation of a strong interfacial bond. The reactions at ≈500°C in Kaowool- and Saffil-reinforced 339 aluminum casting alloys were measured by differential isothermal calorimetry. The reaction exotherm is superimposed upon an endotherm associated with dissolution of magnesium-containing precipitates in the aluminum matrix. The time constants of the exotherm and endotherm were determined separately to characterize their kinetics. The extent of the reaction, as measured by the total exothermic heat, is controlled by the kinetics of the magnesium dissolution. The reaction with Kaowool is a two-step process, and is confined primarily to the surface of the fibers. The principal reaction is the conversion of SiO2 to MgO. This is the rate-limiting step, and has an activation energy of ≈280 kJ mol−1, corresponding to the diffusion of oxygen and/or magnesium through MgO. At fiber surface locations where SiO2 is totally converted to MgO, the MgO reacts with Al2O3 in the fiber to form spinel. The distribution of spinel is not uniform because of variations in the thickness of the SiO2.