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

The fiber/matrix reaction in aluminum composites reinforced with discontinu ous ceramic fibers is critical to the formation of a strong interfacial bon d. The reactions at approximate to 500 degrees C in Kaowool- and Saffil-rei nforced 339 aluminum casting alloys were measured by differential isotherma l calorimetry. The reaction exotherm is superimposed upon an endotherm asso ciated with dissolution of magnesium-containing precipitates in the aluminu m matrix. The time constants of the exotherm and endotherm were determined separately to characterize their kinetics. The extent of the reaction, as m easured 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 approximate to 280 kJ mol(-1), corresponding to the diffusion of oxygen and/or magnesium through MgO. At fiber surface locatio ns 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. (C) 2000 Elsevier Science S.A. Al l rights reserved.