KINETICS OF THE GROWTH OF SPINEL, MGAL2O4, ON ALUMINA PARTICULATE IN ALUMINUM-ALLOYS CONTAINING MAGNESIUM

The growth of spinel in melts of alumina-reinforced composites not only consumes the magnesium required for age hardening, but also acts to increase the viscosity and thus adversely affects the castability of the material. The kinetics of this growth have been studied in molten Al-1 pct Mg alloys in the temperature range of 948 to 1073 K. The results are shown to fit an equation of the form ln (1 - alpha) = k(t0 - t) describing deceleratory growth on fine particles, where alpha is a dimensionless reaction parameter. The rate constant, k, fits an Arrhenius equation giving an activation energy of 103 +/- 7 kJ and a time constant of 50 s-1. The incubation time, t0, which is about 2000 seconds for T < 1000 K, drops to below 500 seconds for T > 1000 K.