The kinetics of isothermal β′ precipitation in Al-Mg alloys

Isothermal beta' precipitation in Al-14 at % Mg and Al-16 at % Mg is studied by isothermal calorimetry, differential scanning calorimetry, and optical and transmission electron microscopy. The characteristic length of the beta' precipitates varies strongly with ageing temperature, indicating that the definition of a state variable is not possible. As a consequence the commonly used equation: d alpha/dt = g(T)f(alpha) with f(alpha) a function depending only on the fraction transformed, and g(T) a function of temperature, is not valid for this reaction. The isothermal calorimetry curves can be fitted over the entire range of transformation by a recently developed variant of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. For T 160 degrees C and T > 200 degrees C the reaction exponent n is larger than 2.5, indicating an autocatalytic precipitation process. The mechanisms for this autocatalytic process are discussed. (C) 1999 Kluwer Academic Publishers.