Modelling of the Transition Kinetics in Al-Alloys

Abstract

Transformation, that proceed via nucleation and growth are treated by the classical Johnson-Mehl-Avrami-Kolmogorov (JMAK) theory - the validity of which was criticised but reasserted in the case of random nucleation. Of physical interest are the rate constant leading to information about the activation energy, about the Arrhenian equation, and the Avrami parameter which expresses as the sum of two contributions nucleation and different growth processes. In Aluminium alloys different transition processes may take place. In most of the cases models of nucleation and growth are qualified for the description of the transition kinetic. The paper will start using the classical JMAK (Johnson-Mehl-Avrami-Kolmokorov) model and develop different extension of this model. As an extension there will be discussed: ○ A general model (the JMAK model with the impingment parameter η), It will be shown, that the kinetic exponent n η of the general model derived for a constant η can be transformed into the physical explainable kinetic exponents of the JMAK theory. ○ The local Avramiparameter and ○ the activation energy for isothermal ageing. An interpretation of the model extension on the basis of different transitions (interface, eutectic, phase transition first and second order) on selected alloys (AlCu(Mg), AlSc(Li, Mg)) after casting and rapid quenching as well as different amorphous AlLnT alloys is given.