Modelling of dispersoid and constituent particle evolution in 3XXX alloys

A kinetic model is developed for constituent and dispersoid particle evolution during preheating of 3XXX alloys. Dispersoid modelling combines the parallel processes of nucleation, growth and coarsening, resulting in evolution equations for particle size, number density and volume fraction of dispersoid. Constituent modelling combines the parallel processes of growth (or dissolution) and transformation from Al-6[Fe,Mn] to Al-12[Fe,Mn](3)Si phases, resulting in evolution equations for volume fractions of the two constituent phases, as well as Mn/Fe ratios. The evolution equations for dispersoids and constituents are coupled to each other through the matrix concentrations of manganese and silicon. The formation of new dispersoid, and the growth and transformation of the constituents are processes competing for available manganese and silicon. Model parameters relate to diffusivity, interfacial energy, nucleation kinetics and equilibrium solvi for the Al-6 and Al-12 phases. The model is fit to conductivity data, dispersoid size measurements, constituent proportion which is Al-12 type, and Mn/Fe ratios in either phase. Data used in fitting and testing this model consist primarily of internal Alcoa data, supplemented by published, external results. These data contain variations in solidification rate, alloy composition, namely Fe, Mn, Si and Mg (i.e. either 3003 or 3004), as well as time and temperature of thermal treatments. All alloys have silicon contents large enough that dispersoids are calculated as being exclusively Al-12 type. The resulting model enables predictions of the effects of process excursions, or the search for new combinations of alloy composition and thermal practice, on the basis of physical principles and prior data.