Calorimetric Studies of Precipitation and Dissolution Kinetics in Aluminum Alloys 2219 and 7075

Abstract

Differential scanning calorimetry (DSC) was used to study the kinetics of precipitation and dissolution of metastable and stable phases in aluminum alloys 2219 and 7075. A comparison of DSC scans obtained at heating rates of 1, 5, 10, and 20 K per minute showed that, during a DSC scan, the rates of precipitation of θ′ and θ in 2219 and η′ and η in 7075 were limited by their reaction kinetics. Likewise, the rates of dissolution of GP zones, θ′ and η′, were found to be dominated by kinetics. In contrast, the dissolution of θ and η was dominated by the thermodynamic equilibrium between these phases and the matrix. Analysis of the kinetically dominated reaction peaks and their dependence on heating rate and particle size showed that the GP zone dissolution reaction could best be described by a three-dimensional volume diffusion limited rate expression with an activation energy equal to that for diffusion. The rate of formation of θ′ was best described by an Avrami expression withn = 1.1, indicating that nucleation was not the rate controlling step. A pronounced dependence of the θ′ formation rate on prior plastic deformation was observed and ascribed to the influence of the matrix dislocation density on diffusivity.