Computational analysis of heterogeneous nucleation and precipitation in AA6005 Al-alloy during continuous cooling DSC experiments

Abstract

A physically-based model for the computational evaluation of differential scanning calorimetry (DSC) curves during continuous cooling of Al alloys from solution annealing temperature is developed. The model is particularly suitable for predicting the nucleation and growth of quench induced precipitates at heterogeneous nucleation sites, such as grain boundaries, the stress field of dislocations or particles like primary phases or dispersoids. The thermokinetic model incorporates an extended formulation of the precipitate nucleation barrier and considers β-Mg2Si and B’-Al4Mg8Si7 phase precipitates. These are the dominating, experimentally observed quench induced phases in an aluminium AA6005 alloy within a wide range of cooling rates. The model is implemented in the thermokinetic software MatCalc, which simulates the precipitation processes and the evolution of excess specific heat capacities during the heat treatments. The Generalized Broken Bond model, incorporating the effects of interface curvature and diffuse interfaces, is used for interface energy prediction. Comparison of experiment and simulation demonstrates that the impact of heterogeneous nucleation sites must be adequately considered in the nucleation energy expression to obtain accurate predictions of the precipitate evolution, particularly for precipitation at low and very low undercooling as present during continuous cooling, particularly at slower cooling rates.