Modelling Age Hardening of Aluminium Alloys with Consideration of GP Zones or Clusters

Abstract

The work reported here follows the theme of our previous research presented at the ICAA16 and ICAA17 conferences, where material models have been developed to predict the microstructure evolution and precipitation hardening of aluminium alloys during cooling and ageing. The models have two important features: one is the consideration of quenched-in vacancies and their effect on precipitation kinetics, which was discussed in past conferences; the other is the formation of GP zones or clusters and their transition to other hardening phases during ageing, which is the focus of this paper. GP zones or clusters are known to serve as precursor phases for various hardening phases. Depending on the ageing temperature being above or below the solvus temperature of the precursor phase, precipitates can either form on their own from the matrix (heterogeneous or HET type) or form on existing GP zones or clusters (homogeneous or HOM type). In the latter case, GP zones or clusters are allowed to form first from the matrix, which will later transform to their metastable counterparts. Both HET and HOM types of precipitates are considered in the current strengthening model for predicting age hardening curves for aluminium alloys. The age hardening model has been validated over a wide range of commercial aluminium alloys and demonstrated good agreement with experimental data in the form of age hardening curve and T5/T6 peak position and strength.