A combined experimental-analytical modeling study of the artificial aging response of Al–Mg–Si alloys

The isothermal aging response at 170 °C of three extruded Al–Mg–Si alloys that had been solution-treated and quenched was experimentally tracked to understand the influence of Mg + Si content and the Mg/Si ratio on microstructure and mechanical behavior. Transmission electron microscopy (TEM) was employed to quantitatively characterize the needle-shaped β'' precipitates in the underaged, peak-aged and overaged conditions; while the alloy with a high Mg + Si content raised the number density of β'' precipitates, a higher Mg/Si ratio increased the number density but decreased the size of β'' precipitates in the peak-aged condition. In the overaged condition, coarsening of β'' precipitates occurred primarily along its length. Single crystal micropillar compression tests were conducted in the naturally-aged, under-aged, peak-aged and over-aged conditions to correlate the microstructure to the critical resolved shear stress (CRSS). In the peak-aged condition, the alloy with the high Mg + Si content and a Mg/Si ratio corresponding to the β'' stoichiometry (Mg5Si6) exhibited the highest CRSS. A precipitation kinetics model that assumed a cylindrical morphology for the β'' phase with a fixed aspect ratio was developed to describe β'' phase microstructure evolution as a function of alloy chemistry, solution treatment temperature and artificial aging parameters. The results from this model served as input to a dislocation-precipitate interaction model to calculate single crystal yield strength. The predictions from these analytical models were validated with experimental results from this study and from the literature and fundamental insights were obtained on the role of microstructural parameters in affecting yield strength for non-spherical precipitates.