The effect of chemical patterning induced by cyclic plasticity on the formation of precipitates during aging of an Al–Mg–Si alloy

Room temperature cyclic deformation of solution treated AA6061 led to the development of a high number density of Mg/Si clusters. The fine dispersion of clusters resulted in a strength comparable to that obtained using the T6 condition. Subsequent heat-treatment has the potential to further increase the strength of the alloy. Slow heating from room temperature to 177 °C allowed the clusters to catalyse the creation of a very high number density of precipitates. Isothermal aging for 2 h at this temperature produced a tensile strength which is 10% greater than that obtained using the T6 heat-treatment. The uniform and total elongation of cyclically deformed and heat-treated material were both greater than those obtained through the standard T6 treatment. In addition, the aging time needed to achieve the above properties in the cyclically deformed material (2 h) is significantly lower than the time needed to achieve peak strength in the traditional T6 treatment (25.5 h). Detailed observations of the microstructure using Transmission Electron Microscopy and Atom Probe Tomography suggest that the increased strength is due to the presence of a larger number density of fine precipitates in the cyclically hardened and aged material compared to the material that underwent traditional aging. The clusters formed during cyclic deformation can therefore play an important role in the nucleation of precipitates during subsequent aging. A simple model that captures the size evolution of clusters is presented to rationalize the experimental results and demonstrate the existence of a critical heating rate, above which, the clusters dissolve and are ineffective at promoting a high number density of precipitates.