The Localization of Plastic Deformation in the Precipitate Free Zone of an Al-Mg-Si-Mn Alloy

Precipitate free zones (PFZ) near grain boundaries are known to have important effects on the fracture of aluminum alloys. It has been shown qualitatively that deformation localized in the PFZ leads to the acceleration of damage initiation, growth and coalescence in this region, i.e. intergranular fracture. The objective of the current study was to quantify strain localization in the PFZ using a combination of digital image correlation (DIC) and crystal plasticity simulations for an Al-Mg-Si-Mn alloy. Two different cooling paths after solution treatment were considered: i) water quenched and ii) air cooled followed in both cases by artificially aged to the peak strength. The samples were deformed in tension with strain increments of ≈ 0.01 to strains of 0.01-0.05 with the local strain calculated from the displacement field measured by DIC. It was found that the level of strain localization depends on i) the PFZ thickness and ii) the orientation of the trace of the grain boundary to the loading axis. Further, the state of strain was characterized in terms of the level of shear parallel to and stretch perpendicular to the grain boundary. Experiments and simulations demonstrated that localization of plastic strain in the PFZ was favoured for i) thicker PFZ’s and ii) grain boundaries inclined at angles of 15-75 o with respect to the loading axis. The state of strain in the PFZ was dominated by shear parallel to the grain boundary for grain boundaries inclined to the loading axis.