Determining the strength of GP zones in Mg alloy AXM10304, both parallel and perpendicular to the zone

Abstract

Microalloyed, age-hardenable Mg alloys can exhibit an excellent balance between strength and ductility. One such alloy, AXM10304 (Mg-1.31Al-0.33Ca-0.46Mn in wt. %), possesses high extrudability with mechanical properties comparable to 6xxx-series Al alloys at considerably lower density. The mechanical properties are due to the presence of a high number density of disc-shaped GP zones parallel to the basal planes. The present work focuses on providing a mechanistic understanding of the effect of these GP zones, on both basal and prismatic <a> slip. Using crystal plasticity simulations in conjunction with analytical strength modeling, the anisotropy in shear resistance of disc-shaped GP zones is quantitatively evaluated for the first time. It is shown that the passage of dislocations parallel to the zone (basal slip) experience a lower resistance compared to the case when the shearing occurs perpendicular to the zone (prismatic slip). This is not simply a geometrical effect; in fact, the geometry of basal discs favors the strengthening of basal slip when dislocations are not required to bow around the obstacles. Rather, it is hypothesized that the GP zone coherency strain fields contribute less resistance to basal slip as compared to prismatic slip. Furthermore, the possibility of a chemical effect is only present for the prismatic. Despite the fact that the harder mode (prismatic slip) is strengthened more than the soft, it is shown that the net strengthening effects still cause a decrease in the overall anisotropy of the individual grains, which may help to explain why the ductility is good, despite the high strength.