Effect of precipitate shapes on fracture toughness in extruded Mg–Zn–Zr magnesium alloys

A commercial Mg–6 wt% Zn–0.5 wt% Zr (ZK60) alloy was used to investigate the effect of precipitate shapes on fracture toughness. The ZK60 alloy was extruded at a temperature of 653 K (extruded alloy). The extruded alloy was annealed at 633 K for 86.4 ks (annealed alloy), and then the annealed alloy was aged at 448 K for 100 ks (aged alloy). The average grain size in all the conditions was about the same, 13.5 ± 1.0 μm. The extruded and aged alloys had different shaped precipitates: spherical and rod shaped precipitates, respectively. The plane-strain fracture toughness K_IC of the extruded, annealed, and aged alloys were estimated to be 22.4, 20.2, and 21.0 MPam^1/2, respectively, by the stretched zone analysis. Transmission electron microscopy (TEM) observations showed that the deformation during the fracture toughness test was dominated by a dislocations on the basal slip planes in all the conditions. Such dislocations are commonly activated in magnesium alloys during the tensile and compression tests. The spherical shaped precipitates were found to be more effective than the rod shaped precipitates for improving the fracture toughness in the magnesium alloy.