Microfracture behaviour of extruded Mg–Zn–Y alloys containing long-period stacking ordered structure at room and elevated temperatures

We studied the fracture behaviour of extruded Mg–Zn–Y alloys at room temperature (RT) and at 523 K using microfracture testing. An Mg97Zn1Y2 alloy was used to obtain two-phase specimens consisting of α-Mg and long-period stacking ordered (LPSO) structure phases, and an Mg88Zn5Y7 alloy was used to obtain specimens consisting of an LPSO phase. The microfracture testing of the two-phase specimen revealed that the fracture behaviour changed from brittle to ductile as the testing temperature increased. By contrast, the LPSO-phase specimen remained brittle even at the elevated temperature and the intrinsic fracture toughness values obtained at both testing temperatures were nearly identical. Ex situ transmission electron microscopy of the two-phase specimen showed that mechanical twinning in the α-Mg phase did not occur at the elevated temperature, although it was activated at RT. This suggests that the plastic deformation mode in the α-Mg phase plays a crucial part in the enhanced crack growth resistance of the two-phase alloy at the elevated temperature.