The interaction of grain refinement and ageing in magnesium–zinc–zirconium (ZK) alloys

Magnesium–zinc–zirconium (ZK) alloys are precipitation strengthened by the addition of zinc and grain refined by zirconium. In ZK alloys these mechanisms interact since some Zn can be precipitated into Zn–Zr intermetallics. This reduces the fraction of age hardening precipitates. An experimental study has confirmed the presence of these two particle types in ZK60, a commercial Mg–Zn–Zr alloy. A model has been developed to consider precipitation and strengthening in ZK alloys accounting for this interaction. It has been shown that whilst Zr additions will increase strength by grain refinement this is balanced by a loss of age hardening response. It is predicted that if the same fine grain size could be achieved without Zr then a significant strength increase of around 15% would be expected. Therefore, in wrought applications where the final grain size is determined by recrystallisation, the use of Zr grain refiner in ZK alloys does not lead to optimum strengthening. The model was then applied to explore strategies to increase strengthening in Mg–Zn alloys.