Effects of Sr and Ca on the microstructure and properties of Mg–12Zn–4Al–0.3Mn alloy

Microstructures and properties of the Mg–12Zn–4Al alloy with calcium and strontium addition have been investigated. The results indicate that the as-cast microstructure of the ZA124 (Mg–12Zn–4Al) alloy consists of the α-Mg matrix and block shaped quasi-crystal at grain boundaries. Calcium addition to the ZA124 alloy results in the formation of a lamellar eutectic phase ϕ (Al2Mg5Zn2) and the transition of the non-equilibrium quasi-crystalline phase Q to the equilibrium τ (Mg32(Al,Zn)49) phase. The as-cast microstructure of the alloy with Sr addition consists of a binary dense eutectic ɛ (Mg51Zn20) phase and equilibrium τ phase. A small amount of calcium addition to the base alloy results in decrease of ultimate strength as well as ductility, whereas addition of strontium causes increase of both ultimate and yield strengths but decrease of ductility. The creep resistance of the ZA124 alloy is significantly improved by a small amount of calcium addition due to the formation of a grain boundary network consisting of the ϕ phase. However, strontium addition to the ZA124 alloy causes reduction on the creep resistance. Microstructure observations performed on the sample with strontium addition after creep test reveal that the ɛ phase is distorted during creep, reflecting its formation in the as-cast microstructure is not beneficial to creep properties of the ZA124 alloys. The creep mechanism responsible for the alloys studied has also been illustrated based on stress exponent and activation energy obtained from a series of creep tests.