Effect of Ca and CaO on the microstructure and hot compressive deformation behavior of Mg-9.5Zn-2.0Y alloy

In this study, the effect of adding 0.3 wt% Ca to a Mg-9.5Zn-2.0Y alloy with icosahedral (I) phase (Mg3Zn6Y) was examined. The Ca alloying was done by two different routes, either through a master alloy produced with Mg and CaO (which is convenient from the points of view of safety and cost) or by direct addition of powdered Ca to the molten alloy. The small addition of Ca (CaO) effectively increased the ignition temperature of the alloys (by about 90 K), which is promising for fire suppression and safe handling with the molten alloys, but it led to the formation of Ca2Mg5Zn13 phase at grain boundaries or interdendritic interfaces, causing partial melting and intergranular cracking during compressive deformation near ∼615 K. As a result, the upper limit of hot workability temperature decreased to 598 K, which is lower than that of the Mg-9.5Zn-2.0Y alloy by 125 K. The cast Mg-9.5Zn-2.0Y-0.3Ca alloys prepared using the two different routes exhibited high similarity in ignition temperature, chemical composition, microstructure, hot compressive behaviors and processing maps, indicating that the use of CaO is as effective as the use of Ca in producing the same quality of Ca containing Mg alloys. The flow stresses during hot compressive deformation and the activation energy for plastic flow remained unaffected by the Ca (CaO) alloying, but decreased the strain rate sensitivity at high strain rates over 10−3 s−1 due to the initiation of either power-law breakdown or partial melting by adiabatic heating. Because of this, the Ca containing alloy lost hot workability at high strain rates by comparison to the Ca-free alloy.