Effects of Mg and RE additions on the semi-solid microstructure of a zinc alloy ZA27

The effects of 2 mass% Mg and 0.8 mass% RE additions on the microstructural evolution of a Zr modified zinc alloy ZA27 during isothermal holding at semi-solid temperature of 460 °C have been studied by optical microscope and scanning electron microscope. Results show that these two elements all decrease the coarsening rate of solid primary particles, and improve the uniformity of particle size. The addition of Mg greatly decreases the solid fraction, and then increases the distance between particles. However, the element of RE mainly concentrates at the liquid regions between particles, and hinders both the atom diffusion and the welding of contact particles. In addition, the behaviors of particle coarsening of these two alloys, together with that of the Zr modified ZA27 alloy without the other elements, obey the LSW law after the three semi-solid systems reach at their solid-liquid equilibrium states. The additions of these two elements have no visible effect on the shape factor of particles.