Microstructure Evolution and Growth Orientation of Directionally Solidified Mg-4 wt% Zn Alloy with Different Growth Rates

The microstructure evolution and growth orientation of directionally solidified Mg-4 wt% Zn alloy in the growth rate range from 20 to 200 mu m/s were investigated. A typical cellular structure was observed with a growth rate of 20 mu m/s, and the cellular spacing was 115 mu m. When the growth rate increased to 60 mu m/s, cellular structure with some developed perturbations was obtained and the cellular spacing was 145 mu m, suggesting that the cell-to-dendrite transition happened at the growth rate lower than 60 mu m/s. As the growth rate further increased, the microstructure was dendritic and the primary dendritic arm spacing decreased. The relationship between the primary dendritic arm spacings and the growth rates was in good agreement with Trivedi model during dendritic growth. Besides, X-ray diffraction and transmission electron microscopy analyses showed that the growth direction of directionally solidified Mg-4 wt% Zn alloy was < 11 (2) over bar0 > ilay in{0002} crystal plane, and the preferred orientation was explained with the lattice vibration model for one-dimensional monatomic chain.