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

摘要

The microstructure evolution and growth orientation of directionally solidified Mg–4 wt%Zn alloy in the growth rate range from 20 to 200 lm/s were investigated.A typical cellular structure was observed with a growth rate of 20 lm/s,and the cellular spacing was 115 lm.When the growth rate increased to 60 lm/s,cellular structure with some developed perturbations was obtained and the cellular spacing was 145 lm,suggesting that the cell-to-dendrite transition happened at the growth rate lower than 60 lm/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 h112"0i lay in f0002g crystal plane,and the preferred orientation was explained with the lattice vibration model for one-dimensional monatomic chain.