Microstructure and microsegregation in directionally solidified Mg-4Al alloy

Directional solidification (DS) of a binary Mg-4Al (wt.%) alloy was carried out to investigate the microstructures and microsegregation under controlled solidification conditions. In directional solidification the microstructure depends on the growth rate V because the cooling rate, which governs the solidification microstructure, is the product of the growth rate and the temperature gradient. The ability to produce simple and uniform microstructures in directional solidification enables us to correlate the formation of the microstructure and its characteristic length scales quantitatively with processing parameters. The morphology of the solid-liquid interface and the microstructure of both the mushy zone and the steady-state region were characterized at different levels of growth rates. With the help of an electron microprobe, microsegregation was determined in a specimen directionally solidified with cooling rates ranging from 0.06 to 0.8 K/s. The calculated microsegregation results based on the Scheil model deviated significantly from the experimental data, which is anticipated since back diffusion was not included due to the lack of diffusivity data.