Experimental and Modeling Studies of the Lamellar Eutectic Growth of Mg-Al Alloy

Abstract

Directionally solidified samples of Mg-32.3 wt pct Al eutectic alloy were produced under an argon atmosphere in a vacuum Bridgman-type furnace to study the eutectic growth with different growth velocities. Typical features such as steady-state lamellar eutectic growth, lamellar branching at the quenching interface, and the formation of colony structures due to the impurity of the Mg-Al binary alloy were observed using a JEOL 6301F scanning electron microscope (JEOL Ltd., Tokyo, Japan). The lamellar spacing of the two eutectic phases was measured on the transverse sections of the samples. It was found that the relationship between the measured lamellar spacing and growth velocity agreed well with the prediction of the Jackson-Hunt model. Subsequent studies of Mg-Al eutectic growth were conducted using a numerical model based on the cellular automaton (CA) method. Taking account of the solute diffusion, constitutional undercooling, and curvature undercooling, modeling of steady-state lamellar eutectic growth was achieved. A systematic investigation of the eutectic growth morphology and lamellar spacing of the Mg-Al eutectic was carried out under directional solidification with different undercoolings, initial lamellar spacings, temperature gradients, and growth velocities. The results showed that under the interaction between solute diffusion and surface energy, the adjustment of eutectic lamellar spacing was accomplished by nucleation, lamellar branching, lamellar termination, and overgrowth. The simulated results were consistent with both the experimental results and the Jackson-Hunt eutectic theory.