The formation of secondary phase in sub-rapid solidification process of Al-Mg-Si alloys

Abstract

The formation of secondary phases (Mg2Si and (Si)) in Al-Mg-Si hypoeutectic alloys during the sub-rapid solidification was investigated via the CALPHAD-coupled pseudo-front tracking (PFT) model [Du Q, Jacot A. Acta Mater 2005; 53:3479–93] and V-shaped solidification experiment. The calculated and experimental results show that the amount of secondary phases initially increases and then decreases, as the cooling rate increases. We found from the calculated results that the local equilibrium at liquid-solid interface exhibits two different modes within the range of cooling rate of 0.2–300 K/s. It starts as the Scheil local equilibrium (Scheil-LE) and gradually transforms into a mode like the Non-partition Local Equilibrium (NPLE) encountered in solid-solid phase transformation. The NPLE-like mode at the solid/liquid interface can lead to the decrease of the volume fraction of secondary phases. At lower cooling rate, the solute diffusion in the liquid is more uniform, there is Scheil-LE mode without concentration peak at the front of solid/liquid interface, and the volume fraction of secondary phases increases with the increase of cooling rate. At higher cooling rate, the effect of NPLE-like is more significant and the volume fraction of secondary phases decreases with the increase of cooling rate. The results indicate that the finite diffusion of solute in the liquid plays a key role in solute distribution and formation of secondary phases during the solidification process.