Effects of Bi addition on solidification behavior of Mg–Al–Zn casting alloys using experiments and CALPHAD calculations

Abstract

A comprehensive understanding of solidification behavior of Mg–Al–Zn–Bi alloys is necessary for designing novel casting Bi-modified AZ alloys. In this paper, 21 Mg–Al–Zn–Bi alloys were prepared to investigate the phase constituents, morphologies, grain sizes and area fractions of precipitates in as-cast microstructures, as well as their phase transition temperatures and solidification paths by using X-ray diffraction, scanning electron microscopy, optical microscopy and differential scanning calorimetric techniques. It was found that the Bi addition leads to the formation of α-Mg3Bi2 phase, which became coarse flake as Bi content increased. Besides, the Bi addition can modify the morphology of eutectic γ-Al12Mg17 phase and refine the grain size of primary phase. Moreover, the measured area fractions of precipitates increased steadily as Bi content increased, which were comparable with the model-predicated values. After that, the calculated liquidus projections in the Mg-rich corner of Mg–Al–Zn–Bi quaternary system were employed for demonstrating the effects of Bi addition on the solidification sequences. Additionally, the phase transition temperatures predicted via Scheil solidification simulations agreed well with measured values. The present work indicated that the combination of experimental investigations and thermodynamic calculations can serve as an efficient method to obtain accurate and comprehensive solidification behavior of Bi-modified Mg–Al–Zn alloys.