Directional Solidification of Mg-Al Alloys and Microsegregation Study of Mg Alloys AZ31 and AM50: Part II. Comparison between AZ31 and AM50

Directional solidification (DS) of magnesium alloys AZ31 and AM50 was carried out to investigate microstructures and microsegregation under controlled solidification conditions. The methodology used is based on a customized DS technique elaborated on in Part I of this work. Dendritic microstructures observed in longitudinal sections of the quenched mushy zone, X-ray maps of fully directionally solidified cross sections, and quantitative solute profiles reveal the impact of cooling rate and alloy type. The solute-solvent correlation discloses the opposite segregation behavior of manganese compared to both aluminum and zinc and provides an impartial basis for the advanced weighted-interval ranking sort (WIRS) sorting scheme of electron probe microanalysis (EPMA) data. Precipitates Al₈Mn₅ and γ-Mg₁₇Al₁₂, predicted by thermodynamic calculations, are identified in the microstructure. The effect of back-diffusion during solidification, most pronounced for the component zinc, was clearly observed for both alloys in comparison with dedicated Scheil-total calculations.