Microsegregation and microstructure in directionally solidified Mg4Al, Mg5Al3Ca and AXJ530 alloys

Microsegregation and microstructure in Mg-rich alloys Mg4Al, Mg5A13Ca and AXJ530 have been investigated in directionally solidification. The area scan method was used to measure the solute redistribution. The fraction of solid used to normalize the experimental data was acquired using quantitative image analysis. Microsegregation was calculated based on the Scheil model using the preliminary thermodynamic description of the ternary Mg-Al-Ca system. Discrepancies between the calculated and experimental results were observed as anticipated since back diffusion was not taken into consideration. With increasing cooling rate, the discrepancy became smaller as also anticipated. The microsegregation of Ca in the α(Mg) primary dendrite was negligible due to the low solubilities in α(Mg). The transverse and longitudinal cross-sections of directionally solidified samples were examined. Both alloys Mg5A13Ca and AXJ530 exhibited two types of eutectic-like microstructures in the steady-state interdendritic regions, C14 (Mg₂Ca)+α(Mg) and C36 ((Mg,Al)₂Ca)+α(Mg).