Characterisation of three-dimensional dendritic morphology and orientation selection of α-Mg in Mg–Ca alloy using synchrotron X-ray tomography

• The morphologies of primary α-Mg dendrites in two growth conditions were studied.
• Five secondary arms were found in the columnar growth condition.
• Three types of α-Mg branching structures were observed in equiaxed growth.
• The dominate growth directions in Mg–Ca alloys are < 112¯0 > and < 224¯5 >.

The dendritic morphology and branching structure of α-Mg dendrites were investigated in both columnar and equiaxed growth Mg–9 wt.%Ca alloys. Using synchrotron X-ray tomography, dendrite morphologies were observed in Mg–Ca alloys from columnar and equiaxed growth, revealing 5, 4 and 3 side arms growing around the primary trunk. The crystallographic growth directions of the main trunk and branch arms were determined, through the topological analysis of the dendritic branching structure, as well as the computed angles between them. The results clearly showed that the preferential crystallographic orientations of primary α-Mg dendrites in Mg–Ca alloy are dominated by < 112¯0 > and < 224¯5 > orientations.