Anomalous α-Mg Dendrite Growth During Directional Solidification of a Mg-Zn Alloy

Dendritic morphology was investigated in a directionally solidified magnesium-zinc alloy using synchrotron X-ray tomography and electron backscattered diffraction. Unexpectedly, primary dendrites grew along 213̄1 ⟨ 21 3 ¯ 1 ⟩ , rather than the previously reported 112̄ 0 ⟨ 11 2 ¯ 0 ⟩ and 224̄5 ⟨ 22 4 ¯ 5 ⟩ directions. Further, seven asymmetric sets of side branches formed, instead of six-fold symmetric arms, evolving with three coexisting morphologies per trunk of: traditional, seaweed structure, and free growth. The anomalous growth is attributed to the imposed thermal gradient and zinc-induced interfacial energy anisotropy variations.