α-Mg primary phase formation and dendritic morphology transition in solidification of a Mg-Nd-Gd-Zn-Zr casting alloy

Microstructure evolution in the commercial Mg-Nd-Gd-Zn-Zr alloy Elektron 21, solidified under nearly isothermal conditions, has been studied via in-situ X-ray radiography. For cooling rates T <= 0.075 K/s, primary equiaxed alpha-Mg dendrites undergo a morphological transition after nucleation and an initial stage of growth. The growth regime is observed to change abruptly from a 3D to a more pronounced anisotropic sheet-like growth occurring predominantly along (11 (2) over bar0) direction, with a 4-5 times increase in the growth velocity. The experimental results together with thermodynamic calculations and density functional theory simulations give support to relate the morphology transition to the formation of ordered rare earth-zinc dimers in the {0001} basal plane and {10 (1) over bar1} pyramidal plane of a-Mg lattice. At the temperature where the morphological transition occurs, it is found that both the solute concentration and zinc diffusivity in alpha-Mg are high enough for dimer formation to occur within a diffusive layer extending a few micrometres from the solid-liquid interface into alpha-Mg, and thereby open for increased solute-partitioning at the growth front. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.