Abstract
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 View the MathML source≤0.075 K/s, primary equiaxed α-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 View the MathML source 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 View the MathML source pyramidal plane of α-Mg lattice. At the temperature where the morphological transition occurs, it is found that both the solute concentration and zinc diffusivity in α-Mg are high enough for dimer formation to occur within a diffusive layer extending a few micrometres from the solid-liquid interface into α-Mg, and thereby open for increased solute-partitioning at the growth front.