Morphological Transition of α-Mg Dendrites During Near-Isothermal Solidification of a Mg–Nd–Gd–Zn–Zr Casting Alloy

Abstract

Microstructure evolution in the Mg–Nd–Gd–Zn–Zr commercial casting alloy Elektron21 and in a Zn-free alloy variant, solidified under near-isothermal conditions at six constant cooling rates, has been studied via in situ X-ray radiography. In the Zn-free alloy, equiaxed α-Mg primary dendrites are always observed to develop with a steady growth rate. Conversely, in the Elektron21 alloy, primary dendrites undergo a morphological transition after nucleation and an initial transient growth for cooling rates \( \dot{T} \) ≤ 0.075 K/s. Such transition leads to a change in the growth morphology from volume spanning 3D to anisotropic sheet-like growth occurring mainly along \( \left\langle {11\bar{2}0} \right\rangle \) directions, with 4–5 times increase in the growth velocity. Experiments and simulations highlight the pivotal role of Zn, indicating that the morphological transition occurs due to the formation of ordered rare earth-zinc arrangements in the \( \{ 10\bar{1}1\} \) pyramidal and \( \{ 0001\} \) basal planes of the α-Mg lattice within a layer extending a few micrometres from the solid-liquid interface into α-Mg.