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

摘要

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 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 〈1120〉 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 {1011} 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.