Microstructure characterization of a directionally solidified Mg-12wt.%Zn alloy: Equiaxed dendrites, eutectic mixture and type/ morphology of intermetallics

Abstract

Over the last years, magnesium and its alloys have deserved special attention due to their good mechanical properties and promising biomedical applications. In the present investigation the Mg-12 wt.%Zn alloy has been directionally solidified (DS) under an extensive range of cooling rates. Based on the scarceness of characterization works related to both the morphology of the eutectic mixture and the development of equilibrium and non-equilibrium intermetallic particles (IMCs) in Mg-Zn alloys, a number of analyses are performed emphasizing and discussing such aspects. Rod and lamellar eutectic spacings are experimentally correlated with the growth rate and the cooling rate. The volume fraction of rod-like eutectic is shown to increase with the decrease in cooling rate during solidification. The characterization on the DS Mg-12 wt.%Zn samples is carried out using X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopies, which allowed the identification of two different non-equilibrium IMCs, MgZn2 and Mg4Zn7, which are distributed as nanoparticles throughout the α-Mg matrix. The Mg-Zn eutectic morphologies (lamellar and rod-like) are found to be constituted by α-Mg/Mg21Zn25 phases, the latter being formed by the decomposition of the eutectic IMC Mg51Zn20 through a eutectoid reaction.