Effect of Zn on the microstructure evolution of extruded Mg–3Nd (–Zn)–Zr (wt.%) alloys

This paper presents the results of a transmission electron microscopy study of the microstructure of extruded Mg–Nd–Zn alloys that exhibit improvement in strength, making them potential candidates for bumper beam applications. Three sets of extruded rods with 0%, 0.2% and 0.5% Zn addition are prepared to examine the effect of Zn on the precipitates in the as-extruded and aged condition. At least five different precipitate phases with different structures, morphologies and distributions were identified and correlated with the mechanical properties. Zn is seen to have no effect on the precipitation sequence of Nd-rich phases but seems to control the growth of those phases. It is found that small amount of Zn can lead to drastic changes in the nature of the precipitate phase due to the strong affinity between Zn and Nd in the alloy to form Mg–Nd–Zn intermetallics, affecting the strength and ductility. Control of both the Nd and Zn content are necessary to take advantage of the “rare earth texture” induced by Nd to improve ductility and precipitation strengthening by the Mg–Nd–Zn intermetallics when Zn is present.