(1. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Science,Shenyang 110016, China)
(2. Ji Hua Laboratory, Foshan 528200, China)
The effect of Zr addition on microstructures and their evolution during heat treatments of a kind of Mg-Zn-Nd alloy was studied using various electron microscopy techniques. The minor addition of Zr not only provid heterogenous nucleation sites to refine grain sizes of Mg during solidification, but also influence microstructural evolution upon solution treatment and aging. Zn2Zr3 precipitates were formed in Mg-2.7Nd-0.6Zn-0.5Zr (wt%) during solution treatment at 798 K, inducing remarkable hardness increment of the alloy. The atomic structure of nanometer Zn-Zr precipitates is determined by combining tilt series of micro-beam electron diffraction with atomic resolution imaging and chemical measurement. The stoichiometry of the Zn-Zr precipitates is Zn2Zr3 with a primitive tetragonal structure (space group P42/mnm, a=b=0.761 nm, c=0.682 nm). Moreover, decrease in Zn concentration in Mg matrix due to the formation of Zn2Zr3 precipitates increased the density ratio of β1-Mg3Nd prismatic rod precipitates to γ″-Mg7(Nd, Zn)3 basal plates in peak aged samples. Our results may shed light on optimization of microstructures and mechanical properties of Mg alloys with Zr addition. Our present finding shed new light on the role of Zr micro-alloying in microstructures and mechanical properties of Mg alloys.