Study on Directional Solidification Microstructures of Mg-Zn-Gd Ternary Magnesium Alloy

Abstract

Mg-5.5Zn-2.0Gd ternary magnesium alloy was solidified under a constant temperature gradient (G=40K/mm) at different growth rates (v=6~35μm/s) using a directional solidification (DS) technique, and the directional solidification microstructures were studied. It was confirmed experimentally that the directional solidified alloys were mainly consisted of a(Mg) matrix and α(Mg)+W(Mg3Zn3Gd2) binary eutectic and α(Mg)+I(Mg3Zn6Gd) ternary eutectic at all of the three growth rates (v=6μm/s, 20μm/s and 35μm/s), which just fell inside the limit of computational thermodynamics solidification path calculated using the Scheil model. α(Mg) primary arm spacing (λ1) and its secondary arm spacing (λ2) could be expressed as a functions of the growth rate (v) in the form, λ1=k1v −a (k1=1×10–5, a=0.225) and λ2=k2v −b (k2=111.89, b=0.369) separately, and the growth velocity indexes (a=0.225 and b=0.369) were very close to the values (a=1/4, b=1/3) predicted by the Hunt-model and the Kurz-Fisher model.