Study on directional solidification microstructures of Mg-Zn-Gd ternary magnesium alloy

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 α(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, 6=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.