Microstructural evolution, phase constitution and mechanical properties of directionally solidified Mg-5.5Zn-xGd (x = 0.8, 2.0, and 4.0) alloys

The microstructural evolution, phase constitution and mechanical properties of directionally solidified Mg-5.5Zn-xGd (x = 0.8, 2.0, and 4.0) alloys were firstly investigated under G = 30 K/mm at a wide range of V (10 μm/s - 100 μm/s). It was confirmed that there existed α(Mg) + I(Mg3Zn6Gd) in Mg-5.5Zn-0.8Gd alloy, and α(Mg) + I(Mg3Zn6Gd) + W(Mg3Zn3Gd2) in Mg-5.5Zn-(2.0, 4.0)Gd alloys, respectively. The criterion growth rate for cellular-columnar dendrite transition (CDT) for Mg-5.5Zn-xGd alloys decreased with the increase of Gd content. The relationships between the microstructural parameters (λ1, λ2) and V for Mg-5.5Zn-xGd alloys were established using a linear regression analysis. The values of λ1 and λ2 decreased exponentially with the increase of V and the exponent values were found to be close to the theory values of 1/4 and 1/3, respectively. The tensile test showed that the room temperature ultimate tensile strength (UTS) increased and the elongation decreased with the increase of the growth rate for a certain composition of Mg-5.5Zn-xGd alloy. For a certain growth rate, UTS first increased from 0 wt % to 2.0 wt % of Gd content, and then decreased with the further increase of Gd content. The directionally solidified Mg-5.5Zn-2.0Gd experimental alloy showed the maximum ultimate tensile strength.