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The influences of cooling rate on the phase constitution, microstructural length scale, and microhardness of directionally solidified Galvalume (Zn-55Al-1.6Si) alloy were investigated by directional solidification experiments at different withdrawal speeds (5, 10, 20, 50, 100, 200, and 400 µm·s-1). The results show that the microstructure of directionally solidified Galvalume alloys is composed of primary Al dendrites, Si-rich phase and (Zn-Al-Si) ternary eutectics at the withdrawal speed ranging from 5 to 400 µm·s-1. As the withdrawal speed increases, the segregation of Si element intensifies, resulting in an increase in the area fraction of the Si-rich phase. In addition, the primary Al dendrites show significant refinement with an increase in the withdrawal speed. The relationship between the primary dendrite arm spacing (λ1) and the thermal parameters of solidification is obtained: λ1=127.3V-0.31. Moreover, as the withdrawal speed increases from 5 to 400 µm·s-1, the microhardness of the alloy increases from 90 HV to 151 HV. This is a combined effect of grain refinement and second-phase strengthening. [ABSTRACT FROM AUTHOR]

In the process of hot-dip Zn-Al-Mg alloy coating, the plating solution dissipates heat in the direction perpendicular to the steel plate, which is considered to be a process of directional solidification. To understand the relationship between microstructure and cooling rate of Zn-Al-Mg alloys, both the phase constitution and microstructure characteristic length scales of Zn-9.5Al-3Mg-0.01Ce (wt.%) alloy were investigated by the directional solidification experiments at different growth velocities (V=40, 80, 160, 250 µm·s−1). The experimental results show that the microstructure of directionally solidified Zn-9.5Al-3Mg-0.01Ce alloy is composed of primary Al dendrites and (Zn-Al-Mg2Zn11) ternary eutectics at the growth velocities ranging from 40 to 250 µm·s−1. The primary Al dendrites are aligned regularly along the growth direction, accompanied with obvious secondary dendrites. The relationship between the microstructure length scale and the thermal parameters of solidification is obtained: λ1=374.66V−0.383, and λ2=167.5V−0.563 (λ1 is the primary dendrite arm spacing, and λ2 is the secondary dendrit arm spacing). In addition, through the interface response function (IRF) and the nucleation and constitutional undercooling (NCU), the phase selection of Zn-9.5Al-3Mg-0.01Ce is obtained: (Zn+Al+Mg2Zn11) ternary eutectics in the Zn-9.5Al-3Mg-0.01Ce alloy will be replaced by ternary eutectics (Zn+Al+MgZn2) when the growth rate is lower than 7.53 µm·s−1. [ABSTRACT FROM AUTHOR]