The influence of growth rate on the microstructure and properties of directionally solidified Zn-11Al-3Mg-0.2Si alloy

Zn-11Al-3Mg-0.2Si alloy was prepared via directional solidication with growth rates ranging from 1 to 200 μm/s. The growth rate dependence of the microstructure, corrosion resistance and microhardness of the alloy were investigated in detail. The results show that the microstructure of the alloy consists mainly of the primary Al phase and MgZn ₂ + Al binary eutectic at the growth rates of 1–5 μm/s. This can be attributed to the further growth of the Al and MgZn ₂ phases formed in the mushy zone during the heat-holding period. However, a microstructure of primary Al phase and Zn+ Al+Mg ₂Zn ₁₁ ternary eutectic was obtained when the growth rate is 10 μm/s and higher. The reason for the microstructural change is that the increase in undercooling caused by the growth rate at the solid-liquid interface promotes the nucleation of the Zn+Al+Mg ₂Zn ₁₁ ternary eutectic phase instead of MgZn ₂ phase. Electrochemical and weightless corrosion experiments show that the alloy obtained at the growth rate of 10 μm/s possesses the best corrosion resistance. The ternary eutectic structure facilitates the formation of dense corrosion products, while the grain refinement caused by the increase in growth rate will aggravate intergranular corrosion. The nanoindentation tests demonstrate that the microhardness of each phase increases with grain refinement, and the microhardness of MgZn ₂ phase is much higher than that of the ternary eutectic phase. Under the combined effects of intermetallic compound phase strengthening and grain refinement strengthing, the alloy obtained at the growth rate of 5 μm/s shows the highest microhardness.