Effects of the Growth Rate on Microstructures and Room Temperature Mechanical Properties of Directionally Solidified Mg-5.2Zn Alloy

Effects of the growth rate on the microstructures and room temperature mechanical properties of Mg-5.2Zn alloy were investigated using Bridgman method at a constant temperature gradient 30 K/mm with different growth rates (v = 10 ~ 100 μm/s). The microstructure of directionally solidified Mg-5.2Zn alloy is composed of dendrite primary α(Mg) phase and interdendritic α(Mg) + Mg₇Zn₃ eutectic, which agrees well with the predicted microstructure using Scheil model. The morphology of the primary α(Mg) phase transforms from cellular, to cellular-dendritic, and then to dendritic with the increase of growth rate from 10 μm/s to 100 μm/s. According to the Kurz–Fisher model, the approximate criterion growth rate for cellular/dendrite transition is determined to be about 12.7 μm/s, which just lies in the experimental result interval. Using non-linear fitting analysis, λ₁ (the primary dendrite arm spacing) and λ₂ (secondary dendrite arm spacing) were found to be dependent on v (growth rate) in the form of λ₁ = 8.6964 × 10⁻⁶v^−0.23983, λ₂ = 1.7703 × 10⁻⁶v^−0.34161, which is in good agreement with the calculated values by the Trivedi model and Kattamis–Flemings model, respectively. Furthermore, tensile test shows that the directional solidified experimental Mg-5.2Zn alloy shows higher strength than the non-directional solidified alloy under the same cooling rate. The dendritic structure shows higher strength than the cellular structure due to the fact that brittle interdendrite eutectic was refined in dendritic structures.