Selection of the solidification path of Mg-Zn-Gd ternary casting alloy

Mg-Zn-Gd base alloys possess much superiority, such as, high strength, light weight, low cost, etc., and favorable for the application in various airframe components. Two kinds of eutectic phases, such as, W(Mg₃Zn₃Gd₂) and I(Mg₃Zn₆Gd) can be usually found in Mg-Zn-Gd alloy under the traditional casting conditions. The interface between W phase and α(Mg) is incoherent and thus weak. However, I phase has quasiperiodic lattice leading to a coherent interface between I phase and α(Mg). Therefore, compared with W phase, I phase is more effective to obstruct dislocations slipping and so efficiently strengthening the alloy. So, controlling the solidification path, i. e., controlling the relative amount of I phase and W phase, is critical for increasing the heat resistant of Mg- Zn-Gd magnesium alloy. In this work, the solidification path of Mg-4.58Zn-2.6Gd ternary casting alloy was investigated by experiments and numerical analysis. Experimental results showed that at lower cooling rate (≤0.75 K/s) α+W(Mg₃Zn₃Gd₂) eutectic will be formed first, while at higher cooling rate (≥7.71 K/s) α(Mg)+I(Mg₃Zn₆Gd) eutectic will be formed first. A numerical model for predicting solidification path of the primary phase in multi-component alloy with considering the effects of solute diffusion in liquid phase and the cooling rate was developed. The thermodynamic data in the computation model was obtained by using the database of Thermo-Calc. The numerical results were in favorable agreement with the experimental ones. The numerical model established in this work provides a direct and easy way to predict solidification path of Mg-Zn-Gd alloy for different casting conditions. The validity of this model was further confirmed by other three different Mg-Zn-Gd alloys, i. e., Mg-3.8Zn-2.0Gd, Mg-5.5Zn-2.0Gd and Mg-5.5Zn-4Gd. It is also found that for Mg-Zn-Gd alloy, the higher Zn-content and the higher cooling rate will promote the formation of I phase. However, higher Gd-content and the lower cooling rate is favorable for the formation of W phase.