Microstructure evolution and mechanical properties of directionally solidified Mg-xGd (x=0.8, 1.5, and 2.5) alloys

The microstructure evolution and mechanical properties of directionally solidified Mg-0.60, 1.38 and 2.35 wt% Gd alloys were investigated as a function of temperature gradients (G), growth rates (V), cooling rates (R) and solute (Gd) contents. A typical cellular microstructure with varied morphologies was observed in three different Mg-Gd alloys under steady states with three different G (20, 25, and 30 K/mm) at the fixed V (10 µm/s) or with different V (10-200 µm/s) at the fixed G (30 K/mm). The formation of cellular microstructure can be attributed to the combined effects of the lower solute content and the higher interfacial tension. The cellular spacing (λ) decreases not only with increasing G or V, but also with increasing Gd content for the fixed G and V. The measured λ values are in good agreement with Trivedi model and the previous experimental results. Meanwhile, the ultimate tensile strength of directionally solidified Mg-Gd alloys decreases with increasing λ, but it increases with increasing cooling rates. Furthermore, the relationship between the ultimate tensile strength, structure parameter and cooling rate was also discussed and compared with the previous experimental results.