Dependence of Microstructures and Mechanical Properties on the Growth Rate and Composition in Directionally Solidified Mg-Gd Alloys

Microstructures and mechanical properties of directionally solidified Mg-xGd (5.21, 7.96 and 9.58 wt.%) alloys were investigated at a wide range of growth rates (V = 10-200 μm/s) under the constant temperature gradient (G = 30 K/mm). The results showed that when the growth rate was 10 µm/s, different interface morphologies were observed in three tested alloys: cellular morphology for Mg-5.21Gd alloy, a mixed morphology of cellular structure and dendritic structure for Mg-7.96Gd alloy and dendrite morphology for Mg-9.58Gd alloy, respectively. Upon further increasing the growth rate, only dendrite morphology was exhibited in all experimental alloys. The microstructural parameters (λ1, λ2) decreased with increasing the growth rate for all the experimental alloy, and the measured λ1 and λ2 values were in good agreement with Trivedi model and Kattamis-Flemings model, respectively. Vickers hardness and the ultimate tensile strength increased with the increase of the growth rate and Gd content, while the elongation decreased gradually. Furthermore, the relationships between the hardness, ultimate tensile strength, the growth rate and the microstructural parameters were discussed and compared with the previous experimental results.