Grain Morphology and Orientation Effect on the High-Temperature Tensile Behavior of Directionally Solidified Magnesium Alloy

Columnar-structured Mg_97.27Zn_2.54Y_0.19 alloy with the preferred growth direction of 〈22$\overline{4}$5〉 and 〈12$\overline{3}$3〉, and Mg_98.01Zn_1.84Y_0.13Zr_0.03 alloy with the preferred growth direction of 〈11$\overline{2}$0〉 were prepared using the directional solidification technique and then tensile tested at 250–300 °C. The results showed that the Mg_97.27Zn_2.54Y_0.19 alloy with columnar dendritic structure experienced a significant deformation strengthening period when stretched at 250 °C, and a typical dynamic recrystallization stage at 300 °C, which were mainly accounted for its well-developed secondary dendrites, a large number of branch-shaped second phases distributed in the longitudinal grain boundary, and less granular second phases existing in the crystal. While in the whole test temperature range (250–300 °C), the stress–strain curves of Mg_98.01Zn_1.84Y_0.13Zr_0.03 alloys with cellular dendritic crystals showed a steady deformation state because of the second phases with strip distributing in the longitudinal grain boundary, which was induced by a balance between the deformation strengthening and the dynamic recrystallization. Especially at 300 °C, this steady deformation state existed in the strain of 5.6%–36%. The good uniform plastic deformation was closely related to the small misorientations between columnar grains, the associated movement of grains by grain boundary slipping, and the less percentage of dynamically recrystallized grains which could do harm to the orientation continuity of the columnar crystals.