Strengthening of αMg and long-period stacking ordered phases in a Mg-Zn-Y alloy by hot-extrusion with low extrusion ratio

An as-cast sample and two hot-extruded samples with different extrusion ratios (R) of Mg ₉₇Zn ₁Y ₂ alloy containing the HCP α matrix (αMg) and the long-period stacking ordered phase (LPSO) of about 25-vol%, were used in tensile deformation in situ neutron diffraction experiments, to elucidate the effects of uniquely different microstructural evolutions in αMg and LPSO with varying the R value to the mechanical properties. αMg behaved as the soft phase and LPSO as the hard phase, and hot-extrusion improved the strength of both. At the R value of 5.0, dynamic recrystallization partly occurred in αMg creating a bimodal microstructure, consisting of a deformed αMg component and a recrystallized αMg component. The yield strength of αMg was significantly improved due to the optimal effects of dislocation density accumulation, texture development in the deformed αMg component and grain refinement in the recrystallized αMg component. With increasing the R value to 12.5, grain growth as well as dynamic recrystallization were promoted in αMg, causing a decrease in the yield strength of αMg, but the elongation was improved due to the presence of weakly textured grains. In contrast, the strength of LPSO increased monotonously with increasing the R value due to the developments of kink bands and texture. The density of kink bands saturated at R=5.0, but the texture of LPSO became stronger monotonously with increasing the R value. At R=12.5, the strength improvement of LPSO compensated for the strength decrease of αMg, and maintained the high ultimate tensile strength of the alloy.