Effects of Yb element on microstructure and mechanical properties of high-strength Mg-Sm-Gd(-Yb)-Zr extruded alloys

A high-strength magnesium alloy containing Yb was prepared through a simple hot extrusion process. The effect of Yb addition on dynamic precipitation, texture evolution, dynamic recrystallization mechanisms, deformation mechanisms, and strengthening mechanisms in as-extruded Mg-4Sm-3Gd(-2Yb)-0.5Zr (SGY0, SGY2) alloys was systematically investigated. The results indicated that the average grain size decreased from 4.17 µm to 1.48 µm with the addition of Yb. This extreme grain refinement greatly enhances the strength. The addition of Yb significantly facilitated the phase precipitation, but did not change the texture type. The non-dynamic recrystallized (unDRXed) grains exhibited a strong basal plane texture of <0110> parallel to the extrusion direction (ED), while the dynamic recrystallized (DRXed) grains showed a weaker rare earth texture, characterized by <1212> // ED. Moreover, the as-extruded SGY0 and SGY2 alloys predominantly undergo continuous dynamic recrystallization (CDRX), and some DRXed grains exhibit a discontinuous dynamic recrystallization mechanism (DDRX). In addition, the addition of Yb facilitates the activation of non-basal plane slip. The dislocation types in the as-extruded SGY0 and SGY2 alloys include 〈a〉, 〈c〉 and 〈c + a〉 dislocations. However, the SGY2 alloy exhibits a relatively high dislocation density, which contributes to the enhancement of the strength. Extreme grain refinement and the dispersion of nanoscale second-phase particles are key factors in increasing the strength.