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The effects of Samarium (Sm) on the microstructures and mechanical properties of as-extruded AZ31 Mg alloy have been investigated. It is shown that the grain size changes in a non-monotonic mode of firstly coarsening and then refining, where the alloys with 0 $ 1.18wt.%Sm have bimodal grain structure consisted of dynamic recrystallized (DRXed) grains and unDRXed grains, while the alloys with 2.17 $ 3.13wt.%Sm have fully DRXed fine grains. A fine as-cast microstructure produces a finer extruded microstructure, and a large amount of tiny Al 11 Sm 3 particle is necessary to generate DRXed grains through particle-stimulated nucleation (PSN) effect. However, bigger Al 2 Sm particles can lead to increase in both area fraction of larger grains (above 40 lm) and texture, due to less effective in both PSN effect and suppressing DRXed grain growth. Grain refinement, secondary phase distribution and texture are important factors determining the mechanical properties of extruded alloys. Due to the fine grains, large amount of tiny secondary phase and suitable texture intensity, alloy with 3.13%Sm has the best mechanical property (i.e., YS 198 MPa, UTS 272 MPa and elongation 10%).

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