Enhancing pyramidal slip in magnesium by yttrium through mobile screw-steps

Magnesium (Mg), the lightest structural metal, suffers from limited plasticity, which severely restricts its industrial application. Alloying elements, especially yttrium (Y), can improve the plasticity of Mg effectively, which is often linked to enhanced ⟨c + a⟩ dislocation activity. However, direct experimental investigations of how Y affects ⟨c + a⟩ dislocations remain scarce, obscuring the underlying mechanism. Here, by employing in-situ transmission electron microscopy testing, we quantify ⟨c + a⟩ dislocation dynamics in Mg and Mg-Y. The Mg-Y samples exhibit higher strain hardening and smoother plastic flow, supported by a higher density of uniformly distributed ⟨c + a⟩ dislocations. Crucially, we find that edge and screw ⟨c + a⟩ segments exhibit comparable density and mobility in the Mg-Y alloy, in stark contrast to their marked disparity observed in Mg. Further real-time tracking on individual dislocation motion reveals that the lateral migration of highly mobile screw-steps drives the glide of hard edge segments. Atomistic simulations reveal that Y locally transforms the edge core to a non-planar structure that moves sluggishly, causing the adjacent edge segment to bow out and become a mixed character with screw-step which exhibits higher mobility. The reduced edge/screw mobility gap promotes dislocation motion and multiplication, thereby improving plasticity. Our work elucidates how Y enhances plasticity in Mg and provides a guideline for the design of high-performance alloys.