Effect of extrusion and cold rotary swaging on the microstructure of Mg–Y–Zn alloy containing needle-shaped LPSO phases

Wrought magnesium (Mg) alloys containing long-period stacking ordered (LPSO) phases exhibit outstanding comprehensive mechanical properties. However, the role and influence of intragranular needle-shaped structures on plastic deformation in such alloys remain poorly understood, particularly in LPSO-containing Mg alloys subjected to severe deformation. Therefore, this study investigates needle-shaped structures in as-extruded and rapidly cold rotary swaged Mg-3.16Y-1.85Zn-0.12Zr (wt.%) alloys. The results indicate that the intragranular needle-shaped structures in the as-extruded alloy consist of needle-shaped 14H-type LPSO phases and stacking faults (SFs). Following fast cold rotary swaging deformation, the needle-shaped LPSO phases exhibit significant bending and kinking deformation. This deformation results from the interaction between deformation twins and needle-shaped LPSO phases, where twins grow and traverse the LPSO during swaging, as well as from localized stress concentrations caused by dislocation pile-ups. Stress concentration at the tips of needle-shaped SFs promotes the formation of cellular structures in the adjacent α-Mg matrix, refining its grain size. Moreover, the needle-shaped phase areas form nanolamellar structures after cold rotary swaging, composed of needle-shaped LPSO phase and α-Mg nanoslices with orientation differences both between and within the nanoslices.