Break through the strength-ductility trade-off dilemma in Mg-Bi-Al alloy via dynamic precipitation controlling during extrusion

Precipitation is one of the key microstructural features that governs mechanical performance of wrought Mg alloys. By tailoring dynamic precipitation during thermomechanical processing, the inherent strength-ductility trade-off can be effectively overcome. In this study, dynamic precipitation of Mg 3 Bi 2 phase is successfully regulated during extrusion of Mg-Bi-Al alloys through Cu alloying, achieving synergy improvement between strength and ductility. The newly developed Mg-5Bi-3Al-1Cu alloy exhibits an excellent balance of high strength (yield strength of 347 MPa) and ductility (elongation of 14.3%). During extrusion, dissolved Cu atoms and hard MgAlCu particles provide abundant nucleation sites for Mg 3 Bi 2 precipitates, thereby strengthening alloy via the Orowan mechanism. In-situ synchrotron X-ray diffraction and tomography during mechanical testing reveal that Mg-5Bi-3Al-1Cu alloy exhibits faster dislocation multiplication and lower defect growth rates compared with the base alloy before Cu alloying. High-density low-angle grain boundaries and MgAlCu particles act as potential sources for new dislocations, enhancing dislocation multiplication capability. Moreover, the formation of precipitation free zone around MgAlCu particles provides storage space for proliferating dislocations, preventing them from serving as crack initiation sites. These mechanisms collectively enable an outstanding strength-ductility synergy. This work provides valuable insights into the design of high-performance Mg alloys with superior strength-ductility coordination through dynamic precipitation control.