Developing a low-alloyed fine-grained Mg alloy with high strength-ductility based on dislocation evolution and grain boundary segregation

A new low-alloyed Mg-2Sm-0.8Mn-0.6Ca-0.5Zn (wt.%) alloy is prepared by low-temperature and low-speed extrusion. The as-extruded alloy has ultra-high yield strength (YS, 453 MPa) but poor elongation (3.2%) mainly due to the formation of a fine-grained structure containing high-density residual dislocations and Mn nanoparticles. More importantly, after subsequent simple annealing, the alloy exhibits an excellent combination of high-strength and high-ductility, with the YS of 403 MPa and elongation of 15.5%. The effective inhibition of grain growth by grain boundary (GB) co-segregation of Sm/Zn/Ca is crucial for the annealed alloy to maintain high strength. Appropriately decreased dislocation density, especially the evolution of immovable long S- dislocations towards new GBs, is a key factor for the remarkable increase of ductility for the annealed alloy. Thus, we put forward a new strategy for developing low-alloyed Mg alloy with high strength-ductility mainly based on dislocation evolution and GB segregation.