Improvement of strength and ductility synergy and the elimination of the yield point phenomenon of ultrafine-grained Mg–Al–Zn–Ag alloy sheet

Abstract

Strength and ductility synergy in ultrafine-grained AZQ310 alloy sheet was successfully improved at the expense of grain boundary segregation via multi-pass rolling. Before rolling, the as-extruded alloy has high yield strength (TYS, 382 MPa) but poor elongation (4.1%) mainly due to the ultra-fine grains (414 nm) and the co-segregation of Al/Zn atoms at grain boundaries. More importantly, this non-equilibrium segregation dies away during the rolling process, and the sheet exhibits a tensile yield strength of 370 MPa, an ultimate strength of 422 MPa, and an elongation of 9.8% along the rolling direction after 5th pass rolling. The results show that the high strength-ductility synergy is mainly attributed to the ultra-fine grains and residual dislocations. Furthermore, it is surprising that the yield point phenomenon (YPP) prominently in as-extruded sample is weakened and finally eliminated in as-rolled sheets. Microstructure evolution in the room temperature tensile test manifests that the underlying reason for the YPP is the stress-induced grain growth and grain boundary segregation, which in turn expounds the difficulty in the refining of ultrafine-grained Mg alloys by traditional processing methods.