An ultra-fine grained (UFG) Mg-1Zn-0.2Ca-1.0MgO composite with an average grain size of 0.49 µm as well as an excellent combination of yield strength (379 MPa) and ductility (10.1%) was produced by one-step extrusion. Subsequent heat treatment at 200 ℃ for 30 min further improved its yield strength to 420 MPa and elongation to 12.1% with a slight grain growth to 0.76 µm. Microstructure observations revealed that the precipitation of high number density of Ca₂Mg₆Zn₃ phase was the main reason for the enhanced strength after ageing treatment. Grain coarsening and recovery of dislocations during ageing improved the work-hardening capability of the extruded sample, leading to improved ductility. Our work provides a pathway for the mass production of strong and ductile Mg-based materials without excessive addition of alloying elements.

The effects of nano-CaO contents on the microstructure, mechanical properties and corrosion resistance of lean Mg-1Zn alloy were investigated. The results showed that the addition of nano-CaO significantly refined the grain size and improved mechanical properties of the Mg-1Zn alloy. At the same time, CaO reacted with molten Mg in situ to form nano-MgO, whose corrosion product in SBF solution was the same with the degradation product of Mg matrix, resulting in the enhanced compactness of the Mg(OH)₂ layer and reduced corrosion rate of matrix. The Mg-1Zn alloy had lower corrosion resistance due to excessively large grain size and shedding of corrosion products. The composite with 0.5 wt.% CaO had the best corrosion resistance with a weight loss of 9.875 mg·y⁻¹·mm⁻² due to the small number of Ca₂Mg₆Zn₃ phase and suitable grain size. While for composites with high content of CaO (0.7 wt.% and 1.0 wt.%), they had lower corrosion resistance due to the coexistence of large number of Ca₂Mg₆Zn₃ and Mg₂Ca at grain boundaries, especially for 1.0 wt.% CaO composite, resulting from the strong micro-galvanic corrosion.