Trace Ca-induced trade-off between mechanical strengthening and corrosion degradation in as-extruded Mg-Sm-Zn alloys

This study systematically evaluated the impact of trace Ca additions on the microstructural characteristics, mechanical performance, and corrosion behavior of Mg-1.5Sm-1Zn alloy. The alloys, designated as SZX210 (Mg-1.5Sm-1Zn-0.2Ca) and SZX211 (Mg-1.5Sm-1Zn-0.6Ca), were compared with the base SZ21 (Mg-1.5Sm-1Zn) alloy. Microstructural characterization revealed that Ca addition promoted the area fraction of secondary phases and slightly refines grain size. The SZX210 exhibited a highly homogeneous dispersion of secondary phases with Ca primarily in solid solution, while the SZX211 alloy contained blocky Mg 2 Ca precipitates with heterogeneous phase distribution. Compressive tests demonstrated significant improvements in mechanical properties with Ca addition: the compressive yield strength (CYS) of SZX210 and SZX211 increased by 39.38 % and 62.87 %, respectively, compared to SZ21. In addition, the plasticity performance of SZX210 alloy reached its optimal level. Crystal plasticity finite element method (CPFEM) simulations confirmed that the enhanced strength originated from increased critical resolved shear stress (CRSS) for basal slip and tensile twinning. Corrosion assessments, including hydrogen evolution, electrochemical measurements, and immersion tests, demonstrated that the SZX210 alloy possessed the superior anti-corrosion performance, attributed to its uniform microstructure and formation of a stable Ca(OH) 2 -enriched surface film. In contrast, the SZX211 alloy suffered from severe localized corrosion attributed to galvanic coupling between the Mg 2 Ca phase and the Mg matrix, as well as inhomogeneous secondary phase distribution.