Mg-Zn-RE alloys typically exhibit non-basal texture, weak dispersion hardening, and low yield strength. In this study, we designed a dilute quaternary Mg-0.6Zn-0.4Nd-0.2Ca (wt.%) alloy. Then, applied pre-strain and heat treatment to investigate the balance of yield strength and ductility via solute atom segregation at twin boundaries and nanophase modifications. The results indicated that Zn and Nd elements tend to co-segregate at the twin boundary, while Ca element segregates alone and presents a discontinuous distribution. Nanoscale precipitates of (Mg, Zn)₃Nd and Mg₂Ca with the size of 5∼20 nm underwent significant preferential precipitation at the twin boundary. Moreover, compared with Ca atoms, Zn atoms tend to combine with Nd atoms more preferentially towards the stacking faults, thereby forming a high-density monolayer Guinier-Preston (G.P.) zone. The segregation and precipitation of solute atoms at the twin boundary and the stacking faults increased the friction stress of twinning dislocations and lattice dislocations, thus improving the strength. Pinned twin boundaries facilitate the transition from basal 〈a〉 slip to pyramidal 〈c + a〉 slip due to the small geometrical compatibility factor (m′) value as well as the Schmid factor (SF) incompatibility. As a result, the pre-strained and heat-treated specimen’s yield strength exhibits a 141% increase relative to the initial state specimen, accompanied by a modest improvement in ductility. The mechanism of multi-element segregation and precipitation at twin boundaries, and G.P. zone formation was discussed in detail.

The effects of small additions of calcium (0.1% and 0.5%¹) on the dynamic recrystallization behavior and mechanical properties of as-extruded Mg-1Mn-0.5Al alloys were investigated. Calcium microalloying led to the formation of Al₂Ca in as-cast Mg-1Mn-0.5Al-0.1Ca alloy and both Mg₂Ca and Al₂Ca phases in Mg-1Mn-0.5Al-0.5Ca alloy. The formed Al₂Ca particles were fractured during extrusion process and distributed at grain boundary along extrusion direction (ED). The Mg₂Ca phase was dynamically precipitated during extrusion process, hindering dislocation movement and reducing dislocation accumulation in low angle grain boundaries (LAGBs) and hindering the transformation of high density of LAGBs into high angle grain boundaries (HAGBs). Therefore, a bimodal structure composed of fine dynamically recrystallized (DRXed) grains and coarse unDRXed regions was formed in Ca-microalloyed Mg-1Mn-0.5Al alloys. The bimodal structure resulted in effective hetero-deformation-induced (HDI) strengthening. Additionally, the fine grains in DRXed regions and the coarse grains in unDRXed regions and the dynamically precipitated Mg₂Ca phase significantly enhanced the tensile yield strength from 224 MPa in Mg-1Mn-0.5Al to 335 MPa and 352 MPa in Mg-1Mn-0.5Al-0.1Ca and Mg-1Mn-0.5Al-0.5Ca, respectively. Finally, a yield point phenomenon was observed in as-extruded Mg-1Mn-0.5Al-xCa alloys, more profound with 0.5% Ca addition, which was due to the formation of (1012) extension twins in unDRXed regions.