Improving Tension/Compression Asymmetry of a Hot-Extruded Mg–Zn–Y–Zr Alloy via Yttrium Addition

In this study, effects of different yttrium (Y) contents ranging from 0 to 2.5 wt% on the microstructure and tension/compression asymmetry of as-extruded Mg–5.5Zn–0.6Zr alloys are investigated. The influence of Y on improving the tension/compression yield strength is been studied. The results show that the volume fractions of the microscale I (Mg₃YZn₆) and W (Mg₃Zn₃Y₂) phases remarkably increase with Y addition. Simultaneously, the nanoscale MgZn and Mg–Zn–Y phases uniformly precipitate during extrusion. Benefiting from the pinning of second phases, the microstructure is strikingly refined from 16.3 to 5.5 μm. Among them, the Y element promotes grain refinement most effectively when the Y content is 2.0 wt%. Second phase strengthening and fine-grain strengthening enhance the hardening rate and suppress twinning, which leads to the improvement of the tension/compression asymmetry. Specifically, yield strength of 231 MPa and ultimate tensile strength of 312 MPa are achieved with 2.5 wt% Y addition. The relative increment is about 33.0% and 65.3% compared to the unmodified alloy, respectively. In addition, the tensile yield strength to compressive yield strength ratio (TYS/CYS) is reduced from 1.30 to 1.09.