Microstructure and mechanical properties of double continuously extruded Mg-Zn-Gd-based magnesium alloys

Double continuous extrusion (DCE) is proposed here as a novel thermomechanical process which means that the sample is continuously extruded twice by one process. The microstructures and mechanical properties of Mg98Zn1.92Gd0.08 (at%, Alloy 1) and Mg96.83Zn2.7Gd0.47 (at%, Alloy 2) after conventional extrusion and DCE at 373 K were investigated. The results show that the major secondary phase in Alloy 1 is Mg2Zn3 and that in Alloy 2 is icosahedral quasicrystals phase (I-phase). After DCE, the microstructure is refined, and the average grain sizes of the both alloys are smaller than 1 μm, and this further improves the mechanical properties. Both the alloys exhibit excellent mechanical properties, and the ultimate tensile strength, yield strength and elongation to failure are 434 MPa, 410 MPa and 14.6%, respectively, for Alloy 1; and 444 MPa, 417 MPa and 13.1%, respectively, for Alloy 2. The possible strengthening mechanisms were discussed and it was found that DCE is an effective method for Mg-based alloys strengthening by grain refinement and secondary phase dispersion.