Investigation on microstructure, mechanical properties and work hardening behavior of as-extruded Mg-4.5Zn-0.75Y-xZr alloys

The effects of Zr content on microstructures, mechanical properties and work hardening behavior of the as-extruded Mg-4.5Zn-0.75Y-xZr (x=0.14, 0.28, 0.42, in at%) alloys were investigated with the assistance of optical microscope (OM), x-ray diffraction (XRD), scanning electron microscope (SEM) and material testing machine. The results show that the matrix microstructures of the as-extruded Mg-4.5Zn-0.75Y-xZr alloys present as typical bimodal structures, which are characterized by coarse ?-Mg grains surrounded by many relatively fine dynamically recrystallized grains. The average grain sizes (GSs) of the extruded alloys show a trend of first decreasing and then slightly increasing with increasing Zr content, but the I-phase (Mg3Zn6Y) volume fractions keep almost constant. Among the three alloys, Mg-4.5Zn-0.75Y-0.28Zr alloy exhibits the optimum comprehensive mechanical properties (the ultimate tensile strength, yield strength and elongation to failure are 353 MPa, 296 MPa and 26.5%, respectively), which is corresponded to its smallest average GS. Meanwhile, with the reduction of the average GS, the work hardening (WH) rate, hardening capacity and WH exponent of the extruded alloys decrease. The WH rate decreases linearly with increasing deformation at the third stage of WH. Moreover, all the studied alloys show good ductility, and their ductile fracture features can be observed from the morphologies of tensile fracture surfaces at room temperature.