Abstract:
Due to the increasing demand to reduce greenhouse gas emissions, magnesium (Mg) alloys have received renewed attention as a means to reduce the weight of vehicles. However, the cast products account for most of Mg alloy usage and the most intensively studied wrought Mg alloys are those based on AZ31 alloy. Obviously, the lack of choice limits the scope of applications of wrought Mg alloy. Mg-Zn-based alloys show a pronounced age hardening response; and many alloying elements have been added into the system to improve its mechanical performance. However, the application of Mg-Zn alloys is still limited due to the high cost of rare earth alloying elements and processing technologies. Therefore, it has been a challenge to develop high strength Mg alloys with improved ductility. Previous studies at the University of Auckland have discovered a new alloy system containing a small amount of tin (Sn), lead (Pb) and zirconium (Zr), which shows very high ductility. Motivated by this discovery, the thesis project investigates the effect of alloying elements Pb and Sn addition on the microstructure and mechanical properties of Mg-Zn alloy. This work commenced with a critically evaluatation of the thermodynamic and phase diagram data of Mg-Zn-Pb and Mg-Zn-Sn phase diagram. Based on the existing literatures, the Mg–Pb binary system was optimised using the CALPHAD method through Thermo-calc® software package. The calculated results are in good agreement with the reported experimental data. Combined with the previous assessments of the Mg–Zn and Pb–Zn binary systems, the experimentally unexplored ternary phase diagram of the Mg–Zn-Pb system was predicted for the first time. Based on the predicted Mg-Zn-Pb and existing Mg-Zn-Sn ternary phase diagram, small amount additions of Pb or Sn, 1wt.% and 3 wt.%, has been chosen to study their effect on the microstructure and mechanical properties of Mg-4Zn alloys. Tensile and nanoindentation tests were conducted to evaluate the mechanical properties of the as-cast Mg-4Zn and Pb-containing alloys and the nanomechanical properties of primary α-Mg phase in these alloys, respectively. Significant improvements of elongation (E) were achieved from 14% to 20% by adding only 1.0 wt.% Pb into the Mg-4Zn alloy. However, strengths of Pb-containing Mg-4Zn alloys are relatively low, especially, the yield strength. Additions of Pb modify the morphology of intermetallic phases in cast Mg-4%Zn alloy and suppress the formation of the intermetallic compounds. In contrast, Sn additions enhance the strengths of as-cast Mg-4Zn alloy with sacrificing the ductility. Improvements of the mechanical properties in Sn-containing alloys are attributed to the increasing volume fraction of secondary phases and also modifying the type of intermetallic phases. Furthermore, the effect of Sn addition on the microstructure and mechanical properties of as-rolled and subsequent annealed Mg-4Zn alloys are also investigated. The addition of Sn increases the twinning density and modifies the annealing texture in as-rolled Mg-4Zn alloys. Comparing with the rolled-annealed Mg- 4Zn, numerous nanosized Zn-rich particles and rectangular-shaped Mg2Sn particles are dispersed in the matrix of rolled-annealed Mg-4Zn-3Sn alloy. As a result of dispersion strengthening effect of nanoparticles, the rolled Mg-4Zn-3Sn sheet alloy exhibits very high strength. The yield stress (YS) and ultimate tensile stress (UTS) of the rolled alloy are 305 MPa and 334 MPa, respectively. Although the strength decreased after annealing treated for 20 min at 400°C, the ductility of the alloy significantly increased. The UTS, YS and ductility of rolled-annealed Mg-4Zn-3Sn alloy are 273 MPa, 160 MPa and 16%, which is above the requirement for many structural applications (yield strength > 120 MPa and UTS > 210 MPa). The as-cast Mg-Zn-Pb and Mg-Zn-Sn alloys have been ageing treated at 200°C to further improve the performance of the alloys. The age-hardening behaviors and transmission electron microscopy (TEM) images have shown that Sn addition significantly refines the precipitates and enhances age-hardening response of Mg-4Zn alloys. However, addition of Pb coarsens the precipitates and slows down the ageing kinetics. The tensile strength of peak-aged Mg-4Zn has been significantly improved through the addition of Sn, but slightly decreased with addition of Pb. Finally, conclusions and recommended future works are suggested in the last part of this thesis.