The development of micro-alloyed magnesium-zinc based ternary alloy
Magnesium (Mg) alloys have been widely studied for applications in 3C and transport areas. However, the intrinsically high susceptibility to corrosion and the inadequate ductility at room temperature largely limit their wider practical applications. The poor creep resistance of commonly commercial magnesium-aluminum (Mg-Al) based alloys at elevated temperature drives the demand for Al-free Mg alloys, among which magnesium-zinc system (Mg-Zn) shows great potential for the development of low-cost Mg alloys with higher strength and good corrosion performance. In this thesis, low-Zn containing Mg-Zn alloys micro-alloyed with different ternary alloying elements were developed and comprehensively studied, aiming at achieving a good combination of corrosion performance and mechanical properties. By investigating the influence of ternary alloying elements on the microstructures and corrosion behavior of Mg0.5Zn0.2X and Mg4Zn0.2X alloys (in wt.%), Mg0.5Zn0.2Ca, Mg0.5Zn0.2Ge and Mg4Zn0.2Sn alloys were identified as promising alloys with possible good combination of corrosion performance and mechanical strength. Afterwards, hot extrusion at different speed was applied to the three alloys to further improve corrosion resistance and strength. The extrusion speed showed little influence on the corrosion resistance of the three optimized alloys because of the slight alternation of the microstructures. Affected by the chemistry of the bulk materials, the corrosion rates of Mg0.5Zn0.2Ge and Mg4Zn0.2Sn alloys were reduced after extrusion while that of Mg0.5Zn0.2Ca alloy was not clearly affected. The corrosion mechanism of Mg0.5Zn0.2Ge alloy changed from localized corrosion to uniform corrosion after extrusion owing to the refined microstructure and the increased participation of Zn in the corrosion product layer. In comparison, both as-cast and extruded Mg0.5Zn0.2Ca alloys revealed uniform corrosion, while Mg4Zn0.2Sn alloys in both conditions suffered from localized corrosion in corrosive electrolytes due to the heterogeneous microstructures. Deionized water based sodium chloride (NaCl) solutions at different concentrations did not affect the corrosion mechanism of the alloys, while artificial tap water based NaCl solution significantly enhanced the corrosion resistance of the alloys owing the formation of an additional calcium carbonate layer on the top of the primary oxide/hydroxide layer. The stronger textures of Mg0.5Zn0.2Ge and Mg4Zn0.2Sn alloys conferred higher tensile strength but higher mechanical anisotropy on the alloys compared with Mg0.5Zn0.2Ca alloy. The tensile properties of all alloys deteriorated with exposure time in salt spray because of corrosion, especially when localized corrosion happened. However, the variation tendency of the tensile properties was closely related to the corrosion resistance of the alloys in salt fog. The fatigue behavior (S-N curves) of the optimized alloys deviated from near-linear trend in air. In the presence of corrosive electrolytes, the fatigue lives and fatigue limits of the alloys decreased. Again, the corrosion fatigue behavior of the alloys were strongly related to the corrosion behavior in different solutions, especially for Mg4Zn0.2Sn alloy. The susceptibility of the alloys to stress corrosion cracking (SCC) in four different electrolytes were studied by constant load tests. Mg0.5Zn0.2Ca and Mg4Zn0.2Sn alloys were resistant to SCC in all environments. While Mg0.5Zn0.2Ge alloy exhibited susceptibility to SCC in all environments, especially in deionized water. This was because of the different corrosion products/substrate interfaces formed in different solutions, which influenced the development of cracks. The results emphasized the influence of corrosion on the fatigue behavior and mechanical properties of Mg-Zn alloys, and also highlighted the importance of the investigation of the overall properties (corrosion, mechanical, fatigue and stress corrosion properties) of Mg alloys during practical alloy development.