| Mg alloys have many excellent properties,such as low density,high specific strength,high thermal conductivity and remarkable dimensional stability,which have been used in aerospace area,automotive lightweight area and the manufacture of lightweight electronic products.However,due to the inferior mechanical properties at room temperature and poor corrosion resistance,the difficulty in room temperature forming and the untimely rupture failure of the components during the application involved with the fast corrosion rate,the combination of improving the comprehensive mechanical properties and ensuring excellent corrosion resistance has become the key to explore the high-performance Mg alloys.The Mg-Ga alloy system is one of the novel promising Mg alloys with high performance and is lack of the systematic deep study.Previous studies have shown that Mg-Ga alloy has excellent ductility and is age hardenable.Therefore,the microstructure,mechanical properties and corrosion resistance of the Mg-Ga-Zn alloy system are investigated in the thesis with the aim of developing a novel high-performance Mg alloy.In this paper,the effects of Zn alloying(0,0.3,0.6,0.9 and 1.2 wt.%)and aging treatment on the microstructure,mechanical and corrosion properties of Mg-5Ga alloy are systematically studied.The role of Zn element in the process of aging precipitation of Mg-5Ga alloy is deeply discussed and the correlations between the mechanical and corrosion properties and microstructure are revealed.The main conclusions are as follows.(1)The effects of Zn alloying on the microstructure and mechanical properties of as-cast and as-solutionized Mg-5Ga alloy are studied.The results show that the grains of as-cast and as-solutionized alloys can be refined by adding Zn,and the average grain sizes decrease at first and then increase with the increase of Zn content,wherein Mg-5Ga-0.9Zn alloy has the smallest grain size.After solution treatment,the divorced eutectic structure composed of MgsGa2 second phase and ?-Mg in the as-cast alloy is transformed into the uniform ?-Mg single-phase structure.The yield strength(YTS)gradually increases,the ultimate tensile strength(UTS)and elongation(El)of the as-cast Mg-5Ga-xZn alloys increase at first and then decrease with the increase of Zn content,and the Mg-5Ga-0.9Zn alloy has the best comprehensive mechanical properties with UTS,YTS and El of 230MPa,81MPa and 25.1%,respectively.The UTS,YTS and El of the as-solutionized Mg-5Ga-xZn alloys monotonically increase with the increase of Zn content,and Mg-5Ga-1.2Zn alloy has the best comprehensive mechanical properties with UTS,YTS and El of 260MPa,97MPa and 40.3%,respectively.The increase in the strength of the as-solutionized alloys can be attributed to the synergistic effect of solid solution strengthening and grain refinement strengthening.(2)The effects of Zn alloying on the corrosion resistance of the as-cast and as-solutionized Mg-5Ga alloys are studied.The results show that the corrosion rates of the as-cast and as-solutionized Mg-5Ga-xZn alloys in 3.5%NaCl solution all decrease at first and then increase with the increase of Zn content.In the as-cast alloys,Mg-5Ga-0.6Zn alloy has the best corrosion resistance and its 7-day weight loss rate is 1.571 mg·cm-2·d-1.In the as-solutionized alloys,Mg-5Ga-0.9Zn alloy has the optimal corrosion resistance and its 7-day weight loss rate is only 0.835 mg·cm-2·d-1.Zn addition can obviously improve the compactness of the corrosion product film on the alloy surface and can inhibit the cathodic reaction of hydrogen evolution.Compared with the as-cast alloys,the as-solutionized alloys show the greatly improved corrosion resistance due to the obvious weakening of the galvanic corrosion caused by the dissolution of the second phases.(3)The age-hardening behaviors of the Mg-5Ga-xZn alloys at 250? are studied.The results show that Zn addition can accelerate the aging kinetics and can improve the age-hardening ability of the Mg-5Ga alloy.Wherein Mg-5Ga-1.2Zn has the optimal age-hardening effect,its peak hardness is 14%higher than that of Mg-5Ga alloy,and the time reached to the peak hardness is shortened by 4 h.The precipitates in peak aged Mg-5Ga and Mg-5Ga-1.2Zn alloys are mainly rod-like MgsGa2,and their length directions are parallel to[0110]Mg.Moreover,the orientation relationship between Mg5Ga2 and a-Mg is[002]Mg5Ga2//[1120]Mg·The Zn addition can refine the rod-like MgsGa2 particles and can also increase their number due to the Zn segregation at the interphase boundary between ?-Mg and MgsGa2,which is beneficial to reduce the interface free energy and increases the nucleation rate of Mg5Ga2 precipitates.(4)The correlations between the mechanical and corrosion properties of the peak aged Mg-5Ga-xZn alloys and the characteristics of the precipitates are investigated.The results show that the strength of the alloy montonically increases with the increase of Zn content,while the elongation gradually decreases.Among five alloys,Mg-5Ga-1.2Zn alloy has the highest UTS and YTS,which are 268MPa and 131MPa,respectively.Compared with the corresponding as-solutionized alloy,the yield strength increment of the peak aged Mg-5Ga alloy is only 15MPa and that of Mg-5Ga-1.2Zn alloy is 34MPa,which can be attributed to the significant increase of the precipitation strengthening effect by adding Zn.The corrosion resistance of the peak-aged Mg-5Ga-xZn alloys becomes better at first and then deteriorates with the increase of Zn content.The Mg-5Ga-0.9Zn alloy has the optimal corrosion resistance,with the 7-day weight loss rate in 3.5%NaCl solution of 0.994 mg·cm-2-d-1,the corrosion potential of-1.514 V and the corrosion current density of 15.14?A/cm2.The reason for the improved corrosion resistance of the Zn-containing alloys is that Zn can refine the precipitate size of the alloy and increase the number of precipitates,which is conducive to improve the uniformity of pitting corrosion and film-like corrosion.Moreover,the proper Zn addition is beneficial to elevate the uniformity and the compactness of the corrosion product film. |
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