A Study On Microstructures And Mechanical Properties Of High Strength Mg-Zn-Y-Zr Alloys

Being the lightest metal alloy material, magnesium alloy has been paid much attention to in recent years. However the low strength characteristic restrains its application to secondary stressed part. At present high strength magnesium alloy used for structural material is made by high rare earth alloying, mostly Mg-Gd-Y-Zr alloy. Rare earth is expensive and hardly possible for civil use, except for a few military projects. Our research is based on the Mg-Zn-Zr alloy which is widely used in commercial applications. The influence of varied amounts of Zn and Y on the solidified structure of Mg-Zn-Y-Zr alloy is investigated by adding Y element and adjusting the concentration of Zn and Y. The indirect extrusion and extrusion after heat treatment of Mg-Zn-Y-Zr alloy with different component is also measured.Conclusions of the research are as follows:The solidification organization phase composition of Mg-Zn-Y-Zr alloy is irrelevant with the content of Zn or Y respectively, but relevant with the mole ratio of Zn/Y. The phase composition gradually changes from α-Mg phase plus W(Mg3Y2Zn3) phase to a-Mg phase plus X(Mg12YZn) phase in order with the addition of Y/Zn mole ratio.For Mg-Zn-Y-Zr alloy, no obvious effect of varied Y concentration on the content and appearance of second phase in solidification organization inconspicuous is observed under low Zn concentration, while notable influence is observed with content variation of Y in high Zn concentration. In the later condition, the second phase changes from slender areolar tissue into a broad lamellar structure.Mg-Zn-Y-Zr alloy has good extrusion forming property. The higher the content of Y is, the more striking the effects of Mg-Zn-Y-Zr alloy strengthening are. For Mg-Zn-Y-Zr alloy with the same content of Zn, the alloy extrude strength increased with the content of Y. There’s a distinct gap in strength between alloy with low and high content of Y with maximum difference being 120MPa. For Mg-Zn-Y-Zr alloy with fixed concentration of Y, strength of alloy extrude increases with the concentration of Zn, the maximum gap is 20MPa, which is relatively low when compared with Y-varied alloy. In brief, the impact of Zn concentration on the alloy property is smaller than Y concentration.Generally there are 3 types of Mg-Zn-Y-Zr alloy whose tensile strength is over 400MPa, with Mg-5Zn-11Y-0.6Zr being best in applications. Its tensile strength, yield strength and elongation are 429MPa,351MPa and 2%, respectively. No obvious improvement in property is observed when extruded Mg-Zn-Y-Zr alloy bar went through T5 treatment.