Research On Microstructures And Mechanical Properties Of Mg-Zn-Cu Alloys

Magnesium alloy is one of the lightest metal structure materials with excellent performances. Magnesium alloys have attracted great attention for applications in automotive and aero-space industries because of their low density, high specific strength and high specific stiffness, however, its low strength, the bad plasticity, and the poor corrosion resistance had become limited its development and applications.In this experiment, the microstructures and mechanical properties of the Mg-Zn-Cu ternary alloys and the Mg-Zn-Cu-Y-Zr qiunary alloy in as-extrusion and in heat treatment conditions were investigated through changing the concentrations of Zn, Cu or Y. Finally, the optimum conpositions were achivered and the optimu preparing technical conditions were determined:(1) The main precipitated phases of ternary Mg-Zn-Cu alloys are a-Mg、Mg(Zn, Cu)2, CuMgZn, Mg2Cu and Cu5Zn8. With the increase of Cu and Zn content, the content of MgZn2、 MgZnCu and Cu5Zn8 increase and the grain size decrease. The main precipitated phases of Mg-Zn-Cu-Y-Zr alloys are α-Mg, Mg(Zn, Cu)2, CuMgZn, Mg2Cu, Cu5Zn8, X(Mg12 Y Zn) and I(Mg3YZn6). With the increase of Zn content, the alloy grain refines first and then coarsened, the intergranular precipitates gradually increased. As compared with mould casting, distributions of the second phases of semi-continuous casting were more uniform.(2) The comtents of Zn and Cu and extrusion temperature had significant effects on the mechanical properties of as-extruded ternary and quinary alloys. In ternary alloys, Mg-4Zn-1Cu and Mg-8Zn-5Cu alloys had high mechanical properties when extruded at 250℃, and the other alloys had higher mechanical properties when extruded at 150℃. In quinary alloys, Mg-(4,6)Zn-1Cu-0.4Y-0.6Zr and Mg-8Zn-5Cu-0.4Y-0.6Zr had high mechanical properties when extruded at 250℃, and the other alloys had higher mechanical properties when extruded at 150℃.(3) After T5 heat treatment, in ternary alloys, except the strength of Mg-8Zn-5Cu alloys extruded at 150℃ increased, and those of the other compositions or in other extrusion conditions had a little increment or decreased. In quinary alloys, except the strengths of Mg-4Zn-5Cu-0.4Y-0.6Zr and Mg-8Zn-5Cu-0.4Y-0.6Zr alloys extruded at 150℃ increased, the others had a little increment or decreased.(4) In ternary alloys, the Mg-6Zn-5Cu alloy extruded at 150℃ has the highest strength. Its tensile strength, the yield strength and the elongation is 304.4MPa,285.1MPa and 18.6% respectively. In quinary alloys, the Mg-4Zn-3Cu-0.4Y-0.6Zr alloy has high strengths in both extrusion temperatures and there was no significant difference for strength in both conditions, however, higher elongation was obtained in high temperature extrusion. Its tensile strength was 325.9MPa, the yield strength was 322.2MPa and the elongation was 15.4% after extruded at 250℃. After T5 heat treatment, the Mg-4Zn-5Cu-0.4Y-0.6Zr alloys extruded at 150℃ had the highest strength, the tensile strength, yield strength and elongation were respectively 370MPa、354.2MPa and 15.4%.(5) The as-extruded Mg-6Zn-5Cu-lY-0.6Zr alloy at 150"C when its billet prepared by DC casting processing had better mechanical properties and there was no obversely varation after T5 treatment. Its tensile strength, yield strength and elongation were 402.1MPa, 366.5MPa and 7.1% respectively.