Research On Microstructure And Mechanical Properties Of Mg-Zn-Y Alloy

Mg-Zn-Y alloys attracted significant interest because they had both high strength at both room and elevated temperatures. This dissertation studied the effects of yttrium on microstructures and mechanical properties of Mg-Zn-Y alloys. Four kinds of Mg-3Zn-xY(x=0.5, 1.5, 3, 6) alloys were studied to investigate microstructures and mechanical properties evolution in both cast and heat-treatment state.Solidification behaviors, microstructures and mechanical properties of the Mg-Zn-Y alloys were studied by experimental methods as computer data collection system, differential scanning calorimeter(DSC), optical microscopy, X-ray diffractometer(XRD), scanning electron microscopy(SEM) and transition electron microscopy(TEM). Results indicate that primary magnesium crystals nucleate and grow up first, discharging redundant Zn and Y element to interdendritic spaces in the process of solidification. After the contents of Zn and Y reach to eutectic component, interdendritic liquid phase begins to transform to second phase of MgZnY and eutecticα-Mg by eutectic-reaction. As Y content increases, the shape of eutectic structure in the alloys transforms from punctate, short rodlike to reticular and the second phases coarsen. The types of the second phase in the alloys depend upon the Zn/Y ratio. As Y content increases, the second phases transform from Mg7Zn3+I-phase to I-phase+W-phase, to W-phase+H-phase and H-phase. When Zn/Y≥2, icosahedral phase which has fivefold, threefold, twofold symmetrical structures forms in the alloys. Mechanical testing results indicate that Y improves tensile strength, hardness and impact ductility of Mg-3Zn alloys remarkably. However, tensile strength, percentage elongation after fracture and impact ductility of Mg-3Zn-xY alloys decrease when Y content comes to 6%.Microstructures of Mg-3Zn-xY alloys after heat-treatment indicate that dissolvability of interdendritic phases in Mg-3Zn-xY alloys increases, second phases are passivated, and hardness grows up while temperature of solution treatment rises, gradually, the second phase of alloys coarsens and hardness decreases obviously when temperature of solution treatment is up to 550℃. Mg-3Zn-xY alloys become age-harden as aging time rises. As Y content increases, MgZn phase precipitated in the Mg-3Zn-xY alloys decreases gradually. While Y content increases, the best hardness of Mg-3Zn-xY alloys increases gradually after solution and aging treated.