| Magnesium alloy have great potentials to be used as structural materials in the fields of automobile, electronic and airspace industries due to their low densities, high specific strellgth, electro magnetism shield etc. But its poor creep resistance limits its application in the core components of structural materials. Mg-Zn-Y alloy series have very high mechanical properties at room and high temperature, which was considered as a typical second phase strengthening alloy. The micro structure, aging behavior, mechanical properties at room temperature and high temperature creep properties of Mg-5.5Zn-(0.7,1.5,3.5,7.5) Y gravity cast magnesium alloys were investigated in this paper. And the micro structure and creep mechanism were analyzed by building up the creep physical model.The results show that:(1) With increasing yttrium content in Mg-5.5Zn(wt%)alloy, three second phases which are Mg7Zn3+I-phase (tiny)、I-phase+Mg7Zn3(modicum)、W-phase and W-phase+X-phase are found in Mg-5.5Zn-(0.7,1.5,3.5,7.5) Y four alloys respectively, the second phase volume fraction increases and the grain size decreases with Y addition in Mg-5.5%Zn binary alloy. Particularly, with the increase of Y addition in Mg-5.5%Zn alloy, the obvious transformation about the composition, amount, size, shape, and distribution of the second phase is observed. The volume fraction is1.9%、2.8%、3.3%、4.5%、5.7%, respectively for the as-cast Mg-5.5Zn-(0.7,1.5,3.5)Y alloys. The distribution characteristic of the second phase is transferred from discontinuous to continuous. The second phases distribution of (0.7,1.5,3.5)%Y added alloys present discontinuous. But7.5%Y added alloy contains network or skeletal second phases structure. The tensile strength and yield strength of the alloy increased first and then decreased with the content of Y When the Y content is1.5%, the alloy exhibits a highest tensile strength and yield strength,which is186MPa and100MPa respectively.(2) Mg-5.5Zn-(0.7,1.5,3.5)Y creep resistance mainly depend on the second phase component, distribution, morphology and volume fraction.The creep resistance of Mg-5.5%Zn-(0.7,1.5,3.5,7.5)%Y alloys increases with the increase of Y. Moreover, compared to Mg7Zn3phase, high melting point rare earth phases (Mg3ZnY、Mg3Z16Y、 Mg3Zn3Y2) have a stronger effect for improving the creep resistance of alloy. So the (1.5,3.5)%Y added alloys exhibit good creep resistance properties in the range of temperature from175℃to200℃and stress from70MPa to55MPa range. The7.5%Y alloy exhibits excellent creep resistance at the condition of225-275℃/70-55MPa range.(3) After crept for100h at175℃and70MPa, the preferentia second phase is observed in (0.7,1.5,3.5)%Y added three alloys. Simultaneously, after crept for100h at200℃and55MPa, the build up of particles on longitudinal boundaries and elongated grains are formed at grain boundary of (1.5,3.5)%Y added alloys. Moreover, the thick network or skeletal continuous second phase strcture has better blocking effect on the dislocation motion, GBS and grain boundary migrationthe creep rate of alloy in the same given creep conditions.(4) The calculated stress exponent n for the as-cast Mg-5.5Zn-(0.7,1.5,3.5)Y is5.2、32、2.2at175℃/50-60MPa and is11、3.8、2.9at200℃/50-60MPa. The results indicate that the creep rate controlling mechanism of Mg-5.5Zn-0.7Y alloy at175℃/50-60MPa and200℃/50-60MPa is respectively the dislocation creep and power law breakdown, and the creep mechanism of Mg-5.5Zn-(1.5,3.5)Y two alloys is dislocation viscous motion at the applied stresses50-60MPa and the temperature175-200℃. |
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