Preparation And Study On Microstructure And Properties Of Quasicrystal Reinforced AM50 Alloy

The AM50 alloy has good ductility,better castability and low cost.However,its low strength cannot satisfy the property requirements of some key automobile structural parts.In this work,the methods of quasicrystal strengthening and heat treatment strengthening were used to improve the mechanical properties of the AM50 alloy.The OM,SEM,EDS,XRD,TEM,thermal analysis,tensile test and hardness test were used in the experiment.Firstly,the effects of Zn and Y elements on microstructure and mechanical properties of ordinary solidification Mg-Zn-Y quasicrystal alloys were studied to obtain the optimum composition of quasicrystal alloy.Secondly,the effects of different additions of the MZY quasicrystal alloy on the microstructure and mechanical properties of AM50 alloys were studied by using the external addition method.Lastly,the effects of Zn and Y on microstructure,mechanical properties and solidification behavior of AM50 alloys were studied by using the in situ method,and discussed the influence of different heat treatment process on microstructure and mechanical properties of the optimum composition alloy.The results show that the I-phase can nucleate and grow directly from the melts under the condition of ordinary solidification.The hardness of the Mg-xZn-4.5Y alloys increases gradually with the increase of Zn content.The morphology of I-phase changes from petal-like to strip,and the lamellar eutectic structure??-Mg+I-phase?and ?-Mg disappear gradually in the microstructures of the Mg-xZn-4.5Y alloys.The hardness of the Mg-48Zn-yY alloys and the amount of petal-like I-phase increase gradually with the increase of Y content.The size of the petal-like I-phase decreases gradually,and its distribution is more uniform.Meanwhile,the lamellar eutectic structure becomes more compact.The Mg₃Zn₆ Y quasicrystal phase can be left well in AM50 matrix alloy,and the microstructures are obviously refined after adding the MZY quasicrystal alloy to the AM50 alloy.The coarse and continuous netlike ? phase changes to discontinuous strip and granular shape,and the amount of ? phase decreases.When the addition amount of the MZY quasicrystal alloy is 6wt.%,the microstructure is most fine.The ultimate tensile strength,yield strength and elongation of the AM50-6%?Mg-48Zn-13Y?alloy reach to the maximum values of 202.92 MPa,100.57 MPa and 10.8%,which is improved by 24.59%,74.9% and 66.15% respectively compared with those of AM50 alloy.The mechanical properties of AM50 alloy could be improved significantly by the addition of the MZY quasicrystal alloy,which results from the microstructure refinement,the optimization of the morphology and amount of the ? phase and the dispersion strengthening effect of the quasicrystal phase.When the Zn and Y elements are added into the AM50 alloy in the form of atomic ratio of 6,the Y elements combine with Al and Mn to form Al₆YMn₆ phase and Al₂ Y phase.For the Zn elements,a part dissolves into the ? phase and the other part combines with Mg and Al to form lamellar ?-Mg₂₁?Zn,Al?₁₇ phase around the ? phase,but the quasicrystal I-phase?Mg₃Zn₆Y?cannot form in the microstructures.Thermal analysis results show that the ?-Mg₂₁?Zn,Al?₁₇ phase forms at 354 oC by the peritectic reaction,in which the precipitation temperatures of ?-Mg and ? phase decrease with the increase of Zn and Y content.When the addition amount of Zn and Y elements is 4wt.%,the microstructure is most fine,and the ultimate tensile strength,yield strength and elongation of the AM50-4%?Zn,Y?alloy reach to the maximum values of 206.63 MPa,92.5MPa and 10.04%,which is improved by 26.87%,60.87% and 54.46% respectively compared with those of AM50 alloy.After T4 I heat treatment at 345 oC for 16 h,the ? phase dissolves and the ? phase is retained in the AM50-4%?Zn,Y?alloy.While after T4 II heat treatment at 345 oC for 16 h and 375 oC for 6h,both the ? phase and the ? phase dissolve.During the following aging process at 180 oC for two alloys with different T4 states,the phase composition of the alloys does not change,and a large number of fine precipitates are distributed in the microstructures.The two alloys with different T6 states all reach to the peak-aging at 12 h.The ultimate tensile strength,yield strength and elongation of AM50-4%?Zn,Y?alloy with T6 I and T6 II heat treatment are 221.78 MPa,108.88 MPa,8.1% and 230.63 MPa,126.23 MPa,8.5%,respectively.