| Rare earth magnesium al oy has a great prospect in the application of structural parts due to their light weight,high specific strength and stiffness and good machinability.Second phase reinforcement is the most direct and effective way to improve the strength of magnesium al oys.Thereout,the Mg-Zn-Y based alloy with long period effective strengthening phase has attracted more and more researchers attention.At present,a lot of researches have been carried out on the preparation technology and alloy composition of Mg-Zn-Y al oy at home and abroad,however,there are few studies on Mg-Zn-Y-Al al oy and its heat treatment strengthening.As is well-known,heat treatment is an effective way to improve the performance of alloy,the solubility of Zn,Y and Al in Mg varies greatly under the influence of temperature.At the same time,al oying can change the reinforcing body type,morphology,distribution.Therefore,improving the microstructure and mechanical properties of Mg-Zn-Y-Al by al oying and optimizing the heat treatment process is of great significance for expanding the production and application of rare-earth magnesium al oy.In order to further develop the Mg-Zn-Y based alloy,optimize the al oy composition and heat treatment regimes,in this paper,the equilibrium microstructure of the solid solution alloy was analyzed by phase diagram calculation and experiment.Based on Thermo-calc,this work has been calculated the binary marginal phase diagram and ternary isothermal section of Mg-Zn-Y based alloy,thereby predicted possible phases and their solidification processes in al oys.The microstructure evolution of Mg-2Zn-3Y-xAl al oy with various solid solution and aging treatment regimes was analyzed by metallographic microscope,scanning electron microscope,energy spectrum analysis and XRD diffraction.The experimental results shows that the abundant second phases of Mg-2Zn-3Y-xAl al oys are largely dissolved in the matrix after solution treatment,the reticular W phase is spheroidized coarsed,and the part of skeletal LPSO phase fuse into rods.As solution temperature increases,the grain gradually coarsened and grew.The microstructure was basically saturated when the temperature reaches 450??470?,and over-burned at 490?,moreover,the more Al element was added,the less severe of the over-burn.After aging treatment,the original second phase was re-precipitated from the supersaturated matrix,and the amount of precipitates increase with aging time and temperature,which plays a role of precipitation strengthening.When reach 250?,the second phase begin to coarsen.Among the al oys with different Al content,Mg-2Zn-3Y-1Al exhibits the best solutio n mechanical properties.However,in the Mg-2Zn-3Y-2Al al oy with more Al content,the low-melting eutectic phase almost disappear,the Al2Y bulk phase agglomerated,resulting in poor mechanical properties.The effects of solid solution and aging treatment on tensile properties and microhard ne ss of Mg-2Zn-3Y-xAl al oy were tested by universal tensile testing machine and vickers microhardness tester.The influnce of the type,distribution,size,morphology,and quantity of the second phase on mechanical properties during the solution and aging processes was mainly analyzed from the perspective of microstructure evolution.The results indicates that the tensile properties and microhardness of Mg-2Zn-3Y-xAl al oys can be effectively improved through solid solution and aging treatment.With the increase of aging temperature and time,the comprehensive mechanical properties exhibite a trend of increasing first and then decreasing.The main strengthening factors include solid solution strengthening,second phase reinforcement,dispersion strengthening and precipitation strengthening.Moreover,it is concluded that the optimal solid solution treatment process for Mg-2Zn-3Y-1Al al oys is solid solution at 450?for 12 hours followed by hot water quenching,and the optimal aging process is aging at 215?for 20 hours and then air-cooled under the previuos conditions. |
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