Study On Preparation Of High Volume Fraction Spherical Quasicrystal Mg₃Zn₆RE-contained Master Alloy

Magnesium alloys with high strength and high toughness are urgently needed in applications including national defense weapons and equipment,aerospace,automobiles and 3C,which have urgent demanding in energy conservation and weight reduction.Generally,strength and toughness of metallic materials are two contradictory parties.However,nano-reinforced particles can improve the strength of magnesium alloys without reducing their toughness.Mg₃Zn₆Y quasi-crystalline（quasi-crystalline I-phase）particle reinforcement can be formed spontaneously during solidification process.It exhibits high hardness,good thermodynamic stability,low surface energy and good wettability with magnesium alloy matrix.Therefore,nanosized Mg₃Zn₆Y quasi-crystalline I-phase is an ideal reinforcement for the preparation magnesium alloys with high strength and high toughness.It is known that the properties of magnesium alloys are closely related to the morphology,size and volume fraction of the quasi-crystalline particles which are influenced by factors such as alloy composition and preparation process.In order to improve the strength and toughness of magnesium alloys,this paper optimizes the alloy composition and preparation process and prepares master alloy containing high volume fraction spherical quasi-crystalline particles,which is added to the alloy melt to form magnesium alloys and composites enhanced by quasi-crystalline.In this paper,Mg-Zn-Y alloy with Zn/Y（atomic fraction,the same below）=6:1（3≦Y≦10）was prepared by steel mould casting,water-cooled copper mould（Φ2 mm,Φ4 mm,Φ6 mm andΦ8 mm）and melt spinning technique（with rolling speed at 1000 r/min,1500 r/min and2000 r/min）.The effects of alloy composition and preparation process(ultrasonic process,heat treatment,cooling rate（water-cooled copper mold and melt spinning technique and overheating treatment）on the phase constituent,the morphology,size and volume fraction of quasi-crystalline I-phase in the Mg-Zn-Y alloys were studied by using OM,XRD,SEM,EDS,TEM and DSC.Volume fraction of the phases in the alloys was calculated with Image-Pro Plus6.0 software.Results are shown as followings:（1）Alloy composition.The formation reaction of quasi-crystalline I-phase transfers from single peritectic reaction,to the combination of peritectic and eutectic reactions,and to the completely eutectic reaction with the decrease of Zn and Y content in the Mg-Zn-Y alloys with a Zn/Y（at.%）=6:1.When the Y content is and more than 7%,alloys are composed of（Mg,Zn）₅Y,quasi-crystalline I-phase,Mg₂Zn₃ and Mg₇Zn₃ phases and they distribute in a layered form,and the quasi-crystalline I-phase takes the polygonal form by peritectic reaction during solidification.When the Y content is less than 7%,Mg phase precipitate in the alloys and their microstructure consists of（Mg,Zn）₅Y,quasi-crystalline I-phase,Mg₄Zn₇ and Mg phases.When the Y content is between 5%to 6%,the quasi-crystalline I-phase precipitated by peritectic and eutectic reactions,and eutectic reaction is primarily in charge of the precipitation of quasi-crystalline I-phase.When the Y content is and less than 4%,the quasi-crystalline I-phase forms totally a（Mg+I-phase）lamellar eutectic microstructure by eutectic reaction.The V_f of quasi-crystalline I-phase is all greater than 27%,and in Mg₃₀Zn₆₀Y₁₀0 alloy is the maximum（about 77%）among the studied alloys.（2）Ultrasonic treatment.The effect of ultrasonic treatment at different powers on the microstructure of Mg₅₁Zn₄₂Y₇ alloy,especially the volume fraction and size of the quasi-crystalline I-phase were studied.Results demonstrate that increment of ultrasonic power has little effect on the phase constituent of the alloy and the volume fraction of the quasi-crystalline I-phase.But it has a crucial impact on the morphology and size of the quasi-crystalline I-phase.As the ultrasonic power increases from 600 W to 900 W,the size of the quasi-crystalline I-phase is reduced from 319μm（for the as-cast sample）to 258μm to 192μm,correspondingly.The size of quasi-crystalline I-phase in the sample ultrasonic treated at900 W is decreased by 40%compared to that of the unsonicated sample.The morphology of quasi-crystalline I-phase changes from an irregular large block adhered together to a small block with smooth edges and they dispersively distribute in the matrix.While the morphology of the Mg phase varies from coarse dendrites to a granular form.（3）Heat treatment.The effect of solid solution heat treatment on the microstructure of Mg₅₁Zn₄₂Y₇ alloy,especially on the morphology,size and volume fraction of the quasi-crystalline I-phase was investigated in order to round up the morphology of quasi-crystalline I-phase.The results show that the phase constitution is still composed of（Mg,Zn）₅Y,quasi-crystalline I-phase,Mg₂Zn₃ and Mg₇Zn₃ phases after solid solution heat treatment at 400℃and 450℃.With the increase of solid solution temperature,the coarse round shape Mg phase in the as-cast state changes into dendritic shape,and finally completely solid solutioned into the matrix.Thereby,the microstructure of the alloy is composed of（Mg,Zn）₅Y,quasi-crystalline I-phase,Mg₂Zn₃ and Mg₇Zn₃ phases heat treated at 480℃.The quasi-crystalline I-phase demonstrates good thermal stability during heat treatment at high temperature.The morphology of quasi-crystalline I-phase is rounded rather than the sharp edges in the as-cast state.It is considered that 450℃×4 h is the optimum solution heat treatment process by considering the combination of morphology,particle size and volume fraction of quasi-crystalline I-phase.The sample heat treated at the optimum parameters contains 61%（in volume fraction）quasi-crystalline I-phase with an average size of 77μm.（4）Cooling rate.The phase constitution of the sub-rapidly solidified Mg₅₁Zn₄₂Y₇ rod alloy is the same as that of the as-cast alloy,which consists of（Mg,Zn）₅Y,quasi-crystalline I-phase,Mg₇Zn₃ and Mg phases.The morphology of quasi-crystalline I-phase is mainly in dendritic form,some of them are granular and petal-like.It is abnormally developed dendritic crystal when the diameter of the rod alloy is 2 mm.With the increase of cooling rate,the phase constitution in the rapid solidified Mg₅₁Zn₄₂Y₇ strip alloy are composed of quasi-crystalline I-phase and Mg phase.Particle size of quasi-crystalline I-phase can reach nanometer scale and its and volume fraction is up to 80%.The quasi-crystalline I-phase takes near spherical and spherical morphology.（5）Overheating treatment.The morphology of the quasi-crystalline I-phase in the heat-treated Mg₅₁Zn₄₂Y₇ alloy is mainly equiaxed crystals.With overheating power increasing,the morphology of quasi-crystalline I-phase is dendrites at the same position of the rod alloy,and its size tend to coarse.It the center position of the copper mould,the dendritic quasi-crystalline I-phase have a significant melting phenomenon,the size of the quasi-crystalline I-phase becomes smaller and the number is increasing as the increasing overheating power.For the Mg₅₁Zn₄₂Y₇ alloy,the ideal melt overheating treatment power is 21kW,the alloy distribution of the quasi-crystalline I-phase is uniform,the size is small,and the morphology is equiaxed.