| In recent years, the magnesium alloys with LPSO(long period stacking ordered, LPSO) phase has attracted more and more attention because of its excellent mechanical properties. High-property magnesium alloys with LPSO phase could be made through a series of methods including alloying, heat treatment, deformation and aging strengthening. As for the present study rare earth element is essential in the formation of LPSO, but costly rare earth element will increase the cost of this magnesium alloys and limits its application. Therefore, it is an important question to find the elements that can partly replace the rare earth elements. Based on Mg100-2x-Yx-Znx(x=2, 3, 2.5, 3.5) alloys, valuable exploration has been made on this aspect in the paper.Mg-Y-Zn, Mg-Y-Zn-Ti, Mg-Y-Zn-Zr and Mg-Y-Zn-Ti-Zr alloys were prepared by conventional casting method in this experiment. Firstly, effect of Y、Zn contents on the microstructure and mechanical properties of Mg-Y-Zn alloys was studied on condition that the atomic ratio of Y:Zn is 1. Then the effect of three most important parameters(temperature, time and cooling rate) on the microstructure was explored. Secondly, effect of single and composite additions of Ti and Zr on the microstructure and mechanical properties of Mg-Y-Zn alloy was investigated. Finally, the relationship between microstructure and mechanical properties of the different original microstructure after extrusion was discussed thrugh comparative study of the effect of direct extrusion on microstructure and mechanical properties of Mg95Y2.5Zn2.5 alloy and Mg93.1Y2.5Zn2.5Ti1.6Zr0.3 alloy. The results of the study are as follows:(1) The microstructure of Mg-Y-Zn alloy was significantly affected by the content of Y and Zn. With the increase of the content of Y and Zn, the amount of second phase in the alloys increases, and the grain size of α-Mg decreases. LPSO phase was thick and presented a rod-shaped morphology. The overall size of W phase became larger, but the microstructure still liked fishbone.(2) The microstructures of Mg100-2xYxZnx(x=2, 2.5, 3, 3.5) alloys are composed of α-Mg, 18R-LPSO phase and fishbone-like W phase under conventional casting condition. Mg95Y2.5Zn2.5 alloy exhibited the best comprehensive mechanical properties(3) The solid solution treatment of 530 ℃ for 10 h or 500℃ for 40 h was favorable for the dissolution of 18 R and the spheroidization of W phase, and the slow cooling rate is conducive to the precipitation of 14 H. The spherical Wˊ phase(Mg YZn2) has a trend of coarsening with the extension of time.(4) After the addition of Ti and Zr in Mg95Y2.5Zn2.5 alloy, the average size of α-Mg grains was decreased from 25 μm to 15 μm and 20 μm, respectively. The average size of α-Mg grains was reduced from 25 μm to 13 μm after compound addition of Ti and Zr.(5) More importantly, Ti and Zr promoted the formation of LPSO phase and inhibited the formation of W phase in Mg95Y2.5Zn2.5 alloy. By compound addition of Ti and Zr, the Mg93.1Y2.5Zn2.5Ti1.6Zr0.3 alloy with finer α-Mg grains and larger area fraction of LPSO phase was obtained.(6) Three different original microstructure of Mg95Y2.5Zn2.5 alloy(as-cast, solid solution treated with water cooling and furnace cooling) were extruded. It was found that the solution treated alloy obtained the good elongation(17% ~22%), but the ultimate tensile strength was lower(320~330 MPa); as-cast alloy possessed higher ultimate tensile strength and slightly lower elongation(345 mpa, 16%).(7) Mechanical properties of as-cast Mg93.1Y2.5Zn2.5Ti1.6Zr0.3 alloy was improved from 112 MPa(ultimate tensile strength, UTS) and 3.5%(elongation) to 196 MPa and 7.5% respectively after solid solution treatment. Three different original microstructure of alloy Mg93.1Y2.5Zn2.5Ti1.6Zr0.3(as-cast, solid solution treated with water cooling and furnace cooling) was extruded. It was found that the alloy treated by solid solution with water cooling obtained optimal comprehensive mechanical properties(UTS=375 MPa, elongation= 9.5%).(8) The microstructure of the alloys showed fibrous morphology after direct extrusion, and basal plane textures of magnesium matrix was strengthened. Fishbone W phase was broken into small particles, which has the dispersion strengthening effect. LPSO phase occured distortion, which can improve strength and plasticity of the alloy. Dynamic recrystallization grains enhanced ductility of the alloy. The improvement of mechanical properties of the alloy after extrusion is the result of the coordination of the above factors. |
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