The Effect Of Y And Gd On The Microstructure And Properties Of Mg-Li Alloy

As the lightest metallic structural materials in the engineering applications, coMPared with other metallic materials, Mg-Li alloy has many superior properties, such as low density, high specific stiffness and good casting performance, mechanical processing performance and easy recycling, etc. Mg-Li alloy is widely applied in the fields of defense, aerospace, automotive and electronics industry, which makes it garnered great interests and extensive investigation. However, the poor strength property limits the application and the development of Mg-Li alloy. Research shows that it is very effective to improve integrated mechanical properties of Mg-Li alloy by adding rare earth elements, it is also a traditional and common way to solve this problem. The research of this graduation thesis has achieved some valuable consequences as follow:With the protection of surface covering agents and filling argon, the Mg-5Li-xGd (x = 2, 4,6), Mg-5Li-xY (x= 1,2,3,4) and Mg-5Li-xY-yGd (x= 1,2,3; y= 4,6,8) alloys are cast by conventional casting methods, respectively. The samples are prepared by homogenization treatment, hot extrude treatment and annealing process, respectively. We coMPared the influence on the micro structure, phases tructure, mechanical properties of the alloy by adding different content of Y and Gd. In addition, the strengthening mechanism of Y and Gd on the Mg-5Li alloy was also discussed. Furthermore, the effects and mechanism of extrusion temperature on the micro structure and mechanical properties are investigated.The as-cast Mg-5Li alloy is composed of a-Mg substrate and a small content of β-Li phase. The RE compound Mg5Gd generates and the grains size decrease after adding Gd in the Mg-5Li alloy. The dynamic recrystallization arises during hot extrusion process, leading to the formation of the equiaxed fine grains and refining grains. The Mg5Gd phase was crashed and distributed along extrusion direction after hot extrusion treatment. The strength property of as-extruded sample improves with the increase of Gd content, and the Mg-5Li-6Gd achieves the highest tensile strength,325MPa.The RE compound Mg24Y5 generates and the size of grains decreases after adding Y into the Mg-5Li alloy, and the majority parts of Mg24Y5 phase distribute on grain boundaries. The dynamic recrystallization arises during hot extrusion process, and the dispersive distribution of Mg24Y5 phase hindered the growth of grains in the process of dynamic recrystallization. When the content of Y is 3%, the grain refining effect of alloy reaches its best, and the average grain size reduced to 3 μm. The Mg-5Li-3Y alloy achieves the best mechanical property, the tensile strength and elongation reach to 230MPa and 16%, respectively.The RE compound Mgs(Gd, Y) and Mg24(Y, Gd)5 generates and the effect of refining grain is stronger after adding both Y and Gd. When the contents of Y and Gd reached to 3% and 6%, respectively, the organization of as-cast alloy is consisted of the equiaxed fine grains, and the grains size are becoming continuously smaller with the increase of the Y and Gd content. The as-cast Mg-5Li-4Y-8Gd alloy exhibits the smallest grain size, about 20μm, and the tensile strength and elongation reach to 160MPa and 7.3%, respectively. The dynamic recrystallization arises during hot extrusion, leading to grain refinement, and the maximum tensile strength of the as-extruded alloy is gained with the adding content of Y and Gd are to 3% and 6%, respectively.The grain size of Mg-5Li-3Y-6Gd alloy is significantly refined by reducing the extruding temperature, when the extrusion temperature descends from 300℃ to 250℃, the average size of grains decreases from 10μm to 5μm. The work hardening is becoming more serious with the decrease of extruding temperature, which leads to the increase of hardness and decrease of ductility. Through comprehensive analysis, the Mg-5Li-3Y-6Gd alloy extruded under the temperature of 280℃ achieves excellent mechanical properties. Its tensile strength and elongation reach to 230MPa and 16%, respectively.