Research Of Metal Alloying And Deformation Treatment Of Mg-Sn Magnesium Alloys

The application of Magnesium alloys is dramatically limited by the low mechanical properties, few alloys systems and the high cost of Rare Earth Magnesium alloys. Author wants to find new alloying elements and supply a new strengthening design method of magnesium alloys.After2000, there has been a renewed global interest in Mg-Sn alloys which are believed to have potential for applications at elevated temperatures. Solubility of Sn in α-Mg alloys drops sharply that can provide powerful aging hardening strength. The eutectic Mg2Sn phase has a melting temperature of771.5℃, which is higher than that of Mg17l12(462℃) in Mg-Al alloys and MgZn (347℃) in Mg-Zn alloys. Overall study of the Mg-Sn alloys has just begun, a lot of work needs to continue in depth.This thesis presents a study of microstructure and tensile properties of as-cast Mg-xSn-yZn-1.0Al (wt.%) alloys, which shows that the Mg-5.0Sn-1.0Zn-1.0Al alloy offers good comprehensive properties. Based on the results, we study the microstructure and tensile properties of the as-cast Mg-5.0Sn-1.0Zn-1.0Al alloy with different Ca and Gd addition. Microstructure and properties of as-extruded Mg-5.0Sn-xZn-1.0Al alloy is also studied. The primary conclusions were listed as follows:(1) The as-cast Mg-Sn-Zn-Al alloy is composed of a-Mg matrix, Mg2Sn phase and Mg32(Al,Zn)49phase. Zn can refine the microstructure of the alloy. Zn can significantly improve the tensile strength of Mg-Sn-Zn-Al alloy. But the elongation will be significantly reduced when the Zn content reaches a certain value. Sn can improve the microstructure of the alloy and make it equiaxed. Mg2Sn eutectic phase gradually changed from granular in the grains to reticular at the boundaries. The Mg2Sn eutectic phase which distributed along the grain boundaries play adversely affect of the mechanical properties of the Mg-Sn-Zn-Al alloy.(2) Under our experimental condition, the Mg-5.0Sn-1.0Zn-1.0Al alloy showes the best mechanical properties, which yield strength and tensile strength provide11OMPa and182MPa, respectively and the elongation gets to9.7%.(3) The as-cast Mg-5.0Sn-1.0Zn-1.0Al-xCa alloys are composed of a-Mg matrix, Mg2Sn phase, Mg32(Al,Zn)49phase and CaMgSn phase. Ca plays a role of solid solution strengthening and refinement strengthening in the alloys. Ca can enhance the strength of the alloys. CaMgSn phase has a needle-like or rod-like shape. A little Ca forms the needle-like CaMgSn phase. Too much Ca forms the rod-like CaMgSn phase. The needle-like CaMgSn phase will lower the elongation of the alloys. The rod-like CaMgSn phase will lower the strength and elongation of the alloys. The as-cast Mg-5.0Sn-1.0Zn-1.0A1-0.2Ca alloy showes the best mechanical property in three compositions of Gd-containing alloys, with the tensile strength of184MPa and the elongation of12.7%.(4) The as-cast Mg-5.0Sn-1.0Zn-1.0Al-xGd alloys are composed of a-Mg matrix, Mg2Sn phase, Mg32(Al,Zn)49phase and Mg-Sn-Gd ternary Rare Earth phase. Gd plays a role of solid solution strengthening and refinement strengthening in the alloys. Gd can enhance the strength of the alloy. Mg-Sn-Gd phase has a shape of irregular sheet form at grain boundaries. With Gd addtion, the granular Mg2Sn eutectic phase reduces in the grain and the strip Mg2Sn eutectic phase at grain boundaries. The as-cast Mg-5.0Sn-1.0Zn-1.0Al-0.2Gd alloy showes the best mechanical property in three compositions of Gd-containing alloys, with the tensile strength of193MPa and the elongation of14.3%.(5) Mg-Sn-Zn-Al alloy has good extrusion properties. The extruded alloys form many dynamic recrystallized regions after extrusion at310℃. The dynamically recrystallized grains have diameters of2～8um. Many Mg2Sn nanoparticles with a diameter of100～300nm are found at grain boundaries, and also a small amount of nanoparticles found in the grains. Mg2Sn nanoparticles have better precipitation hardening effect with more Zn addition. With different Zn addition, the as-extruded Mg-Sn-Zn-Al alloys have a different fracture behavior by mixed fracture and then quasi-cleavage fracture and brittle cleavage fracture.