| 其他摘要 | Magnesium alloys are particularly attractive for the aerospace and automotive industries due to their low densities, high strength to weight ratio and good machinability. However, magnesium alloys have not been widely used as a main structural material because of their low tensile strength, low deformation ability and bad corrosion resistance. Therefore, it is necessary to design a new type of magnesium alloy which exhibits good properties. For developing a new magnesium alloy, the composition design is the precondition and foundation for the subsequent processing to improve properties. The present paper is to study the microstructures and mechanical properties of a new series of Mg-Sn-Zn(Al) magnesium alloys as well as the strengthening mechanism for the alloys.
The microstructure of as-cast Mg-7Sn alloy consists of -Mg primary, Mg+Mg2Sn eutectic, divorced eutectic Mg2Sn and secondary precipitation Mg2Sn. The solidification sequence of Mg-7Sn alloy is as follows: L L1 + -Mg -Mg + Divorced eutectic (Mg2Sn) + Eutectic (Mg+Mg2Sn) -Mg + Divorced eutectic (Mg2Sn) + Eutectic (Mg+Mg2Sn) + Secondary precipitation (Mg2Sn). It is observed there exists dendritic segregation of Sn in this alloy.
The microstructures and tensile properties of as-cast Mg-xSn-5Zn alloys are investigated. The solidified microstructure of Mg-5Sn-5Zn (TZ55) alloy is composed of -Mg primary, Mg+MgZn eutectic, divorced eutectic Mg2Sn and secondary precipitation Mg2Sn. Some divorced eutectic Mg2Sn phase adheres to the eutectic MgZn phase, and some distributes alone. Secondary precipitation Mg2Sn appears within the -Mg grains or adhering to the eutectic MgZn and divorced Mg2Sn. The tensile strength, yield strength and the elongation of the TZ55 alloy are 236.5MPa, 90.5MPa, 12.7% at room temperature and 141.3MPa, 75.8MPa, 39.2% at 150℃, respectively, which indicates that TZ55 alloy has high ductility.
The microstructures and tensile properties of as-cast Mg-Sn-Al alloys are investigated as well. The solidified microstructure of Mg-4Sn-5Al (TA45) alloy consists of -Mg primary, Mg+β-Mg17Al12 eutectic, divorced eutectic Mg2Sn and secondary precipitation Mg2Sn. Some eutectic Mg2Sn adheres to Mg+β-Mg17Al12 eutectic or divorced eutectic β-Mg17Al12, and some distributes alone. Secondary precipitation Mg2Sn appears within grains or adheres to β-Mg17Al12 and divorced Mg2Sn. The tensile strength, yield strength and the elongation of TA45 alloy at room temperature and 150℃ are 242.5MPa, 102.4MPa, 14.4% and 167.0MPa, 87.0MPa, 22.0%, respectively.
The solution treatment and solution-aging treatment are carried out for TZ55 and TA45 alloys. After solution treatment, all the MgZn (or -Mg17Al12 ) particles and the majority of Mg2Sn particles are dissolved into the matrix, and the solute distribution within the matrix is homogeneous. The tensile strength and the elongation of TZ55 and TA45 alloys are improved obviously to 270.1MPa and 20.0%, respectively, for TZ55, and 260.1MPa and 17.0%, respectively, for TA45 alloy. However, the yield strength of the two alloys is decreased. After aging, the yield strength and the tensile strength of TZ55 alloy increase to 183.3MPa and 291.6MPa, respectively, while the elongation decreases to 10.5%. The yield strength and the tensile strength of the aged TA45 alloy increase to 163.7MPa and 282.6MPa, respectively, while the elongation decreases to 8.2%.
The microstructures and tensile properties of as-extruded TZ55 and TA45 alloys as well as the two aged alloys are investigated. The results show that the tensile properties of TZ55 and TA45 alloys are significantly improved due to the extrusion texture and fine microstructures. The tensile strength, yield strength and the elongation of the extruded TZ55 alloy reach to 294.2MPa, 188.2MPa and 20.0%, respectively, and that of the extruded TA45 alloy are 283.9MPa, 187.7MPa and 20.0%, respectively. After aging, many fine precipitates distributes within the matrix and along the grain boundaries in the extruded TZ55 and TA45 alloys and these fine precipitates can strengthen the alloys by blocking dislocation slipping. The tensile strength, yield strength and the elongation of the extruded and aged TZ55 alloy reach to 329.3MPa, 294.7MPa, 8.8%, respectively, and 288.9MPa, 195.8MPa, 16.0%, respectively, for TA45 alloy.
The addition of Sr can refine the grains of TZ55, TA45, as well as AZ91D, AZ80, and AS31 alloys, obviously. The grain refinement mechanism is attributed to the enrichment of Sr at the solidification interface front which restricts the growth of α-Mg. The tensile properties of these alloys are improved by Sr addition. |
修改评论