Investigations Of Microstructure And Mechanical Behaviors Of Mg-Sn-Zn-Ca Alloys

Magnesium alloys with high specific stiffness and strength have good electric and heat conductive, good damping and electromagnetic shielding capability. Besides, magnesium alloys can be casted, machined and recoveried easily. Therefore, they are very attractive in various applications in military, communication, aerospace industries, and civil fields. However, due to the low absolute strength, poor mechanical properties at high temperature, poor plastic deformation ability at room temperature and easy corrosion, the application of the materials is severely limited. Compared with traditional cast magnesium alloys, wrought magnesium alloys have superior performance in all aspects of strength, dimensional accuracy, ductility and so on. Developing a new type of wrought magnesium alloy and its production process has become an important issue in the field of development of magnesium alloys.Aiming at some aspects mentioned above, research works were focused on the following aspects:(1)Composition design of the Mg-Sn-Zn-Ca alloys and preparation of the alloys by vacuum induction melting; (2)Microstructure evolution of as-cast Mg-Sn-Zn-Ca alloys; (3)Study on the effect of temperature and strain rate on microstructure and characteristics of the stress-strain behavior of the alloys by Gleeble thermal simulation experiment. (4)Examination of mechanical properties, characteristics of microstructure and texture in Mg-Sn-Zn-Ca alloys fabricated by hot extrusion; (5)Effect of solution and aging treatment on microstructure and mechanical properties of the Mg-Sn-Zn-Ca alloys; The primary conclusions were as follows:The as-cast Mg-Sn-Zn-Ca alloys is composed of a-Mg matrix, Mg2Sn phase, CaMgSn phase and a small amount of divorced eutectic (a-Mg+Mg4Zn7). Sn can improve the microstructure of the alloy and make grains equiaxed. Mg2Sn eutectic phase gradually changed from granular in the grains to reticular at the grain boundaries. Ca increases the precipitation of CaMgSn phase which has a higher thermodynamic stability and make it gradually changed from needle-like shape to rod-like shape, inhibiting the formation of Mg2Sn.During hot compression process of the series of cast Mg-5Sn-3Zn alloys, the flow stress of the materials decreased with the increasing of the temperature and increased with the increasing of strain rate, it means that the alloys are positive strain rate sensitive materials. All phases distributed along the direction perpendicular to the compression. At the same temperature, dynamic recrystallization carried out more fully and the grain sizes become more uniform with a decreasing strain rate. The increase of Ca addition plays adversely affect on the plastic deformation ability of the Mg-Sn-Zn-Ca alloys at low temperature. Peak stress of the alloys decreased with the increasing of Ca content under thermo-compression at low temperatures such as 200℃ or 300℃. At high temperature of 400℃, the addition of 0.5 wt.% Ca improve the mechanical properties of the alloys significantly.Compared with the as-cast alloys, the microstructrue of the extruded Mg-Sn-Zn-Ca alloys changed from dendrites and coarse equiaxed grains to fine equiaxed grains and coarse elongated grains, form fibrous structure along the extrusion direction, presented as{0002} fiber texture. With addition of Sn and Ca, intensity of{0002} fiber texture decreases. The average grain sizes of the alloys are 15～20 μm and with the increase of Sn from 3.5 wt.% to 5wt.% the average grain size is decreased 2-4 urn. When the Ca content increases, the average grain size of the alloys decreases firstly and then increases. Compared with the as-cast alloys, the amount and sizes of the second phases decreased, and their shape and distribution changed. Mg2Sn eutectic phase changed from reticular at the boundaries to fine granular, irregular flake or granular in the grains. Divorced eutectic (a-Mg+Mg4Zn7) with a spherical or irregular spherical spotted like internal structure disappears. Needle-like or rod-like shaped CaMgSn changed to chrysanthemum-shaped and broken needle-like or rod-like shaped precipitates. All precipitates are distributed linearly along the extrusion direction. The grain sizes of the alloys increase to 20～25 μm after solution treatment. The original coarse grains are replaced by recrystallized grains and the grain sizes become uniform. After solution treatment, the second phases of the alloys substantially dispersely distributed inside Mg-Sn-Zn-Ca alloys. Compare with the alloys after simple solution treatment, the grain sizes of the alloys after aging are more uniform and the second phases are smaller and dispersed in the alloy.The hardness of the extruded alloys are about 53-59 HV0.1, tensile strength are about 264～279 MPa and the elongation at room temperature are about 15.2%-21.9%. With addition of Ca, the tensile strength of the alloy increased first and then decreased with a higher Ca content, and the elongation reduced gradually with the increasing Ca concentration. Solution and aging treatment can significantly enhance the hardness of Mg-Sn-Zn alloys. However, aging treatment decrease the hardness of Mg-Sn-Zn-Ca alloys. There is no significant improvement in tensile strength of the extruded alloys after solution and aging treatment, and the elongation decreased.