Effects Of Sr,Sn On The Microstructure And Properties Of Mg-2.0%Si Magnesium Alloy

As the lightest metal structure materials, magnesium alloys take some important advantages of low density, good casting formability, good thermal conductivity and electrical conductivity, vibration noise, excellent electromagnetic shielding ability and easy to recycle. However, the absolute strength of magnesium alloy is lower, this has had limited the production and application of magnesium alloys. Though scholars have developed some new magnesium alloys with excellent properties by adding rare earth and rare metal element, due to the high cost, these alloys can not be applied in daily life. Thus researching and designing some casting magnesium alloys with high strengthe has been having very important practical meanings.Based on the research of predecessors, the microstructures and properties of Mg-Si alloys were investigated in this paper, through the alloying process to refine the microstructures and change the configuration, distribution and quantity of the strengthening phases in the alloys, then to achieve the purpose of improving performance of magnesium alloy. In this paper, the Sr and Sn elements were added into Mg-2.0%Si alloys then, by using metallographic analysis（OM）, X-ray diffraction（XRD）, scanning electron microscopy（SEM）, energy spectrum analysis（EDS）, and the universal tensile testing machine et al to study the as-cast microstructure and mechanical properties of different alloying magnesium alloy.The results are mainly concluded as below:（1） The microstructure of Mg-2.0%Si alloy consists of α-Mg matrix and Mg₂ Si phase,and Mg-2.0%Si-x Sr alloys are also composed of α-Mg matrix, the primary Mg₂ Si phase,eutectic Mg₂ Si phase. When adding 0.04% and 0.08% Sr, the size of primary block Mg₂ Si decreases, the edges and corners of ones passivate and the number increases. What’s more,the size of dendritic α-Mg decreases and tends to be homogeneous, which reduces the mixed crystal phenomenon, while the microstructure of eutectic Mg₂ Si does not change,but remains lamellar. When the Sr content increases to 0.16%, the size of primary Mg₂ Si is the minimum and the shape changes toward round and the distribution in the α-Mg matrix becomes uniformly.Meanwhile, the microstructure of eutectic Mg₂ Si is no longer lamellar,but becames into irregular short rod in the grain boundary.Although the adding of Sr element has the effect of refining microstructure,the tensile strength of Mg-2.0%Si-x%Sr alloys does not have any obvious enhancement. The different is that, the elongation of Mg-2.0%Si-x%Sr alloys has been improved significantly, among these alloys, theMg-2.0%Si-0.16%Sr alloy is up to 179%.（2） Adding Sn element to Mg-2.0%Si alloy can effectively reduce the size of primary Mg₂ Si phase and change the microstructure of eutectic Mg₂ Si from lamellar to Chinese script. In the Mg-2.0%Si-x%Sn alloys, the overall dimensions of Chinese script eutectic Mg₂ Si reduces with the increasing of Sn content, at the same time, the bodyies of Chineses scripts gradually decompose into rod and granular. The Mg₂ Sn phase exists around eutectic Mg₂ Si in a granular shape with increasing of Sn content is less than 3.5%; however, the Mg₂ Sn phase changes into irregular and thick when the Sn content is up to 4.5%. The tensile strength and elongation of alloy increase with the increasing of Sn content and reache the maximums at 3.5% Sn content, which is 148.5 MPa and 11.85% respectively.After solid solution treatment of Mg-2.0%Si-4.5%Sn, the coarse Mg₂ Sn has disappear. The eutectic Mg₂ Si changes from Chinese script to small granule. Compared to 160℃/4h and240℃/4h aging treatment, after 160℃ /8h and 240℃/8h aging treatment, the edges of primary Mg₂ Si passivate.（3） The microstructures of Mg-2.0%Si-3.5%Sn-x%Sr alloys mainly consist of α-Mg matrix, Mg₂ Si phase, Mg₂ Sn phase and ternary alloy MgSnSr phase. When adding Sr to Mg-2.0%Si-3.5%Sr alloy, the microstructure of eutectic Mg₂ Si changes from Chinese scripts into slender rods and small blocks, what’s more, the size of primary Mg₂ Si decreases and its quantity decreases with the increase of Sr element.When the Sr content is less than 0.5%, MgSnSr phase mainly exists in tiny acicular structures and acrosses the grain boundaries, however, when the Sr content reaches 1.0%, the thick needle MgSnSr begins to form.With further increase of Sr element,the quantity of needle MgSnSr increased, so does the thickness of needle MgSnSr phase.