Effect Of In-situ Endogenous Mg₂Si Phase On The Microstructure And Properties Of Mg-Zn Magnesium Alloy

Magnesium alloy has the advantages of low density,high specific strength/stiffness,excellent damping and shock absorption performance and electromagnetic shielding ability,and has good application prospects in military,aerospace,automotive,3C electronic products and other fields.However,magnesium alloy itself has low modulus of elasticity,low absolute strength and poor plasticity,which restricts the further promotion and application of magnesium alloy.The research and development of magnesium alloys with high modulus to improve their elastic deformation resistance and improve their toughness are of great significance for expanding the application range of magnesium alloys.Traditionally studied high-modulus magnesium alloys are usually external reinforcement particles,but due to the huge difference in physical and chemical properties between the reinforcement and the Mg matrix,the interface between the two is poorly bonded,which is easy to generate holes at the interface and reduce the elongation of the material.Alloying can avoid this defect,but the cost of commonly studied high-rare earth magnesium alloys is high and difficult to achieve commercialization,and this paper aims to develop low-cost high-strength magnesium alloys.Therefore,Si elements were added to Mg-Zn alloys to form Mg₂Si phases with high strength and high modulus.However,the Mg₂Si phase in the Mg-Zn-Si alloy prepared by conventional casting process mostly presents a coarse dendrite and a Chinese character morphology,in which the edge of the Chinese character phase is sharp,easy to cut the matrix,and cracks germinate inside the alloy,thereby reducing the mechanical properties of the alloy.Deterioration treatment is to add a metamorphic agent in the melting process,change the nucleation and growth of Mg₂Si through the metamorphic agent,thereby improving the morphology of Mg₂Si,simple operation,good effect,low cost,has become the main method for improving the morphology of Mg₂Si at home and abroad.Therefore,improving the morphology of Mg₂Si generated during the casting process is an effective way to improve the mechanical properties of alloysIn this paper,the method of in-situ endogenous Mg₂Si phase is used to introduce Mg₂Si reinforced phase into the alloy to try to obtain magnesium alloy with high strength and high modulus.By adding Sb metamorphism during melting,the nucleation and growth of the Mg₂Si phase are changed to improve its size and distribution,and then improve the properties of the alloy.In the experiment,the effects of different Si contents on the morphology and properties of Mg₂Si were first explored.Then,for the coarse Mg₂Si phase in magnesium alloy with high Si content,Sb element was introduced as metamorphic agent,the influence of Sb element on the microstructure and properties of Mg-8Zn-1Al-0.5Cu-x Si alloy was discussed,and the mechanism of Sb element metamorphosis and its influence mechanism on the mechanical properties and damping properties of solid solution and extruded alloys were analyzed.The main conclusions are as follows:（1）As-Cast Mg-8Zn-1Al-0.5Cu-x Si alloy is composed ofα-Mg phase,Mg Zn phase,Mg Zn Cu phase and Mg₂Si phase.With the addition of Si content,lumpy Mg₂Si phases appear at grain boundaries and gradually coarse.When the Si content reaches 1.5 wt.%,a primary Mg₂Si phase with a size of about 200μm begins to appear.The yield strength of the alloy continues to increase and the elongation decreases.When the Si content is 2.5wt.%,the coarse primary Mg₂Si phase leads to brittle fracture of the alloy.（2）After extrusion,the grain of Mg-8Zn-1Al-0.5Cu-x Si alloy is obviously refined and fine.Compared with the casting state,the alloy mechanics is significantly improved,among which Mg-8Zn-1Al-0.5Cu-2.0Si alloy has the best comprehensive properties,the ultimate tensile strength is 284.8MPa,the yield strength is 198.8MPa,the elongation is 5.81%,and the elastic modulus can reach 45.4GPa,which is 17.9%higher than that without Si.（3）After the introduction of Sb element into Mg-8Zn-1Al-0.5Cu-x Si alloy,the primary Mg2Si phase changed from coarse dendrite to fine regular polygon,and the average size was reduced to about 20μm.After solution treatment,the edge passivation of the primary Mg₂Si phase becomes rounded,and part of it is fractured into punctate particles by Chinese characters,which achieves the purpose of refining the second phase,and the mechanical properties of the alloy are improved after the introduction of Sb element,and the elongation of the alloy is the best.（4）After the introduction of Sb element into Mg-8Zn-1Al-0.5Cu-x Si alloy,the primary Mg₂Si phase changed from coarse dendrendrites or blocks to fine polygons,and the average size was reduced to about 20μm.After solution treatment,the edge passivation of the primary Mg₂Si phase becomes rounded,and part of it is fractured into punctate particles by Chinese characters,which achieves the purpose of refining the second phase,and the mechanical properties of the alloy are improved after the introduction of Sb element,and the elongation of the alloy is the best.After heat treatment of deteriorated extruded alloy T6,the tensile strength of the alloy increased by about 30MPa and the yield strength increased by about20MPa.