Effect Of Sb,Ce,Nd Composite Modification On The Microstructure And Properties Of Mg-4Si Alloy

Magnesium-silicon heat-resistant alloys had excellent creep resistance because Mg₂Si phase was introduced at the grain boundaries.Meanwhile,Mg₂Si phase had some adantages such as low density（1.99×10³ Kg/m3）,high elastic modulus（120GPa）,and high melting point（1085℃）,high hardness（4500 MN/m²）.However,Mg₂Si solidified under ordinary gravity conditions appeard as coarse dendrites and Chinese characters.When it came into contact with the substrate,stress concentration will occur,which will cause the formation of internal cracks and reduce the mechanical properties of the alloy.Therefore,improving the morphology of Mg₂Si was an effective approach to improve alloy mechanical properties.In recent years,many domestic and foreign scholars had developed some methods to improve the morphology of Mg₂Si,including rapid solidification technology,centrifugal casting technology,hot extrusion technology,directional solidification,solid-phase reaction synthesis and alloying,etc.However,these methods were difficult to be operated,and the process was complicated to control.The most important thing was that the morphological improvement of Mg₂Si was not obvious,and the process of these methods needed to be further improved.Modification treatment with simple operation,good effect and low cost was carried out during the smelting process,so it had become the main method for improving the morphology of Mg₂Si at home and abroad.Researchers had carried out a lot of research on the modification of Mg₂Si.It was generally believed that rare earth elements（RE）could effectively improve the morphology of Mg₂Si,which could refine coarse dendrites into regular polygons,and improve the mechanical properties of alloys.However,most scholars were studying the effect of single element on Mg₂Si phase,and there were few studies on the composite modification of two elements.Therefore,it was of great significance to further study the effect of composite modification of two elements on Mg₂Si phase.This article mainly studied the effect of Nd and Sb composite modification on the Mg₂Si phase in Mg-4Si alloy,and explored the mechanism of Nd and Sb composite modification and the changes in alloy mechanical properties.It also studied the Ce and Sb composite modification treatment the Mg₂Si phase in Mg-4Si alloy,and the mechanism of Ce and Sb composite modification and the changes in the mechanical properties of the alloy were explored.Then the effect of the solution treatment on the Mg₂Si phase in the alloy after the composite modification was studied,and the optimal solution temperature and time were found.Finally,the effect of extrusion treatment on the Mg₂Si phase in the 4.0wt.%Si AZ91 alloy was studied,and the changes in mechanical properties before and after extrusion were studied;the purpose of these studies was to find a way to improve the morphology of Mg₂Si with the simplest and most effective effect.The research conclusions of this article are as follows:（1）The optimal addition amount of Ce and Sb composite modification was1.0wt.%Sb and 1.5wt.%Ce,which could effectively improve the morphology of the primary Mg₂Si phase in Mg-4Si alloy.The average size of the primary Mg₂Si was reduced from 81.5μm to 7.3μm.The hardness was increased from 64.3 HB to 93.2HB,the tensile strength was increased from 103.5 MPa to 167.8 MPa,and the mechanism of Ce and Sb composite modification was that the Mg₃Sb₂ will be formed when Sb was added to Mg-4Si alloy,which could be used as the heterogeneous nucleation core of Mg₂Si.And the addition of Ce was enriched on the surface of the primary Mg₂Si particles,which prevents the primary Mg₂Si particles from growing.And finally the primary Mg₂Si became a regular octahedron.（2）The optimal addition amount of Nd and Sb composite modification is 1.0 wt.%Sb and 1.0 wt.%Nd,which could effectively improve the morphology of primary Mg₂Si from coarse dendrites to regular multi-deformation in Mg-4Si alloy.The average size of primary Mg₂Si particles decreased from 78.3μm to 6.5μm.The Brinell hardness increased from 65.74 HB to 87.68 HB.The tensile strength increased from 112.3 MPa to 175.8 MPa,and the elongation increased from 2.23%to 4.61%.The addition of 1.0 wt.%Sb and 1.0 wt.%Nd to Mg-4Si alloy can form the Nd₄Sb₃phase which can act as the nucleation core of the Mg₂Si phase,and the nucleation rate of the Mg₂Si phase increases.Thereby the morphology of Mg₂Si was improved with the increasing nucleation rate,and the mechanical properties of the alloy were improved.（3）The solution treatment can effectively improve the sharp edges of the primary Mg₂Si phase in the alloy after Ce and Sb composite modification,and it can make the Chinese eutectic Mg₂Si transforme into a point shape.The mechanical properties of the alloy after solution treatment had been improved,mainly due to the change of the Mg₂Si phase in the alloy.The solution treatment at 500°C for 24 hours will cause overheating,which will damage the alloy structure and reduce the mechanical properties of the alloy.（4）Extrusion treatment cannot break the primary Mg₂Si particles in 4.0wt.%Si-AZ91 alloy which was modificated with 1.0wt.%Sb and 1.0 wt.%Nd.The size difference was not large before and after extrusion,indicating that the extrusion treatment has a small effect on the primary Mg₂Si particles.This was mainly due to the high elastic modulus and high hardness of the Mg₂Si phase.At the same time,the size of the primary Mg₂Si particles after composite modification was small,and it appeared as a regular octahedron in the alloy.The interface with the matrix was strong,and it was not easy to break during extrusion.The second phase Mg₁₇Al₁₂ precipitated during the extrusion treatment of the alloy restricts the growth of the matrix grains,made the matrix grains fine,and improved the plasticity and toughness of the alloy.