The Effect Of B On The Microstructure And Property Of AM50,AM60 And AZ91 Magnesium Alloys

In twenty-first century,due to the over exploitation and utilization of resources such as oil and coal,people pay more and more attention to energy saving and emission reduction and lightweight structural materials are attracting more and more attention.Magnesium alloy is the lightest structural metal material with good damping properties,electromagnetic shielding,and high specific strength and stiffness.It shows great potential to be used in aerospace,3C（Computer,Communication,Consumer electronics）products,automotive and other fields.However,the comprehensive properties and corrosion properties of Mg alloys needs to be further improved.Since grain refinement can enhance the strength and plasticity simultaneously,it is of great importance to develop an effective and low-cost grain refiner to further improve the comprehensive properties of magnesium alloys.This thesis studied AM50,AM60 and AZ91 magnesium alloy with methods,such as the X-ray diffraction（XRD）,optical microscopy（OM）,mechanical properties test,scanning electron microscope（SEM）with energy dispersive spectroscope（EDS）analysis and Electron probe micro-analyzer（EPMA）.The Boron（B）was added to AM50,AM60and AZ91 magnesium alloys,to investigate the microstructures and mechanical properties under two or three heat treatment（as cast,as solution and ageing）.The results of this study are listed as following:（1）Adding 0-0.15wt.%B to AM50,AM60 and AZ91 magnesium alloys,the grain size of the cast alloy has been obviously reduced,and the grain size of the three alloys adding 0.15wt.%B is the smallest,which is reduced by 82%,50%,44%respectively.B is an effective grain refiner for Mg-Al alloy.（2）Adding Al-3B master alloy,the AlB₂ particle will react in the magnesium melt to produce MgB₂,which as a heterotype nucleus,significantly refines the grain size of AM50,AM60 and AZ91 magnesium alloys.The mismatch between MgB₂ and Mg is only 3.86%,and the formation enthalpy of MgB₂ is lower than AlB₂.Because of the lowest enthalpy of formation of Mn-B compounds,MgB₂ is mainly associated with Al₄Mn phase in AM system,but only MgB₂ compounds can be formed in the melting condition.（3）Adding 0-0.15wt.%B to AM50,AM60 and AZ91 magnesium alloys,the yield strength,tensile strength and elongation of the three alloys as cast and heat treated alloys have been greatly improved,the main reason is the decrease of the average grain size.（4）Adding 0-0.15wt.%B to AM50,AM60 and AZ91 magnesium alloys,the original Mg₁₇Al₁₂ in the alloy cast microstructure almost completely dissolved into the matrix after 420°C×24h solution treatment.Al₄Mn is high temperature phase in AM50 and AM60,so it does not dissolve into the matrix.Among them,AM60+0.15wt.%B found a few B chemicals attached to Al₄Mn around,further proving that B compounds exist as high temperature phase.（5）Adding 0-0.15wt.%B to AZ91 magnesium alloy,after 420°C×24h solution treatment,after aging treatment at 200°C,Mg₁₇Al₁₂ is precipitated in the structure of peak aging magnesium alloy,which is mainly divided into two kinds of discontinuous precipitation phase（DP）and continuous precipitate phase（CP）.The discontinuous precipitates precipitate at the grain boundary first and precipitate in the grain after continuous precipitation.The yield strength and tensile strength of the mechanical properties of AZ91 after T6 have been obviously improved,and the elongation has a significant decrease compared with the T4.On the one hand,the precipitation of Mg₁₇Al₁₂leads to the strength increase,On the other hand,the precipitation of Mg₁₇Al₁₂2 as brittle phase causes the crack growth to become faster.With the addition of B,there is no obvious effect on the precipitation of AZ91.