Strengthening Of High Toughness Value Mg-Al-Zn Alloy And The Research Of Its Microstructure And Mechanical Properties

Mg-Al-Zn based alloys are one of the most widely used magnesium alloy series currently,but at present what blocks it from further utilization is as follows:â‘ Casting magnesium alloys(for example AZ91D)suffers from low creep resistance at temperatures above 120℃,which makes it unsuitable for many of components that requiring excellent creep resistance at high temperature;â‘¡Wrought magnesium alloys(for example AZ31)have poor plastic deformation property at room temperature,so they can not meet different needs in different situations.In order to solve the existing problems of Mg-Al-Zn based magnesium alloys,lots of research about alloying,grain refinement,melt treatment and heattreatment and so on,had been carried out to improve the properties.Because the properties of alloys are determined by their microstructures,it is of great importance to carry out investigation on the microstructure of Mg-Al-Zn based magnesium alloys,especially mechanism of forming and transforming of alloy phases in Mg-Al-Zn based magnesium alloys. The morphology,category,amount and distribution of alloy phases in the Mg-Al-Zn magnesium alloys with different amount of Al and Zn were investigated.Choosing AZ54 alloy with better property as the based alloy and the effects of the addition of spherical Mg-Zn-Y-Mn quasicrystal on the microstructure and mechanical properties of Mg-5%Al-4%Zn(AZ54)were investigated by metallurgical phase analysis,scanning electron microscopy (SEM)analysis,XRD analysis and energy diffraction(EDS)analysis in this paper.The research results that can be used to guide the alloy phases controlling and the optimization of composition and processing parameters that expected to obtain.The research results indicate that:The microstructure of the casting Mg-Al-Zn magnesium alloys consists of the matrix(α-phase)and the second phases whose morphology are bone-like, lump-like,particle-like and layer-like,they mainly distribute at grain boundaries and only a few are inside the grains;The change of Al and Zn content has very obvious effect on the amount and distribution of alloy phases,With the increase of Al or Zn content,the amount of primaryα-phase decrease,the amount of compounds increases and their distribution become more continuous;In Mg-5%Al-X%Zn(X≥3)alloy,the amount ofÏ„(Mg₃₂(Al,Zn)₄₉)phases increase with the increase of Zn addition,while theφ(Al₂Mg₅Zn₂)phases decrease.The mechanical properties of Mg-5%Al-4%Zn cast alloy at room temperature is:The impact toughness is 14.3J/cm²,impact tensile strength is 155.7MPa and brinell hardness is 65.53.By orthogonal design,we acquire a relationship between the addition of Mn,Y and the size&amount of quasicrystals.According with the microstructure, the result indicate:In the circumstances that Mn is confined,the amount of quasicrystals increase with the increase of amount of Y,to a certain extent,less Mn can promote the quasi-crystal formation,though the effect of Mn on the number of quasicrystal is little.On the contrary the Mn High it will inhibit the formation of quasicrystals.Mn,Y and the interaction of the two make a great impact on the size of quasicrystals.If the addition of Mn is relatively low,Mn plays a core role in the process of spherical Quasicrystals' growth,as a result, the size of Quasicrystals is large.On the contrary,relatively high amount of Mn can cause an instable interface and lead the quasicrystals to grow like a petal, and its size become larger.When the addition of Y is 3.9%～4.0%and Mn 4.0%～4.3%,we can acquire better quasicrystals.After Mg-Zn-Y-Mn master alloy being added into Mg-5%Al-4%Zn alloy, the microstructure of AZ54 cast alloy are consisted of matrixα-Mg phases,Ï„(Mg₃₂(Al,Zn)₄₉)phases,φ(Al₂Mg₅Zn₂)phases and i(Mg₄₅Zn₄₇Y₃Mn₅)phase. The matrix microstructure of AZ54 alloy can be refined obviously by the addition of quasicrystal master alloy,and the morphology ofÏ„(Mg₃₂(Al,Zn)₄₉) phases andφ(Al₂Mg₅Zn₂)phases on the grain boundaries changes from continuous net shape to discontinuous shape,even to particles.When 2.5% MZYM master alloy being added,the impact toughness enhanced by 28.2%, reach to 18.3J/cm²;after 3.5%MZYM being added,the tensile strength at room temperature enhanced by 16.4%,reach to 181.2MPa;when the addition of MZYM reached to 5.5%,the ultimate strength at temperature of 200℃of AZ54 alloy respectively increase by 18.1%,reach to 166.5MPa.The results of heattreatment on AZ54 alloy with addition of Mg-Zn-Y-Mn master alloy show that:after solution treatment,part of theÏ„(Mg₃₂(Al,Zn)₄₉) phases andφ(Al₂Mg₅Zn₂)phases dissolve into the matrix,and the morphology of quasicrystal phase didn't obviously change.After aging treatment,theÏ„(Mg₃₂(Al,Zn)₄₉)phases were precipitated again,but the amount,size and morphology changed.The addition of quasicrystal master alloy to AZ54 alloy prolongs precipitation time of theÏ„(Mg₃₂(Al,Zn)₄₉)phases,and suppressed the discontinuous precipition.In aging temper,the mechanical properties of alloys promote obviously at room temperature and elevated temperature are significantly improved.Compared with AZ54 alloy,the ultimate strength at room temperature and elevated temperature of 200℃of AZ54+3.5%MZYM alloy,increase by 30.96%and 28.36%,reach to 237.3MPa and 199.6MPa respectively.Macrohardness at room temperature increase by 44.4%,reach to 97.5.But impact toughness decreases.