Study On Microstructure And Properties Of Mg-Er-Zn Alloys With LPSO Phase

Magnesium alloys,as the lightest structural metal material used in engineering application,not only posess the characteristics of low density,but also has a series of advantages such as high specific strength,high specific stiffness,good damping effect,good electromagnetic shielding effect,etc.So it has been widely used in the aerospace industry,automobile industry,electronics industry.However,the absolute strength of magnesium alloy is low,which limits its further development in the field of industry.The scientists have been looking for suitable strengthening phase to enhance the mechanical properties of magnesium alloys.In recent years,it has been found that the long period ordered structure?LPSO?phase,which has a good coherent interface with the magnesium matrix,can act as effective strengthening phase in magnesium alloy.In this paper,the as-cast Mg-Er-Zn alloys with LPSO phases were prepared by conventional casting method,and explored the effect of heat treatment and adding alloying elements of Al on the phase distribution,volume fraction,and morphology of LPSO phases and the mechanical properties of alloys.In this paper,the microstructure,chemical composition,phase composition,phase distribution,atomic stacking modes were observated by means of metallographic microscope,scanning electron microscopy,energy spectrum analyzer,X ray diffraction,transmission electron microscopy.And then explore the reasons for the change of mechanical properties.The main results of this paper are as follows:The Mg-Er-Zn alloy with high volume fraction of LPSO phase was prepared successfuly by controlling the ratio of Er/Zn atoms.The results show that when Er/Zn=1,no LPSO phases formed;when Er/Zn=1.5,the LPSO phases and herringbone Mg8Er1Zn1 eutectic phases formed,they interact with each other and distribute at the grain boundaries;when Er/Zn=2,only a small amount of LPSO phases formed,and in the same atoms ratio,with the increase of Er and Zn the number of LPSO phases increased in alloys;when Er/Zn=3,the volume fraction of LPSO phases reach maximum,when Er/Zn=4,the volume fraction of LPSO phases slightly decreased,and the LPSO phase begins to become coarse.Because the Mg96Er2Zn2 alloy contain high volume fraction of hard brittle Mg3Er2Zn3 eutectic phases,the hardness of the alloy is the highest,up to 85.3HB;the tensile results show that Mg₉₆Er₃Zn₁ alloy has good comprehensive mechanical properties,which the tensile strength,yield strength and elongation reach 194 MPa,154MPa and 9.7%,respectively.The improvement of the overall mechanical properties of the alloy is mainly the effect of grain refinement and the strengthening of the second phase of the LPSO phase.Mg₉₆Er₃Zn₁ alloy after 500?-12 h solid solution treatment,many parallel distribution of fine acicular phase appears in the matrix,through the TEM bright field area picture observation and selected area electron diffraction spots that fine acicular second phase is 14H-LPSO,18R-LPSO bulk phase volume fraction decreased gradually.The 14H-LPSO structure is mainly generated by precipitation from ?-Mg matrix directly.Because the alloy has many stacking fault,the 14 H structure nucleate and growth in the stacking fault continuously.Solid solution treatment resulted in the decrease of the volume fraction of eutectic phase and the decrease of hardness.But in the new generation of large volume fraction of fine acicular 14H-LPSO phase formend in Mg₉₆Er₃Zn₁ alloy,the tensile strength,yield strength and elongation increased to 207 MPa,189 MPa and 11.2% respectively,increased by 6.7%,22.7% and 1.5%,the comprehensive mechanical properties have been improved obviously.The Mg₉₆Er₃Zn₁ alloy exhibited double-peak ageing behavior.It reached maximun hardeness values being with 83.4 HB at 8h,compared to the hardness value of the alloy without aging increased significantly,about 11.9%;then it showed the trend of decline,at 24 h hardness decreased to minimal 77.3 HB;but as the time going on,the alloy miraculously reached another maximun peak values being with 82.2 HB at 32 h once again.The microstructures determine the performance.The microstructures of Mg₉₆Er₃Zn₁ alloy under different aging time are very different.in the 8h Mg3Er2Zn3 phase volume fraction reached the maximum at 24 h Mg3Er2Zn3 phase basically disappeared,generating Everfount dissolved Er,Zn solute atoms supply 14H-LPSO structure phase,promote the increase of 14H-LPSO structure phase volume fraction,volume fraction of 14H-LPSO reached the highest in the 32 h alloy.Influenced by the microstructure of the alloy,the aging behavior of Mg₉₆Er₃Zn₁ alloy appeared in Shuangfeng.At 8h,the first peak hardness of the alloy was obtained,which was mainly affected by the precipitation phase strengthening.The Mg3Er2Zn3 phase disappeared in the alloy at 4h,leading to the minimum hardness of the alloy.With the increase of aging time,the volume fraction of 14H-LPSO phase in the alloy reached the maximum at 32 h,and the effect of the strengthening of the 14H-LPSO structure was obtained.At the same time,the second aging peak hardness was obtained.Adding Al in Mg₉₆Er₃Zn₁ alloy,the microstructure changed significantly when adding 1 at.%Al,the alloy morphology transition from cellular crystal to petal shaped dendritic morphology obviously,the length of a dendrite is about more than 60 m,two times the dendrite grain size is very fine,and lamellar LPSO at grain boundaries where the distribution of high volume fraction of phase;adding 3 at.%Al,the alloy dendrite size becomes coarse grain boundary decrease in lamellar LPSO grain boundary distribution of phase volume fraction is also reduced,in addition,a small amount of dendritic al rich phase occurred in the alloy along the crystal axis is divergent;with the increase of Al content.LPSO in the alloy phase volume fraction continue to reduce dendritic aluminum rich phase volume fraction increases gradually,when the Al content reached 7 at.%,LPSO in the alloy phase disappears,the second phase is only the high volume fraction of dendritic Aluminum rich phase,which shows a network structure in the grain boundaries.In Mg95Er3Zn1Al1 alloy due to higher volume fraction of the LPSO phase,so that the hardness value of the alloy increases from 76.3 HB to 80.2 HB,Mg93Er3Zn1Al3 alloy due to the reduction of LPSO volume fraction,the hardness decreased,but Mg89Er3Zn1Al7 alloy produced in the high volume fraction of Al rich phase,the macro Brinell hardness value reached the highest 84.8 HB was added,Al increased by 12%.