Effect Of Rare Earth Elements(Dy,Ho,Yb,Sm) On Microstructure Refinement Of ZMT613 Alloy

The addition of rare earth element(RE)to magnesium alloy has obvious effect on dispersion strengthening,solution strengthening and aging strengthening.At the same time,it can effectively refine the grain size and improve the mechanical properties of magnesium alloy at room or high temperature.It is of great significance to research and develop new type magnesium alloy and expand its industrial application.The aim of this paper is to study the effect of RE on microstructure refinement of wrought magnesium alloy with high zinc content.0.5wt.%RE(RE=Dy,Ho,Yb,Sm)was added into the matrix of Mg-6Zn-1Mn-3Sn(ZMT613)magnesium alloy by alloying method.By means of optical microscope(OM),scanning electron microscope(SEM),x-ray diffraction(XRD)and differential thermal analysis(DSC),the effects of four rare earth elements on microstructure refinement of ZMT613-0.5Dy/Ho/Yb/Sm magnesium alloys as-cast,extruded and solid-solution were investigated.The results are as follows:The effect of Dy,Ho,Yb,Sm on the grain size of ZMT613 alloy in as-cast,extruded and solid-solution state is different.For as-cast alloy,the secondary dendrite arm spacing of as-cast alloy decreases with the addition of rare earth elements.The order of grain refinement effect is Ho>Dy>Yb>Sm.After hot extrusion,dynamic recrystallization took place in all the microstructures,but the dynamic recrystallization degree was higher and the grain refinement was obvious in the recrystallized region.The average grain size of extruded ZMT613-0.5Dy/Ho/Yb/Sm alloy increases and decreases as compared with that of ZMT613 alloy.Combined with the mechanism of particle stimulation nucleation and the synergistic effect of the second phase,Yb and Ho can refine the microstructure of extruded alloy,Dy and Sm can make the microstructure coarsened slightly.For solid-solution alloy,the effect of four rare earth elements on the grain size of the alloy is not significant,but compared with the ZMT613 matrix alloy,the grain size of alloy is generally fine.Dy,Ho,Yb and Sm change the type,content and morphology of the second phase in ZMT613 alloy.The as-cast ZMT613-0.5Dy alloy has rod-like Mg₂Dy phase with increased second phase continuity.The second phase of ZMT613-0.5Ho alloy is more dispersed and fine,with granular Mg₂₄Ho₅phase.The second phase of ZMT613-0.5Yb alloy becomes continuous and coarse,and there is a massive Mg Sn Yb phase.The second phase of ZMT613-0.5Sm alloy is semi-continuous and large in size with skeletal Mg₄₁Sm₅ phase.These rare earth phases have high melting point,good thermal stability,can strengthen the matrix,improve the heat resistance and creep resistance of the alloy.The second phase in ZMT613-0.5Dy/Ho/Yb/Sm alloy was significantly fragmented during extrusion deformation.The second phase is mainly distributed in the dynamic recrystallization region of the alloy.Increasing the solution temperature and prolonging the solution time have different effects on the solution effect.The amount of the second phase precipitates did not decrease obviously.When the solid solution process was set at470°C+2 h,the five alloys all had obvious"over-burning"phenomenon.Dy,Ho,Yb and Sm can affect the mechanical properties of as-cast,extruded and solid-solution ZMT613 alloys at room temperature.These four rare earth elements can improve the microhardness of as-cast and solid-solution alloys.The microhardness of solid-solution ZMT613-0.5Dy/Ho/Yb/Sm alloy decreases with the increase of solution temperature and solution time.But the strength and plasticity of extruded alloys are affected by these four rare earth elements differently.Based on the analysis of fine grain strengthening and solid solution strengthening.The results show that 0.5wt.%Ho and Yb can improve the strength and plasticity of the extruded alloy.And add 0.5wt.%Dy and Sm do not refine the grains,so the strength and plasticity of the extruded alloy do not increase.