Effects Of Erbium On The Microstructures And Mechanical Properties Of As-Extruded ZK21Wrought Magnesium Alloys

As the lighting metallic structural material, magnesium alloys have beenextensively applied in many filed, especially the wrought ones. However, due to theinadequate mechanical properties and poor thermal stability, it is a critical issue todevelop new Mg alloys of high performance for industry applications. Previousinvestigation has been revealed that Mg alloy with rare earth (RE) shown outstandingmechanical properties both at room and elevated temperatures, they are of greatpotential to cater to higher demand of the future industry．In this work, ZK21(Mg-1.5%Zn-0.6%Zr，wt.%) wrought magnesium alloy wasmodified with the addition of Er. Optical microscope (OM), scanning electronmicroscopy (SEM), transmission electron microscope (TEM) and energy dispersivespectroscopy (EDS) were used to study the microstructure of alloys after deformationprocessing and heat-treatment. Existing form of Er in the alloys and its effect onmicrostructure and thermal stability were studied under different conditions．Influenceof different heat treatment processes on microstructure and mechanical properties ofas-extruded alloy was investigated．The results indicate that there were two forms of Mg-Zn phase existed in ZK21alloy．Addition of Er changed the type and quantity of second phases of the alloy, twotypes of Mg-Zn-Er phases, irregular quadrilateral and rod-like respectively, appeared,and the amount of Mg-Zn phase was decreased. Moreover，a stripe-shaped constituentpresented in the Er-bearing alloy.A clear yield phenomena with apparent upper and lower yield points can beobserved in the extruded alloys with rare earth addition. The alloy was tensiledeformed to the lower yield point, unloaded and then immediately reloaded till reaching15%strain and there was still a yield phenomenon. But the second one was not asapparent as the first one:(a) the gap between the upper and lower yield point wasnarrowed;(b) yield platform was less wide compared with the first one.Addition of Er promoted the dynamic recrystallization during hot extrusion at350℃, but the critical temperature for static crystallization during the subsequent heattreatment was higher. Volume fraction of recrystallized grains was increased withincreasing Er addition in extruded alloy, whereas, critical temperature for staticrecrystallization was gradually improved with the increase of Er content.The growth activation energy of static recrystallized grains was increasesed with Er addition in the heat treatment after thermal deformation. With the same heattreatment condition, the higher Er content is, the more difficult of the staticrecrystallized grains to grow up. Grain size of the heat-treated alloy was lager butmicrostructure changed little compare to extrusion one. It was further noticed that thegrain size of the Er-bearing alloys basically decreased with increasing Er addition.Addition of Er suppressed recrystallization grain growth and improved the thermalstability of the alloy.Addition of Er led to a more uniform static recrystallization. The Er-free alloy andalloy with lower Er content manifested an obvious grain growth and uneven graindistribution; further increase of Er content to2and4wt.%led to equiaxed grains whichwere fine and uniformly distributed without apparent growth.Compared with the alloys of different Er content, hardness values of the alloy with2%Er after hot extrusion and subsequent heat treatment under different temperatureschanged much slightly, indicating alloy with2%Er performed the best thermal stabilityunder the experimental conditions.