Effect Of Zn Content On Dynamic Recrystallization Of Mg-Zn-Gd Alloys Under Equal Channel Angular Pressing

The Mg-Zn-Gd alloy with different Zn content was extruded and deformed by equal channel angular pressing.The purposes is to improve the properties of magnesium alloy materials and promote the application of magnesium alloy in various fields.The microstructure characteristics of different Zn content of Mg-Zn-Gd alloys,such as microstructure morphology,twinning microstructure and distribution,were analyzed by optical microscopy(OM)and scanning electron microscopy(SEM).Texture and slip system activation of Mg-Zn-Gd alloys with different Zn content were analyzed by Channel5,OIM software and electron backscatter diffraction(EBSD)upon ECAP.Characteristics of the second phase distribution and particles-stimulated nucleation(PSN)of Mg-Zn-Gd alloys were analyzed by transmission electron microscopy(TEM),SEM and EBSD.All the research results are as follows:1.The coarse original grains were gradually broken and refined by increasing extrusion passes of the Mg-Zn-Gd alloy with different Zn contents.The number of twins increases and then gradually evolves into chain twins,finally the chain twins are replaced by continuous dynamic recrystallization.The microstructure is composed of smaller size recrystallized grains,which the main feature is that the grains are equiaxed grains and the microstructure become more uniform after extrusion.2.As the Zn content increasing and shear force acting on Mg-Zn-Gd alloys,the dislocation slip occurs in the coarse grains and a large number of dislocations accumulate at grain boundaries.Then,all the recrystallized regions were consumed to generate the equiaxed grains at the 16 th passes.At the same time,more Zn content will inhibit the production of recrystallized grains.3.Mg-1Zn-1Gd(wt.%)alloy exhibits nanoscale dynamic precipitates after different extrusion passes.Most of the precipitated phases are spherical or disk-shaped but there are small number of irregular nanoparticles.These precipitated phases are pinned to recrystallized grain boundaries,which hinder grain boundary migration.These precipitated phases can effectively prevent grain growth when grains nucleation and recrystallization.4.Stress field gradient was found around the second phase with a size of2?m-3?m,which can promote nucleation of recrystallized grains and transform grain boundaries at large angles.The results show that there is high efficient dynamic recrystallization in the microstructure and gradually develop from small angle grain boundary to subgrain structure.The second phase particles promote the nucleation ofrecrystallized grains and the grains provide nucleation sites,which gradually develop into continuous dynamic recrystallization.5.With the increasing Zn content,the texture strength of Mg-Zn-Gd alloy,such as recrystallized grain,deformed grain and sub-grain,strengthen obviously.Excess second phases inhibit the nucleation of dynamic recrystallized grains and strengthen the grain orientation,which weakens the degree of recrystallization.6.Schmitt factor(SF)of basal plane slip increases with the increasing extrusion passes.A large number of dynamic recrystallization change the original grain orientation to form a place gradually,which conducive to grain recrystallization nucleation and growth.Furthermore,the formation of a larger size second phase will destroy the compatibility of different grain boundaries,leading to the obstruction of basal plane slip.