Study On Microstructure Evolution And Corrosion Behavior Of Mg-4Y-2Zn Alloy

In the wake of enhancement of national economic strength and development of science and technology,the application of various alloys in engineering experienced day by day from initial birth to mature.In recent years,with higher requirements for metal parts in lightweight,environmental protection and performance,magnesium alloy has broad development prospects in the field of engineering projects with higher requirements for structural lightweight based on its advantages of low density,high specific strength and high specific stiffness.Surface mechanical attrition treatment(SMAT)is a kind of severe plastic deformation technology on the alloy surface,which can form gradient nanostructures closely combined with the matrix on the alloy surface.It can improve the strength of the alloy and maintain good plasticity at the same time,which plays an important role in improving the surface properties of the alloy.So far,most of the researches on SMAT of magnesium alloy are focused on mechanical properties,but the researches on the corrosion properties are limited.Mg-Y-Zn alloy with long period stacking order(LPSO)structure will get better mechanical properties after SMAT treatment.However,the change of LPSO phase during deformation and the change of corrosion resistance after SMAT treatment need to be further studied.In this paper,the research object selected Mg-4Y-2Zn alloy,Mg-4Y-2Zn alloy was pretreated by solution treatment to optimization microstructure,Then SMAT technology was applied to as-cast alloy and solution alloy.The microstructure evolution of Mg-4Y-2Zn alloy before and after solution treatment and SMAT treatment was studied by OM,SEM,EDS and TEM.The effects of solid solution treatment and SMAT treatment on the corrosion behavior of the alloy were investigated using hydrogen precipitation immersion experiments and electrochemical tests,and the factors influencing the changes in the corrosion behavior of the alloy were discussed.The main results are as follows:The as-cast Mg-4Y-2Zn alloy contains W-Mg₃Y₂Zn₃ phase and lamellar14H-LPSO phase.After solution treatment,the grain size of the alloy increases with the increase of solution temperature;The number of W phase gradually decreased from continuous bony to semi-continuous short stick and spherical.When the solution temperature is higher than 400?,14H-LPSO phase disappears and transforms into long needle like stacking faults(SFs).The order of corrosion resistance of the three alloys is 500?solution alloy>as-cast alloy>400?solution alloy.The increase in corrosion resistance of solution alloy at 500?is due to the reduction of grain boundary and second phase,which leads to the decrease of corrosion active sites of alloy.However,the decrease of corrosion resistance of the alloy at 400?is due to the decrease of grain boundaries,which is far less beneficial than the harmful effect of a large number of spindle-shaped nano-YZn-rich phases.After SMAT treatment,gradient nanostructures are formed on the surface of the three alloys,and the grain size of the alloy surface is about 30-100nm.The deformation mechanism of as-cast alloy and 500?solution alloy is similar:in the initial stage of deformation,twins play a leading role;in the middle stage of high stress level,sub-grains are formed by the delivery between twins and the accumulation of dislocations at twin boundary and phase boundary;in the later stage of deformation,sub-grains with small angle grain boundary are refined into nanocrystals with large angle grain boundary by dynamic recrystallization.The deformation mechanism of 400?solution alloy is different from these above:the precipitation of a large amount of YZn-rich phase in 400?solution alloy leads to the increase of stacking fault energy of magnesium matrix,and a large amount of YZn-rich phase boundary can be used as the dislocation source to coordinate the deformation process.Therefore,the deformation of 400?solution alloy is mainly dominated by dislocation movement.Dislocations pile up at the grain boundary and YZn-rich phase boundary,resulting in a large number of dislocation configurations such as dislocation entanglement and dislocation wall.Under the action of stress,the dislocation configuration changes to subgrain,and finally to nanocrystalline through dynamic recrystallization.After SMAT treatment,the corrosion resistance of the three alloys decreases remarkably,which is mainly due to the increase in the density of defects such as grain boundaries and dislocations in the surface layer of the alloy,the decrease of second phase size,the rise of surface roughness and the surface Fe contamination.The improvement of corrosion resistance of SMAT treated alloy after grinding50?m is mainly due to the competition between defect density and residual compressive stress.