Investigation On Microstructure And Corrosion Behavior Of Mg-Zn-Y Alloy

Lightweight magnesium alloys as the most promising and environmental-friendly engineering material,with the advantages of light weight,high strength,good shock absorption,easy recovery and so on,have a wide application in the automobile industry,aerospace,electronic communications and energy storage materials and other fields.Mg-Zn-Y alloys have become the focus as one of the most promising high strength magnesium ones owing to the advantages of high temperature strength,high temperature resistance and creep resistance.However,owing to its own shortcomings,poor corrosion resistance has greatly restricted their wide applications in related fields.Improving their corrosion resistance has become the core for their wider applications urgently.In this paper as the analysis and test methods,optical microscope?OM?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,energy spectrum?EDS?,X ray diffraction?XRD?,immersion test,hydrogen evolution test and electrochemical test were used,the quasicrystal reinforced Mg-4Zn-1Y alloy as the research objects,to study systematically the microstructure and influence of corrosion time,NaCl concentration,pH on the corrosion behavior of as-cast alloy,and to explore the corrosion rule of as-cast Mg-4Zn-1Y alloy.Based on the extrusion deformation of the as-cast Mg-4Zn-1Y alloy,the variety and influence mechanism of the microstructure and corrosion behavior of the alloy were investigated during extrusion.The extruded Mg-4Zn-1Y alloy treated by solid solution T4 and aged T5respectively,the variety and influence mechanism of microstructure and corrosion behavior of the alloy during heat treatment were studied.The influence and mechanism of Y on the microstructure and corrosion behavior of as-cast alloy were investigated by adjusting the contents of Y elements in the Mg-4Zn-x Y alloys,the composition rations of the best alloy corrosion resistance were determined.The purpose is to better the corrosion resistance of Mg-Zn-Y alloys.The results of the experiment will lay theoretical and practical basis for the practical applications of the new magnesium alloy.Main results are as follows.The microstructure of the as-cast Mg-4Zn-1Y alloy was relatively coarse whose average grain size was about 150?m.A great deal of Mg₃Zn₆Y phases in spherical or short rod,a few amount of Mg₃Zn₃Y₂ phases in strip or net were randomly distributed in the as-cast structure.The corrosion behavior of the as-cast Mg-4Zn-1Y alloy in NaCl solution was mainly affected by the incomplete surface coverage,galvanic corrosion of the cathode second phase and selective corrosion of Cl^-.The corrosion process of the alloy was pitting process which were caused by galvanic corrosion in the corrosion sensitive area,and the pitting formation included two stages:pitting occurrence and pitting development.The corrosion process went through three stages:the formation of pitting corrosion,the growth of pitting and the formation of corrosion pits and the growth of corrosion pits.The prolongation of corrosion time increased the density of the corrosion product film.The increase of NaCl concentration strengthened the breakdown,lattice substitution and corrosion acceleration of Cl^-.The increase of medium p H reduced the thermodynamic driving force of corrosion and increased the stability of corrosion product film.The as-cast Mg-4Zn-1Y alloy showed better corrosion resistance in a longer time,lower NaCl concentration and higher pH corrosion medium.The as-cast Mg-4Zn-1Y alloy dynamically recrystallized after hot extrusion,and the microstructure of the alloy was obviously refined.The grain boundaries increased,which enhanced the intergranular corrosion of the alloy.Extrusion made that the organization of the alloy produced anisotropy.The second phase of the cross section which uniformly distributed and dispersed in the alloy increased the uniformity and randomicity of galvanic corrosion,and the second phase of the longitudinal section distributed along the extrusion direction.The anisotropy rendered the corrosion morphology of the cross section to be uniform corrosion morphology of galvanic corrosion and intergranular corrosion co-existing,and the corrosion morphology of longitudinal section to be the corrosion strip along the extrusion direction.The longitudinal section showed a typical basal texture after the extrusion,and required much more energy to destroy the base surface atomic binding energy and dissolve Mg.Therefore,the corrosion resistance of the longitudinal section was superior to that of the cross section.The second phase which dispersed in the crystal and the grain inside the alloy was dissolved back into the matrix after the solid solution treatment of the extruded Mg-4Zn-1Y alloy.The decrease of cathode phase reduced the driving force of corrosion,which was beneficial to improve the corrosion resistance of the alloy.The higher the solid solution temperature or the longer the solid solution time,the better the effect of redissolution,the more uniform distribution of matrix elements,the more obvious static recovery effect.Hence the corrosion resistance of the alloy increased with increase of solution temperature or time.A nanometer MgZn₂ phase was precipitated in the alloys after the aging treatment of the extruded Mg-4Zn-1Y alloy.The second phases in aged alloy had dual roles:A few amount of granular second phases as cathode accelerated corrosion at lower aging temperature or shorter time.Semi continuous distribution second phases as the barrier inhibited corrosion at higher temperature or longer time.Meanwhile with increase of heat treatment temperature and time,the static recovery on the positive role to improve the corrosion resistant of the alloy was also much more obvious,and the corrosion resistance of the aged alloy decreased first and then increased with increase of aging temperature and time.The corrosion mechanisms of Mg-4Zn-1Y alloy of different states were the same.The corrosion process was caused by galvanic corrosion between the second phase and the magnesium matrix because of the potential differences.The initial morphology of corrosion was pitting corrosion,and the corrosion morphologies of the as-cast alloy and aged alloy were the pitting and local corrosion co-existing,and the corrosion morphologies of the extruded alloy and solution treated alloy were uniform corrosion.The corrosion process of extruded and subsequent heat treated alloys were with different degrees of intergranular corrosion,surface erosion and abscission.The main chemical compositions of the corrosion products were Mg?OH?₂.The change of corrosion rate had three stages:rapid corrosion,slow corrosion and stable corrosion.The corrosion resistance of Mg-4Zn-1Y alloy decreased progressively as following:solution treated>extruded>aged>as-cast.The corrosion resistance of solution treated alloy was the best,and the best the solution process was 400?×12h.The as-cast Mg-4Zn-x Y alloys were mainly composed of?-Mg matrix and Mg-Zn-Y phase.The microstructure of the alloys was gradually refined with the increase of Y.The Mg₃Zn₆Y phase gradually reduced to disappear completely,and Mg₃Zn₃Y₂ phase increased gradually until it became the only second phase in the alloy.And the second phases in the alloy were gradually transformed from mix morphology of spherical,short rod,strip and net into a single continuous net morphology.With increase of Y,so did the effect of the inhibiting corrosion of Mg₃Zn₃Y₂ phase in net increase,and local corrosion tendency reduced after grain refinement,and the density of corrosion product film increased,therefore the corrosion resistance of the alloys was strengthened.The corrosion resistance of as-cast Mg-4Zn-x Y alloys could be improved by adding rare earth element Y,to the best with Y content of 3%.