Effect Of Micro-alloying On Microstructure And Corrosion Behavior Of Sensitized Al-Mg-Mn Alloy

Al-Mg alloys have the advantages of high specific strength,weldability and easy molding,which is widely used in the field of rail transportation and ship manufacturing.However,when the alloy serves in a corrosive environment at a certain temperature,it is easy to precipitateβphase at the grain boundary,resulting in problems such as intergranular corrosion and stress corrosion,which reduces the service life of the alloy.Micro-alloying is one of the important means to improve the corrosion resistance of alloy.In this thesis,Al-Mg-Mn-Y-Zr and Al-Mg-Mn-Sc-Zr alloys were prepared by adding trace elements such as Y and Sc on the basis of Al-Mg-Mn alloy.The effects of micro-alloying on the microstructure and corrosion behavior of sensitized Al-Mg-Mn alloy were studied by tensile mechanical properties test,electrochemical corrosion,intergranular corrosion and stress corrosion test combined with OM,EBSD,SEM and TEM.The research results indicate that:（1）The Al-Mg-Mn alloy is prone to segregation during the casting process,and the grain size is large,with the average grain size of the as-cast alloy is 180μm.Y and Zr micro-alloying can refine the microstructure of alloy ingots and effectively reduce the composition segregation of alloy ingot.The average grain size of Al-Mg-Mn-Y-Zr ingot is125μm.The effect of Sc and Zr micro-alloying on refining ingot grain and reducing composition segregation is more obvious.The average grain size of Al-Mg-Mn-Sc-Zr alloy is refined to 25μm.After rolling deformation,the grains of the three alloys are elongated along the rolling direction without recrystallization,showing typical deformation structure.（2）After 175°C/1000 h sensitization heat treatment,the degree of recovery and recrystallization of Al-Mg-Mn alloy increases to 4.23%,and the proportion of small angle grain boundary is 0.75.The Al₃（Y,Zr）particles precipitated in the homogenization process of Al-Mg-Mn-Y-Zr alloy can inhibit the recrystallization of the alloy and increase the proportion of low angle grain boundaries.The degree of recrystallization is 2.85%and the proportion of low angle grain boundaries is 0.79.The Al₃（Sc,Zr）particles precipitated by homogenization of Al-Mg-Mn-Sc-Zr alloy have stronger thermal stability,stronger ability to inhibit recrystallization,and can retain the proportion of small angle grain boundaries to a greater extent.The degree of recrystallization is reduced to 2.36%,and the proportion of small angle grain boundaries is 0.81.（3）The more coarse Al₆（Fe,Mn）second phase was produced during the casting of Al-Mg-Mn alloy.After sensitization heat treatment,the degree of recrystallization of the alloy becomes larger,and the proportion of high-angle grain boundaries increases,while Mg atoms are easy to aggregate near high-angle grain boundaries and Al₆（Fe,Mn）particles,resulting in continuous distribution and coarsening ofβphase on grain boundaries.The Al₃（Y,Zr）particles precipitated from Al-Mg-Mn-Y-Zr alloy can effectively inhibit the formation of Al₆（Fe,Mn）phase,increase the proportion of low-angle grain boundaries of the alloy,and attract Mg atoms to accumulate near it,inhibiting the nucleation and growth ofβphase at grain boundaries.The ability of Al₃（Sc,Zr）particles precipitated in Al-Mg-Mn-Sc-Zr alloy to increase dislocation density is better,and the ability to inhibit the continuous distribution and coarsening ofβphase at grain boundary is stronger.（4）After sensitization heat treatment,theβphase is continuously distributed and coarsened on the grain boundary of Al-Mg-Mn alloy,and the strength and corrosion resistance decrease obviously.The Al₃（Y,Zr）particles precipitated from the Al-Mg-Mn-Y-Zr alloy can hinder the continuous distribution,coarsening of the grain boundaryβphase,improve the electrochemical corrosion resistance,intergranular corrosion resistance of the alloy and play a role in inhibiting the generation,propagation of stress corrosion cracks at low temperatures.Due to the low thermal stability of Al₃（Y,Zr）particles,the effect of improving the performance of the alloy at higher sensitization temperatures is weakened.The Al₃（Sc,Zr）particles precipitated from Al-Mg-Mn-Sc-Zr alloy have stronger thermal stability,which can better improve the precipitation and distribution ofβphase,and have a stronger effect on the mechanical properties and corrosion resistance of the alloy.